GTE Long Range Transport: TRACE-A and PEM-Expeditions

Tropospheric chemistry and transport govern the distribution of trace species within the atmosphere and alter the chemical state in which trace elements move through the atmosphere. The resulting changes in global atmospheric chemical composition can affect our climate. We have long known, for example, that increases in carbon dioxide (CO₂) in the atmosphere from burning fossil fuels can contribute to a global warming through the "greenhouse effect". We now realize that other trace gases and species that can also contribute to the greenhouse effect are originating from man's activities and now appear to be accumulating in the troposphere. It is now very apparent that there are no boundaries within the earth's atmosphere. Gases products produced on one continent can easily be transported around the globe, often in a matter of days.

Within GTE the impact of long range transport of trace gases on the chemistry of the free troposphere has been studied in this decade in two campaigns consisting of four separate field missions. The first of these, denoted as the Pacific Exploratory Missions (PEM), focused on the vast regions of the Pacific ocean. Two of the PEM's, PEM-West A & B, studied the impact of natural and anthropogenic emissions from the Asian continent on the chemistry of the troposphere over the western north Pacific ocean. These two missions were conducted during the two major contrasting meteorological regimes in the north western Pacific. The first, PEM-West(A) conducted in the late summer of 1991, was during a period of minimum out-flow from the Asian continent. PEM-West(B) was conducted during the spring of 1994, a period of maximum outflow from the Asian continent. A third PEM, PEM-Tropics, was conducted in September, 1996 in the south central regions of the Pacific to characterize the natural oxidation power of the remote troposphere.

The second campaign in this series of GTE missions to characterize the impact of long range transport was the TRansport And Chemistry near the Equator in the Atlantic (TRACE-A) experiment, conducted in September/October 1992 to investigate the distribution of atmospheric trace gases over the tropical south Atlantic. This portion of the Atlantic troposphere is coupled by atmospheric-transport of emissions from both South America and southern Africa. The TRACE-A experiment was conducted to study the impact of emission from these two continents on a seasonal enhancement of tropospheric O₃ over a large region of the South Atlantic ocean off the coast of Brazil and southern Africa. The Objectives of TRACE-A focused on determining the relative importance of natural versus anthropogenic emission processes on the formation of this O₃ enhancement.