Mark C. Shipham

Pacific Exploratory Mission-West A (PEM-West A): September–October 1991

The NASA Pacific Exploratory Mission-West (PEM-West) is a major component of the East Asia/North Pacific Regional Study (APARE), a project within the International Global Atmospheric Chemistry (IGAC) program. The broad objective of the PEM-West/APARE initiative is to study chemical processes and long-range transport over the northwestern Pacific Ocean and to estimate the magnitude of the human impact on the oceanic atmosphere over this region particularly for tropospheric ozone and its precursors as well as for sulfur species. The first phase of this mission, PEM-West A, was conducted during September–October 1991. The PEM-West A included intensive airborne measurements of trace gases from the NASA DC-8 aircraft coordinated with measurements at PEM-West A surface sites as well as with measurements obtained from collaborating APARE ground and airborne platforms. This paper reports the experimental design for PEM-West A with a brief summary of the general content and focus of companion papers in this special issue.

Summertime photochemistry of the troposphere at high northern latitudes

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 O3 in 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 O3 photochemical loss appreciably relative to a NOx-free atmosphere; if no NOx were present, the lifetime of O3 in the 0–6 km column would decrease from 46 to 26 days because of faster photochemical loss. The small amounts of NOx present in the Arctic troposphere have thus a major impact on the regional O3 budget. Decomposition of peroxyacetyl nitrate (PAN) can account for most of the NOx below 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 NOx could 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 NOx emissions in the fires, and in part to rapid conversion of NOx to PAN promoted by low atmospheric temperatures. It appears that fires make little contribution to the regional O3 budget. Only 30% of the acetic acid concentrations measured during ABLE 3A can be accounted for by reactions of CH3CO3 with HO2 and CH3O2. There remains a major unidentified source of acetic acid in the atmosphere.

The Arctic Boundary Layer Expedition (ABLE 3A): July–August 1988

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 CH 4 m -2 d -1 . Soil moisture and temperature were positively correlated with methane emission rates, indicating quanti- tative 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 0 3 against photochemical loss may have already doubled. A doubling of NOx concentration from present levels would lead to net photochemical production of 03 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.

Biomass burning signatures in the atmosphere and snow at Summit, Greenland: An event on 5 August 1994

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.

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.

PEM-West A: Meteorological overview

Phase A of the NASA Pacific Exploratory Mission in the western North Pacific (PEM- West A) region was conducted during September-October 1991. The background meteorology of eastern Asia and the western North Pacific region during the PEM-West A study is described. Mean large-scale flow patterns are discussed along with transient synoptic scale features (e.g., midlatitude cyclones, anticyclones, and frontal systems) responsible for long-range transport of trace species over the study region. Synoptic summaries are given for each of the 18 data flights, together with selected examples of meteorological processes that gave rise to some of the changes observed in the measured trace gases. Examples of large-scale ozone features observed above and below the DC-8 flight altitude by an onboard lidar system are also related to meteorological processes such as stratospheric-tropospheric exchange and upward transport of air from the boundary layer. The broad objectives of the National Aeronautics and Space Administrations (NASA) Pacific Exploratory Mission in the west- ern North Pacific (PEM-West) region are to study the chemical processes and long-range transport of trace gas species over the Pacific Ocean and to estimate the human impact on chemistry of the troposphere in this region. Specifically, the major objectives of PEM-West are to understand the factors influencing the budgets of ozone and sulfur. The overall experimental design of PEM-West encompassed two intensive airborne field studies positioned in time such that contrasting meteorological regimes in the western North Pacific could be sampled. The firat phase, PEM-West A, was conducted during September-October 1991, a period in which the lower tropospheric airflow was dominated by flow from the mid- Pacific regions. The phase B was conducted in March 1994, a period characterized by maximum outflow from the Asian continent.

Chemical characteristics of tropospheric air over the Pacific Ocean as measured during PEM‐West B: Relationship to Asian outflow and trajectory history

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.

Meteorological overview of the Arctic Boundary Layer Expedition (ABLE 3A) flight series

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.

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.

Stratospheric/tropospheric exchange affecting the northern wetlands regions of Canada during summer 1990

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.