Robert J. McNeal

The Pacific Exploratory Mission-West Phase B: February-March, 1994

The NASA Pacific Exploratory Mission in the Western Pacific Ocean (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 objectives of the PEM-West/APARE initiative are to study chemical processes and long-range transport of atmospheric trace species over the north-west Pacific Ocean and to estimate the magnitude of the human impact on these species over this region. The first phase of PEM-West (PEM-West A) was conducted in September-October 1991, a period characterized by minimum outflow from the Asian continent. The second phase of this mission, PEM-West B, was conducted during February-March 1994, a period characterized by enhanced outflow from the Asian continent. Both field campaigns of PEM-West included intensive airborne measurements of trace gases and aerosols from the NASA DC-8 aircraft coordinated with measurements at surface sites. This paper reports the experimental design for PEM-West B and provides a brief summary of the salient results of the PEM-West B campaign with particular emphases on the difference/similarities between phases A and B. Results from the two campaigns clearly quantify, from a trace gas perspective, the seasonal differences in the continental outflow that were qualitatively anticipated based upon meteorological considerations, and show the impact of major meteorological features within the region on the quality of tropospheric air over the North Pacific Ocean regions. The PEM-West database provides a “baseline” tool by which future assessments of a continuing impact of Asian emissions on remote Pacific regions can be judged. [These data are currently available through the Global Troposhperic Experiment Data Archive at NASAs Langley Research Center (http://www-gte.larc.nasa.gov) and the Langley Distributed Archive Center (http://eosdis.larc.nasa.gov)].

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.

Pacific Exploratory Mission in the tropical Pacific: PEM‐Tropics A, August‐September 1996

The NASA Pacific Exploratory Mission to the Pacific tropics (PEM-Tropics) is the third major field campaign of NASAs Global Tropospheric Experiment (GTE) to study the impact of human and natural processes on the chemistry of the troposphere over the Pacific basin. The first two campaigns, PEM-West A and B were conducted over the northwestern regions of the Pacific and focused on the impact of emissions from the Asian continent. The broad objectives of PEM-Tropics included improving our understanding of the oxidizing power of the tropical atmosphere as well as investigating oceanic sulfur compounds and their conversion to aerosols. Phase A of the PEM-Tropics program, conducted between August-September 1996, involved the NASA DC-8 and P-3B aircraft. Phase B of this program is scheduled for March/April 1999. During PEM-Tropics A, the flight tracks of the two aircraft extended zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand. Both aircraft were instrumented for airborne measurements of trace gases and aerosols and meteorological parameters. The DC-8, given its long-range and high-altitude capabilities coupled with the lidar instrument in its payload, focused on transport issues and ozone photochemistry, while the P-3B, with its sulfur-oriented instrument payload and more limited range, focused on detailed sulfur process studies. Among its accomplishments, the PEM-Tropics A field campaign has provided a unique set of atmospheric measurements in a heretofore data sparse region; demonstrated the capability of several new or improved instruments for measuring OH, H2SO4, NO, NO2, and actinic fluxes; and conducted experiments which tested our understanding of HOx and NOx photochemistry, as well as sulfur oxidation and aerosol formation processes. In addition, PEM-Tropics A documented for the first time the considerable and widespread influence of biomass burning pollution over the South Pacific, and identified the South Pacific Convergence Zone as a major barrier for atmospheric transport in the southern hemisphere.

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.

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.