Global Tropospheric Experiment Research Articles

Ethane, ethyne and carbon monoxide concentrations in the upper troposphere and lower stratosphere from ACE and GEOS-Chem: a comparison study

Abstract. Near global upper tropospheric concentrations of carbon monoxide (CO), ethane (C2H6) and ethyne (C2H2) from ACE (Atmospheric Chemistry Experiment) Fourier transform spectrometer on board the Canadian satellite SCISAT-1 are presented and compared with the output from the Chemical Transport Model (CTM) GEOS-Chem. The retrievals of ethane and ethyne from ACE have been improved for this paper by using new sets of microwindows compared with those for previous versions of ACE data. With the improved ethyne retrieval we have been able to produce a near global upper tropospheric distribution of C2H2 from space. Carbon monoxide, ethane and ethyne concentrations retrieved using ACE spectra show the expected seasonality linked to variations in the anthropogenic emissions and destruction rates as well as seasonal biomass burning activity. The GEOS-Chem model was run using the dicarbonyl chemistry suite, an extended chemical mechanism in which ethyne is treated explicitly. Seasonal cycles observed from satellite data are well reproduced by the model output, however the simulated CO concentrations are found to be systematically biased low over the Northern Hemisphere. An average negative global mean bias of 12% and 7% of the model relative to the satellite observations has been found for CO and C2H6 respectively and a positive global mean bias of 1% has been found for C2H2. ACE data are compared for validation purposes with MkIV spectrometer data and Global Tropospheric Experiment (GTE) TRACE-A campaign data showing good agreement with all of them.

Long‐term and seasonal variations in the levels of hydrogen peroxide, methylhydroperoxide, and selected compounds over the Pacific Ocean

The National Aeronautics and Space Administration's Global Tropospheric Experiment program has conducted five sampling programs examining the Pacific troposphere from 1991 to 2001. Flights on two aircraft, a P3‐B and DC‐8, sampled the Pacific troposphere at altitudes from 0.5 km to 13 km and from 130°E to 115°W and from 65°N to 70°S. Long‐term trends in the observed species mixing ratios in different geographic regions are presented. Changes in species mixing ratios are discussed with respect to changes in source, transport, and photochemical production and loss processes over the decade study period. CH4 and CO2 exhibited average annual mixing ratio increases of 6.39 ppbv yr−1 and 1.92 ppmv yr−1, respectively. O3, CO, and NO all exhibit significantly higher mixing ratios in the north Pacific troposphere in the spring than in the fall. However, there is no clear annual trend for these species. H2O2 mixing ratios were 38 ± 16% higher (Δ(fall–spring) 258 ± 34 pptv), and CH3OOH was 61 ± 15% higher (Δ343 ± 76 pptv) throughout the Pacific troposphere in the fall than in the spring. During 2001, SO2 loading in the North Pacific is 2–3 times larger than during earlier years. Anthropogenic tracer gases, C2H6, C3H8, C2H2, and C2Cl4, do not exhibit comparable increases in their average mixing ratios during the decade study period. CH3I, a tracer for oceanic emission, also does not exhibit an increase in 2001. Sulfur loading characterized by SO2 also does not exhibit a clear annual trend. The enhanced sulfur loading during 2001 appears to have resulted in a suppression of H2O2 mixing ratios in the lower troposphere throughout the midlatitude North Pacific.

Regional Air Quality Modeling System (RAQMS) predictions of the tropospheric ozone budget over east Asia

The NASA Langley Research Center and University of Wisconsin Regional Air Quality Modeling System (RAQMS) is used to estimate the tropospheric ozone budget over east Asia during the NASA Global Tropospheric Experiment (GTE) Transport and Chemical Evolution over the Pacific (TRACE‐P) mission. The computed ozone budget explicitly accounts for stratosphere/troposphere exchange (STE) and in situ ozone production using on‐line chemical calculations. The east Asian O3 budget is computed during the period from 7 March to 12 April 2001. Gross formation dominates STE by a ratio of 7 to 1 in east Asia during TRACE‐P. However, this ratio is strongly influenced by altitude of the tropopause. Approximately 30% of the ozone that is advected across the tropopause over east Asia is subsequently advected out over the western Pacific within the upper 4 km of the troposphere by the Japan jet. The average net photochemical production (gross formation‐gross destruction) within the regional domain is 0.37 Tg d−1 or 7% of the average flux at the eastern boundary of the domain during the TRACE‐P time period. The budget analysis shows a very close balance between sources and sinks within the RAQMS regional domain during the TRACE‐P time period. This balance results in very small average accumulation (∼1 Tg) of O3 in the east Asian region and very little net export averaged over the period (0.03 Tg d−1). The low ozone export from east Asia predicted by RAQMS during TRACE‐P is a consequence of relatively high dry deposition rates, which are 37% of the gross ozone formation (1.469 Tg d−1) within the TRACE‐P regional domain.

A fast‐GC/MS system to measure C2 to C4 carbonyls and methanol aboard aircraft

A fast response gas chromatograph/mass spectrometer (FGCMS) instrument to measure ≤C2 through C4 carbonyl compounds and methanol was developed for the NASA Global Tropospheric Experiment (GTE) Transport and Chemical Evolution over the Pacific (TRACE‐P) mission. The system consists of four major components: sample inlet, preconcentration system, gas chromatograph (GC), and detector. The preconcentration system is a custom‐built cryogen‐conservative system. The GC is a compact, custom‐built unit that can be temperature programmed and rapidly cooled. Detection is accomplished with an Agilent Technologies 5973 mass spectrometer. The FGCMS instrument provides positive identification because the compounds are chromatographically separated and mass selected. During TRACE‐P, a sample was analyzed every 5 min. The FGCMS limit of detection was between 5 and 75 pptv, depending on the compound. The entire instrument package is contained in a standard NASA instrument rack (106 cm × 61 cm × 135 cm), consumes less than 1200 watts and is fully automated with LabVIEW™ 6i. Methods were developed for producing highly accurate gas phase standards for the target compounds and for testing the system in the presence of potential interferents. This paper presents data on these tests and on the general overall performance of the system in the laboratory and aboard the DC‐8 aircraft during the mission. Vertical profiles for acetaldehyde, methanol, acetone, propanal, methyl ethyl ketone, and butanal from FGCMS data collected over the entire mission are also presented.

Air motion intercomparison flights during Transport and Chemical Evolution in the Pacific (TRACE‐P)/ACE‐ASIA

Intercomparisons of chemical, aerosol, and meteorological measurement systems were conducted in the spring of 2001 between the NASA Wallops P3‐B and the NCAR EC‐130Q aircraft during overlapping portions of the concurrent tropospheric missions: the Global Tropospheric Experiment's (GTE) Transport and Chemical Evolution in the Pacific (TRACE‐P) and Aerosol Characterization Experiment's ACE‐ASIA mission, respectively. Both aircraft were equipped with similar air‐motion measurement systems and in situ meteorology sensors designed to measure the eddy‐correlation fluxes of momentum, heat, and water vapor. This paper presents the results of the informal intercomparison flight legs at two altitudes within the marine boundary layer performed over the Sea of Japan. The variances and spectra of the three‐dimensional winds and temperature are presented along with the cospectra of the vertical velocities and various parameters. The results show good agreement between the measurements obtained from the two aircraft. Discrepancies in the data are analyzed and discussed.

Preface to the NASA Global Tropospheric Experiment Transport and Chemical Evolution Over the Pacific (TRACE‐P): Measurements and analysis

Preface to the NASA Global Tropospheric Experiment Transport and Chemical Evolution Over the Pacific (TRACE‐P): Measurements and analysis

Regional chemical weather forecasting system CFORS: Model descriptions and analysis of surface observations at Japanese island stations during the ACE‐Asia experiment

The Chemical Weather Forecast System (CFORS) is designed to aid in the design of field experiments and in the interpretation/postanalysis of observed data. The system integrates a regional chemical transport model with a multitracer, online system built within the Regional Atmospheric Modeling System (RAMS) mesoscale model. CFORS was deployed in forecast and postanalysis modes during the NASA Global Tropospheric Experiment (GTE)‐Transport and Chemical Evolution over the Pacific (TRACE‐P), International Global Atmospheric Chemistry project (IGAC)‐International Geosphere‐Biosphere Programme (IGBP) Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), and National Oceanic and Atmospheric Administration Intercontinental Transport and Chemical Transformation of Anthropogenic Pollution 2002 (ITCT 2K2) field studies. A description of the CFORS model system is presented. The model is used to help interpret the Variability of Maritime Aerosol Properties (VMAP) surface observation data. The CFORS model results help to explain the time variation of both anthropogenic pollutants (sulfate, black carbon, and CO) and natural constituents including radon and mineral dust. Time series and time‐height cross‐section analysis of gases and aerosols are presented to help identify key processes. Synoptic‐scale weather changes are found to play an important role in the continental‐scale transport of pollution in the springtime in East Asia. The complex vertical and horizontal structure of pollutants in these outflow events is also presented and discussed.

Long‐range transport of Asian outflow to the equatorial Pacific

The second Pacific Exploratory Mission to the Tropics (PEM T‐B) was conducted as part of NASA's Global Tropospheric Experiment (GTE) from 25 February through 19 April 1999. Long‐range pollution signatures are examined during four PEM T‐B flights that ranged as far northeast as California and as far southwest as coastal New Guinea. The signatures are studied to determine their ages and chemical evolution during transport. The chemical species examined include nonmethane hydrocarbons, halocarbons, and carbon monoxide. Wind data from the European Centre for Medium Range Weather Forecasts (ECMWF) are used to calculate backward trajectories along the four flight tracks. Results show that some pollutants originating from the Asian continent, and even farther west, are transported across the Pacific by the middle latitude westerly winds and reach the subtropical Pacific anticyclone where they subside and turn southwestward under the influence of the low level trade winds. The parcels ultimately reach the western Pacific near coastal New Guinea after 20–25 days of transit.

Tropospheric ozone layers observed during PEM‐Tropics B

In this paper we analyze distinct layers seen in the tropospheric ozone and water vapor profiles taken during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission in the Tropical Pacific (PEM‐Tropics) Phase B campaign. In summary, fewer layers were observed in this campaign than during the PEM‐Tropics A campaign. However, of those layers found, there were relatively more ozone‐rich‐water‐vapor‐poor layers (68% versus 52%). The percentage of the sampled troposphere occupied by layers during PEM‐Tropics B was less than half of that found during PEM‐Tropics A (8% versus 20%). The differences between these two campaigns suggest a seasonal variation in the occurrence of layers. This is confirmed using measurements made by the Southern Hemisphere Additional Ozonesondes (SHADOZ) network.

Large‐scale latitudinal and vertical distributions of NMHCs and selected halocarbons in the troposphere over the Pacific Ocean during the March‐April 1999 Pacific Exploratory Mission (PEM‐Tropics B)

Nonmethane hydrocarbons (NMHCs) and selected halocarbons were measured in whole air samples collected over the remote Pacific Ocean during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics B (PEM‐Tropics B) in March and early April 1999. The large‐scale spatial distributions of NMHCs and C2Cl4 reveal a much more pronounced north‐south interhemispheric gradient, with higher concentrations in the north and lower levels in the south, than for the late August to early October 1996 PEM‐Tropics A experiment. Strong continental outflow and winter‐long accumulation of pollutants led to seasonally high Northern Hemisphere trace gas levels during PEM‐Tropics B. Observations of enhanced levels of Halon 1211 (from developing Asian nations such as the PRC) and CH3Cl (from SE Asian biomass burning) support a significant southern Asian influence at altitudes above 1 km and north of 10°N. By contrast, at low altitude over the North Pacific the dominance of urban/industrial tracers, combined with low levels of Halon 1211 and CH3Cl, indicate a greater influence from developed nations such as Japan, Europe, and North America. Penetration of air exhibiting aged northern hemisphere characteristics was frequently observed at low altitudes over the equatorial central and western Pacific south to ∼5°S. The relative lack of southern hemisphere biomass burning sources and the westerly position of the South Pacific convergence zone contributed to significantly lower PEM‐Tropics B mixing ratios of the NMHCs and CH3Cl south of 10°S compared to PEM‐Tropics A. Therefore the trace gas composition of the South Pacific troposphere was considerably more representative of minimally polluted tropospheric conditions during PEM‐Tropics B.

Pacific Exploratory Mission in the Tropical Pacific: PEM‐Tropics B, March‐April 1999

The Pacific Exploratory Mission ‐ Tropics B (PEM‐Tropics B) was conducted by the NASA Global Tropospheric Experiment (GTE) over the tropical Pacific Ocean in March‐April 1999. It used the NASA DC‐8 and P‐3B aircraft equipped with extensive instrumentation for measuring numerous chemical compounds and gases. Its central objective was to improve knowledge of the factors controlling ozone, OH, aerosols, and related species over the tropical Pacific. Geographical coverage ranged from 38°N to 36°S and 148°W to 76°E. Major deployment sites included Hilo, Hawaii, Christmas Island, Tahiti, Fiji, and Easter Island. PEM‐Tropics B was a sequel to PEM‐Tropics A, which was conducted in September‐October 1996 and encountered considerable biomass burning. PEM‐Tropics B, conducted in the wet season of the southern tropics, observed an exceedingly clean atmosphere over the South Pacific but a variety of pollution influences over the tropical North Pacific. Photochemical ozone loss over both the North and the South Pacific exceeded local photochemical production by about a factor of 2, implying a major deficit in the tropospheric ozone budget. Dedicated flights investigated the sharp air mass transitions at the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Extensive OH observations permitted the first large‐scale comparisons with photochemical model predictions. High concentrations of oxygenated organics were observed ubiquitously in the tropical Pacific atmosphere and may have important implications for global HOx and NOx budgets. Extensive equatorial measurements of dimethyl sulfide and OH suggest that important aspects of marine sulfur chemistry are still poorly understood.

Global distribution of carbon monoxide

This study explores the evolution and distribution of carbon monoxide (CO) using the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory three‐dimensional global chemical transport model (GFDL GCTM). The work aims to gain an improved understanding of the global carbon monoxide budget, specifically focusing on the contribution of each of the four source terms to the seasonal variability of CO. The sum of all CO sources in the model is 2.5 Pg CO/yr (1 Pg = 103Tg), including fossil fuel use (300 Tg CO/yr), biomass burning (748 Tg CO/yr), oxidation of biogenic hydrocarbons (683 Tg CO/yr), and methane oxidation (760 Tg CO/yr). The main sink for CO is destruction by the hydroxyl radical, and we assume a hydroxyl distribution based on three‐dimensional monthly varying fields given bySpivakovsky et al. [1990], but we increase this field by 15% uniformly to agree with a methyl chloroform lifetime of 4.8 years [Prinn et al, 1995]. Our simulation produces a carbon monoxide field that agrees well with available measurements from the NOAA/Climate Monitoring and Diagnostics Laboratory global cooperative flask sampling network and from the Jungfraujoch observing station of the Swiss Federal Laboratories for Materials Testing and Research (EMPA) (93% of seasonal‐average data points agree within ±25%) and flight data from measurement campaigns of the NASA Global Tropospheric Experiment (79% of regional‐average data points agree within ±25%). For all 34 ground‐based measurement sites we have calculated the percentage contribution of each CO source term to the total model‐simulated distribution and examined how these contributions vary seasonally due to transport, changes in OH concentration, and seasonality of emission sources. CO from all four sources contributes to the total magnitude of CO in all regions. Seasonality, however, is usually governed by the transport and destruction by OH of CO emitted by fossil fuel and/or biomass burning. The sensitivity to the hydroxyl field varies spatially, with a 30% increase in OH yielding decreases in CO ranging from 4–23%, with lower sensitivities near emission regions where advection acts as a strong local sink. The lifetime of CO varies from 10 days over summer continental regions to well over a year at the winter poles, where we define lifetime as the turnover time in the troposphere due to reaction with OH.

Chemical NOx budget in the upper troposphere over the tropical South Pacific

The chemical NOx budget in the upper troposphere over the tropical South Pacific is analyzed using aircraft measurements made at 6–12 km altitude in September 1996 during the Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A campaign. Chemical loss and production rates of NOx along the aircraft flight tracks are calculated with a photochemical model constrained by observations. Calculations using a standard chemical mechanism show a large missing source for NOx; chemical loss exceeds chemical production by a factor of 2.4 on average. Similar or greater NOx budget imbalances have been reported in analyses of data from previous field studies. Ammonium aerosol concentrations in PEM‐Tropics A generally exceeded sulfate on a charge equivalent basis, and relative humidities were low (median 25% relative to ice). This implies that the aerosol could be dry in which case N2O5 hydrolysis would be suppressed as a sink for NOx. Suppression of NO2O5 hydrolysis and adoption of new measurements of the reaction rate constants for NO2 + OH + M and HNO3 + OH reduces the median chemical imbalance in the NOx budget for PEM‐Tropics A from 2.4 to 1.9. The remaining imbalance cannot be easily explained from known chemistry or long‐range transport of primary NOx and may imply a major gap in our understanding of the chemical cycling of NOx in the free troposphere.

Observed distributions of nitrogen oxides in the remote free troposphere from the Nasa Global Tropospheric Experiment Programs

Reactive oxides of nitrogen are critical catalysts that can limit the formation rates of tropospheric oxidants. As a result, they control the efficiency with which the troposphere cleanses itself of the many natural and artificial compounds that would otherwise reach toxic levels. They play a pivotal role in the troposphere's capacity to generate O3via photochemical processes. Even at concentrations of a few parts per trillion by volume, as are typically seen in the remote tropical Pacific and Arctic/Antarctic regions, reactive nitrogen oxides lead to ozone formation at rates estimated to be several times larger than the influx of ozone from the stratosphere. Reflecting its multilevel importance in our evolving chemical environment, significant effort has been expended toward defining the factors that control the global distribution of reactive nitrogen oxides. Developing this understanding, however, has not been a simple task. The chemical reactivities that are inherently associated with these compounds allow them to undergo transformations on timescales that range from minutes to months. In addition, a wide variety of sources exist for this family. Some of these are near the Earth's surface (e.g., by‐products of emissions from fossil fuel combustion, biomass burning, and nitrification/denitrification of soils), whereas others are within the troposphere itself (i.e., lightning, emissions from subsonic aircraft, stratospheric intrusions, and the oxidation of reduced nitrogen compounds). As a result, large temporal and regional variations can be expected and in fact are found from this array of sources. The variations are dependent on factors ranging from the effects of soil moisture on microbial activity to the convective potential of cumulus clouds. Recent airborne field measurement campaigns have made significant progress toward unraveling many of the NOxdistribution controlling factors, particularly as they relate to the remote troposphere. This review concentrates on the progress that has been made during the last 15 years. The major focus is on compounds within the reactive nitrogen oxide family, together with associated chemically coupled species. The measurements reported are predominately those from airborne platforms that have been part of NASA's Global Tropospheric Experiment (GTE) program. However, comparison is also made with other prominent airborne studies including NASA's Airborne Arctic Stratospheric Experiment (AASE) I and II as well as the non‐U.S. programs Stratospheric Ozone Experiment (STRATOZ) II and III and International Stratospheric Chemistry (INSTAC) 1. Major findings from these programs as reflected in this review can be summarized as follows: (1) Throughout much of the remote troposphere, NOxlevels (inferred from NO observations) are at sufficient levels to significantly impact on the photochemistry of this region. This was found to be particularly true in regard to the O3production. Thus average column production of O3in the remote troposphere is estimated to be several times the average stratospheric intrusion flux of O3. (2) A persistent characteristic of observed NO is that mixing ratios in the remote upper troposphere are generally enhanced by a factor of 3 or more compared with middle and lower altitudes. This typically results in net O3production in the upper troposphere and net loss at the lowest tropospheric altitudes. (3) The largest source of NOxin the upper troposphere appears to be from lightning. However, convection of surface sources such as fossil fuel combustion and biomass burning were also found to be important. In addition, recycling of NOxthrough other odd nitrogen species may contribute significantly to maintaining the level of NOx, particularly in the upper troposphere, but much uncertainty remains concerning the mechanisms responsible for this. (4) Substantial uncertainties also still exist in the NOxproduction rate from lightning and from oxidation of NH3.

Pacific Exploratory Mission‐Tropics carbon monoxide measurements in historical context

The three‐dimensional (3‐D) distribution of carbon monoxide (CO) over the southern Pacific during the NASA Global Tropospheric Experiment Pacific Exploratory Mission‐Tropics (PEM‐T) (August‐October 1996) has been analyzed in comparison to other CO measurements. The following data sets have been used in the study: National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostic Laboratory surface level sampling; Commonwealth Scientific and Industrial Research Organization aircraft measurements over Cape Grim, Tasmania; solar spectroscopic measurements at Lauder, New Zealand; and data from two spaceborne Measurement of Air Pollution From Satellite experiments. For the PEM‐T mission back trajectories analysis and 3‐D modeling of the CO transport have been performed. It has been demonstrated that CO measurements obtained by different in situ and remote techniques can be used to build the picture of the CO climatology over the large geographical area. The structure of the CO distribution over the western part of the southern Pacific during the austral spring is mainly controlled by emission from biomass burning in Australia and Africa and subsequent long‐range transport. The prevailing westerly transport occurs in the middle and upper troposphere, whereas the marine boundary layer remains relatively clean and uniform. Barriers in the form of the Intertropical Convergence Zone and South Pacific Convergence Zone protect the equatorial area (equator to 10°S) from direct impact of biomass burning plumes from north and southwest. Consistency between the measurements taken in different years and modeling results indicates that the observed feature is a stable phenomenon. Outside the equatorial area the CO vertical distribution has a broad distinctive maximum at the altitude range 5–8 km and latitudes between 20°S and 30°S. This maximum is a stable feature, and its location indicates the area where the most intensive westerly transport occurs.

Distributions of brominated organic compounds in the troposphere and lower stratosphere

A comprehensive suite of brominated organic compounds was measured from whole air samples collected during the 1996 NASA Stratospheric Tracers of Atmospheric Transport aircraft campaign and the 1996 NASA Global Tropospheric Experiment Pacific Exploratory Mission‐Tropics aircraft campaign. Measurements of individual species and total organic bromine were utilized to describe latitudinal and vertical distributions in the troposphere and lower stratosphere, fractional contributions to total organic bromine by individual species, fractional dissociation of the long‐lived species relative to CFC‐11, and the Ozone Depletion Potential of the halons and CH3Br. Spatial differences in the various organic brominated compounds were related to their respective sources and chemical lifetimes. The difference between tropospheric mixing ratios in the Northern and Southern Hemispheres for halons was approximately equivalent to their annual tropospheric growth rates, while the interhemispheric ratio of CH3Br was 1.18. The shorter‐lived brominated organic species showed larger tropospheric mixing ratios in the tropics relative to midlatitudes, which may reflect marine biogenic sources. Significant vertical gradients in the troposphere were observed for the short‐lived species with upper troposphere values 40–70% of the lower troposphere values. Much smaller vertical gradients (3–14%) were observed for CH3Br, and no significant vertical gradients were observed for the halons. Above the tropopause, the decrease in organic bromine compounds was found to have some seasonal and latitudinal differences. The combined losses of the individual compounds resulted in a loss of total organic bromine between the tropopause and 20 km of 38–40% in the tropics and 75–85% in midlatitudes. The fractional dissociation of the halons and CH3Br relative to CFC‐11 showed latitudinal differences, with larger values in the tropics.

Influence of southern hemispheric biomass burning on midtropospheric distributions of nonmethane hydrocarbons and selected halocarbons over the remote South Pacific.

Aircraft measurements of nonmethane hydrocarbons (NMHCs) and halocarbons were made over the remote South Pacific Ocean during late August‐early October 1996 for NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A). This paper discusses the large‐scale spatial distributions of selected trace gases encountered during PEM‐Tropics A. The PEM‐Tropics A observations are compared to measurements made over the southwestern pacific in early November 1995 as part of Aerosol Characterization Experiment (ACE 1). Continental pollution in the form of layers containing elevated levels of O3 was observed during a majority of PEM‐Tropics flights, as well as during several ACE 1 flights. The chemical composition of these air masses indicates that they were not fresh and were derived from nonurban combustion sources. The substantial impact of biomass burning on the vertical structure of the South Pacific troposphere is discussed.

Ozone and aerosol distributions and air mass characteristics over the South Pacific during the burning season

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to establish a baseline chemical signature of the atmosphere above the South Pacific Ocean during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A) conducted in August‐October 1996. This paper discusses general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large‐scale variations in ozone (O3) and aerosol distributions across the troposphere, calculated potential vorticity (PV) from the European Centre for Medium‐Range Weather Forecasting (ECMWF), and in situ measurements for comprehensive air mass composition. Between 8°S and 52°S, biomass burning plumes containing elevated levels of O3, over 100 ppbv, were frequently encountered by the aircraft at altitudes ranging from 2 to 9 km. Air with elevated O3 was also observed remotely up to the tropopause, and these air masses were observed to have no enhanced aerosol loading. Frequently, these air masses had some enhanced PV associated with them, but not enough to explain the observed O3 levels. A relationship between PV and O3 was developed from cases of clearly defined O3 from stratospheric origin, and this relationship was used to estimate the stratospheric contribution to the air masses containing elevated O3 in the troposphere. The frequency of observation of the different air mass types and their average chemical composition is discussed in this paper.

Characterization of the chemical signatures of air masses observed during the PEM experiments over the western Pacific

Extensive observations of tropospheric trace species during the second NASA Global Tropospheric Experiment Western Pacific Exploratory Mission (PEM‐West B) in February‐March 1994 showed significant seasonal variability in comparison with the first mission (PEM‐West A), conducted in September‐October 1991. In this study we adopt a previously established analytical method, i.e., the ratio C2H2/CO as a measure of the relative degree of atmospheric processing, to elucidate the key similarities and variations between the two missions. In addition, the C2H2/CO ratio scheme is combined with the back‐trajectory‐based and the LIDAR‐based air mass classification schemes, respectively, to make in‐depth analysis of the seasonal variation between PEM‐West A and PEM‐West B (hereinafter referred to as PEM‐WA and PEM‐WB). A large number of compounds, including long‐lived NMHCs, CH4, and CO2, are, as expected, well correlated with the ratio C2H2/CO. In comparison with PEM‐WA, a significantly larger range of observed C2H2/CO values at the high end for the PEM‐WB period indicates that the western Pacific was more impacted by “fresher” source emissions, i.e., faster or more efficient continental outflow. As in the case of PEM‐WA, the C2H2/CO scheme complements the back‐trajectory air mass classification scheme very well. By combining the two schemes, we found that the atmospheric processing in the region is dominated by atmospheric mixing for the trace species analyzed. This PEM‐WB wintertime result is similar to that found in PEM‐WA for the autumn. In both cases, photochemical reactions are found to play a significant role in determining the background mixing ratios of trace gases, and in this way the two processes are directly related and dependent upon each other. This analysis also indicates that many of the upper tropospheric air masses encountered over the western Pacific during PEM‐WB may have had little impact from eastern Asia's continental surface sources. NOx mixing ratios were significantly enhanced during PEM‐WB when compared with PEM‐WA, in the upper troposphere's more atmospherically processed air masses. These high levels of NOx resulted in a substantial amount of photochemical production of O3. A lack of corresponding enhancements in surface emission tracers strongly implies that in situ atmospheric sources such as lightning are responsible for the enhanced upper tropospheric NOx. The similarity in NOx values between the northern (higher air traffic) and southern continental air masses together with the indications of a large seasonal shift suggests that aircraft emissions are not the dominant source. However, photochemical recycling cannot be ruled out as this in situ source of NOx.

Stable carbon isotopic composition of atmospheric methane: A comparison of surface level and free tropospheric air

We report CH4 mixing ratios and δ13C of CH4 values for remote air at two ground‐based atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)‐Tropics A. Surface values of δ13C‐CH4 ranged from −47.02 to −47.52‰ at Niwot Ridge, Colorado (40°N, 105°W), and from −46.81 to −47.64‰ at Montaña de Oro, California (35°N, 121°W). Samples for isotopic analysis were taken from 2° to 27°S latitude and 81° to 158°W longitude and from sea level to 11.3 km in altitude during the PEM‐Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At ∼11 km, δ13C‐CH4 was ∼1‰ greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in δ13C‐CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two‐dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model‐calculated values for δ13C‐CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups.