James G. Walega

National Center for Atmospheric Research, Boulder, Colorado, United States

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Publications (59)119.06 Total impact

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    ABSTRACT: We report on the development and airborne field deployment of a mid-IR laser-based spectrometer. The instrument was configured for the simultaneous in situ detection of formaldehyde (CH2O) and ethane (C2H6). Numerous mechanical, optical, electronic, and software improvements over a previous instrument design resulted in reliable highly sensitive airborne operation with long sta- bility times yielding 90 % airborne measurement coverage during the recent air quality study over the Colorado Front Range, FRAPPÉ 2014. Airborne detection sensitivities of ~15 pptv (C2H6) and ~40 pptv (CH2O) were generally obtained for 1 s of averaging for simultaneous detection.
    Applied Physics B 02/2015; 119(1). DOI:10.1007/s00340-015-6038-8 · 1.78 Impact Factor
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    ABSTRACT: Many important atmospheric constituents can be detected with infrared laser absorption spectroscopy. This talk reviews the engineering challenges, opportunities, and selected scientific results from recent airborne campaigns.
    CLEO: Applications and Technology; 06/2013
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    ABSTRACT: We report on the design and performance of a new Difference Frequency Generation spectrometer operated aboard a Gulfstream-V jet during the Deep Convective Cloud and Chemistry research study (DC3) during the spring of 2012.
    CLEO: Science and Innovations; 06/2013
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    ABSTRACT: Observations of chemical constituents and meteorological quantities obtained during the two Arctic phases of the airborne campaign ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) are analyzed using an observationally constrained steady state box model. Measurements of OH and HO2 from the Penn State ATHOS instrument are compared to model predictions. Forty percent of OH measurements below 2 km are at the limit of detection during the spring phase (ARCTAS-A). While the median observed-to-calculated ratio is near one, both the scatter of observations and the model uncertainty for OH are at the magnitude of ambient values. During the summer phase (ARCTAS-B), model predictions of OH are biased low relative to observations and demonstrate a high sensitivity to the level of uncertainty in NO observations. Predictions of HO2 using observed CH2O and H2O2 as model constraints are up to a factor of two larger than observed. A temperature-dependent terminal loss rate of HO2 to aerosol recently proposed in the literature is shown to be insufficient to reconcile these differences. A comparison of ARCTAS-A to the high latitude springtime portion of the 2000 TOPSE campaign (Tropospheric Ozone Production about the Spring Equinox) shows similar meteorological and chemical environments with the exception of peroxides; observations of H2O2 during ARCTAS-A were 2.5 to 3 times larger than those during TOPSE. The cause of this difference in peroxides remains unresolved and has important implications for the Arctic HOx budget. Unconstrained model predictions for both phases indicate photochemistry alone is unable to simultaneously sustain observed levels of CH2O and H2O2; however when the model is constrained with observed CH2O, H2O2 predictions from a range of rainout parameterizations bracket its observations. A mechanism suitable to explain observed concentrations of CH2O is uncertain. Free tropospheric observations of acetaldehyde (CH3CHO) are 2-3 times larger than its predictions, though constraint of the model to those observations is sufficient to account for less than half of the deficit in predicted CH2O. The box model calculates gross O3 formation during spring to maximize from 1-4 km at 0.8 ppbv d-1, in agreement with estimates from TOPSE, and a gross production of 2-4 ppbv d-1 in the boundary layer and upper troposphere during summer. Use of the lower observed levels of HO2 in place of model predictions decreases the gross production by 25-50%. Net O3 production is near zero throughout the ARCTAS-A troposphere, and is 1-2 ppbv in the boundary layer and upper altitudes during ARCTAS-B.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 08/2012; 12(15):6799-6825. DOI:10.5194/acp-12-6799-2012 · 5.30 Impact Factor
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    ABSTRACT: Inorganic bromine plays a critical role in ozone and mercury depletions events (ODEs and MDEs) in the Arctic marine boundary layer. Direct observations of bromine species other than bromine oxide (BrO) during ODEs are very limited. Here we report the first direct measurements of hypobromous acid (HOBr) as well as observations of BrO and molecular bromine (Br2) by chemical ionization mass spectrometry at Barrow, Alaska in spring 2009 during the Ocean-Atmospheric-Sea Ice-Snowpack (OASIS) campaign. Diurnal profiles of HOBr with maximum concentrations near local noon and no significant concentrations at night were observed. The measured average daytime HOBr mixing ratio was 10 pptv with a maximum value of 26 pptv. The observed HOBr was reasonably well correlated (R2 = 0.57) with predictions from a simple steady state photochemical model constrained to observed BrO and HO2 at wind speeds <6 m s-1. However, predicted HOBr levels were considerably higher than observations at higher wind speeds. This may be due to enhanced heterogeneous loss of HOBr on blowing snow coincident with higher wind speeds. BrO levels were also found to be higher at elevated wind speeds. Br2 was observed in significant mixing ratios (maximum = 46 pptv; average = 13 pptv) at night and was strongly anti-correlated with ozone. The diurnal speciation of observed gas phase inorganic bromine species can be predicted by a time-dependent box model that includes efficient heterogeneous recycling of HOBr, hydrogen bromide (HBr), and bromine nitrate (BrONO2) back to more reactive forms of bromine.
    Journal of Geophysical Research Atmospheres 07/2012; 117(D14). DOI:10.1029/2011JD016641 · 3.44 Impact Factor
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    ABSTRACT: Observations of chemical constituents and meteorological quantities obtained during the two Arctic phases of the airborne campaign ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) are analyzed using an observationally constrained steady state box model. Measurements of OH and HO2 from the Penn State ATHOS instrument are compared to model predictions. Forty percent of OH measurements below 2 km are at the limit of detection during the spring phase (ARCTAS-A). While the median observed-to-calculated ratio is near one, both the scatter of observations and the model uncertainty for OH are at the magnitude of ambient values. During the summer phase (ARCTAS-B), model predictions of OH are biased low relative to observations and demonstrate a high sensitivity to the level of uncertainty in NO observations. Predictions of HO2 using observed CH2O and H2O2 as model constraints are up to a factor of two larger than observed. A temperature-dependent terminal loss rate of HO2 to aerosol recently proposed in the literature is shown to be insufficient to reconcile these differences. A comparison of ARCTAS-A to the high latitude springtime portion of the 2000 TOPSE campaign (Tropospheric Ozone Production about the Spring Equinox) shows similar meteorological and chemical environments with the exception of peroxides; observations of H2O2 during ARCTAS-A were 2.5 to 3 times larger than those during TOPSE. The cause of this difference in peroxides remains unresolved and has important implications for the Arctic HOx budget. Unconstrained model predictions for both phases indicate photochemistry alone is unable to simultaneously sustain observed levels of CH2O and H2O2; however when the model is constrained with observed CH2O, H2O2 predictions from a range of rainout parameterizations bracket its observations. A mechanism suitable to explain observed concentrations of CH2O is uncertain. Free tropospheric observations of acetaldehyde (CH3CHO) are 2-3 times larger than its predictions, though constraint of the model to those observations is sufficient to account for less than half of the deficit in predicted CH2O. The box model calculates gross O3 formation during spring to maximize from 1-4 km at 0.8 ppbv d-1, in agreement with estimates from TOPSE, and a gross production of 2-4 ppbv d-1 in the boundary layer and upper troposphere during summer. Use of the lower observed levels of HO2 in place of model predictions decreases the gross production by 25-50%. Net O3 production is near zero throughout the ARCTAS-A troposphere, and is 1-2 ppbv in the boundary layer and upper altitudes during ARCTAS-B.
    Atmospheric Chemistry and Physics 04/2012; 12(4):9377-9450. DOI:10.5194/acpd-12-9377-2012 · 4.88 Impact Factor
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    ABSTRACT: Aircraft observations of constituents and meteorological quantities observed during the two seasonal Arctic phases of ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) and during the 2000 TOPSE (Tropospheric Ozone Production About the Spring Equinox) are analyzed using an observationally-constrained steady state box model. An examination of the springtime Arctic portion of the 2000 TOPSE program shows a highly similar meteorological background and chemical composition relative to ARCTAS-A, with the exception of peroxides. Concentrations of H2O2 observed during ARCTAS-A were 2-3 times larger than those during TOPSE. The cause of this discrepancy is unresolved, and it will be shown to have important implications for conclusions related to the Arctic HOx budget. Measurements of HOx from the Penn State ATHOS instrument are available during ARCTAS and are compared to box model predictions. Model predictions show striking inconsistencies during both phases of ARCTAS between observed concentrations of HO2 and of HOx precursors, primarily H2O2 and CH2O. Using observations of precursors in the box model results in predictions of HO2 that are up to nearly a factor of 2 larger than observed. An estimated temperature-dependent terminal loss rate of HO2 to aerosol [Mao et al., 2010] was shown to be insufficient to reconcile model predictions and observations of HO2. When the terminal losses from GEOS-Chem are directly inserted into the fully constrained boxmodel, predictions of upper tropospheric HO2 decrease by no more than 15-25%. Steady state predictions of upper tropospheric CH2O are lower than observations by factors of 2-4 during both phases of ARCTAS. Likewise, steady state predictions of H2O2 are lower than observations by factors of 2-3, and are similar to concentrations measured during TOPSE. Global models suggest that there is an important transport component to the Arctic H2O2 budget not captured by steady state models. An examination of back-trajectories and observations from ARCTAS for in-situ evidence of transport on Arctic peroxide concentrations does not find a widespread persistent signal from transport, although short-lived transient features are evident. Additional possible explanations for the inconsistencies between HO2 and precursor observations are explored. A comparison of calculated and observed OH during the spring (ARCTAS-A) phase shows that median observed-to-calculated ratios are near one, but have large scatter. 40% of OH measurements below 2 km were at the limit of detection (LOD) during the spring, and analysis indicates that the scatter of raw observations at these very low concentrations is larger than the ambient variability of OH, limiting the practicality of further analysis such as finding observational evidence of the impact of halogens on the HO2/OH ratio. Alternately, during ARCTAS-B, model predictions of OH were persistently lower than observations.
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    ABSTRACT: A box model using a highly modified version of the NCAR Master Mechanism is employed to investigate the unique springtime surface layer photochemistry occurring in the high Arctic. The most comprehensive suite of atmospheric chemistry measurements made in the high Arctic to date were obtained during the Ocean-Atmosphere-Sea-Ice-Snowpack (OASIS) Barrow, AK field campaign, conducted in March and April of 2009. These measurements are examined with the help of our model, as well as laboratory experiments, with respect to sources, reactions, and fate of NOx and VOC, as well as halogen-, HOx-, and RO2-radical chemistry. Special emphasis is placed on the sequestration of NOx and the formation of acyl peroxy nitrates, as well as ozone photochemistry. This work is ongoing and the presentation will summarize first results of this effort.
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    ABSTRACT: We may have considerable optimism that concentrations of pollutants retrievable from space are quite relevant in mapping near-surface O3 smog pollution. A general relationship obtains between O3 in near-surface layers - important for air pollution exposure assessment - and ozone in a deeper region of the troposphere - the thinnest layer to be retrieved potentially by remote sensing. This relationship has been observed where plentiful measurements of North American ozonesonde network. [Chatfield ..., AE, 2011]. Does it hold for complex urbanized and industrial regions? Does it hold for O3 precursors and for the O3 chemical production rate P(O3)? The answers determine the usefulness of proposed missions like NASA's GEO-CAPE geostationary observations. The DISCOVER-AQ airborne study repeatedly made spirals over various urban, industrial, transportation, and rural sites in detail around the Baltimore-Washington area in July, 2011. We compare mixing ratios appropriately averaged over a 0.2-3 km altitude and those measured at the bottom of the spirals, 0.2-0.5 km. The "retrievable" layer 0.2-3 km was set by GEO-CAPE remote-sensing sensitivity analyses for ozone [Natraj ..., AE submitted, 2011]. Correlations were quite good, ~0.9. Detailed comparison of various sites reveals more complexity. Comparisons for NO2 and HCHO m.r. and especially log(m.r.) have similar correlation but reflect a steeper m.r. decrease with altitude. These species allows us to estimate P(O3) and its controls with useful accuracy [Chatfield ..., AE, 2010]. Comparison all layer means to O3 monitors at the surface sites is more complex, since near-surface monitors suffer very local removal processes.
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    ABSTRACT: Investigators at the National Center for Atmospheric Research have developed and deployed a state-of-the-art instrument based upon difference frequency generation absorption spectroscopy to carry out such investigations on various airborne platforms.
    Optical Instrumentation for Energy and Environmental Applications; 11/2011
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    ABSTRACT: HONO was measured by a LOPAP instrument (LOng Path Absorption Photometer) for one month during the OASIS spring 2009 campaign in Barrow, Alaska. HONO concentrations between ≤ 0.4 pptv (DL) and ˜500 pptv were measured. The very high concentrations observed on several days were caused by local direct emissions and were highly correlated with the NOx and CO data. When only "clean days" were considered, average HONO concentrations varied between ≤ 0.4 - 10 pptv. Average HONO/NOx and HONO/NOy ratios of ˜6% and ˜1% were observed, respectively, in good agreement with other remote LOPAP measurement data, but lower than measured in most other polar regions by other methods. The strong correlation between sharp peaks of OH and HONO during daytime, which was not observed for any other measured radical precursor, suggested that HONO photolysis was a major source of OH radicals in Barrow. This was supported by calculated net OH radical production by HONO and O3 photolysis for which the contribution of O3 (2%) could be neglected compared to that of HONO (98%). A net extra HONO/OH source necessary to explain elevated HONO levels during daytime of up to 90 pptv/h was determined, which was highly correlated with the actinic flux. Accordingly, a photochemical HONO source is proposed here, in good agreement with recent studies. From the higher correlation of the net HONO source with ? and [NO2] compared to ? and [NO3-], photosensitized conversion of NO2 on humic acid containing snow surfaces may be a more likely source of HONO in the polar atmosphere of Barrow than nitrate photolysis.
    Journal of Geophysical Research Atmospheres 07/2011; 116(D14). DOI:10.1029/2011JD016643 · 3.44 Impact Factor
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    ABSTRACT: We combine aircraft measurements (Second Texas Air Quality Study, Megacity Initiative: Local and Global Research Observations, Intercontinental Chemical Transport Experiment: Phase B) over the United States, Mexico, and the Pacific with a 3-D model (GEOS-Chem) to evaluate formaldehyde column (ΩHCHO) retrievals from the Ozone Monitoring Instrument (OMI) and assess the information they provide on HCHO across local to regional scales and urban to background regimes. OMI ΩHCHO correlates well with columns derived from aircraft measurements and GEOS-Chem (R = 0.80). For the full data ensemble, OMI's mean bias is −3% relative to aircraft-derived ΩHCHO (−17% where ΩHCHO > 5 � 1015 molecules cm−2) and −8% relative to GEOS-Chem, within expected uncertainty for the retrieval. Some negative bias is expected for the satellite and model, given the plume sampling of many flights and averaging over the satellite and model footprints. Major axis regression for OMI versus aircraft and model columns yields slopes (95% confidence intervals) of 0.80 (0.62–1.03) and 0.98 (0.73–1.35), respectively, with no significant intercept. Aircraft measurements indicate that the normalized vertical HCHO distribution, required by the satellite retrieval, is well captured by GEOS-Chem, except near Mexico City. Using measured HCHO profiles in the retrieval algorithm does not improve satellite-aircraft agreement, suggesting that use of a global model to specify shape factors does not substantially degrade retrievals over polluted areas. While the OMI measurements show that biogenic volatile organic compounds dominate intra-annual and regional ΩHCHO variability across the United States, smaller anthropogenic ΩHCHO gradients are detectable at finer spatial scales (∼20–200 km) near many urban areas.
    Journal of Geophysical Research Atmospheres 01/2011; 116(D5):D05303. DOI:10.1029/2010JD014870 · 3.44 Impact Factor
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    Atmospheric Chemistry and Physics 01/2011; 11(21). DOI:10.5194/acp-11-11103-2011 · 5.51 Impact Factor
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    ABSTRACT: The snowpack is a photochemically active medium which produces numerous key reactive species involved in the atmospheric chemistry of polar regions. Formaldehyde (HCHO) is one such reactive species produced in the snow, and which can be released to the atmospheric boundary layer. Based on atmospheric and snow measurements, this study investigates the physical processes involved in the HCHO air-snow exchanges observed during the OASIS 2009 field campaign at Barrow, Alaska. HCHO concentration changes in a fresh diamond dust layer are quantitatively explained by the equilibration of a solid solution of HCHO in ice, through solid-state diffusion of HCHO within snow crystals. Because diffusion of HCHO in ice is slow, the size of snow crystals is a major variable in the kinetics of exchange and the knowledge of the snow specific surface area is therefore crucial. Air-snow exchanges of HCHO can thus be explained without having to consider processes taking place in the quasi-liquid layer present at the surface of ice crystals. A flux of HCHO to the atmosphere was observed simultaneously with an increase of HCHO concentration in snow, indicating photochemical production in surface snow. This study also suggests that the difference in bromine chemistry between Alert (Canadian Arctic) and Barrow leads to different snow composition and post-deposition evolutions. The highly active bromine chemistry at Barrow probably leads to low HCHO concentrations at the altitude where diamond dust formed. Precipitated diamond dust was subsequently undersaturated with respect to thermodynamic equilibrium, which contrasts to what was observed elsewhere in previous studies.
    Journal of Geophysical Research Atmospheres 01/2011; 116:D00R03. DOI:10.1029/2011JD016038 · 3.44 Impact Factor
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    ABSTRACT: Approximately 3-20% of NOy in the flame front of biomass burning, as determined from field and lab studies, is HONO. Because HONO is readily photolyzed, it serves as a short-term reservoir for HOx and NOx. As the HONO is lost to photolysis, it affects NOy partitioning as well as O3 formation/loss. Using data from the NASA DC-8 payload during the summer 2008 ARCTAS campaign for model initialization, we have previously implemented a gas phase chemical model to evaluate the impact of HONO on the chemical evolution of one plume on 1 July 2008. We present results incorporating the ARCTAS particle phase data with the Aerosol Simulation Program and a Lagrangian parcel model to more completely represent the plume evolution.
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    ABSTRACT: Formaldehyde (HCHO) is an oxidation intermediate of hydrocarbon oxidation and can be an important source of oxidants (HOx) when photolysed. Gaseous HCHO is exchanged between the snowpack and atmosphere. Processes involved include snowpack (1) production from the photo-oxidation of organic matter, presumably present as scavenged aerosol particles or (2) exchanges of HCHO dissolved within snow crystals. Testing the relative importance of both these processes is difficult in part because we know neither the solubility of HCHO in ice as a function of partial pressure of formaldehyde (PHCHO) and temperature, nor the diffusion rate of HCHO in ice. We have therefore studied the diffusion and solubility of HCHO in ice by exposing large (8 cm) single crystals of ice to known PHCHO for several weeks. Experiments were performed between -30° C and -7° C and allowed the construction of the phase diagram of the solid solution of HCHO in ice. The diffusion coefficient was also measured and values are in the range of 10-12 to 10-11 cm2.s-1. Results from this experimental work will be compared to data obtained during the OASIS 2009 field campaign in Barrow where HCHO concentrations were measured both in snow and atmosphere.
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    ABSTRACT: We present new ground and airborne instruments for atmospheric measurements based on fiber and diode laser sources. This versatile optical technology can be configured to provide high resolution, sensitive, selective, and real-time measurements. In particular we will present current and planned instruments to measure important trace gas species, including isotopes, and 3D wind-speeds from an aircraft platform. All the instruments presented leverage technology advances made in the photonics and optical telecommunication industry. We have developed a set of tools based around these technological building blocks and used them to design a suite of measurement capabilities for use by the atmospheric research community. Optical technologies have been accumulating a proven record of robust performance, and enable one to built more lightweight and compact instrumentation for easy deployment for traditional ground, advanced sea, and airborne measurement platforms. We will present how these enabling optical technologies have served as the foundation for select instruments, and provide a roadmap for future development opportunities.
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    ABSTRACT: Aldehydes (RCHO) are key reactive intermediates in hydrocarbon oxidation and in OH cycling. They are also emitted and taken up by the snowpack and a combination of both physical and photochemical processes are likely involved. Since the photolysis of aldehydes is a source of HOx radicals, these exchanges can modify the oxidative capacity of the overlying air. Formaldehyde (HCHO), acetaldehyde (MeCHO), glyoxal (CHOCHO) and methylglyoxal (MeCOCHO) concentrations were measured in over 250 snow samples collected during the Barrow 2009 campaign between late February and mid April 2009. Both continental and marine snowpacks were studied as well as frost flowers on sea ice. We found that HCHO was the most abundant aldehyde (1 to 9 µg/L), but significant concentrations of dicarbonyls glyoxal and methylglyoxal were also measured for the first time in Arctic snow. Similar concentrations were measured for the continental and marine snowpacks but some frost flowers exhibited HCHO concentrations as high as 150 µg/L. Daily cycles in the surface snow were observed for HCHO and CH3CHO but also for the dicarbonyls and we concluded to a photochemical production of these species from organic precursors. Additional data such as gas phase concentrations for the measured aldehydes and snow physical properties (specific surface area, density ...) will be used to discuss on the location of aldehydes in the snow. This is essential to identify and quantify the physical processes that occur during the exchange of trace gases between the snow and the atmosphere.
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    ABSTRACT: We will review the state of development and applications of difference-frequency generation based laser spectrometers to atmospheric research and discuss the operating conditions and techniques that enable high precision performance for ground and airborne environments.
    Laser Applications to Chemical, Security and Environmental Analysis; 01/2010
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    ABSTRACT: Measurements of the concentrations of hydrogen peroxide (H2O2) in the gas-phase and in cloud water were obtained in the vicinity of the Carolina Coast of the United States between late January and early March, 1986. The gas-phase concentrations were always less than 2.4 ppbv and generally less than 1 ppbv. Vertical profiles of H2O2 in the clear air around clouds and storm systems were highly variable. The concentrations of H2O2 in the cloud water ranged from the detection limit of 0.3 μM to 112 μM, with the higher values generally occurring in the vicinity of lightning activity. Hydrogen peroxide concentrations in cloud water were well below those calculated to be in Henry's law equilibrium with the gas-phase concentrations of H2O2 in the cloudy air. This is attributed to the rapid depletion of aqueous-phase H2O2 as it oxidizes S(IV).
    Tellus B 01/2010; 41B(1):61 - 69. DOI:10.1111/j.1600-0889.1989.tb00125.x · 3.76 Impact Factor