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OMI Satellite and Ground-Based Pandora Observations and Their Application to Surface NO2 Estimations at Terrestrial and Marine Sites
Abstract
The Pandora spectrometer that uses direct-Sun measurements to derive total column amounts of gases provides an approach for (1) validation of satellite instruments and (2) monitoring of total column (TC) ozone (O 3) and nitrogen dioxide (NO 2). We use for the first time Pandora and Ozone Monitoring Instrument (OMI) observations to estimate surface NO 2 over marine and terrestrial sites downwind of urban pollution and compared with in situ measurements during campaigns in contrasting regions: (1) the South African Highveld (at Welgegund, 26°34 0 10″S, 26°56 0 21″E, 1,480 m asl, ~120 km southwest of the Johannesburg-Pretoria megacity) and (2) shipboard U.S. mid-Atlantic coast during the 2014 Deposition of Atmospheric Nitrogen to Coastal Ecosystems (DANCE) cruise. In both cases, there were no local NO x sources but intermittent regional pollution influences. For TC NO 2 , OMI and Pandora difference is ~20%, with Pandora higher most times. Surface NO 2 values estimated from OMI and Pandora columns are compared to in situ NO 2 for both locations. For Welgegund, the planetary boundary layer (PBL) height, used in converting column to surface NO 2 value, has been estimated by three methods: co-located Atmospheric Infrared Sounder (AIRS) observations; a model simulation; and radiosonde data from Irene, 150 km northeast of the site. AIRS PBL heights agree within 10% of radiosonde-derived values. Absolute differences between Pandora-and OMI-estimated surface NO 2 and the in situ data are better at the terrestrial site (~0.5 ppbv and ~1 ppbv or greater, respectively) than under clean marine air conditions, with differences usually >3 ppbv. Cloud cover and PBL variability influence these estimations.
... The atmospheric NO 2 columns from the Ozone Monitoring Instrument (OMI) have widely been used to analyse spatiotemporal patterns of NO 2 pollution in China, focusing mainly on hotspot areas such as the North China Plain (NCP), YRD, and PRD Qin et al., 2020). Although some studies on urban pollution have revealed a certain correlation between the surface NO 2 concentrations and OMI NO 2 columns (Duncan et al., 2013;Anand and Monks, 2017;Kollonige et al., 2018), some uncertainties still exist in the variation of near-surface NO 2 concentrations derived from anthropogenic emissions (Wang et al., 2011). ...
... These results show that the OMI-MOZART simulations caused high uncertainties in winter. The effects of complex weather conditions on the surface albedo and the effects of PBL (Planetary Boundary Layer) height on the distribution of vertical NO 2 profiles are the main sources for the high uncertainties in the winter (Kollonige et al., 2018). The planetary boundary layer height (PBL) defines the vertical dispersion property and was known to be influential to the ground-level concentrations of air pollutant (Huang et al., 2021). ...
The long-term trends of the ground-level NO2 concentrations over coastal areas in China were analysed from 2007 to 2019 in this study, based on the measurements from Ozone Monitoring Instrument (OMI) and the simulations from the Model for Ozone and Related Chemical Tracers (MOZART)-4. Multiple Gaussian functions were used to simulate the atmospheric profiles of NO2 concentrations from the MOZART-4, and the ratio of the ground-level concentrations to the columns estimated from this Gaussian function was used to convert the OMI NO2 columns to that at the ground level. The estimation results showed that the ground-level NO2 concentrations were well consistent with those of ground measurements in 2017 (the correlation coefficients ranging from 0.84 to 0.87 in the four seasons). The NO2 concentrations presented an increasing rate of 0.6 μg/m³/yr during 2007–2011 and then a decreasing rate of 0.5 μg/m³/yr until 2019. Most areas in the China Sea suffered from the maximum NO2 concentrations during 2011–2013. The temporal variations of NO2 over coastal lands were consistent with those over the ocean during 2007–2016, but they presented different after the implementation of coastal emission control area (ECA) policies in 2016. The ECA policies significantly decreased the NO2 concentrations in the Yangtze River Delta (the YRD) and the Bohai Rim, and they inhibited the increase of NO2 in the Pearl River Delta (the PRD). These results might be helpful in understanding the current situation of ground-level NO2 pollution over oceans and coastal land in China and provide guidance for further development of ECA policies.
... Surface NO 2 has been a focus of scientific studies due to its strong correlation with air quality (AQ) and health issues (ECCC, 2016), with NO 2 as one of the three components (along with ozone and PM 2.5 ) used to 30 compute the Air Quality Health Index (AQHI: Stieb et al., 2008) in Canada's AQ public awareness programs. Efforts to link total column NO 2 with its surface concentrations have been made by many researchers (Flynn et al., 2014;Knepp et al., 2015;Kollonige et al., 2017;Lamsal et al., 2008Lamsal et al., , 2014McLinden et al., 2014). For example, Knepp et al. (2015) proposed a method to estimate NO 2 surface mixing ratios from Pandora direct-sun total column NO 2 via application of a planetary Atmos. ...
... boundary-layer (PBL) height correction factor. Kollonige et al. (2017) adapted this method and compared Pandora direct-sun surface NO 2 and OMI surface NO 2 . They concluded that the two main sources of error for the conversion of the total column NO 2 to surface NO 2 are (1) poor weather conditions (e.g., cloud cover and precipitation) and (2) PBL height estimation, both of which affect the NO 2 column-surface relationship and instrument sensitivities to boundary layer NO 2 . ...
Pandora spectrometers can retrieve nitrogen dioxide (NO2) vertical column densities (VCDs) via two viewing geometries: direct-sun and zenith-sky. The direct-sun NO2 VCD measurements have high quality (0.1DU accuracy in clear-sky conditions) and do not rely on any radiative transfer model to calculate air mass factors (AMFs); however, they are not available when the sun is obscured by clouds. To perform NO2 measurements in cloudy conditions, a simple but robust NO2 retrieval algorithm is developed for Pandora zenith-sky measurements. This algorithm derives empirical zenith-sky NO2 AMFs from coincident high-quality direct-sun NO2 observations. Moreover, the retrieved Pandora zenith-sky NO2 VCD data are converted to surface NO2 concentrations with a scaling algorithm that uses chemical-transport-model predictions and satellite measurements as inputs. NO2 VCDs and surface concentrations are retrieved from Pandora zenith-sky measurements made in Toronto, Canada, from 2015 to 2017. The retrieved Pandora zenith-sky NO2 data (VCD and surface concentration) show good agreement with both satellite and in situ measurements. The diurnal and seasonal variations of derived Pandora zenith-sky surface NO2 data also agree well with in situ measurements (diurnal difference within ±2ppbv). Overall, this work shows that the new Pandora zenith-sky NO2 products have the potential to be used in various applications such as future satellite validation in moderate cloudy scenes and air quality monitoring.
... In addition, measurements of tropospheric NO 2 from satellites or aircraft are also influenced and limited by clouds (Bovensmann et al., 1999;Liang et al., 2017). Ground-based measurements of column NO 2 from instruments such as Pandora using differential optical absorption spectroscopy (DOAS) are often used for the validation of satellite instruments (Herman et al., 2009;Lamsal et al., 2014;Kollonige et al., 2018). In situ measurements of near-surface NO 2 can best monitor local emissions. ...
The conventional two-wavelength differential absorption lidar (DIAL) has
measured air pollutants such as nitrogen dioxide (NO2). However, high
concentrations of aerosol within the planetary boundary layer (PBL) can
cause significant retrieval errors using only a two-wavelength DIAL
technique to measure NO2. We proposed a new technique to obtain more
accurate measurements of NO2 using a three-wavelength DIAL technique
based on an optical parametric oscillator (OPO) laser. This study derives
the three-wavelength DIAL retrieval equations necessary to retrieve vertical
profiles of NO2 in the troposphere. Additionally, two rules to obtain
the optimum choice of the three wavelengths applied in the retrieval are
designed to help increase the differences in the NO2 absorption cross-sections and reduce aerosol interference. NO2 retrieval relative
uncertainties caused by aerosol extinction, molecular extinction, absorption
of gases other than the gas of interest and backscattering are calculated
using two-wavelength DIAL (438 and 439.5 nm) and three-wavelength DIAL
(438, 439.5 and 441 nm) techniques. The retrieval uncertainties in
aerosol extinction using the three-wavelength DIAL technique are reduced to
less than 2 % of those when using the two-wavelength DIAL technique. Moreover, the
retrieval uncertainty analysis indicates that the three-wavelength DIAL
technique can reduce more fluctuation caused by aerosol backscattering than
the two-wavelength DIAL technique. This study presents NO2 concentration
profiles which were obtained using the HU (Hampton University)
three-wavelength OPO DIAL. As a first step to assess the accuracy of the HU
lidar NO2 profiles, we compared the NO2 profiles to simulated data
from the Weather Research and
Forecasting Chemistry (WRF-Chem) model. This comparison suggests that the NO2 profiles
retrieved with the three-wavelength DIAL technique have similar vertical
structure and magnitudes typically within ±0.1 ppb compared to modeled
profiles.
... Pandora data have been used before to validate satellite NO 2 measurements from Aura OMI (Herman et al., 2009;Tzortziou et al., 2014;Kollonige et al., 2018;Choi et al., 2019;Judd et al., 2019;Griffin et al., 2019;Herman et al., 2019;Pinardi et al., 2020) and TROPOMI Ialongo et al., 2020;Zhao et al., 2020). ...
This paper reports on consolidated ground-based validation results of the atmospheric NO2 data produced operationally since April 2018 by the TROPOspheric Monitoring Instrument (TROPOMI) on board of the ESA/EU Copernicus Sentinel-5 Precursor (S5P) satellite. Tropospheric, stratospheric, and total NO2 column data from S5P are compared to correlative measurements collected from, respectively, 19 Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), 26 Network for the Detection of Atmospheric Composition Change (NDACC) Zenith-Scattered-Light DOAS (ZSL-DOAS), and 25 Pandonia Global Network (PGN)/Pandora instruments distributed globally. The validation methodology gives special care to minimizing mismatch errors due to imperfect spatio-temporal co-location of the satellite and correlative data, e.g. by using tailored observation operators to account for differences in smoothing and in sampling of atmospheric structures and variability and photochemical modelling to reduce diurnal cycle effects. Compared to the ground-based measurements, S5P data show, on average, (i) a negative bias for the tropospheric column data, of typically −23 % to −37 % in clean to slightly polluted conditions but reaching values as high as −51 % over highly polluted areas; (ii) a slight negative median difference for the stratospheric column data, of about −0.2 Pmolec cm−2, i.e. approx. −2 % in summer to −15 % in winter; and (iii) a bias ranging from zero to −50 % for the total column data, found to depend on the amplitude of the total NO2 column, with small to slightly positive bias values for columns below 6 Pmolec cm−2 and negative values above. The dispersion between S5P and correlative measurements contains mostly random components, which remain within mission requirements for the stratospheric column data (0.5 Pmolec cm−2) but exceed those for the tropospheric column data (0.7 Pmolec cm−2). While a part of the biases and dispersion may be due to representativeness differences such as different area averaging and measurement times, it is known that errors in the S5P tropospheric columns exist due to shortcomings in the (horizontally coarse) a priori profile representation in the TM5-MP chemical transport model used in the S5P retrieval and, to a lesser extent, to the treatment of cloud effects and aerosols. Although considerable differences (up to 2 Pmolec cm−2 and more) are observed at single ground-pixel level, the near-real-time (NRTI) and offline (OFFL) versions of the S5P NO2 operational data processor provide similar NO2 column values and validation results when globally averaged, with the NRTI values being on average 0.79 % larger than the OFFL values.
... Pandora data have been used before to validate satellite NO 2 measurements from Aura OMI (Herman et al., 2009;Tzortziou et al., 2014;Kollonige et al., 2018;Choi et al., 2019;Judd et al., 2019;Griffin et al., 2019;Herman et al., 2019;Pinardi et al., 2020) and TROPOMI Ialongo et al., 2020;Zhao et al., 2020). ...
... Lin et al., 2014;van Geffen et al., 2015;Krotkov et al., 2016), by the outcome of validation studies showing that various state-of-science retrievals have biases of the order of tens of percents (e.g. Jin et al., 2016;Drosoglou et al., 2017;Kollonige et al., 2018), and the considerable structural uncertainty in retrieved tropospheric NO 2 columns emerging when different retrieval methodologies are applied to the exact same satellite observations (e.g. van Noije et al., 2006;Lorente et al., 2017). ...
Global observations of tropospheric nitrogen dioxide (NO2) columns have been shown to be feasible from space, but consistent multi-sensor records do not yet exist, nor are they covered by planned activities at the international level. Harmonised, multi-decadal records of NO2 columns and their associated uncertainties can provide crucial information on how the emissions and concentrations of nitrogen oxides evolve over time. Here we describe the development of a new, community best-practice NO2 retrieval algorithm based on a synthesis of existing approaches. Detailed comparisons of these approaches led us to implement an enhanced spectral fitting method for NO2, a 1° × 1° TM5-MP data assimilation scheme to estimate the stratospheric background and improve air mass factor calculations. Guided by the needs expressed by data users, producers, and WMO GCOS guidelines, we incorporated detailed per-pixel uncertainty information in the data product, along with easily traceable information on the relevant quality aspects of the retrieval. We applied the improved QA4ECV NO2 algorithm to the most current level-1 data sets to produce a complete 22-year data record that includes GOME (1995–2003), SCIAMACHY (2002–2012), GOME-2(A) (2007 onwards) and OMI (2004 onwards). The QA4ECV NO2 spectral fitting recommendations and TM5-MP stratospheric column and air mass factor approach are currently also applied to S5P-TROPOMI. The uncertainties in the QA4ECV tropospheric NO2 columns amount to typically 40% over polluted scenes. The first validation results of the QA4ECV OMI NO2 columns and their uncertainties over Tai'an, China, in June 2006 suggest a small bias (−2%) and better precision than suggested by uncertainty propagation. We conclude that our improved QA4ECV NO2 long-term data record is providing valuable information to quantitatively constrain emissions, deposition, and trends in nitrogen oxides on a global scale.
Coastal areas are some of the most densely populated and economically important regions in the world. As such, protecting the health of the human population and ecosystems at the coastal interface and understanding the impacts of environmental stressors such as air pollutants provides wide‐ranging benefits. Air quality (AQ) processes within coastal regions have been studied using ground and space‐based platforms, with intensive field campaigns focused on addressing key science questions that are typically partitioned into either direct atmospheric effects (e.g., anthropogenic emissions creating air pollution) or indirect processes and feedback loops (e.g., terrestrial/marine biogenic processes modifying atmospheric properties). The atmospheric planetary boundary layer (PBL) and its depth (or height) connect land, air, and the water surface via many pathways, especially with transport and exchange processes tied to the complexities of the coastal interface. We still cannot accurately characterize—through field, aircraft, or space‐based observations—the spatial and temporal PBL variability and processes within the PBL that couple together coastal dynamics and air quality. Several upcoming geostationary and polar‐orbiting satellite missions are likely to make significant progress in characterizing these air/land/water interactions over the next decade. Here, we present a framework of the current understanding of the PBL's role in coastal regions, primarily regarding air quality and atmospheric deposition, to motivate future concerted efforts from ground‐ and space‐based platforms to achieve a holistic understanding of the coastal interface.
We use a multi‐year record of Pandora‐derived NO2 total column abundance in Boston to examine the influence of atmospheric transport on column NO2 and its surface concentrations during the warm season in a coastal urban environment. We derive tropospheric NO2 estimates from the total column with a measurement‐model fusion approach using near‐real‐time estimates of stratospheric NO2 from NASA's Goddard Earth Observing System Composition Forecast model system and find the average influence of stratospheric NO2 at this urban site can be 30%–70% depending on season and time of day. Sea breeze days tend to exhibit rapid temporal variability in the column that which can go in the opposite direction of changes in surface NO2 concentrations. By comparing tropospheric NO2 with surface concentrations, we constrain the role of boundary layer entrainment processes in the evolution of surface NO2 concentrations, while highlighting the value of column measurements in identifying sea breeze frontal dynamics. We estimate an apparent equal mixing layer height of NO2 and infer that surface NOx emissions remain concentrated near the surface regardless of atmospheric stability regime. When comparing the Pandora‐ to TROPOMI‐derived column NO2 measurements, we find that sea breeze days present a unique challenge likely due to higher spatial heterogeneity in NO2 and the meteorology involved that is not well represented in retrieval inputs. Our observations provide new insights into column and surface variability of NO2 which will be relevant to interpreting geostationary observations, especially in coastal urban locations.
Abstract The Satellite Coastal and Oceanic Atmospheric Pollution Experiment (SCOAPE) cruise in the Gulf of Mexico was conducted in May 2019 by NASA and the Bureau of Ocean Energy Management to determine the feasibility of using satellite data to measure air quality in a region of concentrated oil and natural gas (ONG) operations. SCOAPE addressed both technological and scientific issues related to measuring NO2 columns over the outer continental shelf. Featured were nitrogen dioxide (NO2) instruments (Pandora, Teledyne API analyzer) at Cocodrie, LA (29.26°, −90.66°), and on the Research Vessel Point Sur operating off the Louisiana coast with measurements of ozone, carbon monoxide, and volatile organic compounds (VOCs). The findings: (a) all NO2 observations revealed two atmospheric regimes over the Gulf, the first influenced by tropical air in 10–14 May, the second influenced by flow from urban areas on 15–17 May; (b) comparisons of OMI v4 and TROPOMI v1.3 TC (total column) NO2 data with shipboard Pandora NO2 column observations averaged 13% agreement with the largest difference during 15–17 May (∼20%). At Cocodrie, the satellite–Pandora agreement was ∼5%. (c) Three new‐model Pandora instruments displayed a TC NO2 precision of 0.01 Dobson Units (∼5%); (d) regions of smaller, older natural gas operations showed high methane readings from leakage; elevated VOCs were also detected. Neither satellite nor spectrometer captured the magnitude of ambient NO2 variability near ONG platforms. Given an absence of regular air quality monitoring over the Gulf of Mexico, SCOAPE data constitute a baseline against which future observations can be compared.
Near-surface air quality (AQ) observations over coastal waters are scarce, a situation that limits our capacity to monitor pollution events at land-water interfaces. Satellite measurements of total column (TC) nitrogen dioxide (NO2) observations are a useful proxy for combustion sources but the once daily snapshots available from most sensors are insufficient for tracking the diurnal evolution and transport of pollution. Ground-based remote sensors like the Pandora Spectrometer Instrument (PSI) that have been developed to verify space-based total column NO2 and other trace gases are being tested for routine use as certified AQ monitors. The KORUS-OC (Korea-United States Ocean Color) cruise aboard the R/V Onnuri in May-June 2016 represented an opportunity to study AQ near the South Korean coast, a region affected by both local/regional and long-distance pollution sources. Using PSI data in direct-sun mode and in situ sensors for shipboard ozone, CO and NO2, we explore, for the first time, relationships between TC NO2 and surface AQ in this coastal region. Three case studies illustrate the value of the PSI as well as complexities in the surface-column NO2 relationship caused by varying meteorological conditions. Case Study 1 (25-26 May 2016) exhibited a high correlation of surface NO2 to TC NO2 measured by both PSI and Aura's Ozone Monitoring Instrument (OMI) but two other cases displayed poor relationships between in situ and TC NO2 due to decoupling of pollution layers from the surface. With suitable interpretation the PSI TC NO2 measurement demonstrates good potential for working with upcoming geostationary satellites to advance diurnal tracking of pollution.
The Pandora spectrometer that uses direct-sun measurements to derive total column amounts of gases provides an approach for (1) validation of satellite instruments and (2) monitoring of total column (TC) ozone (O3) and nitrogen dioxide (NO2). We use for the first time Pandora and OMI observations to estimate surface NO2 over marine and terrestrial sites downwind of urban pollution and compared with in situ measurements during campaigns in contrasting regions: (1) the South African Highveld (at Welgegund, 26°34'10"S, 26°56'21"E, 1480 m asl, ~120 km south-west of the Johannesburg-Pretoria megacity); (2) shipboard US mid-Atlantic coast during the 2014 Deposition of Atmospheric Nitrogen to Coastal Ecosystems (DANCE) cruise. In both cases there were no local NOx sources, but intermittent regional pollution influences. For TC NO2, OMI and Pandora difference is ~ 20% with Pandora higher most times. Surface NO2 values estimated from OMI and Pandora columns are compared to in situ NO2 for both locations. For Welgegund, the planetary boundary layer (PBL) height, used in converting column to surface NO2 value, has been estimated by three methods: co-located Atmospheric InfraRed Sounder (AIRS) observations; a model simulation; radiosonde data from Irene, 150 km northeast of the site. AIRS PBL heights agree within 10% of radiosonde-derived values. Absolute differences between Pandora- and OMI-estimated surface NO2 and the in situ data are better at the terrestrial site (~0.5ppbv and ~1 ppbv or greater, respectively) than under clean marine air conditions, with differences usually >3 ppbv. Cloud cover and PBL variability can influence these estimations.
This paper presents the operational cloud retrieval algorithms for the TROPOspheric Monitoring Instrument (TROPOMI) on board the European Space Agency Sentinel-5 Precursor (S5P) mission scheduled for launch in 2017.
Two algorithms working in tandem are used for retrieving cloud properties: OCRA (Optical Cloud Recognition Algorithm) and ROCINN (Retrieval of Cloud Information using Neural Networks). OCRA retrieves the cloud fraction using TROPOMI measurements in the UV/VIS spectral regions and ROCINN retrieves the cloud top height (pressure) and optical thickness (albedo) using TROPOMI measurements in and around the oxygen A-band in the NIR.
Cloud parameters from TROPOMI/S5P will be used not only for enhancing the accuracy of trace gas retrievals, but also for extending the satellite data record of cloud information derived from oxygen A-band measurements, a record initiated with GOME/ERS-2 over twenty years ago. Use of the oxygen A-band generates complementary cloud information (especially for low clouds), as compared to traditional thermal infrared sensors (as used in most meteorological satellites) that are less sensitive to low clouds due to reduced thermal contrast.
The OCRA and ROCINN algorithms are integrated in the S5P operational processor UPAS (Universal Processor for UV/VIS/NIR Atmospheric Spectrometers), and we present here UPAS cloud results using OMI and GOME-2 measurements. In addition, we examine anticipated challenges for the TROPOMI/S5P cloud retrieval algorithms and we discuss the future validation needs for OCRA and ROCINN.
Biogenic volatile organic compounds (BVOCs) play an important role in the chemistry of the troposphere, especially in the formation of tropospheric ozone (O3) and secondary organic aerosols (SOA). Ecosystems produce and emit a large number of BVOCs. It is estimated on a global scale that approximately 90 % of annual BVOC emissions are from terrestrial sources. In this study, measurements of BVOCs were conducted at the Welgegund measurement station (South Africa), which is considered to be a regionally representative background site situated in savannah grasslands. Very few BVOC measurements exist for savannah grasslands and results presented in this study are the most extensive for this type of landscape. Samples were collected twice a week for 2 h during the daytime and 2 h during the night-time through two long-term sampling campaigns from February 2011 to February 2012 and from December 2013 to February 2015, respectively. Individual BVOCs were identified and quantified using a thermal desorption instrument, which was connected to a gas chromatograph and a mass selective detector. The annual median concentrations of isoprene, 2-methyl-3-butene-2-ol (MBO), monoterpene and sesquiterpene (SQT) during the first campaign were 14, 7, 120 and 8 pptv, respectively, and 14, 4, 83 and 4 pptv, respectively, during the second campaign. The sum of the concentrations of the monoterpenes were at least an order of magnitude higher than the concentrations of other BVOC species during both sampling campaigns, with α-pinene being the most abundant species. The highest BVOC concentrations were observed during the wet season and elevated soil moisture was associated with increased BVOC concentrations. However, comparisons with measurements conducted at other landscapes in southern Africa and the rest of the world that have more woody vegetation indicated that BVOC concentrations were, in general, significantly lower for savannah grasslands. Furthermore, BVOC concentrations were an order of magnitude lower compared to total aromatic concentrations measured at Welgegund. An analysis of concentrations by wind direction indicated that isoprene concentrations were higher from the western sector that is considered to be a relatively clean regional background region with no large anthropogenic point sources, while wind direction did not indicate any significant differences in the concentrations of the other BVOC species. Statistical analysis indicated that soil moisture had the most significant impact on atmospheric levels of MBO, monoterpene and SQT concentrations, whereas temperature had the greatest influence on isoprene levels. The combined O3 formation potentials of all the BVOCs measured calculated with maximum incremental reactivity (MIR) coefficients during the first and second campaign were 1162 and 1022 pptv, respectively. α-Pinene and limonene had the highest reaction rates with O3, whereas isoprene exhibited relatively small contributions to O3 depletion. Limonene, α-pinene and terpinolene had the largest contributions to the OH reactivity of BVOCs measured at Welgegund for all of the months during both sampling campaigns.
Raman lidar data obtained over a 1 year period has been analysed in
relation to aerosol layers in the free troposphere over the Highveld in
South Africa. In total, 375 layers were observed above the boundary layer
during the period 30 January 2010 to 31 January 2011. The seasonal behaviour
of aerosol layer geometrical characteristics, as well as intensive and
extensive optical properties were studied. The highest centre heights of
free-tropospheric layers were observed during the South African spring (2520 ± 970 m a.g.l., also elsewhere). The geometrical layer depth was found
to be maximum during spring, while it did not show any significant
difference for the rest of the seasons. The variability of the analysed
intensive and extensive optical properties was high during all seasons.
Layers were observed at a mean centre height of 2100 ± 1000 m with an
average lidar ratio of 67 ± 25 sr (mean value with 1 standard
deviation) at 355 nm and a mean extinction-related Ångström exponent
of 1.9 ± 0.8 between 355 and 532 nm during the period under study.
Except for the intensive biomass burning period from August to October, the
lidar ratios and Ångström exponents are within the range of previous
observations for urban/industrial aerosols. During Southern Hemispheric
spring, the biomass burning activity is clearly reflected in the optical
properties of the observed free-tropospheric layers. Specifically, lidar
ratios at 355 nm were 89 ± 21, 57 ± 20, 59 ± 22
and 65 ± 23 sr during spring (September–November), summer (December–February), autumn (March–May) and winter (June–August),
respectively. The extinction-related Ångström exponents between 355
and 532 nm measured during spring, summer, autumn and winter were 1.8 ± 0.6, 2.4 ± 0.9, 1.8 ± 0.9 and 1.8 ± 0.6,
respectively. The mean columnar aerosol optical depth (AOD) obtained from
lidar measurements was found to be 0.46 ± 0.35 at 355 nm and 0.25 ± 0.2 at 532 nm. The contribution of free-tropospheric aerosols on the
AOD had a wide range of values with a mean contribution of 46%.
Trends in the composition of the lower atmosphere (0–1500 m altitude) and surface air quality over the Baltimore/Washington area and surrounding states were investigated for the period from 1997 to 2011. We examined emissions of ozone precursors from monitors and inventories as well as ambient ground-level and aircraft measurements to characterize trends in air pollution. The US EPA Continuous Emissions Monitoring System (CEMS) program reported substantial decreases in emission of summertime nitrogen oxides (NOx) from power plants, up to ∼80% in the mid-Atlantic States. These large reductions in emission of NOx are reflected in a sharp decrease of ground-level concentrations of NOx starting around 2003. The decreasing trend of tropospheric column CO observed by aircraft is ∼0.8 Dobson unit (DU) per year, corresponding to ∼35 ppbv yr−1 in the lower troposphere (the surface to 1500 m above ground level). Satellite observations of long-term, near-surface CO show a ∼40% decrease over western Maryland between 2000 and 2011; the same magnitude is indicated by aircraft measurements above these regions upwind of the Baltimore/Washington airshed. With decreasing emissions of ozone precursors, the ground-level ozone in the Baltimore/Washington area shows a 0.6 ppbv yr−1 decrease in the past 15 yr. Since photochemical production of ozone is substantially influenced by ambient temperature, we introduce the climate penalty factor (CPF) into the trend analysis of long-term aircraft measurements. After compensating for inter-annual variations in temperature, historical aircraft measurements indicate that the daily net production of tropospheric ozone over the Baltimore/Washington area decreased from ∼20 ppbv day−1 in the late 1990s to ∼7 ppbv day−1 in the early 2010s during ozone season. A decrease in the long-term column ozone is observed as ∼0.2 DU yr−1 in the lowest 1500 m, corresponding to an improvement of ∼1.3 ppbv yr−1. Our aircraft measurements were conducted on days when severe ozone pollution was forecasted, and these results represent the decreasing trend in high ozone events over the past 15 yr. Back trajectory cluster analysis demonstrates that emissions of air pollutants from Ohio and Pennsylvania through Maryland influence the column abundances of downwind ozone in the lower atmosphere. The trends in air pollutants reveal the success of regulations implemented over the past decades and the importance of region-wide emission controls in the eastern United States.
Nitrogen oxides (NOx=NO+NO2) are produced during combustion processes and, thus may serve as a proxy for fossil fuel-based energy usage and coemitted greenhouse gases and other pollutants. We use high-resolution nitrogen dioxide (NO2) data from the Ozone Monitoring Instrument (OMI) to analyze changes in urban NO2 levels around the world from 2005 to 2014, finding complex heterogeneity in the changes. We discuss several potential factors that seem to determine these NOx changes. First, environmental regulations resulted in large decreases. The only large increases in the United States may be associated with three areas of intensive energy activity. Second, elevated NO2 levels were observed over many Asian, tropical, and subtropical cities that experienced rapid economic growth. Two of the largest increases occurred over recently expanded petrochemical complexes in Jamnagar (India) and Daesan (Korea). Third, pollution transport from China possibly influenced the Republic of Korea and Japan, diminishing the impact of local pollution controls. However, in China, there were large decreases over Beijing, Shanghai, and the Pearl River Delta, which were likely associated with local emission control efforts. Fourth, civil unrest and its effect on energy usage may have resulted in lower NO2 levels in Libya, Iraq, and Syria. Fifth, spatial heterogeneity within several megacities may reflect mixed efforts to cope with air quality degradation. We also show the potential of high-resolution data for identifying NOx emission sources in regions with a complex mix of sources. Intensive monitoring of the world's tropical/subtropical megacities will remain a priority, as their populations and emissions of pollutants and greenhouse gases are expected to increase significantly.
We assess the standard operational nitrogen dioxide (NO2)
data product (OMNO2, version 2.1) retrieved from the Ozone
Monitoring Instrument (OMI) onboard NASA's Aura satellite using
a combination of aircraft and surface in~situ measurements as well
as ground-based column measurements at several locations and
a bottom-up NOx emission inventory over the continental
US. Despite considerable sampling differences, NO2 vertical
column densities from OMI are modestly correlated (r = 0.3–0.8)
with in situ measurements of tropospheric NO2 from aircraft,
ground-based observations of NO2 columns from MAX-DOAS and
Pandora instruments, in situ surface NO2 measurements from
photolytic converter instruments, and a bottom-up NOx
emission inventory. Overall, OMI retrievals tend to be lower in
urban regions and higher in remote areas, but generally agree with
other measurements to within ± 20%. No consistent seasonal
bias is evident. Contrasting results between different data sets
reveal complexities behind NO2 validation. Since
validation data sets are scarce and are limited in space and time,
validation of the global product is still limited in scope by
spatial and temporal coverage and retrieval conditions. Monthly
mean vertical NO2 profile shapes from the Global Modeling
Initiative (GMI) chemistry-transport model (CTM) used in the OMI
retrievals are highly consistent with in situ aircraft measurements,
but these measured profiles exhibit considerable day-to-day
variation, affecting the retrieved daily NO2 columns by up to
40%. This assessment of OMI tropospheric NO2 columns, together
with the comparison of OMI-retrieved and model-simulated NO2
columns, could offer diagnostic evaluation of the model.
The organic fraction of atmospheric aerosols is 20 to 90 % of which only a small percentage has been chemically characterised. Two-dimensional gas chromatography with a time-of-flight mass spectrometer (GCxGC-TOFMS) is a powerful instrument used to chemically characterise organic compounds. Size-resolved characterisation and semi-quantification of ambient organic aerosol compounds were performed with a GCxGC-TOFMS for the first time in South Africa. Twenty-four-hour samples were collected for 1 year for three different size ranges. A combined total of 1056 different organic compounds could be tentatively characterised. The largest number of organic compounds tentatively identified was PM2.5-1 (particles in the size range 1-2.5 μm), while this size fraction also had the highest total number of normalised response factors (∑NRF). On average, 52, 26, 6, 13 and 3 % of species tentatively identified were oxygenated species, hydrocarbons, halogenated compounds, N-containing compounds and S-containing compounds, respectively. Oxygenated compounds were the most abundant species. Alkane and mono-aromatic species were the largest number of hydrocarbons tentatively identified with the highest ∑NRFs. The largest number of oxygenated species tentatively characterised were carboxylic acids and esters, while ether compounds had the highest ∑NRFs. Most of the halogenated compounds tentatively identified were chlorinated species with the highest ∑NRFs in two size fractions. Iodate species had a significantly higher ∑NRF in the PM2.5-1 size fraction. The largest number of N-containing species tentatively characterised with the highest ∑NRFs were amines. A small number of S-containing compounds with low ∑NRFs were tentatively identified. The major sources of organic compounds measured at Welgegund were considered to be biomass burning and air masses moving over the anthropogenically impacted source regions.
Emissions of nitrogen oxides (NOx) and, subsequently, atmospheric levels of nitrogen dioxide (NO2) have decreased over the U.S. due to a combination of environmental policies and technological change. Consequently, NO2 levels have decreased by 30–40% in the last decade. We quantify NO2 trends (2005–2013) over the U.S. using surface measurements from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS) and an improved tropospheric NO2 vertical column density (VCD) data product from the Ozone Monitoring Instrument (OMI) on the Aura satellite. We demonstrate that the current OMI NO2 algorithm is of sufficient maturity to allow a favorable correspondence of trends and variations in OMI and AQS data. Our trend model accounts for the non-linear dependence of NO2 concentration on emissions associated with the seasonal variation of the chemical lifetime, including the change in the amplitude of the seasonal cycle associated with the significant change in NOx emissions that occurred over the last decade. The direct relationship between observations and emissions becomes more robust when one accounts for these non-linear dependencies. We improve the OMI NO2 standard retrieval algorithm and, subsequently, the data product by using monthly vertical concentration profiles, a required algorithm input, from a high-resolution chemistry and transport model (CTM) simulation with varying emissions (2005–2013). The impact of neglecting the time-dependence of the profiles leads to errors in trend estimation, particularly in regions where emissions have changed substantially. For example, trends calculated from retrievals based on time-dependent profiles offer 18% more instances of significant trends and up to 15% larger total NO2 reduction versus the results based on profiles for 2005. Using a CTM, we explore the theoretical relation of the trends estimated from NO2 VCDs to those estimated from ground-level concentrations. The model-simulated trends in VCDs strongly correlate with those estimated from surface concentrations (r = 0.83, N = 355). We then explore the observed correspondence of trends estimated from OMI and AQS data. We find a significant, but slightly weaker, correspondence (i.e., r = 0.68, N = 208) than predicted by the model and discuss some of the important factors affecting the relationship, including known problems (e.g., NOz interferents) associated with the AQS data. This significant correspondence gives confidence in trend and surface concentration estimates from OMI VCDs for locations, such as the majority of the U.S. and globe, that are not covered by surface monitoring networks. Using our improved trend model and our enhanced OMI data product, we find that both OMI and AQS data show substantial downward trends from 2005 to 2013, with an average reduction of 38% for each over the U.S. The annual reduction rates inferred from OMI and AQS measurements are larger (−4.8 ± 1.9%/yr, −3.7 ± 1.5%/yr) from 2005 to 2008 than 2010 to 2013 (−1.2 ± 1.2%/yr, −2.1 ± 1.4%/yr). We quantify NO2 trends for major U.S. cities and power plants; the latter suggest larger negative trend (−4.0 ± 1.5%/yr) between 2005 and 2008 and smaller or insignificant changes (−0.5 ± 1.2%/yr) during 2010–2013.
Multi-AXis Differential Optical Absorption Spec-troscopy (MAX-DOAS) measurements were performed in a rural location of southwestern Ontario during the Border Air Quality and Meteorology Study. Slant column densities (SCDs) of NO 2 and O 4 were determined using the standard DOAS technique. Using a radiative transfer model and the O 4 SCDs, aerosol optical depths were determined for clear sky conditions and compared to OMI, MODIS, AERONET, and local PM 2.5 measurements. This aerosol information was input to a radiative transfer model to calculate NO 2 air mass factors, which were fit to the measured NO 2 SCDs to determine tropospheric vertical column densities (VCDs) of NO 2 . The method of determining NO 2 VCDs in this way was validated for the first time by comparison to compos-ite VCDs derived from aircraft and ground-based measure-ments of NO 2 . The new VCDs were compared to VCDs of NO 2 determined via retrievals from the satellite instru-ments SCIAMACHY and OMI, for overlapping time peri-ods. The satellite-derived VCDs were higher, with a mean bias of +0.5–0.9 × 10 15 molec cm −2 . This last finding is dif-ferent from previous studies whereby MAX-DOAS geomet-ric VCDs were higher than satellite determinations, albeit for urban areas with higher VCDs. An effective boundary layer height, BLH eff , is defined as the ratio of the tropospheric VCD and the ground level concentration of NO 2 . Variations of BLH eff can be linked to time of day, source region, stabil-ity of the atmosphere, and the presence or absence of elevated NO x sources. In particular, a case study is shown where a Correspondence to: R. McLaren (rmclaren@yorku.ca) high VCD and BLH eff were observed when an elevated in-dustrial plume of NO x and SO 2 was fumigated to the sur-face as a lake breeze impacted the measurement site. High BLH eff values (∼1.9 km) were observed during a regional smog event when high winds from the SW and high convec-tion promoted mixing throughout the boundary layer. During this event, the regional line flux of NO 2 through the region was estimated to be greater than 112 kg NO 2 km −1 h −1 .
Aromatic hydrocarbons are associated with direct adverse human health effects and can have negative impacts on ecosystems due to their toxicity, as well as indirect negative effects through the formation of tropospheric ozone and secondary organic aerosol, which affect human health, crop production and regional climate. Measurements of aromatic hydrocarbons were conducted at the Welgegund measurement station (South Africa), which is considered to be a regionally representative background site. However, the site is occasionally impacted by plumes from major anthropogenic source regions in the interior of South Africa, which include the western Bushveld Igneous Complex (e.g. platinum, base metal and ferrochrome smelters), the eastern Bushveld Igneous Complex (platinum and ferrochrome smelters), the Johannesburg-Pretoria metropolitan conurbation (> 10 million people), the Vaal Triangle (e.g. petrochemical and pyrometallurgical industries), the Mpumalanga Highveld.
Satellite data of atmospheric pollutants are becoming more widely used in the decision-making and environmental management activities of public, private sector and non-profit organizations. They are employed for estimating emissions, tracking pollutant plumes, supporting air quality forecasting activities, providing evidence for “exceptional event” declarations, monitoring regional long-term trends, and evaluating air quality model output. However, many air quality managers are not taking full advantage of the data for these applications nor has the full potential of satellite data for air quality applications been realized. A key barrier is the inherent difficulties associated with accessing, processing, and properly interpreting observational data. A degree of technical skill is required on the part of the data end-user, which is often problematic for air quality agencies with limited resources. Therefore, we 1) review the primary uses of satellite data for air quality applications, 2) provide some background information on satellite capabilities for measuring pollutants, 3) discuss the many resources available to the end-user for accessing, processing, and visualizing the data, and 4) provide answers to common questions in plain language.
Atmospheric lidar measurements were carried out
at Elandsfontein measurement station, on the eastern Highveld
approximately 150 km east of Johannesburg in South
Africa throughout 2010. The height of the planetary boundary
layer (PBL) top was continuously measured using a Raman
lidar, PollyXT (POrtabLe Lidar sYstem eXTended).
High atmospheric variability together with a large surface
temperature range and significant seasonal changes in precipitation
were observed, which had an impact on the vertical
mixing of particulate matter, and hence, on the PBL evolution.
The results were compared to radiosondes, CALIOP
(Cloud-Aerosol Lidar with Orthogonal Polarization) spaceborne
lidar measurements and three atmospheric models that
followed different approaches to determine the PBL top
height. These models included two weather forecast models
operated by ECMWF (European Centre for MediumrangeWeather
Forecasts) and SAWS (South AfricanWeather
Service), and one mesoscale prognostic meteorological and
air pollution regulatory model TAPM (The Air Pollution
Model). The ground-based lidar used in this study was operational
for 4935 h during 2010 (49% of the time). The PBL
top height was detected 86% of the total measurement time
(42% of the total time). Large seasonal and diurnal variations
were observed between the different methods utilised.
High variation was found when lidar measurements were
compared to radiosonde measurements. This could be partially
due to the distance between the lidar measurements
and the radiosondes, which were 120 km apart. Comparison
of lidar measurements to the models indicated that the
ECMWF model agreed the best with mean relative difference
of 15.4 %, while the second best correlation was with
the SAWS model with corresponding difference of 20.1 %.
TAPM was found to have a tendency to underestimate the
PBL top height. The wind speeds in the SAWS and TAPM
models were strongly underestimated which probably led to
underestimation of the vertical wind and turbulence and thus
underestimation of the PBL top height. Comparison between
ground-based and satellite lidar shows good agreement with
a correlation coefficient of 0.88. On average, the daily maximum
PBL top height in October (spring) and June (winter)was 2260m and 1480 m, respectively. To our knowledge, this
study is the first long-term study of PBL top heights and PBL
growth rates in South Africa.
We show that Aura Ozone Monitoring Instrument (OMI) nitrogen dioxide
(NO2) tropospheric column data may be used to assess changes
of the emissions of nitrogen oxides (NOx) from power plants
in the United States, though careful interpretation of the data is
necessary. There is a clear response for OMI NO2 data to
NOx emission reductions from power plants associated with the
implementation of mandated emission control devices (ECDs) over the OMI
record (2005-2011). This response is scalar for all intents and
purposes, whether the reduction is rapid or incremental over several
years. However, it is variable among the power plants, even for those
with the greatest absolute decrease in emissions. We document the
primary causes of this variability, presenting case examples for
specific power plants.
Southern Africa is a significant source region of atmospheric pollution, yet
long-term data on pollutant concentrations and properties from this region
are rather limited. A recently established atmospheric measurement station
in South Africa, Welgegund, is strategically situated to capture regional
background concentrations, as well as emissions from the major source
regions in the interior of South Africa. We measured non-refractive
submicron aerosols (NR-PM1) and black carbon over a one year period in
Welgegund, and investigated the seasonal and diurnal patterns of aerosol
concentration levels, chemical composition, acidity and oxidation level.
Based on air mass back trajectories, four distinct source regions were
determined for NR-PM1. Supporting data utilised in our analysis
included particle number size distributions, aerosol absorption, trace gas
concentrations, meteorological variables and the flux of carbon dioxide.
The dominant submicron aerosol constituent during the dry season was organic
aerosol, reflecting high contribution from savannah fires and other
combustion sources. Organic aerosol concentrations were lower during the wet
season, presumably due to wet deposition as well as reduced emissions from
combustion sources. Sulfate concentrations were usually high and exceeded
organic aerosol concentrations when air-masses were transported over regions
containing major point sources. Sulfate and nitrate concentrations peaked
when air masses passed over the industrial Highveld
(iHV) area. In contrast, concentrations were much lower when air masses
passed over the cleaner background (BG) areas. Air masses associated with
the anti-cyclonic recirculation (ACBIC) source region contained largely aged
OA.
Positive Matrix Factorization (PMF) analysis of aerosol mass spectra was
used to characterise the organic aerosol (OA) properties. The factors
identified were oxidized organic aerosols (OOA) and biomass burning organic
aerosols (BBOA) in the dry season and low-volatile (LV-OOA) and
semi-volatile (SV-OOA) organic aerosols in the wet season. The results
highlight the importance of primary BBOA in the dry season, which
represented 33% of the total OA. Aerosol acidity and its potential impact
on the evolution of OOA are also discussed.
Observations of tropospheric NO2 vertical column densities
over the United States (US) for 2005-2011 are evaluated using the OMI
Berkeley High Resolution (BEHR) retrieval algorithm. We assess changes
in NO2 on day-of-week and interannual timescales to assess
the impact of changes in emissions from mobile and non-mobile sources on
the observed trends. We observe consistent decreases in cities across
the US, with an average total reduction of 32 ± 7% across the 7
yr. Changes for large power plants have been more variable (-26 ±
12%) due to regionally-specific regulation policies. An increasing trend
of 10-20% in background NO2 columns in the northwestern US is
observed. We examine the impact of the economic recession on emissions
and find that decreases in NO2 column densities over cities
were moderate prior to the recession (-6 ± 5% yr-1),
larger during the recession (-8 ± 5% yr-1), and then
smaller after the recession (-3 ± 4% yr-1).
Differences in the trends observed on weekdays and weekends indicate
that prior to the economic recession, NO2 reductions were
dominated by technological improvements to the light-duty vehicle fleet
but that a decrease in diesel truck activity has contributed to emission
reductions since the recession. We use the satellite observations to
estimate a 34% decrease in NO2 from mobile sources in cities
for 2005-2011 and use that value to infer changes in non-mobile sources.
We find that reductions in NO2 from non-mobile sources in
cities have been both more modest and more variable than NO2
reductions from mobile sources (-10 ± 13%).
Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS)
measurements were performed in a rural location of southwestern Ontario
during the Border Air Quality and Meteorology Study. Slant column
densities (SCDs) of NO2 and O4 were determined
using the standard DOAS technique. Using a radiative transfer model and
the O4 SCDs, aerosol optical depths were determined for clear
sky conditions and compared to OMI, MODIS, AERONET, and local
PM2.5 measurements. This aerosol information was input to a
radiative transfer model to calculate NO2 air mass factors,
which were fit to the measured NO2 SCDs to determine
tropospheric vertical column densities (VCDs) of NO2. The
method of determining NO2 VCDs in this way was validated for
the first time by comparison to composite VCDs derived from aircraft and
ground-based measurements of NO2. The new VCDs were compared
to VCDs of NO2 determined via retrievals from the satellite
instruments SCIAMACHY and OMI, for overlapping time periods. The
satellite-derived VCDs were higher, with a mean bias of
+0.5-0.9×1015 molec cm-2. This last finding
is different from previous studies whereby MAX-DOAS geometric VCDs were
higher than satellite determinations, albeit for urban areas with higher
VCDs. An effective boundary layer height, BLHeff, is defined
as the ratio of the tropospheric VCD and the ground level concentration
of NO2. Variations of BLHeff can be linked to time
of day, source region, stability of the atmosphere, and the presence or
absence of elevated NOx sources. In particular, a case study
is shown where a high VCD and BLHeff were observed when an
elevated industrial plume of NOx and SO2 was
fumigated to the surface as a lake breeze impacted the measurement site.
High BLHeff values (~1.9 km) were observed during a regional
smog event when high winds from the SW and high convection promoted
mixing throughout the boundary layer. During this event, the regional
line flux of NO2 through the region was estimated to be
greater than 112 kg NO2 km-1 h-1.
South Africa has the largest industrialised economy in Africa, with significant mining and metallurgical activities. A large fraction of the South African mineral assets is concentrated in the Bushveld Igneous Complex (BIC), with the western limb being the most exploited. Because the majority of the world's platinum is produced in the BIC, this area is also of international interest. There are some indications that the western BIC should be considered an air pollution hotspot; however, inadequate data exist to substantiate these claims scientifically. To partially address this knowledge gap, a comprehensive air quality monitoring station was operated for more than 2 years in this area. Meteorological parameters, trace gas concentrations and total mass concentration of particulate matter up to 10 mu m in size (PM10) were measured. Compared with South African and European ambient air quality standards, SO2, NO2 and CO concentrations were generally acceptable. The major sources of SO2 were identified as high-stack industry emissions, while household combustion from semi-formal and informal settlements was identified as the predominant source of NO2 and CO. In contrast, O-3 exceeded the 8-h moving average more than 322 times per year. The main contributing factor was identified to be the influx of regional air masses, with high O-3 precursor concentrations. PM10 exceeded the current South African 24-h standard 6.6 times per year, the future (2015) standard 42.3 times per year and the European standard more than 120 times per year. The main source of PM10 was identified as household combustion from semi-formal and informal settlements. The findings clearly indicate that atmospheric O-3 and PM10 levels in the western BIC need to be addressed to avoid negative environmental and human health impacts.
Globally, numerous pollution hotspots have been identified using satellite-based instruments. One of these hotspots is the prominent NO2 hotspot over the South African Highveld. The tropospheric NO2 column density of this area is comparable to that observed for central and northern Europe, eastern North America and south-east Asia. The most well-known pollution source in this area is a large array of coal-fired power stations. Upon closer inspection, long-term means of satellite observations also show a smaller area, approximately 100 km west of the Highveld hotspot, with a seemingly less substantial NO2 column density. This area correlates with the geographical location of the Johannesburg-Pretoria conurbation or megacity, one of the 40 largest metropolitan areas in the world. Ground-based measurements indicate that NO2 concentrations in the megacity have diurnal peaks in the early morning and late afternoon, which coincide with peak traffic hours and domestic combustion. During these times, NO2 concentrations in the megacity are higher than those in the Highveld hotspot. These diurnal NO2 peaks in the megacity have generally been overlooked by satellite observations because the satellites have fixed local overpass times that do not coincide with these peak periods. Consequently, the importance of NO2 over the megacity has been underestimated. We examined the diurnal cycles of NO2 ground-based measurements for the two areas - the megacity and the Highveld hotspot - and compared them with the satellite-based NO2 observations. Results show that the Highveld hotspot is accompanied by a second hotspot over the megacity, which is of significance for the more than 10 million people living in this megacity.
Total-column nitrogen dioxide (NO2) data collected by a ground-based sun-tracking spectrometer system (Pandora) and an photolytic-converter-based in-situ instrument collocated at NASA’s Langley Research Center in Hampton, Virginia were analyzed to study the relationship between total-column and surface NO2 measurements. The measurements span more than a year and cover all seasons. Surface mixing ratios are estimated via application of a planetary boundary-layer (PBL) height correction factor. This PBL correction factor effectively corrects for boundary-layer variability throughout the day, and accounts for up to ≈75 % of the variability between the NO2 data sets. Previous studies have made monthly and seasonal comparisons of column/surface data, which has shown generally good agreement over these long average times. In the current analysis comparisons of column densities averaged over 90 s and 1 h are made. Applicability of this technique to sulfur dioxide (SO2) is briefly explored. The SO2 correlation is improved by excluding conditions where surface levels are considered background. The analysis is extended to data from the July 2011 DISCOVER-AQ mission over the greater Baltimore, MD area to examine the method’s performance in more-polluted urban conditions where NO2 concentrations are typically much higher.
An analysis is presented for both ground- and satellite-based retrievals of total column ozone and nitrogen dioxide levels from the Washington, D.C., and Baltimore, Maryland, metropolitan area during the NASA-sponsored July 2011 campaign of Deriving Information on Surface COnditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ). Satellite retrievals of total column ozone and nitrogen dioxide from the Ozone Monitoring Instrument (OMI) on the Aura satellite are used, while Pandora spectrometers provide total column ozone and nitrogen dioxide amounts from the ground. We found that OMI and Pandora agree well (residuals within ±25 % for nitrogen dioxide, and ±4.5 % for ozone) for a majority of coincident observations during July 2011. Comparisons with surface nitrogen dioxide from a Teledyne API 200 EU NOx Analyzer showed nitrogen dioxide diurnal variability that was consistent with measurements by Pandora. However, the wide OMI field of view, clouds, and aerosols affected retrievals on certain days, resulting in differences between Pandora and OMI of up to ±65 % for total column nitrogen dioxide, and ±23 % for total column ozone. As expected, significant cloud cover (cloud fraction >0.2) was the most important parameter affecting comparisons of ozone retrievals; however, small, passing cumulus clouds that do not coincide with a high (>0.2) cloud fraction, or low aerosol layers which cause significant backscatter near the ground affected the comparisons of total column nitrogen dioxide retrievals. Our results will impact post-processing satellite retrieval algorithms and quality control procedures.
Anthropogenic emissions of nitrogen oxides (NOx) can change rapidly due to economic growth or control measures. Bottom-up emissions estimated using source-specific emission factors and activity statistics require years to compile and can become quickly outdated. We present a method to use satellite observations of tropospheric NO2 columns to estimate changes in NOx emissions. We use tropospheric NO2 columns retrieved from the SCIAMACHY satellite instrument for 2003–2009, the response of tropospheric NO2 columns to changes in NOx emissions determined from a global chemical transport model (GEOS-Chem), and the bottom-up anthropogenic NOx emissions for 2006 to hindcast and forecast the inventories. We evaluate our approach by comparing bottom-up and hindcast emissions for 2003. The two inventories agree within 6.0% globally and within 8.9% at the regional scale with consistent trends in western Europe, North America, and East Asia. We go on to forecast emissions for 2009. During 2006–2009, anthropogenic NOx emissions over land increase by 9.2% globally and by 18.8% from East Asia. North American emissions decrease by 5.7%.
In this paper, the incorporation of a simple atmospheric boundary layer diffusion scheme into the NCEP Medium-Range Forecast Model is described. A boundary layer diffusion package based on the Troen and Mahrt nonlocal diffusion concept has been tested for possible operational implementation. The results from this approach are compared with those from the local diffusion approach, which is the current operational scheme, and verified against FIFE observations during 9-10 August 1987. The comparisons between local and nonlocal approaches are extended to the forecast for a heavy rain case of 15-17 May 1995. The sensitivity of both the boundary layer development and the precipitation forecast to the tuning parameters in the nonlocal diffusion scheme is also investigated. Special attention is given to the interaction of boundary layer processes with precipitation physics. Some results of parallel runs during August 1995 are also presented.
Tropospheric air trajectories that occurred during the Southern African Fire-Atmosphere Research Initiative (SAFARI) in August-October 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere. Three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season. Of these, the main subsidence-induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km (3.5 km above the interior plateau elevation). This effective obstacle to vertical mixing is observed to persist without break for up to 40 days. Below this feature an absolutely stable layer at 3 km (1.5 km above the surface) prevails on and off at the top of the surface mixing layer for up to 7 days at a time, being broken by the passage of regularly occurring westerly wave disturbances. Above the middle-level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km. It is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern Africa. Dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers. In exiting the anticyclonic circulation, transport on the northern periphery of the system is to the west over the Atlantic Ocean via a semistationary easterly wave over the western part of the subcontinent. On the southern periphery, wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the Indian Ocean. Independently derived data suggest that during SAFARI only 4% of the total transport of air from three locations south of 18°S was into the Atlantic Ocean. Over 90% of the transport was into the Indian Ocean across 35°E. This result reflects circulation fields typical of the extremely dry conditions prevailing in 1992. The integrated effect of the control exerted by atmospheric stability on vertical mixing, on the one hand, and the nature of the horizontal circulation fields, on the other, is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring, as well as over the tropical South Atlantic and southwestern Indian Oceans.
In situ measurements of O3 and nitrogen oxides (NO + NO2 ≡ NOx) and remote sensing measurements of total column NO2 and O3 were collected on a ship in the North Atlantic Ocean as part of the Deposition of Atmospheric Nitrogen to Coastal Ecosystems (DANCE) campaign in July-August 2014, ~100 km east of the mid-Atlantic United States. Relatively clean conditions for both surface in situ mixing ratio and total column O3 and NO2 measurements were observed throughout the campaign. Increased surface and column NO2 and O3 amounts were observed when a terrestrial air mass was advected over the study region. Relative to ship-based total column measurements using a Pandora over the entire study, satellite measurements overestimated total column NO2 under these relatively clean atmospheric conditions over offshore waters by an average of 16%. Differences are most likely due to proximity, or lack thereof, to surface emissions, spatial averaging due to the field of view of the satellite instrument, and the lack of sensitivity of satellite measurements to the surface concentrations of pollutants. Total column O3 measurements from the shipboard Pandora showed good correlation with the satellite measurements (r = 0.96), but satellite measurements were 3% systematically higher than the ship measurements, in agreement with previous studies. Derived values of boundary layer height using the surface in situ and total column measurements of NO2 are much lower than modeled and satellite-retrieved boundary layer heights, which highlight the differences in the vertical distribution between terrestrial and marine environments.
Hexavalent chromium, Cr(VI), is a proven human carcinogen. Of interest in this paper was the regional atmospheric pollution of Cr(VI) from the ferrochromium and related industries located in the western Bushveld Complex (wBC) of South Africa. A significant fraction of the world's ferrochromium and platinum group metals is produced in this region. Particulate matter (PM), in two size fractions, i.e. PM2.5 (≤2.5 μm) and PM2.5−10 (2.5–10 μm), was sampled for a full calendar year at a regional background site, which is situated downwind of the wBC on the dominant anti-cyclonic recirculation route of air mass over the South African interior. Results indicated that Cr(VI) concentrations in air masses that had passed over the regional background were below the detection limit of the analytical technique applied, but that Cr(VI) in air masses that had passed over the wBC were elevated and had a median concentration of 4.6 ng/m3. The majority of Cr(VI) was found to be in the finer size fraction (PM2.5), which could be explained by the nature of Cr(VI)-containing PM being emitted by the sources in the wBC and the atmospheric lifetimes of different PM size fractions. The results further indicated that it is possible that not only pyrometallurgical sources in the wBC, but also other combustion sources outside the wBC contributed to the observed atmospheric Cr(VI) concentrations.
Automated continuous monitoring of atmospheric species is vital. Different monitoring motives require different operational philosophies. For ambient air quality monitoring the species and techniques applied are set by legislation, while for research this will be determined by the scientific question(s) considered. During instrument selection due consideration should be given to on-site technical expertise, ease of use, reliability and availability of backup and/or spares in addition to analytical selectivity and sensitivity. Following standard operating procedures and obtaining accreditation add significant value, but insight into the analytical techniques applied remains critical. The monitoring motive also influences site selection, as well as the type and volume of data logged. Effective data processing is required to extract high-level science from the large data sets associated with comprehensively equipped long-term measurements. Regular online and on-site checks of the instruments/data and the ability to immediately act on concerns, as well as keeping of an electronic diary and data cleaning are also essential.
The thermodynamic structure of the atmosphere over South Africa is examined to determine the occurrence and persistence of stable discontinuities that govern the horizontal and vertical transport of aerosols and trace gases in the troposphere over the region. Absolutely stable layers in which the lapse rate of temperature is less than the saturated adiabatic lapse rate, are shown to occur preferentially at ~700, ~500 and ~300 hPa levels over the whole of the plateau region, with an additional layer occurring at sub-plateau elevations at ~850 hPa between the coast arid Escarpment. The development of the layers is almost constant throughout the year on no-rain days and no appreciable seasonal variation is apparent. Likewise, changing synoptic circulation patterns appear to exert no significant effect on the development of the layers on no-rain days, which over most of South Africa occur with a frequency exceeding 80 per cent. The persistence of the stable layering of the atmosphere over South Africa is shown to be an important aspect of the climate of the region.
Change of the NRC report. The U.S. National Research Council (NRC), at the request of the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Geological Survey, conducted an Earth Science Decadal Survey review to assist in planning the next generation of Earth science satellite missions [NRC 2007; commonly referred to as the “Decadal Survey” (“DS”)]. The Geostationary Coastal and Air Pollution Events (GEO-CAPE) mission measuring tropospheric trace gases and aerosols and coastal ocean phytoplankton, water quality, and biogeochemistry from geostationary orbit was one of 17 recommended missions. Satellites in geostationary orbit provide continuous observations within their field of view, a revolutionary advance for both atmosphere and ocean science disciplines. The NRC placed GEO-CAPE within the second tier of missions, recommended for launch within the 2013–16 time frame. In addition to providing information for addressing scientific questions, the NRC advised that increasing the societal benefits of Earth science research should be a high priority for federal science agencies and policy makers.
The characterization of atmospheric aerosol properties and trace gas concentrations in various environments provides a basis for scientific research on the assessment of their roles in the climate system, as well as regional air quality issues. However, measurements of these face many problems in the developing world. In this study we present the design of a transportable aerosol dynamic and atmospheric research trailer, which relies minimally on the existing infrastructure by making use of wireless data transfer. The instrumentation used in this trailer was originally used in various aerosol formation studies, and has been expanded so that it can also monitor air quality. The instruments include a Differential Mobility Particle Sizer (DMPS) for aerosol number size distribution in the range 10–840 nm, a Tapered Element Oscillating Microbalance (TEOM) for aerosol mass concentration, and Air Ion Spectrometer (AIS) for atmospheric ion measurements in the size range 0.5–40 nm. Sulfur dioxide, nitrogen oxides, carbon monoxide and ozone mixing ratios are also monitored, as well as photosynthetically active radiation (PAR) and meteorological parameters. As we have already operated the trailer in South Africa for several years, we discuss the lessons learned during the first years of use in the field. Keyword: Atmospheric aerosol particles; Atmospheric pollution; Trace gases; Climate change; Air quality.
Frequent, persistent and the spatially-continuous occurrence of absolutely stable layers of air are confined to the features of the Southern African atmospheric environment. The elevated layers occur preferentially at ~850 hPa (over the coastal regions only), ~700 hPa, ~500 hPa and ~300 hPa throughout the troposphere. They are highly effective in acting as upper air boundaries that control the free diffusion of aerosols and trace gases (including water vapour) in the vertical and may have repercussions on scales ranging from local to synoptic. The seasonal stability characteristics and temporal and spatial continuity of the elevated absolutely stable layers are examined over the eastern half of South Africa and related to a previous case study of moisture transport patterns on rain and no-rain days.
Primary and secondary aerosol particles originating from biomass burning contribute significantly to the atmospheric aerosol budget and thereby to both direct and indirect radiative forcing. Based on detailed measurements of a large number of biomass burning plumes of variable age in southern Africa, we show that the size distribution, chemical composition, single scattering albedo and hygroscopicity of biomass burning particles change considerably during the first 2–4 hours of their atmospheric transport. These changes, driven by atmospheric oxidation and subsequent secondary aerosol formation, may reach a factor of 6 for the aerosol scattering coefficient and a factor >10 for the cloud condensation nuclei concentration. Since the observed changes take place over the spatial and temporal scales that are neither covered by emission inventories nor captured by large-scale model simulations the findings reported here point out a significant gap in our understanding on the climatic effects of biomass burning aerosols.
[1] Surface ozone is a secondary air pollutant formed from reactions between nitrogen oxides (NOx = NO + NO2) and volatile organic compounds in the presence of sunlight. In this work we examine effects of the climate pattern known as the El-Niño Southern Oscillation (ENSO) and NOx variability on surface ozone from 1990 to 2007 over the South African Highveld, a heavily populated region in South Africa with numerous industrial facilities. Over summer and autumn (December-May) on the Highveld, El-Niño, as signified by positive sea surface temperature (SST) anomalies over the central Pacific Ocean, is typically associated with drier and warmer than normal conditions favoring ozone formation. Conversely, La-Niña, or negative SST anomalies over the central Pacific Ocean, is typically associated with cloudier and above normal rainfall conditions, hindering ozone production. We use a generalized regression model to identify any linear dependence that the Highveld ozone, measured at five air quality monitoring stations, may have on ENSO and NOx. Our results indicate that four out of the five stations exhibit a statistically significant sensitivity to ENSO at some point over the December-May period where El Niño amplifies ozone formation and La Niña reduces ozone formation. Three out of the five stations reveal statistically significant sensitivity to NOx variability, primarily in winter and spring. Accounting for ENSO and NOx effects throughout the study period of 18 years, two stations exhibit statistically significant negative ozone trends in spring and September, one station displays statistically significant positive trend in August, and two stations show no statistically significant change in surface ozone.
TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch circa 2018. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO measures from Mexico City to the Canadian tar sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2 km N/S×4.5 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry. Measurements are from geostationary (GEO) orbit, to capture the inherent high variability in the diurnal cycle of emissions and chemistry. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies.
TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve O3, NO2, SO2, H2CO, C2H2O2, H2O, aerosols, cloud parameters, and UVB radiation. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O3 chemistry cycle. Multi-spectral observations provide sensitivity to O3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides near-real-time air quality products that will be made widely, publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with European Sentinel-4 and Korean GEMS.
Nitrogen oxides (NOx = NO + NO2) in the atmosphere
are often measured using instruments equipped with molybdenum
converters. NO2 is catalytically converted to NO on a heated
molybdenum surface and subsequently measured by chemiluminescence after
reaction with ozone. The drawback of this technique is that other
oxidized nitrogen compounds such as peroxyacetyl nitrate and nitric acid
are also partly converted to NO. Thus such NO2 measurements
are really surrogate NO2 measurements because the resultant
values systematically overestimate the true value because of
interferences of these compounds, especially when sampling
photochemically aged air masses. However, molybdenum converters are
widely used, and a dense network of surrogate NO2
measurements exists. As an alternative with far less interference,
photolytic converters using ultraviolet light are nowadays applicable
also for long-term measurements. This work presents long-term collocated
NO2 measurements using molybdenum and photolytic converters
at two rural sites in Switzerland. On a relative scale, the molybdenum
converter instruments overestimate the NO2 concentrations
most during spring/summer because of prevalent photochemistry. On a
monthly basis, only 70-83% of the "surrogate" NO2 can be
attributed to "real" NO2 at the non-elevated site and even
less (43-76%) at the elevated one. The observed interferences have to be
taken into account for monitoring and regulatory issues and to be
considered when using these data for ground-truthing of satellite data
or for validation of chemical transport models. Alternatively, an
increased availability of artifact-free data would also be beneficial
for these issues.
We present new, high precision, high temporal resolution measurements of
total column ozone (TCO) amounts derived from ground-based direct-sun
irradiance measurements using our recently deployed Pandora
single-grating spectrometers. Pandora's small size and portability allow
deployment at multiple sites within an urban air-shed and development of
a ground-based monitoring network for studying small-scale atmospheric
dynamics, spatial heterogeneities in trace gas distribution, local
pollution conditions, photochemical processes and interdependencies of
ozone and its major precursors. Results are shown for four mid- to
high-latitude sites where different Pandora instruments were used.
Comparisons with a well calibrated double-grating Brewer spectrometer
over a period of more than a year in Greenbelt MD showed excellent
agreement and a small bias of approximately 2 DU (or, 0.6%). This was
constant with slant column ozone amount over the full range of observed
solar zenith angles (15-80°), indicating adequate Pandora stray
light correction. A small (1-2%) seasonal difference was found,
consistent with sensitivity studies showing that the Pandora spectral
fitting TCO retrieval has a temperature dependence of 1% per 3°K,
with an underestimation in temperature (e.g., during summer) resulting
in an underestimation of TCO. Pandora agreed well with Aura-OMI (Ozone
Measuring Instrument) satellite data, with average residuals of <1%
at the different sites when the OMI view was within 50 km from the
Pandora location and OMI-measured cloud fraction was <0.2. The
frequent and continuous measurements by Pandora revealed significant
short-term (hourly) temporal changes in TCO, not possible to capture by
sun-synchronous satellites, such as OMI, alone.
Megacity emission inventories, based on bottom-up estimates, are still
highly uncertain, in particular in developing countries. Satellite
observations have been demonstrated to allow regional and global
top-down emission estimates of nitrogen oxides (NOx=NO+NO2), but require
poorly quantified a-priori information on the lifetime of NOx.Here we
present a new method for the determination of megacity NOx emissions and
lifetimes from satellite measurements. Mean patterns of NO2 tropospheric
columns are analyzed separately for a set of different wind direction
sectors. From the combined use of the observed total burden and the
downwind evolution of NO2, mean NOx photochemical lifetimes and total
emissions are derived simultaneously. Typical daytime lifetimes of about
4 hours are found for several megacities at low and mid- latitudes,
corresponding to mean OH concentrations of ~6e6 molec/cm3 around noon.
The derived emissions are generally in good agreement with bottom-up
inventories, but are significantly higher in e.g. the case of Riyadh
(Saudi Arabia).The presented method works best for isolated "hot spots"
of NOx emissions. For megacities in the vicinity (in terms of some
hundred km) of other strong sources, like e.g. Paris, modified
approaches are necessary. We will present different approaches, and the
estimated emissions+uncertainties will be discussed in perspective of
existing, bottom-up emission inventories.
A scanning UV-Visible Spectrometer, GEMS (Geostationary Environment
Spectrometer) is planned to be launched in 2018 onboard a geostationary
satellite, GeoKOMPSAT(Geostationary Korea Multi-Purpose SATellite by
KARI(Korea Aerospace Research Institute), together with ABI(Advanced
Baseline Imager) and GOCI-2 (Geostationary Ocean Color Imager).
Synchronous measurements of air pollutants together with the
meteorological variables and ocean color information are expected to
contribute to better scientific understanding on the distribution and
transboundary transportation of air pollution, and on interactions
between meteorology and air chemistry in the Asia-Pacific region. This
mission is expected to improve the accuracy of air quality forecasting
and reduce current discrepancy between the model and observation.
Furthermore, constellation of the GeoKOMPSAT with Senteniel-4 in Europe
and GEOCAPE in America in 2017-2020 time frame can result in great
synergistic outcomes including enhancing significantly our understanding
in globalization of tropospheric pollution.
Satellite remote sensing of air quality has evolved dramatically over
the last decade. Global observations are now available for a wide
range of species including aerosols, tropospheric O3, tropospheric NO2,
CO, HCHO, and SO2. Current capabilities for satellite remote sensing of
these species in the boundary layer are reviewed, along with physical
processes affecting their accuracy and precision. Applications of
satellite observations are discussed for case studies of specific
events, for estimates of surface concentrations, and to improve emission
estimates of trace gases and aerosols. Although atmospheric scattering
and surface emission of thermal radiation generally reduce instrument
sensitivity to trace gases and aerosols in the boundary layer, strong
boundary layer signals arise from large vertical gradients in aerosols,
NO2, and HCHO. Recommendations are presented to continue improving
satellite remote sensing of surface air quality. Highlights from our
recent work will also be included.
Spatial and temporal dynamics in trace gas pollutants were examined over a major urban estuarine ecosystem, using a new network of ground-based Pandora spectrometers deployed at strategic locations along the Washington-Baltimore corridor and the Chesapeake Bay. Total column ozone (TCO3) and nitrogen dioxide (TCNO2) were measured during NASA’s DISCOVER-AQ and GeoCAPE-CBODAQ campaigns in July 2011. The Pandora network provided high-resolution information on air-quality variability, local pollution conditions, large-scale meteorological influences, and interdependencies of ozone and its major precursor, NO2. Measurements were used to compare with air-quality model simulations (CMAQ), evaluate Aura-OMI satellite retrievals, and assess advantages and limitations of space-based observations under a range of conditions. During the campaign, TCNO2 varied by an order of magnitude, both spatially and temporally. Although fairly constant in rural regions, TCNO2 showed clear diurnal and weekly patterns in polluted urban areas caused by changes in near-surface emissions. With a coarse resolution and an overpass at around 13:30 local time, OMI cannot detect this strong variability in NO2, missing pollution peaks from industrial and rush hour activities. Not as highly variable as NO2, TCO3 was mostly affected by large-scale meteorological patterns as observed by OMI. A clear weekly cycle in TCO3, with minima over the weekend, was due to a combination of weekly weather patterns and changes in near-surface NOx emissions. A Pandora instrument intercomparison under the same conditions at GSFC showed excellent agreement, within ±4.8DU for TCO3 and ±0.07DU for TCNO2 with no air-mass-factor dependence, suggesting that observed variability during the campaign was real.
Atmospheric chemistry observations from space have been made for more than 30 years. They have been motivated by the concern about a number of environmental issues. However, most of the space instruments have been designed for scientific research, improving the understanding of processes that govern stratospheric ozone depletion, climate change and the transport of pollutants starting with the BUV instrument on Nimbus-4. Long-term continuous time series of atmospheric trace gas data have been limited to stratospheric ozone and a few related species. According to current planning, meteorological satellites will maintain some of these observations over the next decade. They will also add some measurements of tropospheric climate-relevant gases. As their measurements are motivated by meeting operational meteorology needs, they fall short in meeting requirements for atmospheric chemistry applications.
Nitrogen oxide (NOx) emissions resulting from fossil fuel combustion lead to unhealthy levels of near-surface ozone (Oâ). One of the largest U.S. sources, electric power generation, represented about 25% of the U.S. anthropogenic NOx emissions in 1999. Here we show that space-based instruments observed declining regional NOx levels between 1999 and 2005 in response to the recent implementation of pollution controls by utility companies in the eastern U.S. Satellite-retrieved summertime nitrogen dioxide (NOâ) columns and bottom-up emission estimates show larger decreases in the Ohio River Valley, where power plants dominate NOx emissions, than in the northeast U.S. urban corridor. Model simulations predict lower Oâ across much of the eastern U.S. in response to these emission reductions.
The new generation of remote sensors on board NASA's A-Train constellation offers the possibility of observing the atmospheric boundary layer in different regimes, with or without clouds. In this study we use data from the Atmospheric InfraRed Sounder (AIRS) and of the Rain In Cumulus over the Ocean (RICO) campaign, to verify the accuracy and precision of the AIRS Version 5 Level 2 support product. This AIRS product has an improved vertical sampling that is necessary for the estimation of boundary layer properties. Good agreement is found between AIRS and RICO data, in a regime of oceanic shallow cumulus that is known to be difficult to analyze with other remote sensing data, and also shows a low sensitivity to cloud or land fraction. This suggests that AIRS data may be used for global boundary layer studies to support parameterization development in regions of difficult in-situ observation.
The retrieval of NO2 vertical column densities from satellite measurements presents a challenge because of the difficulty in determining the air mass factor (AMF). This quantity, which relates the measured slant column to the vertical column, is particularly sensitive to the NO2 profile when tropospheric NO2 amounts are large. We describe the algorithm that will be used to obtain vertical column densities from the NO2 slant column densities measured by OMI on the upcoming EOS AURA mission. Our method relies on a tropospheric separation scheme that identifies polluted regions (containing significant tropospheric NO2) and applies different AMFs in polluted and unpolluted regions. We test the method on synthetic NO2 fields and on data from the Global Ozone Monitoring Instrument (GOME). Results from the OMI algorithm are compared to retrievals used in previous studies to analyze GOME data.