NASA Global Tropospheric Research Articles

Large‐scale air mass characteristics observed over western Pacific during summertime

Remote and in situ measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the western Pacific during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission‐West A (PEM‐West A) conducted in September–October 1991. This paper discusses the 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 and airborne in situ instrumentation for comprehensive measurements of air mass composition. In low latitudes of the western Pacific the airflow was generally from the east, and under these conditions the air was observed to have low aerosol loading and low ozone levels throughout the troposphere. Ozone was found to be below 10 parts per billion volume (ppbv) near the surface to 40–50 ppbv in the middle to upper troposphere. In the middle and high latitudes the airflow was mostly westerly, and the background O3 was generally less than 55 ppbv. On 60% of the PEM‐West A flights, O3 was observed to exceed these levels in regions that were determined to be associated with stratospheric intrusions. In convective outflows from typhoons, near‐surface air with low ozone (<25 ppbv) was transported into the upper troposphere (>10 km). Several cases of continental plumes from Asia were observed over the Pacific during westerly flow conditions. These plumes were found in the lower troposphere with ozone levels in the 60–80 ppbv range and enhanced aerosol scattering. At low latitudes over the central Pacific the troposphere primarily contained air with background or low ozone levels; however, stratospherically influenced air with enhanced ozone (40–60 ppbv) was observed several times in the lower troposphere. The frequency of observation of the air masses and their average chemical composition are also discussed in this paper.

Airborne measurements of total sulfur gases during NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3

A metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) technique was used by investigators from the University of Idaho (UI) to measure ambient total sulfur gas concentrations from an aircraft platform during the NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3 (GTE/CITE 3) program. The MFC/FD/FPD technique allowed rapid quantitation of tropospheric background air masses using sample integration times of 1–3 min with little or no gap between measurements. The rapid and continual sampling nature of this technique yielded data covering approximately 75% of the entire CITE 3 program's air track. Ambient air measurement data obtained during northern hemisphere (NH) flights often exhibited relatively high total sulfur gas values (up to 19 ppb) and an extremely high degree of sample heterogeneity, especially in coastal locations. Data from southern hemisphere (SH) flights typically exhibited relatively low total sulfur gas concentrations and a low degree of sample heterogeneity. A bimodal interhemispheric total sulfur gas gradient was observed using data obtained during transit flights between the two CITE 3 program ground bases. Comparisons were made of UI total sulfur gas measurements with composite sulfur gas values generated using speciated sulfur gas measurements from other CITE 3 participants. Only a relatively small number of overlap periods for comparison were obtained from all the available CITE 3 data because of large differences in measurement integration times and lack of synchronization of sample start/stop times for the various investigators. These effects were compounded with extreme sample heterogeneity in the NH and the speed at which the aircraft traversed the air masses being sampled. Despite these constraints, sufficient overlapping data were available for the comparative evaluations. Comparison of NH UI total with composite sulfur gas values showed excellent correlation and linear curve fit, indicating substantial qualitative agreement. Simple linear regression of total on composite sulfur gas data yielded a slope of 1.9 for coastal NH regions and 1.2 for marine NH regions. The marine NH slope is not statistically different from one, indicating substantial quantitative agreement between UI total and composite sulfur gas values in these regions. However, a significant difference was observed when these same data were treated with a paired t test. SH data exhibited no significant correlation or linear regression slope. A paired t test showed a statistically significant difference when all SH flights were used. However, data from three SH flights that were classified into a unique group using discriminant analysis showed no significant difference between UI total and composite sulfur gas values when analyzed with the paired t test.

Summertime tropospheric ozone distributions over central and eastern Canada

Ozone measurements were obtained between the surface and the 6‐km altitude on aircraft flights over central and eastern Canada during the summer 1990 NASA Global Tropospheric Experiment Arctic Boundary Layer Expedition (GTE/ABLE 3B). Tropospheric O3 budgets for these regions were observed to be highly variable and significantly impacted by long‐range transport and regional scale air mass modification processes. For example, integrated O3 abundance below 5‐km altitude averaged 40% and 30% greater in air masses influenced by anthropogenic sources and biomass burning, respectively, than in background (polar) air. Conversely, aged air transported from subtropical areas of the Pacific at times reduced O3 abundance in this height interval by up to 20%. Though intrusion of anthropogenic air was infrequent during the experiment period, the influence of biomass‐burning emissions was particularly notable as two thirds of the flights sampled air influenced by plumes from fires burning in Alaska and western Canada. The impinging pollution, both natural and anthropogenic, not only elevated O3 levels directly but also was a source of reactive nitrogen (and nonmethane hydrocarbons) which generally increases the tropospheric lifetime of O3 via moderation of photochemical destruction rates.

Ozone and aerosol distributions in the summertime troposphere over Canada

Measurements of ozone (O3) and aerosol distributions were made with an airborne lidar system in the lowland and boreal forest regions of eastern Canada during July–August 1990 as part of the NASA Global Tropospheric Experiment/Arctic Boundary Layer Expedition (ABLE) 3B. Aerosol and O3 profiles were measured simultaneously above and below the Electra aircraft from near the surface to above the tropopause on long‐range flights over these important ecosystems. A broad range of atmospheric conditions were encountered during repeated flights over intensive study sites in the Hudson Bay lowlands near Moosonee, Ontario, and over the boreal forest near Schefferville, Quebec. The tropospheric composition in this high‐latitude region was found to be strongly influenced by stratospheric intrusions. Regions of low aerosol scattering and enhanced O3 mixing ratios were correlated with descending air from the lower stratosphere. Over 33% of the troposphere (0–12 km) along our flight track at latitudes from about 45° to 55°N had significantly enhanced O3 due to stratospheric intrusions, and in the middle to upper troposphere the extent of the enhanced O3 generally exceeded 40%. Ozone mixing ratios of 80 parts per billion by volume (ppbv) near 6 km were common in strong intrusions. In the boundary layer over the lowlands, O3 was in the 20–30 ppbv range with a vertical O3 gradient of 6.7 ppbv km−1 to about 45 ppbv at 3 km. Above 6 km the background tropospheric O3 profile was nearly constant with an average value of 53 ppbv. Due to forest fires in Canada and Alaska, plumes from biomass‐burning sources were observed on many flights. Biomass‐burning plumes influenced about 25% of the free troposphere below 4 km, and in some of the plumes, O3 was enhanced by 10–20 ppbv over ambient levels of 30–45 ppbv. Several air masses transported from the tropical Pacific were observed over Canada in the middle to upper troposphere with O3 levels 10–20 ppbv below background values of 50–55 ppbv.

Ocean color and atmospheric dimethyl sulfide: On their mesoscale variability

The mesoscale variability of dimethyl sulfide (DMS) and ocean color is explored to determine the feasibility of a predictive relationship. During NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 3), simultaneous shipboard and aircraft studies were carried out in the North Atlantic, followed by aircraft studies in the South Atlantic. Surface concentrations of chlorophyll a were measured with an airborne spectroradiometer, the Ocean Data Acquisition System (ODAS), with simultaneous determinations of tropospheric DMS. Shipboard measurements of DMS in air and water as well as in situ chlorophyll a were taken in the North Atlantic. No relation was observed between shipboard aquatic DMS and chlorophyll a or primary productivity. Higher levels of aqueous DMS were not always reflected by atmospheric DMS, although shipboard and aircraft measurements of atmospheric DMS agreed very well. A significant relationship between atmospheric DMS and ocean color was seen once at low altitudes in both the North and South Atlantic only under clean air conditions. Atmospheric DMS levels during the North Atlantic experiment were probably lowered by the presence of mostly polluted air masses in the study area and were, overall, probably not representative of the in situ sea‐to‐air flux of DMS. Changes in concentration of aircraft‐sensed chlorophyllous pigments were not reflected by atmospheric DMS. If a predictive algorithm is to be found, phytoplankton blooms should probably be the first place to study an ocean color‐DMS relationship.

CITE 3 meteorological highlights

Meteorological highlights from the third NASA Global Tropospheric Experiment Chemical Instrumentation Test and Evaluation (GTE/CITE 3) are presented. During August and September 1989, research flights were conducted from Wallops Island, Virginia, and Natal, Brazil, and included airborne sampling of air masses over adjacent regions of the Atlantic Ocean. Isentropic backward trajectory calculations, wind vector/streamline fields, rawinsonde data, and GOES and METEOSAT satellite imagery are utilized to examine the meteorological conditions for each flight and to determine the transport paths of the sampled air masses. Some aspects of the chemical signatures of the sampled air are also discussed. During the series of flights based at Wallops Island, Virginia, the flow into the experiment area was governed primarily by the position of the North Atlantic subtropical anticyclone. The large‐scale tropospheric circulation switched from primarily a marine flow during flights 1–4, to a predominantly offshore mid‐latitude continental flow during flights 5–10. During these later flights, the regional influences of large eastern U.S. cities along with vertical mixing by typical summertime convective activity strongly influenced the chemical characteristics of the sampled air. During the series of flights based at Natal, Brazil, the dominant synoptic feature was the South Atlantic subtropical anticyclone which generally transported air across the tropical Atlantic toward eastern Brazil. Pronounced subsidence and a well‐defined trade wind inversion often characterized the lower and middle troposphere over the Natal region. Some high‐altitude recirculation of air from South America was observed, as was cross‐equatorial transport which had come from northern Africa. Biomass burning plumes were observed on segments of all of the flights, the source region being the central and southern savannah regions of Africa.

The impact of U.S. continental outflow on ozone and aerosol distributions over the western Atlantic

Aircraft measurements of selected trace gas species, aerosols, and meteorological parameters were performed in the lower troposphere off the U.S. east coast during August and September 1989 as part of the NASA Global Tropospheric Experiment (GTE) Chemical Instrumentation Test and Evaluation (CITE 3) expedition. In this paper, we examine these data to assess the impact of continental outflow on western Atlantic O3 and small aerosol budgets. Results show that mixed layer (ML) O3 concentrations and small aerosol number densities (Np) were enhanced by factors of 3 and 6, respectively, within air masses of predominantly continental origin compared with clean maritime background air. These enhancements exhibited a marked altitude dependence, declining rapidly above the ML to the point where only slight to moderate differences in O3 and Np, respectively, were notable above 2.4 km. Within continentally influenced ML's, both O3 and Np were correlated with CO, exhibiting linear regression slopes averaging 0.4 ppbv(O3)/ppbv(CO) for O3 and 7.7 (particles cm−3)/ppbv(CO) for Np and indicating a primarily anthropogenic origin for the observed enhancement of these species. Comparisons between profiles in continental and background maritime air masses suggest that photochemical production below 1.4‐km altitude adds over 10% to western Atlantic tropospheric column O3 abundance in continental outflow regimes. For aerosols, eastward advection of low‐level continental air contributes an average net flux of 2.8 metric tons of submicron (accumulation mode) particles per kilometer of shoreline per day to the western Atlantic troposphere.

Beryllium 7 and Lead 210 in the western hemisphere Arctic atmosphere: Observations from three recent aircraft‐based sampling programs

Concentrations of the natural radionuclides 7Be and 210Pb were determined in aerosol samples collected in the western hemisphere Arctic during the recent NOAA Arctic Gas and Aerosol Sampling Program (AGASP 3) and NASA Global Tropospheric Experiment/Arctic Boundary Layer Expeditions (GTE/ABLE 3A and ABLE 3B) missions. Beryllium 7 showed a free tropospheric concentration maximum between 4 and 5 km in the summer of 1990. Previous 7Be data obtained in the late 1950s and early 1960s also indicated a similar vertical distribution of 7Be near 70°N. Injection of stratospheric air through tropopause folds associated with the Arctic jet near 70°N appears to explain the presence of a layer of air near 4–5 km in the high Arctic free troposphere with elevated 7Be concentrations. The vertical distribution of 210Pb showed a distinct difference between the high‐Arctic and sub‐Arctic in the summer of 1988. At latitudes greater than 65°N, 210Pb concentrations at 3–6 km were elevated compared to those below 1 km. The reverse of this trend was observed near 60°N. These same vertical distributions were also apparent in aerosol SO42−, determined in separate aerosol samples collected on the same flights (Talbot et al., this issue). The results for 210Pb suggest that some of the difference between the summer troposphere in the high‐ and sub‐Arctic is also due to enhanced stratosphere‐troposphere exchange in the vicinity of the Arctic jet. These observations, and other findings from ABLE 3A presented in this issue, suggest that for some species the stratosphere may be a principal source influencing their distribution in the Arctic summer troposphere. For example, intrusions of stratospheric air constitute the dominant source term for tropospheric budgets of 7Be and ozone, and may be important in the 210Pb, SO42−, and NOy budgets. Further investigation, including determination of detailed 7Be and 210Pb distributions, is needed to quantify the stratospheric impact on the chemistry of the Arctic troposphere during the summer.

Airborne flux measurements of trace species in an Arctic boundary layer

In situ airborne measurements of the turbulent flux and mean values for O3, CO, and CH4 were obtained in the boundary layer over selected wetland systems in Alaska. These measurements were obtained in July–August 1988 as part of the NASA Global Tropospheric Experiment Program's Arctic Boundary Layer Expedition (ABLE 3A). The flux measurements obtained from this study provide information on the source/sink distribution of O3 and CH4 over the Yukon‐Kuskokwim Delta (YKD) and Alaskan North Slope (ANS) regions of Alaska. The source/sink distribution over the YKD is qualitatively correlated with surface vegetation type, identified from multispectral scanner imagery. Direct measurements of the spatial variation in the CH4 source strength were obtained over the YKD. The CH4 source strength over the YKD ranged from 25 to 85 mg m−2 d−1 during a flux survey flight which spanned a considerable portion of the YKD. A spatially averaged, seasonally adjusted source strength of 51 mg m−2 d−1 was established for the YKD. Indirect CH4 flux estimates obtained over the ANS indicate a much lower (∼10 mg m−2 d−1) source strength. The global CH4 emissions from tundra were estimated to be 44 Tg/a based on (1) the spatially averaged source strength obtained over the YKD, (2) current estimates of the global coverage of tundra, and (3) assuming a similarity between other tundra areas and that of the YKD. This estimate is taken to be an upper limit due to possible sampling inadequacies and because the spatial distribution of the CH4 source function over the YKD may not extend to all other northern wetland regions. This estimate is, however, in reasonable agreement with previous estimates. Airborne CO flux measurements over the YKD indicated low negative flux values over the coastal areas, while some positive fluxes were observed in the inland, sparsely forested regions. An inspection of the cospectrum of CO with vertical velocity for sample runs in coastal areas indicated a minimum at wavelengths which were noticeably shorter (70–400 m) than where any prominent feature could be found for similar cospectrums of heat, moisture, O3, or CH4 with vertical velocity. Similar analyses for transects over inland areas indicated occasional peaks in this same, short wavelength band. These features indicate the possibility of in situ photochemical destruction/production of CO, although the identification of a possible chemical mechanism was not attempted at this time. Tundra surfaces are estimated to be responsible for ∼32% of the total deposition loss of O3 poleward of 60°N. Data from the airborne O3 flux measurements made during this study indicate similar values of Rc for the ANS and YKD regions.

Large‐scale variability of ozone and aerosols in the summertime Arctic and sub‐Arctic troposphere

Measurements of ozone (O3) and aerosol distributions were made with an airborne lidar system in the Arctic and sub‐Arctic during July–August 1988 as part of the NASA Global Tropospheric Experiment/Arctic Boundary Layer Expedition (ABLE 3A). Aerosol and O3 profiles were measured simultaneously above and below the Electra aircraft from near the surface to above the tropopause. In situ measurements of O3 mixing ratios and aerosol size distributions and number densities were also made on the aircraft. Many different atmospheric conditions were investigated on long‐range survey flights in the Arctic and on intensive flights over the tundra, ice, and marine regions near Barrow and Bethel, Alaska. The tropospheric composition at high latitudes was found to be strongly influenced by stratospheric intrusions. Regions of low‐aerosol scattering and enhanced O3 mixing ratios were correlated with descending air from the lower stratosphere. Over 37% of the troposphere along our flight track at latitudes >57°N had significantly enhanced O3 levels due to stratospheric intrusions, and in the 4‐ to 6‐km altitude range the tropospheric extent of the enhanced O3 exceeded 56%. Ozone mixing ratios of 80 ppbv at 6 km were common, with vertical O3 gradients of over 11 ppbv km−1 observed across the base of strong intrusions. In the mixed layer over the tundra, O3 was in the 25–35 ppbv range with a gradient of 5.5 ppbv km−1, while in continental polar air masses, the average gradient in the lower troposphere was 7.4 ppbv km−1, indicating more downward transport of O3 at higher latitudes. Due to the many forest fires that year, plumes from biomass burning sources were observed on several flights over Alaska. Plumes influenced about 10% of the air below 4 km, and in some photochemically active plumes, O3 was enhanced by 10–20 ppbv over ambient levels. Pollution plumes from industrial sources were infrequently observed; however, a few large plumes were found over the North Pacific with greatly enhanced aerosol scattering and with O3 levels exceeding 75 ppbv.

Measurements of nitric acid and aerosol nitrate at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 1988

Ambient air concentrations of nitric acid vapor and aerosol nitrate were measured above the remote North Pacific between May 1 and June 5, 1988, during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). Nighttime (free tropospheric) mean values were 30 pptv nitrate aerosol and 103 pptv nitric acid. Daytime values, representative of island‐modified marine boundary layer air, averaged 60 pptv aerosol and 130 pptv nitric acid. The measurements were generally consistent with previous measurements in the remote North Pacific including free tropospheric measurements of nitric acid during NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 2). MLOPEX observations are smaller than model HNO3/NOx ratios. However, the magnitude of this discrepancy is obscured by uncertainties, mainly in the model OH concentration and nitric acid photolysis rates. Nonetheless, large observed variations in the HNO3/NOx ratio likely indicate significant loss of nitric acid, at least part of the time, by some process in addition to vapor phase photolysis and destruction by OH. Several processes are considered, but no firm conclusion is reached.

Total reactive oxidized nitrogen (NOy) in the remote Pacific troposphere and its correlation with O3 and CO: Mauna Loa Observatory Photochemistry Experiment 1988

As part of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX) total reactive oxidized nitrogen (NOy) was measured during May and early June of 1988 at the Mauna Loa Observatory, the NOAA‐Geophysical Monitoring for Climatic Change Baseline Monitoring Station, located at 3.4‐km elevation on the island of Hawaii. Gold catalytic surface conversion of individual reactive oxidized nitrogen species to NO and subsequent quantification of the NO by NO/O3 chemiluminescence was used to measure the NOy mixing ratio. The NOy abundance at the site was governed by the local downslope/upslope wind systems as well as synoptic‐scale transport. With some exceptions, downslope wind brought air representative of the free troposphere, while upslope winds transported air from below the trade wind inversion to the site. The upslope air masses could be a mix of marine boundary layer air and free tropospheric air modified by anthropogenic and natural emissions from island sources. It was possible to identify free tropospheric air in the downslope flow through meteorological and chemical tracers. Reflecting the remote location, low NOy mixing ratios with median values of 262 and 239 pptv were found in free tropospheric and upslope air masses, respectively. The median NOy levels in free tropospheric air are consistent with airborne NOy measurements made during NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (CITE 2) program over the northeastern Pacific Ocean at corresponding altitudes. The median NOy values in upslope flow are significantly higher than those measured in the remote marine boundary layer during CITE 2, reflecting probably the influence of island source and/or mixing of free tropospheric air with boundary layer air. The low correlation found between NOy and tracers of anthropogenic sources, such as carbon monoxide, tetrachloroethylene, and n‐propyl nitrate, in free tropospheric air samples is consistent with a stratospheric or upper tropospheric source for NOy. Simultaneous particulate nitrate (NO3−) measurements suggest that at times not all aerosol NO3− was quantitatively converted to NO by the Au‐surf ace converter technique. These episodes were usually found during upslope flow and were characterized by high sodium concentrations, suggesting that possibly the sodium nitrate contained in these aerosols was not converted efficiently by the Au converter.

Photochemical ozone production in tropical squall line convection during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A

We have examined the role of convection in trace gas transport and ozone production in a tropical dry season squall line sampled on August 3, 1985, during NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment 2A (NASA GTE/ABLE 2A)in Amazonia, Brazil. Two types of analyses were performed. (1) Transient effects within the cloud are examined with a combination of two‐dimensional cloud and one‐dimensional photochemical modeling. Tracer analyses using the cloud model wind fields yield a series of cross sections of NOx, CO, and O3 distribution during the lifetime of the cloud; these fields are used in the photochemical model to compute the net rate of O3 production. At noon, when the cloud was mature, the instantaneous ozone production potential in the cloud is between 50 and 60% less than in no‐cloud conditions due to reduced photolysis and cloud scavenging of radicals. (2) Analysis of cloud inflows and outflows is used to differentiate between air that is undisturbed and air that has been modified by the storm. These profiles are used in the photochemical model to examine the aftereffects of convective redistribution in the 24‐hour period following the storm. Total tropospheric column O3 production changed little due to convection because so little NOx was available in the lower troposphere. However, the integrated O3 production potential in the 5‐ to 13‐km layer changed from net destruction to net production as a result of the convection. The conditions of the August 3, 1985, event may be typical of the early part of the dry season in Amazonia, when only minimal amounts of pollution from biomass burning have been transported into the region.

Ozone and aerosol distributions over the Amazon Basin during the wet season

Measurements of ozone (O3) and aerosols were made over the tropical rain forest of Brazil during the wet season in April–May 1987 as part of the NASA Global Tropospheric Experiment to study the Amazon boundary layer. Remote and in situ measurements of O3 and aerosols were made from aircraft on flights over Brazil in the vicinity of Manaus and between Manaus and Belem. Ozonesonde data were also obtained near Manaus. Ozone mixing ratios of <12 ppbv were found in the mixed layer during the wet season with no significant evidence of O3 produced from biomass burning or photochemistry. These values are lower than those found during the 1985 dry season by 6–8 ppbv. These low O3 mixing ratios indicate a strong removal process near the surface during the wet season. The region from the mixed layer top to 3 km in altitude had a slowly increasing O3 profile from 12 to 20 ppbv. On long‐range flights between Manaus and Belem, no significant difference was found in the distribution of O3 above the mixed layer between the inland tropical rain forest and the marine conditions near the coast. Within the mixed layer, there was a definite trend to lower O3 levels above the forest compared to over the ocean. This reflects the marked difference in the sinks for O3 over these two regions. The rate of growth of the mixed layer over the rain forest in the wet season was found to be ∼9 cm s−1, which is within the 7–10 cm s−1 range found for the dry season. There was no evidence of the trade wind inversion that was seen during the dry season, and due to frequent precipitation, the background aerosol loading was lower in the wet season than in the dry season.

Aerosol characteristics and sources for the Amazon Basin during the wet season

As a part of the NASA Global Tropospheric Experiment (GTE), aerosols were sampled in the tropical rain forest of the Amazon Basin during the Amazon Boundary Layer Experiment (ABLE 2B) in April and May 1987, in the wet season, when no forest burning occurs. Fine (dp < 2.0 μm) and coarse (2.0 < dp < 15 μm) aerosol fractions were collected using stacked filter units, at three sites under the forest canopy and at three levels of a tower inside the jungle. Particle‐induced X ray emission (PIXE) was used to measure concentrations of 22 elements (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Zr, and Pb). Morphological and trace element measurements of individual particles were carried out by automated electron probe X ray microanalysis. Gravimetric analysis was performed to obtain the fine and coarse aerosol mass concentration. Absolute factor analysis was used to interpret the large data set of the trace element concentrations and to obtain elemental source profiles. Hierarchical cluster analysis was used to derive groups of individual particles. The concentrations of soil dust related elements (Al, Si, Ti, Fe, Mn) were 5 times larger in the wet season compared to the 1985 ABLE 2A dry season experiment. Biogenic aerosol related elements in the fine fraction showed lower concentrations in the wet season. Fine aerosol mass concentration averaged only 2.1±0.7 μg m−3, while the average coarse mass concentration was 6.1±1.8 μg m −3. Sulphur concentrations averaged 76±14 ng m −3 in the fine fraction and 37±9 ng m −3 in the coarse fraction. Biogenic aerosol‐related elements were dominant under the forest canopy, while soil dust dominated at the top of the forest canopy. Only two factors explained about 90% of the data variability for the fine and coarse aerosol fractions. These were soil dust (represented mainly by Al, Si, Ti, Mn, and Fe) and biogenic aerosol (represented by K, P, Cl, S, Zn, and the aerosol mass concentration). Source profiles showed a homogeneous aerosol distribution with similar elemental compositions at the different sampling sites. Enrichment factor calculations revealed a soil dust elemental profile similar to the average bulk soil composition, and a biogenic component similar to the plant bulk elemental composition. Total aerosol mass source apportionment showed that biogenic particles account for 55–95% of the airborne concentrations. The analysis of individual aerosol particles showed that the biogenic particles consist of leaf fragments, pollen grains, fungi, algae, and other types of particles. Several groups of particles with K, Cl, P, S, and Ca as minor elements could easily be identified as biogenic particles on the basis of their morphology. Considering the vast area of tropical rain forests and the concentrations measured in this work, it is possible that biogenic particles can play an important role in the global aerosol budget and in the global biogeochemical cycles of various elements.

Carbon monoxide over the Amazon Basin during the wet season

Measurements of carbon monoxide (CO) were made over the Amazon Basin of Brazil during the 1987 wet season as part of the NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment (ABLE 2B). The distribution of CO over the altitude range sampled (0.15–4.5 km) was influenced by surface emissions from biological sources, long‐range transport of pollutants from northern hemisphere sources, and by transport processes associated with local convective mixing. Surface sources are indicated by a qualitative interpretation of the typical pattern of decreasing concentrations with increasing altitude and increasing concentrations of CO at 0.15‐km altitude during a transect from the Atlantic coast to the central basin. Atmospheric convective activity produced irregular patterns of variability at time scales of less than 1 hour over a localized area. The disruption of mixed layer growth and decay processes has a particularly important influence on CO concentrations in the daytime lower troposphere. Intrusions of northern hemisphere air into the central Amazon resulted in increased CO concentrations in the lower and midtroposphere. The correlation of CO with O3 was positive under conditions interpreted as being influenced by northern hemisphere air, and negative during all other meterological conditions experienced in ABLE 2B.

Reference NO2 calibration system for ground‐based intercomparisons during NASA's GTE/CITE 2 mission

An NO2 calibration system, based upon a permeation device and a two‐stage dynamic dilution system, was designed, constructed, and characterized at the National Bureau of Standards. In this system, calibrant flow entering the second stage was controlled without contacting a metal flow controller, and permeation oven temperature and flow were continuously maintained, even during transport. The system performance and the permeation emission rate were characterized by extensive laboratory tests based on gravimetry, chemiluminescence detection, and tunable diode laser absorption spectroscopy. This system was capable of accurately delivering known NO2 concentrations in the ppbv and sub‐ppbv concentration range with a total uncertainty of approximately 10%. Upon completion of these tests the calibration system was placed on board NASA's research aircraft at both the Wallops Island and Ames research facilities. There it was employed as the reference standard in NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 2 mission in August 1986, as discussed in a companion paper (Gregory et al., this issue).

Meteorological conditions during the summer 1986 CITE 2 flight series

An overview of meteorological conditions during the NASA Global Tropospheric Experiment/Chemical Instrumentation Testing and Evaluation (GTE/CITE 2) summer 1986 flight series is presented. Computer‐generated isentropic trajectories are used to trace the history of air masses encountered along each aircraft flight path. The synoptic‐scale wind fields are depicted based upon Montgomery stream function analyses. Time series of aircraft‐measured temperature, dew point, ozone, and altitude are shown to depict air mass variability. Observed differences between maritime tropical and maritime polar air masses are discussed.

Trace element concentrations and size distribution of biogenic aerosols from the amazon basin during the wet season

Atmospheric aerosols were sampled in the tropical rain forest of the Amazon Basin during April and May, 1987 (wet season) as part of the NASA Global Tropospheric Experiment (GTE), during the Amazon Boundary Layer Experiment (ABLE-2B). A special fine particle aerosol sampler and 6-stage single orifice cascade impactors were used for the aerosol collections. The samples were analyzed for 24 elements (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Se, Br, Rb, Sr, Zr, and Pb) by particle-induced X-ray emission (PIXE). Absolute principal factor analysis was used to interpret the fine particle aerosol elemental concentrations, and to obtain elemental source profiles. The concentrations of the fine mode soil dust related elements (Al, Ti, Mn, and Fe) were more than 10 times larger in the wet season than in the dry season. The biogenic aerosol related elements (e.g. P, S, K, and Zn) in the fine mode showed strongly reduced concentrations in the wet season. Sulphur concentrations averaged 92 ± 54 ng m 3 in the fine mode aerosol. The concentrations of soil dust related elements were 80% higher during daytime than during night, whereas the biogenic related elements showed much lower levels during day than during night. Two factors explained more than 92% of the data variability for the day and night fine particle samples. These factors were soil dust (represented mainly by Al. Si, Ti, Mn. and Fe) and biogenic related aerosol (with K, P, S, and Zn). Source profiles were obtained for the fine mode aerosol, and the soil dust factor had a similar elemental composition as average crustal rock. The size distributions of the soil dust related elements, as measured at three different levels (ground level, 28 and 45 m), were very similar, whereas for the biogenic related elements some differences as a function of height were observed. The sulphur size distribution was clearly bimodal, but the fine mode was significantly reduced at ground level. Considering the concentrations measured in this work and the large area of the tropical rain forests, biogenic aerosol particles can play an important role in the global aerosol budget and in the global biogeochemical cycles of various elements.

Ozone precursors and ozone photochemistry over eastern North Pacific during the spring of 1984 based on the NASA GTE/CITE 1 airborne observations

Simultaneous, high‐resolution measurements of O3, NO, CO, dew point temperature, and UV flux obtained during the NASA Global Tropospheric Experiment Chemical Instrumentation Test and Evaluation (GTE/CITE 1) spring 1984 airborne field exercise over the eastern North Pacific Ocean are analyzed. Mid‐tropospheric CO, O3, and NO mixing ratios averaged about 120 parts per billion by volume (ppbv), 50 ppbv, and 10 parts per trillion by volume (pptv), respectively. Statistical analysis of the high‐resolution data indicates the existence of two ozone sources: one related to the downward transport of ozone‐rich air from the upper troposphere and stratosphere and the other to the transport of ozone‐rich air from the continents. Modeling calculations based on these average levels imply that from the surface to about 8 km, photochemical reactions probably supplied a net sink of ozone to the region overlying the eastern North Pacific Ocean during the sampling period. However, because the NO levels measured during the flights were frequently at or near the detection limit of the instruments and because our results are very sensitive to the absolute NO levels and their temporal variability, our conclusion must be considered provisional.