PEM Tropics Mission Objectives

During the early part of this decade, NASA , as part of it's Mission to Planet Earth (MtPE) program, initiated the Pacific Exploratory Missions (PEM) to understand the impact of human activity on the chemistry of the troposphere (that portion of the atmosphere that lies between the Earth's surface and an altitude of approximately 15 km) over the Pacific ocean. This vast region of the world has traditionally been thought to be relatively free from the impact of human activity. Yet, in the northern hemisphere, the Pacific ocean is bordered by heavily populated continents, many with an increasing industrial potential, and consequently a potential for significantly impacting the quality of the Pacific region, and the global troposphere. To date, the PEM studies have consisted of two airborne expeditions, in contrasting meteorological conditions, in the north western Pacific region conducted September-October 1992 (PEM-West A) and February-March 1994 (PEM-West B), and the PEM-Tropics expedition conducted in September 1996 which focused on the south tropical regions of the Pacific ocean.

The combined PEM expeditions that have been conducted over the past 5 years, including the PEM-Tropics, cover a significant portion o f the Pacific Basin. From the perspective of global tropospheric chemistry, the Pacific Basin is a very large chemical reaction vessel. From Peru to Borneo it is 17,700 km in the east-west direction, and from the southern ice shelf to Alaska it is 13,300 km. It covers 35% of the total surface area of the earth and 50% of the ocean surface. Much of the Pacific Basin is remote from continental influence, and hence provides a particularly sensitive indicator of the global-scale impact of human activity on the chemistry of the troposphere. The measurements from the combined PEM airborne expeditions will represent the most comprehensive data base available for assessing current and future impacts of the atmospheric changes that may be occurring in this region of the world. Already, results from the PEM-West expeditions show that gaseous and aerosol emissions from the Asian continent, as well as from the more distant North American and European continents can be observed in the atmosphere over the North western portions of the Pacific ocean. The PEM-Tropics expedition has been the more comprehensive of the series, in that two NASA aircraft were involved, and measurements were conducted over a much larger portion of the Pacific ocean.

One of the most important issues in global tropospheric chemistry is the sensitivity of the oxidizing power of the troposphere to human influence. Oxidation by the hydroxyl radical (OH) in the troposphere is the main sink for a number of gases important for climate change and stratospheric ozone (O₃) depletion, including methane, methyl bromide, methyl chloroform, and hydrochlorofluorocarbons (HCFC's). A decrease in tropospheric OH would increase the tropospheric concentrations of these gases, and increase their fluxes into the stratosphere. Concentrations of OH in the troposphere are determined by a number of photochemical reactions involving O₃, nitrogen oxides (NOx), carbon monoxide (CO), methane (CH₄), and non-methane hydrocarbons (NMHC's). Ozone, the primary precursor of OH, is supplied to the troposphere by transport from the stratosphere, and is also produced within the troposphere by photochemical oxidation of CH₄, NMHC's and CO in the presence of NOx. Anthropogenic emissions of NOx, CH₄, NMHC's, and CO could potentially have a large effect on the oxidizing power of the troposphere, directly by affecting OH and indirectly by providing a source of O₃.

Tropical regions play a critical role in determining the global oxidizing power of the atmosphere because of the high UV and humidity, which promote the formation of OH from reactions that follow the photolysis of O₃. Previous GTE missions have surveyed the concentrations of O₃ and its precursors over the tropical Atlantic and neighboring continents (ABLE-2A, ABLE-2B, CITE-3, TRACE-A) and over the north tropical Pacific (CITE-2, PEM-West A and PEM-West B). These missions have documented the importance of both natural processes (biogenic emissions, lightning) and anthropogenic processes (biomass and fossil fuel combustion) as sources of NOx, NMHC's, and CO in the tropical troposphere. The PEM-Tropics expedition has filled a pressing need to study these processes over the tropical South Pacific, a region of the tropical troposphere where O₃ chemistry may still be dominated by natural processes. Anthropogenic perturbation of the tropical troposphere, however, is expected to increase greatly over the next decades, as a result of population growth and industrial development, and it is important to provide a present-day baseline against which future change can be evaluated.

There is also a need to improve our understanding of atmospheric sulfur chemistry over the Pacific. Sulfate aerosol affects the earth's radiative balance both through direct back scattering of solar radiation and indirectly as cloud condensation nuclei (CCN). CCN, themselves products of aerosol growth processes, are believed to have their origin in nucleation processes involving gas phase H₂SO₄, the latter species being produced from the oxidation of SO₂ by OH. Sulfate and SO₂ over the Pacific may originate from a number of sources including long-range transport of anthropogenic pollution, marine biogenic releases of dimethlysulfide (DMS), and volcanic emissions. The relative contributions of these sources over different regions of the Pacific are still poorly known, representing a serious limitation in our ability to evaluate the role of sulfur in global climate change.

Existing data for this region are sparse, reflecting the difficulty of access. The GAMETAG aircraft missions in 1977 and 1978 provided some early data over the western part of the Basin; they were, however, restricted by the low ceiling and limited endurance of the aircraft used, and also by the state-of-the-science of the instruments available at the time. The more recent STRATOZ III and PEM-West(A) and (B) missions provided detailed data along the South American and Asian rims of the South Pacific Basin, respectively. Ozonesonde and CO measurements have been made from island sites (SEAREX program) and from ships. Even so, there are virtually no data for the southeast quadrant of the Basin extending from the international dateline to the South American coast.

It is clear from the few observations available that the South Pacific Basin is particularly remote from human influence. Concentrations of NOx, CO, NMHC's, and other trace gases of continental origin over the Pacific Ocean are often unusually low compared to other regions. Tropospheric ozone column densities are the lowest in the world, with values as low as one third those in other regions of the tropics. The South Pacific Basin likely represents the cleanest region of the tropical troposphere, making a detailed baseline survey of great importance.

The two major objectives of PEM-Tropics are:

1. To provide baseline data over the Basin for gases important in controlling the oxidizing power of the atmosphere including ozone, H₂O, NO, CO, and NMHC's;
2. To understand the factors controlling the concentrations of these gases, and to assess the resulting sensitivity of the oxidizing power of the atmosphere to anthropogenic and natural perturbations.

In addition, PEM-Tropics has three secondary objectives:

1. To survey the concentrations of aerosol precursors and ultra fine aerosol particles over the South Pacific Basin;
2. To improve our understanding of sulfur gas-to-particle formation over the region;
3. To provide detailed latitude-altitude transects of long-lived gases for the evaluation of global tropospheric models.

PEM-Tropics employed the NASA DC-8 based at NASA's Ames Research Center, Moffett Field, CA, and P-3B aircraft based at the NASA Wallops Flight Facility, Wallops, VA. The two aircraft covered an area extending zonally across the entire Pacific Basin and meridionally from Hawaii to south of New Zealand. Together they provided significant coverage of the Walker circulation cell over the Pacific Basin including the upwelling region over the western equatorial Pacific, the subsiding region offshore of South America, and the connecting atmosphere in between.