Article

OMI Satellite and Ground-Based Pandora Observations and Their Application to Surface NO2 Estimations at Terrestrial and Marine Sites

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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.

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... 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). ...
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... 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 . ...
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... 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). ...
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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.
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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.
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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.
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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%.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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[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.
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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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.