Preface [to special section on Pacific Exploratory Mission‐Tropics A]

During the early part of this decade, the National Aeronautics and Space Administration's (NASA) Earth Sciences Program, through the Global Tropospheric Experiment (GTE), initiated a long-term study to improve our understanding of the human influences on the chemistry of the troposphere over the Pacific Ocean.A series of campaigns designated as Pacific Exploratory Missions (PEM) were initiated.To date, three airborne PEM campaigns have been conducted.PEM-West A and B were conducted over the northwestern Pacific during September-October 1992 [Hoell et al., 1996] and February-March 1994 [Hoell et al., 1997] respectively to study Asian outflow during contrasting meteorological conditions.The third campaign, PEM-Tropics A, was conducted during August-September 1996 [Hoell et al., 1999], and focused on the central and southern tropical regions of the Pacific Ocean, extending zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand.A fourth campaign, PEM-Tropics B, was conducted in March/April 1999, and, like PEM-West A and B, the combined PEM-Tropics A and B campaigns provide observations during contrasting seasons (i.e., dry versus wet).The Pacific basin, covering 35% of the total surface area of the Earth, and 50% of the ocean surface, has traditionally been thought to be relatively free from the impacts of human activity.Yet in the northern hemisphere, the Pacific Ocean is bordered by heavily populated continents, many with increasing industrial potential, and consequently a potential for significantly impacting the quality of the Pacific region, and the global troposphere.Results from the PEM-West A and B [Hoell et al., 1996, 1997] missions clearly document.the influence that continental emissions from Asia and from regions west of the Asian continent are having in the northwestern Pacific.The PEM-Tropics missions (e.g., A and B) have focused on the central and southern Pacific Ocean regions.The major objectives of PEM-Tropics (Phases A and B) are to provide baseline data over the southern Pacific Ocean for gases important in controlling the oxidizing power of the atmosphere, to assess the resulting sensitivity of the oxidizing power of the atmosphere to anthropogenic and natural perturbations, and to study marine sulfur photochemistry in a remote tropospheric environment.PEM-Tropics A [Hoell et al., 1999] was conducted using a suite of instruments aboard the NASA DC-8 and P-3B research aircraft [Hoell et al., 1999] operating from Christmas Island (P-3B), Easter Island (P-3B and DC-8), Tahiti (P-3B and DC-8), Fiji (DC-8), New Zealand (DC-8), and Ecuador (P-3B).The scientific payloads of the two aircraft differed in some important respects reflecting the operating characteristics of the two platforms.The high-altitude ceiling and long-range of the DC-8 favored extensive geographical surveys of the South Pacific troposphere.By contrast, the low-altitude capabilities of the P-3B favored sulfur process oriented studies within the boundary layer and lower free troposphere.This paper is not subject to U.S.