James Walega

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

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Publications (34)51.72 Total impact

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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: 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: 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. · 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. · 3.44 Impact Factor
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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.
    05/2010;
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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.
    05/2010;
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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.
    05/2010;
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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: We use observations from the April 2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO<sub>x</sub>≡H+OH+peroxy radicals) and their reservoirs (HO<sub>y</sub>≡HO<sub>x</sub>+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> concentrations. Computation of HO<sub>x</sub> and HO<sub>y</sub> gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO<sub>2</sub> uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H<sub>2</sub>O<sub>2</sub>. We implemented such an uptake of HO<sub>2</sub> by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO<sub>2</sub>) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO<sub>x</sub> species and HO<sub>y</sub> reservoirs. HO<sub>2</sub> uptake by aerosol is then a major HO<sub>x</sub> and HO<sub>y</sub> sink, decreasing mean OH and HO<sub>2</sub> concentrations in the Arctic troposphere by 32% and 31% respectively. Better rate and product data for HO<sub>2</sub> uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2010; · 5.51 Impact Factor
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    ABSTRACT: We use observations from the April~2008 NASA ARCTAS aircraft campaign to the North American Arctic, interpreted with a global 3-D chemical transport model (GEOS-Chem), to better understand the sources and cycling of hydrogen oxide radicals (HO<sub>x</sub>≡H+OH+peroxy radicals) and their reservoirs (HO<sub>y</sub>≡HO<sub>x</sub>+peroxides) in the springtime Arctic atmosphere. We find that a standard gas-phase chemical mechanism overestimates the observed HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> concentrations. Computation of HO<sub>x</sub> and HO<sub>y</sub> gas-phase chemical budgets on the basis of the aircraft observations also indicates a large missing sink for both. We hypothesize that this could reflect HO<sub>2</sub> uptake by aerosols, favored by low temperatures and relatively high aerosol loadings, through a mechanism that does not produce H<sub>2</sub>O<sub>2</sub>. Such a mechanism could involve HO<sub>2</sub> aqueous-phase reaction with sulfate (58% of the ARCTAS submicron aerosol by mass) to produce peroxymonosulfate (HSO<sub>5</sub><sup>−</sup>) that would eventually convert back to sulfate and return water. We implemented such an uptake of HO<sub>2</sub> by aerosol in the model using a standard reactive uptake coefficient parameterization with γ(HO<sub>2</sub>) values ranging from 0.02 at 275 K to 0.5 at 220 K. This successfully reproduces the concentrations and vertical distributions of the different HO<sub>x</sub> species and HO<sub>y</sub> reservoirs. HO<sub>2</sub> uptake by aerosol is then a major HO<sub>x</sub> and HO<sub>y</sub> sink, decreasing mean OH and HO<sub>2</sub> concentrations in the Arctic troposphere by 48% and 45% respectively. Circumpolar budget analysis in the model shows that transport of peroxides from northern mid-latitudes contributes 50% of the HO<sub>y</sub> source above 6 km, and cloud chemistry and deposition of H<sub>2</sub>O<sub>2</sub> account together for 40% of the HO<sub>y</sub> sink below 3 km. Better rate and product data for HO<sub>2</sub> uptake by aerosol are needed to understand this role of aerosols in limiting the oxidizing power of the Arctic atmosphere.
    Atmospheric Chemistry and Physics 01/2010; · 4.88 Impact Factor
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    ABSTRACT: A precision laser spectrometer for the detection of CO(2) isotopes is reported. The spectrometer measures the fundamental absorption signatures of (13)C and (12)C isotopes in CO(2) at 4.32 microm using a tunable mid-IR laser source based on difference-frequency generation. The spectrometer attains a precision of up to 0.02 per thousand for 150 s of averaging. An overall accuracy of 0.05 per thousand was obtained when sampling various calibrated reference gases.
    Optics Letters 02/2009; 34(2):172-4. · 3.39 Impact Factor
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    ABSTRACT: Over the past two decades mid-infrared laser spectroscopy has been increasingly utilized during airborne atmospheric studies to improve our understanding of atmospheric processes and transformations. Enhancing such understanding requires a suite of ever more sensitive, selective, versatile, and fast instruments that can measure trace atmospheric constituents at and below mixing ratios of 100-parts-per-trillion-by-volume. Instruments that can carry out such measurements are very challenging, as airborne platforms vibrate, experience accelerations, and undergo large swings in cabin temperature and pressure. These challenges notwithstanding, scientists and engineers at the National Center for Atmospheric Research (NCAR) have long been employing mid-infrared absorption spectroscopy to make atmospheric measurements of important trace gases like formaldehyde (CH2O) on a variety of airborne platforms. The present paper discusses a new airborne spectrometer based upon a difference frequency generation (DFG) mid-IR laser source that was first deployed in 2006. Many of the fundamental components and concepts of this spectrometer closely follow those incorporated in our liquid-nitrogen cooled tunable lead-salt diode laser system, successfully employed for airborne CH2O measurements over the past 10 years. However, a number of significant modifications were incorporated in the new DFG spectrometer and these will be briefly discussed here along with system performance. The DFG spectrometer was recently deployed during the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign, and specific examples of its performance from this study will be discussed, as will prospects for the detection of other trace gases.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    01/2009;
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    ABSTRACT: Over the past two decades mid-infrared laser spectroscopy has been increasingly utilized during airborne atmospheric studies to improve our understanding of atmospheric processes and transformations. Enhancing such understanding requires a suite of ever more sensitive, selective, versatile, and fast instruments that can measure trace atmospheric constituents at and below mixing ratios of 100-parts-per-trillion-by-volume. Instruments that can carry out such measurements are very challenging, as airborne platforms vibrate, experience accelerations, and undergo large swings in cabin temperature and pressure. These challenges notwithstanding, scientists and engineers at the National Center for Atmospheric Research (NCAR) have long been employing mid-infrared absorption spectroscopy to make atmospheric measurements of important trace gases like formaldehyde (CH2O) on a variety of airborne platforms. The present paper discusses a new airborne spectrometer based upon a difference frequency generation (DFG) mid-IR laser source that was first deployed in 2006. Many of the fundamental components and concepts of this spectrometer closely follow those incorporated in our liquid-nitrogen cooled tunable lead-salt diode laser system, successfully employed for airborne CH2O measurements over the past 10 years. However, a number of significant modifications were incorporated in the new DFG spectrometer and these will be briefly discussed here along with system performance. The DFG spectrometer was recently deployed during the 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign, and specific examples of its performance from this study will be discussed, as will prospects for the detection of other trace gases.
    Proc SPIE 01/2009;
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    ABSTRACT: A tunable diode laser absorption spectrometer (TDLAS) was operated on the NASA DC-8 aircraft during the summer INTEX-NA study to acquire ambient formaldehyde (CH2O) measurements over North America and the North Atlantic Ocean from ~0.2 km to ~12.5 km altitude spanning 17 science flights. Measurements of CH2O in the boundary layer and upper troposphere over the southeastern United States were anomalously low compared to studies in other years, and this was attributed to the record low temperatures over this region during the summer of 2004. Formaldehyde is primarily formed over the southeast from isoprene, and isoprene emissions are strongly temperature-dependent. Despite this effect, the median upper tropospheric (UT) CH2O mixing ratio of 159 pptv from the TDLAS over continental North America is about a factor of 4 times higher than the median UT value of 40 pptv observed over remote regions during TRACE-P. These observations together with the higher variability observed in this study all point to the fact that continental CH2O levels in the upper troposphere were significantly perturbed during the summer of 2004 relative to more typical background levels in the upper troposphere over more remote regions. The TDLAS measurements discussed in this paper are employed together with box model results in the companion paper by Fried et al. to further examine enhanced CH2O distributions in the upper troposphere due to convection. Measurements of CH2O on the DC-8 were also acquired by a coil enzyme fluorometric system and compared with measurements from the TDLAS system.
    Journal of Geophysical Research Atmospheres 05/2008; 113(D10):1-16. · 3.44 Impact Factor
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    ABSTRACT: We present the airborne application of a difference frequency laser based absorption spectrometer, discuss important instrument functions and requirements, present examples of airborne measurements and its preliminary interpretation toward understanding the pollution over Houston, Texas.
    Laser Applications to Chemical, Security and Environmental Analysis; 03/2008
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    ABSTRACT: Tunable, mid-infrared lasers based on quasi-phase matched bulk PPLN crystals have successfully been implemented on airborne atmospheric research platforms and enabling a detectable fractional absorbance of about 5E-7, which equates to single digit part-per-trillion detectable concentrations for many atmospherically important trace gases. Emerging development of ridge waveguide type PPLN crystals show promising performance characteristics, including 100 times better conversion efficiency and good beam quality, which enable more compact system designs. In addition, the flexibility afforded by QPM structured materials to generate coherent mid-infrared radiation, permit unique multi-wavelength operation and detection techniques.
    Proc SPIE 03/2008;
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    ABSTRACT: 1] Measurements of formaldehyde (CH 2 O) from a tunable diode laser absorption spectrometer (TDLAS) were acquired onboard the NASA DC-8 aircraft during the summer 2004 INTEX-NA campaign to test our understanding of convection and CH 2 O production mechanisms in the upper troposphere (UT, 6–12 km) over continental North America and the North Atlantic Ocean. The present study utilizes these TDLAS measurements and results from a box model to (1) establish sets of conditions by which to distinguish ''background'' UT CH 2 O levels from those perturbed by convection and other causes; (2) quantify the CH 2 O precursor budgets for both air mass types; (3) quantify the fraction of time that the UT CH 2 O measurements over North America and North Atlantic are perturbed during the summer of 2004; (4) provide estimates for the fraction of time that such perturbed CH 2 O levels are caused by direct convection of boundary layer CH 2 O and/or convection of CH 2 O precursors; (5) assess the ability of box models to reproduce the CH 2 O measurements; and (6) examine CH 2 O and HO 2 relationships in the presence of enhanced NO. Multiple tracers were used to arrive at a set of UT CH 2 O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. In general, production of CH 2 O from CH 4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from methyl hydroperoxide, methanol, PAN-type compounds, and ketones, in descending order of their contribution. At least 70% to 73% of the elevated UT observations were caused by enhanced production from CH 2 O precursors rather than direct transport of CH 2 O from the boundary layer. In the presence of elevated NO, there was a definite trend in the CH 2 O measurement–model discrepancy, and this was highly correlated with HO 2 measurement–model discrepancies in the UT. Citation: Fried, A., et al. (2008), Role of convection in redistributing formaldehyde to the upper troposphere over North America and the North Atlantic during the summer 2004 INTEX campaign, J. Geophys. Res., 113, D17306, doi:10.1029/2007JD009760.
    Journal of Geophysical Research. 01/2008; 113.
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    ABSTRACT: We discuss the first airborne deployment and performance tests of a mid-IR difference frequency spectrometer system for highly sensitive measurements of formaldehyde. The laser system is based upon difference-frequency generation (DFG) at ~3.5 mum by mixing a DFB diode laser at 1562 nm and a distributed feedback (DFB) fiber laser at 1083 nm in a periodically poled LiNbO(3) (PPLN) crystal. Advanced LabVIEW software for lock-in, dual-beam optical noise subtraction, thermal control and active wavelength stabilization, renders a sensitivity of ~20 pptv (Absorbance ~7*10(-7)) for 30s of averaging. The instrument's performance characteristics spanning more than 300 flight hours during three consecutive airborne field missions MIRAGE, IMPEX and TexAQS operating on two airborne platforms, NCAR's C-130 and NOAA's P-3 aircraft are demonstrated.
    Optics Express 11/2007; 15(21):13476-95. · 3.55 Impact Factor
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    ABSTRACT: Tunable mid-IR laser sources based upon fiber optic pumped difference-frequency generation (DFG) have recently made significant progress. Several advances in the design of DFG sources and detection schemes permit ultra-sensitive detection of Amin~1×E-7.
    02/2006;