The Pacific Exploratory Mission (PEM)‐Tropics provided extensive aircraft data to study the atmospheric chemistry of tropospheric air in Pacific Ocean regions, extending from Hawaii to New Zealand and from Fiji to east of Easter Island. This region, especially the tropics, includes some of the cleanest tropospheric air of the world and, as such, is important for studying atmospheric chemical budgets and cycles. The region also provides a sensitive indicator of the global‐scale impact of human activity on the chemistry of the troposphere, and includes such important features as the Pacific “warm pool,” the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), and Walker Cell circulations. PEM‐Tropics was conducted from August to October 1996. The ITCZ and SPCZ are major upwelling regions within the South Pacific and, as such, create boundaries to exchange of tropospheric air between regions to the north and south. Chemical data obtained in the near vicinity of the ITCZ and the SPCZ are examined. Data measured within the convergent zones themselves are not considered. The analyses show that air north and south of the convergent zones have different chemical signatures, and the signatures are reflective of the source regions and transport histories of the air. Air north of the ITCZ shows a modest urban/industrialized signature compared to air south of the ITCZ. The chemical signature of air south of the SPCZ is dominated by combustion emissions from biomass burning, while air north of the SPCZ is relatively clean and of similar composition to ITCZ south air. Chemical signature differences of air north and south of the zones are most pronounced at altitudes below 5 km, and, as such, show that the ITCZ and SPCZ are effective low‐altitude barriers to the transport of tropospheric air. At altitudes of 8 to 10 km, chemical signatures are less dissimilar, and air backward trajectories (to 10 days) show cross‐convergent‐zone flow. At altitudes below about 5 km, little cross‐zonal flow is observed. Chemical signatures presented include over 30 trace chemical species including ultrafine, fine, and heated‐fine (250°C) aerosol.

The Harvard Forest research site located in central New England is influenced by numerous anthropogenic methane sources on a year-round basis. Methane is strongly correlated to other chemical species that have an anthropogenic component, including acetylene, propane, ethane, hexane, and additional short-lived nonmethane hydrocarbons. The correlation between methane and acetylene is due to the colocation of landfills and cities. The correlation between methane and other short-lived species implies that emissions from local and regional rather than distant sources are the primary cause of elevated events. Wind roses of chemical species are examined for annual and seasonal time periods with enhancements in anthropogenic species corresponding to the location of large cities and landfills. The southwest quadrant is subjected to the most severe pollution events and is impacted by outflow from nearby cities in that sector, including Northampton and Springfield, Massachusetts. Emissions from cities in other quadrants, including Boston and Worcester, Massachusetts, Providence, Rhode Island, and the close-by town of Petersham, Massachusetts, also affect the site, but to a lesser degree. Case studies are used to identify atmospheric conditions that lead to high concentrations of methane and other species. The co-occurrence of a persistent wind direction, light wind speed, and stable atmospheric conditions is the ideal scenario in which emissions from nearby cities and landfills are advected to the site. Emissions from local and regional, rather than distant sources, are the primary cause of elevated events.

We have compiled a unique high‐resolution ambient‐air methane data set consisting of approximately 125,000 independently measured data points for the years 1991–1995 that have been collected at a site in the northeastern United States. The annual median mixing ratio of methane for all measurements was 1808 ppbv in 1992, increasing at a variable rate to 1837 ppbv in 1995. The lower 10–30% of the data from each month were defined as representative of background air and were compared with the global Climate Monitoring and Diagnostics Laboratory (CMDL) data set. The background data exhibit a variable upward trend of 5.5 ± 2 ppbv/yr during the 4‐year time period, with most of the increase observed during 1993 and 1994. The seasonal cycle for the background data set is similar to what is observed by CMDL stations and varies from 24 to 35 ppbv. The amplitude of the seasonal cycle for the full data set was larger, ranging from 35 to 44 ppbv. Differences between the full and background mixing ratios vary on a seasonal basis and are largest in the winter and smallest in the summer. These differences appear to be controlled by changes in atmospheric stability and changes in emissions from local and regional sources throughout the year. Diurnal cycles exist in the data, with the magnitude and timing of maximum and minimum values being controlled by inputs from local sources and atmospheric stability. Nearby wetlands contribute to an overnight buildup of methane in the late spring and summer. The magnitude of the daily cycle is largest in July and August (∼23 ppbv), when inputs from wetlands are large and wind speed is generally low. In April, the daily cycle is smallest (∼6 ppbv), when inputs from local sources are low and more vigorous atmospheric mixing limits pollution buildup.

The Pacific Exploratory Missions (PEM) were designed to study the chemistry of tropospheric air within the Pacific Rim region extending from the equator to about 50°N. Missions emphasized the importance of Asian outflow to the chemistry of tropospheric air. PEM‐West A was conducted in September and October 1991, and PEM‐West B was conducted in February and March 1994. The PEM‐West B winter mission coincides with the time of maximum impact of Asian outflow on the Pacific Rim region. This paper examines the chemical composition of air measured during PEM‐West B aircraft ascents/descents. Chemical composition of tropospheric air is related to its history as determined from 5‐ to 10‐day back trajectory calculations at multiple altitudes of the vertical profiles. Locations and the altitudes for trajectory calculations are selected to elucidate relationships between Asian source regions, transport within the region, and the chemical characteristics of tropospheric air. Data are over‐ocean measurements at locations ranging from hundreds of kilometers from the Asian coast to remote ocean sites thousands of kilometers east of Asia. Seasonal differences are illustrated by comparing PEM‐West A and B results. In general, the chemical composition of tropospheric air throughout the Pacific Rim region is influenced by Asian outflow, and transported continental emissions are an important source of pollution to the region during both seasons.

Two recent reports have suggested that thin layers of ice in Greenland cores with anomalously high concentrations of NH4+ K+ and HCOO− represent deposition from biomass burning plumes advected over Greenland. These interpretations were based primarily on the similarity between the suite of enriched species in the ice and several recent characterizations of biomass burning plumes from various regions around the globe. In August 1994 a biomass burning plume was transported to Summit, Greenland (72°N 38°W) from the Hudson Bay lowlands region of Canada. Gas-phase, aerosol and snow samples impacted by this plume had large enhancements of HCOOHHCOO−, CH3COOHCH3COO−, NH3NH4+ and K+. Several other species that have been reported to be enriched in some biomass burning plumes were also enriched in at least one of the three phases (gas, aerosol and snow) at Summit. Comparisons between the plume at Summit and biomass burning plumes sampled in 1990 over the Hudson Bay lowlands suggest that the carboxylic acids may be significantly enhanced by secondary production during the 3–4 days of transport between Canada and Greenland. It also appears that gas to particle conversion during transport may modify the partitioning of the carboxylates, nitrate, and perhaps ammonium and inorganic sulfur between the gas and aerosol phases in the plume. The relative enrichments of these species differ considerably between the atmosphere and snow, but the signal in snow was quite similar to the composition of the anomalous samples previously described in the ice cores.

The DC‐8 mission of September 27, 1991, was designed to sample air flowing into Typhoon Mireille in the boundary layer, air in the upper tropospheric eye region, and air emerging from the typhoon and ahead of the system, also in the upper troposphere. The objective was to find how a typhoon redistributes trace constituents in the West Pacific region and whether any such redistribution is important on the global scale. The boundary layer air (300 m), in a region to the SE of the eye, contained low mixing ratios of the tracer species O3, CO, C2H6, C2H2, C3H8, C6H6 and CS2 but high values of dimethylsulfide (DMS). The eye region relative to the boundary layer, showed somewhat elevated levels of CO, substantially increased levels of O3, CS2 and all nonmethane hydrocarbons (NMHCs), and somewhat reduced levels of DMS. Ahead of the eye, CO and the NMHCs remained unchanged, O3 and CS2 showed a modest decrease, and DMS showed a substantial decrease. There was no evidence from lidar cross sections of ozone for the downward entrainment of stratospheric air into the eye region; these sections show that low ozone values were measured in the troposphere. The DMS data suggest substantial entrainment of boundary layer air into the system, particularly into the eye wall region. Estimates of the DMS sulphur flux between the boundary layer and the free troposphere, based on computations of velocity potential and divergent winds, gave values of about 69 μg S m−2 d−1 averaged over a 17.5° grid square encompassing the typhoon. A few hours after sampling with the DC‐8, Mireille passed over Oki Island, just to the north of Japan, producing surface values of ozone of 5.5 ppbv. These O3 levels are consistent with the low tropospheric values found by lidar and are more typical of equatorial regions. We suggest that the central eye region may act like a Taylor column which has moved poleward from low latitudes. The high‐altitude photochemical environment within Typhoon Mireille was found to be quite active as evidenced by significant levels of measured gas phase H2O2 and CH3OOH and model‐computed levels of OH.

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.

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.

During all eight flights conducted over the equatorial and tropical South Atlantic (27°–35°W, 2°N–11°S; September 9–22, 1989) in the course of the Chemical Instrumentation Test and Evaluation (CITE 3) experiment, we observed haze layers with elevated concentrations of aerosols, O3, CO, and other trace gases related to biomass burning emissions. They occurred at altitudes between 1000 and 5200 m and were usually only some 100–300 m thick. These layers extended horizontally over several 100 km and were marked by the presence of visible brownish haze. These layers strongly influenced the chemical characteristics of the atmosphere over this remote oceanic region. Air mass trajectories indicate that these layers originate in the biomass burning regions of Africa and South America and typically have aged at least 10 days since the time of emission. In the haze layers, O3and CO concentrations up to 90 and 210 ppb were observed, respectively. The two species were highly correlated. The ratio ΔO3/ΔCO (Δ, concentrations in plume minus background concentrations) is typically in the range 0.2–0.7, much higher than the ratios in the less aged plumes investigated previously in Amazonia. In most cases, aerosol (0.12–3 μm diameter) number concentrations were also elevated by up to 400 cm−3in the layers; aerosol enrichments were also strongly correlated with elevated CO levels. Clear correlations between CO and NOxenrichments were not apparent due to the age of the plumes, in which most NOxwould have already reacted away within 1–2 days. Only in some of the plumes could clear correlations between NOyand CO be identified; the absence of a general correlation between NOyand CO may be due to instrumental limitations and to variable sinks for NOy. The average enrichment of ΔNOy/ΔCO was quite high, consistent with the efficient production of ozone observed in the plumes. The chemical characteristics of the haze layers, together with remote sensing information and trajectory calculations, suggest that fire emissions (in Africa and/or South America) are the primary source of the haze layer components.

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted to determine the summertime tropospheric distribution, sources, and sinks of important trace gas and aerosol species over the wetlands and boreal forests of central and eastern Canada. Isentropic trajectories and analyzed midtropospheric circulation patterns were used to group flights according to the transport histories of polar, midlatitude, or tropical air masses which were sampled. These data were then divided into bands of potential temperature levels representing the low, middle, and maximum aircraft altitudes to assess the effects of both local and long distance transport and natural and man‐made pollutants to the measured chemical species. Detailed case studies are provided to depict the complex three‐dimensional airflow regimes that transported air with differing chemical signatures to the study area. Mission 6 details the large‐scale movement of smoke in the generally prevailing west to northwesterly airflow that was observed on the majority of flights. Mission 1 analyzes the horizontal and vertical motions of maritime Pacific air in the upper troposphere that was routinely mixed downward to the aircraft altitude. Finally, mission 14 tracks the far northward excursion of tropical air that had been associated with a Pacific typhoon. The following three factors all had important influences on the collected chemical data sets: (1) local and distant stratospheric in puts into the upper and middle troposphere; (2) biomass‐burning plumes from active fires in Alaska and Canada; (3) a band of“low ozone”upper tropospheric air that was observed by airborne differential absorption lidar (DIAL) above the aircraft maximum altitude. Other modification factors observed on some flights included urban pollution from U.S. and Canadian cities, tropical air that had been associated with a Pacific typhoon, and precipitation scavenging by clouds and rain. Many flights were affected by several of the above factors which led to complex chemical signatures that will be discussed in other companion papers.

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted over the northern wetlands region of Canada during July and August 1990. Several Stratospheric/tropospheric exchange events were noted by zenith‐looking airborne lidar and in situ measurements of ozone and other trace gas species. Isentropic trajectories and potential vorticity analyses are utilized to determine the frequency of stratospheric inputs which would have affected the tropospheric column over the Moosonee and Schefferville regions and to describe the favored pathways of transport of stratospheric air arriving at these locations. At the 310 K potential temperature level (middle troposphere), trajectories having “aged stratospheric” values of potential vorticity at some point in their 5‐day history arrived at Moosonee or Schefferville roughly 40% of the time during the ABLE 3B study period, most often via large‐scale subsidence enroute from “stratospheric input regions” over the Arctic Ocean or northern and central Canada. At 325 K (upper troposphere), “fresh” stratospheric input was evident on about 80% of the trajectories, most often associated with jet streaks within the polar and Arctic jet streams. A case study is presented which illustrates both of these general stratospheric input processes.

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.

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.

A meteorological overview of the Arctic Boundary Layer Expedition (ABLE 3A) flight series is presented. Synoptic analyses of mid‐tropospheric circulation patterns are combined with isentropic back trajectory calculations to describe the long‐range (400–3000 km) atmospheric transport mechanisms and pathways of air masses to the Arctic and sub‐Arctic regions of North America during July and August 1988. Siberia and the northern Pacific Ocean were found to be the two most likely source areas for 3‐day transport to the study areas in Alaska. Transport to the Barrow region was frequently influenced by polar vortices and associated short‐wave troughs over the Arctic Ocean, while the Bethel area was most often affected by lows migrating across the Bering Sea and the Gulf of Alaska, as well as ridges of high pressure which built into interior Alaska. July 1988 was warmer and dryer than normal over much of Alaska. As a result, the 1988 Alaska fire season was one of the most active of the past decade. Airborne lidar measurements verified the presence of biomass burning plumes on many flights, often trapped in thin subsidence layer temperature inversions. Several cases of stratosphere/troposphere exchange were noted, based upon potential vorticity analyses and aircraft lidar data, especially in the Barrow region and during transit flights to and from Alaska.

The Arctic Boundary Layer Expedition (ABLE 3A) used measurements from ground, aircraft, and satellite platforms to characterize the chemistry and dynamics of the lower atmosphere over Arctic and sub‐Arctic regions of North America during July and August 1988. The primary objectives of ABLE 3A were to investigate the magnitude and variability of methane emissions from the tundra ecosystem, and to elucidate factors controlling ozone production and destruction in the Arctic atmosphere. This paper reports the experimental design for ABLE 3A and a summary of results. Methane emissions from the tundra landscape varied widely from −2.1 to 426 mg CH4 m−2 d−1. Soil moisture and temperature were positively correlated with methane emission rates, indicating quantitative linkages between seasonal climate variability and soil metabolism. Enclosure flux measurement techniques, tower‐based eddy correlation, and airborne eddy correlation flux measurements all proved robust for application to methane studies in the tundra ecosystem. Measurements and photochemical modeling of factors involved in ozone production and destruction validated the hypothesized importance of low NOx concentrations as a dominant factor in maintaining the pristine Arctic troposphere as an ozone sink. Stratospheric intrusions, long‐range transport of mid‐latitude pollution, forest fires, lightning, and aircraft are all potential sources of NOx and NOy to Arctic and sub‐Arctic regions. ABLE 3A results indicate that human activities may have already enhanced NOy inputs to the region to the extent that the lifetime of O3 against photochemical loss may have already doubled. A doubling of NOx concentration from present levels would lead to net photochemical production of O3 during summer months in the Arctic (Jacob et al., this issue (a)). The ABLE 3A results indicate that atmospheric chemical changes in the northern high latitudes may serve as unique early warning indicators of the rates and magnitude of global environmental change.

The budgets of O3, NOx(NO+NO2), reactive nitrogen (NOy), and acetic acid in the 0–6 km column over western Alaska in summer are examined by photochemical modeling of aircraft and ground‐based measurements from the Arctic Boundary Layer Expedition (ABLE 3A). It is found that concentrations of O3in the region are regulated mainly by input from the stratosphere, and losses of comparable magnitude from photochemistry and deposition. The concentrations of NOx(10–50 ppt) are sufficiently high to slow down O3photochemical loss appreciably relative to a NOx‐free atmosphere; if no NOxwere present, the lifetime of O3in the 0–6 km column would decrease from 46 to 26 days because of faster photochemical loss. The small amounts of NOxpresent in the Arctic troposphere have thus a major impact on the regional O3budget. Decomposition of peroxyacetyl nitrate (PAN) can account for most of the NOxbelow 4‐km altitude, but for only 20% at 6‐km altitude. Decomposition of other organic nitrates might supply the missing source of NOx. The lifetime of NOy, in the ABLE 3A flight region is estimated at 29 days, implying that organic nitrate precursors of NOxcould be supplied from distant sources including fossil fuel combustion at northern mid‐latitudes. Biomass fire plumes sampled during ABLE 3A were only marginally enriched in O3; this observation is attributed in part to low NOxemissions in the fires, and in part to rapid conversion of NOxto PAN promoted by low atmospheric temperatures. It appears that fires make little contribution to the regional O3budget. Only 30% of the acetic acid concentrations measured during ABLE 3A can be accounted for by reactions of CH3CO3with HO2and CH3O2. There remains a major unidentified source of acetic acid in the atmosphere.

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.