ABLE Summary

Nowhere is the atmosphere-biosphere interaction more pronounced than within the atmospheric boundary layer--the lowest few hundred meters of the atmosphere. Upward through this layer rise trace gases emitted by the biosphere or produced by industrial activity and combustion. And downward through this layer settle gases and aerosols formed by atmospheric chemistry processes, destined for final deposition on land and sea.

While in the atmosphere, such trace materials undergo photochemical reactions that influence the horizontal and vertical distributions of reaction products. Within the lower layers of the atmosphere, powerful dynamical processes are also at work. These mix the air from near-surface layers with the upper troposphere, where long-range transport takes place and local flux processes are transformed into globally significant effects.

Because of the great importance of trace-gas fluxes and their coupling to the global atmosphere, the first extensive GTE field studies were focussed on these processes. The GTE/ABLE investigations have brought international research teams into some of the most important and sensitive ecosystems in the world. Their findings have already revised previous views of the atmosphere-biosphere interaction and the relative importance of different ecosystems in generating atmospheric change. The atmosphere often does not behave in accordance with predictions based on previous, more limited data; sometimes the differences are startling.

Expeditions have now been completed in three ecosystems that are known to exert a major influence over global tropospheric chemistry and that are being profoundly affected by natural processes, human activities, or both. these are the tropical Atlantic Ocean (ABLE-1), the Brazilian rain forest (ABLE-2), and the northern wetlands (ABLE-3).

The first GTE/ABLE expedition utilized the NASA Electra aircraft, extensively arrayed with chemical and meteorological sensors, to survey regions over the tropical Atlantic from a base in Barbados in June 1984. Among the many new results provided by this pioneering field study were the following:

• Confirmation of long-range Saharan dust transport. Soil dust sampled over the tropical Atlantic two decades ago appeared to come from the Saharan Desert; subsequent work provided additional evidence that windborne dust from arid African and Asian regions is indeed the principal source of mineral aerosols found in global troposphere. In a striking and definitive confirmation of this hypothesis, ABLE-1 fights above Barbados observed a massive infusion of Saharan dust at these longitudes. Dense layers of the dust were mapped for the first time by an airborne lidar system and, at peak intensity, by aerial photography as well. Such dust is eventually deposited onto the sea surface, thousands of kilometers from the source. Successive episodes can add mineral nutrients to the ocean in amounts comparable to those injected by the Amazon River.
• Examination of air chemistry over a tropical forest. The world's tropical forest are important sources and sinks for many gas and aerosol species. ABLE-1 flights over Guyana provided the first comprehensive study of the wet tropical forest boundary layer from an airborne platform. They showed that this layer is a source of CO, isoprene (C₅H₈), and dimethyl sulfide (DMS), as well as a sink for O₃. The Guyana forest was also revealed to be a major source of chemically important aerosols.
• Assessment of marine DMS contributions to atmospheric sulfur. Marine phytoplankton are a major source of DMS. Measurements of tropospheric DMS concentrations confirmed earlier conclusions that marine DMS production accounts for fully half of natural sulfur dioxide (SO₂) emissions, are important in clean-air areas of the troposphere.(See figure 1.)

The tropical rain forest have long been recognized as key participants in the atmosphere-biosphere interaction. These vast, still essentially pristine regions, home to a wide array of plant and animal species, are now under assault by encroaching human development. However, the difficulty of access, the logistical barriers to scientific operations, and the lack of accurate instrumentation have prevented their comprehensive, systematic study until very recently.

The ABLE-2 project, carried out jointly by the United States and Brazil, brought new technology to bear on an investigation of the largest rain forest of all, the Amazon. The impetus for this work was Space Shuttle observations of elevated CO concentrations in the upper troposphere off the coast of Brazil-suspected to arise from a combination of biomass burning and oxidation of natural hydrocarbons, notably isoprene, emitted by the forest itself.

The ABLE-2 project consisted of two expeditions--the first in the Amazonian dry season (ABLE-2A, July-August 1985), and the second in the wet season (ABLE-2B, April-May 1987).

The ABLE-2 core research data were gathered by a series of instrumented Electra aircraft flights that stretched from Belem, at the mouth of the Amazon River, to Tabatinga, on the Brazil-Colombia border, from a base at Manaus, in the heart of the forest. These observations were supplemented by ground-based chemical and meteorological measurements in the dry forest, the Amazon floodplain, and tributary rivers through use of enclosures (for flux measurements), an instrumented tower in the jungle, a large tethered balloon, and weather and ozone sondes. At their peak, the ABLE-2 expeditions involved 60 U.S. scientists and more than 100 Brazilian scientist, working together in the spirit of international, interdisciplinary cooperation essential for the study of global change.

The enormous data base gathered by ABLE-2 will be analyzed by researchers around the world for many years to come. However, early results confirm that both the atmospheric chemistry and the meteorology over Amazonia must play significant roles in global atmospheric change. Among the most noteworthy of the initial findings are the following:

• Seasonal degradation of Amazonian air quality. Air above the Amazon jungle is extremely clean during the wet season but deteriorates dramatically during the dry season as the result of biomass burning. (See figure 2.) This degradation is caused mostly by burning at the edges of the forest. Under the worst conditions, trace-gas concentrations at aircraft altitudes approach those typically observed over industrialized regions. This is a spectacular example of long-range transport of pollution into a pristine environment.
• Combustive production of greenhouse gases. Biomass burning is also a copious source of such greenhouse gases as CO₂, and CH₄, as well as other pollutants (e.g. CO and oxides of nitrogen). Satellite observations during the dry season have detected some 6,000 fires at the peak of the burning, with associated haze covering millions of square kilometers.(See figure 3.)
• Natural Sink for tropospheric ozone. The undisturbed rain forest is the most efficient sink for O₃ yet discovered. Amazonian O₃ decomposition rates were found to be 5 to 50 times higher than those previously measured over pine forest and water surfaces. The disappearance of such a strong ozone sink through deforestation would have global implications for atmospheric chemistry.
• Methane emissions from Amazonian wetlands. The Amazon River floodplain is a globally significant source of methane, supplying about 12% of the estimated worldwide total from all wetlands sources.
• Enhancement of tropospheric carbon monoxide. Over Amazonia, CO is enhanced by factors ranging from 1.2 to 2.7 by comparison with adjacent ocean regions. The major Amazonian sources of CO are isoprene oxidation and biomass burning; the latter probably accounts for the most of the CO enhancement observed by Space Shuttle instruments prior to ABLE-2.
• Importance of atmospheric circulation over the rainforest. ABLE-2 studies of spectacular atmospheric circulations over Amazonia have shed new light on links between the Amazon regions and global circulation. Individual convective storms transport 200 megatons of air per hour, of which 3 megatons is water vapor that releases 100.000 megawatts of energy into the atmosphere through condensation into rain. Replacement of forest with wetlands or pasture is likely to have a large impact on this enormous furnace, with attendant effects on atmospheric circulation patterns, and hence climate.

The changes being produced by deforestation and biomass burning may accelerate the greenhouse effect through a positive-feedback mechanism involving the OH radical, as well as through direct changes in greenhouse-gas source and sink strengths. Most tropospheric trace gases are removed through reactions with OH; as biological combustion precesses drive CO, CH₄, and other trace gases to higher aggregate levels, OH levels generally fall. There is thus less OH remaining to retard a growing accumulation of greenhouse gases.

The reduction of the world's tropical rain forest has been extensively documented and publicized. by contrast, one might suppose that the Earth's vast, northerly areas of arctic tundra and boreal forest--apparently remote from human habitation and industry--have remained unaltered by such activities and do not bear the marks of global environmental change observed elsewhere.

Such is not the case. During the late winter and early spring of each year, for example, these northern regions suffer episodes of pollution caused by industrial emissions from Europe and the Soviet Union; air currents normally at midlatitudes frequently intrude into polar latitudes during that season. The Arctic is also adjacent to major North American anthropogenic sources of trace gases and aerosols. Studies of Artcic atmospheric aerosol concentrations have revealed large seasonal variations that can be attributed to human activities to the south. In addition, increasing concentrations of tropospheric O₃ have been observed by a long-term monitoring program at Point Barrow, Alaska.

The ABLE-3 project was designed to investigate the atmosphere-biosphere gas-exchange precesses in Arctic tundra and subarctic boreal regions. This research targets the sources, sinks, and distributions of CH, CO, CO₂, O₃, PAN, NO and other oxides of nitrogen, and certain nonmethane hydrocarbons and organic acids.

The ABLE-3 project consists of two field expeditions: ABLE-3A (summer 1988) and ABLE-3B (summer 1990), designed to survey two different high-latitude ecosystems. The first expedition, ABLE-3A, was conducted during July-August 1988 from bases at Point Barrow, near the northern Alaskan coast, and Bethel, in southwest Alaska. Flights of the NASA instrumented Electra aircraft measured trace-gas fluxes over wide regions of the typical peatlands surrounding these bases. Atmospheric photochemical processes involving CO, NO, O₃ oxides of nitrogen, and other reactive species were also investigated on all flights. Transit flights to and from Alaska also permitted high-altitude studies of air masses originating over the Arctic and northern Pacific Ocean prior to their passage over industrialized regions of North America, thereby providing a baseline for future pollution studies.

Some exploratory studies were carried out as well. For example, the influence of different Arctic surface environments on trace-gas variability in the boundary layer was probed through measurements above polar ice-pack boundaries and the open Arctic Ocean.

A joint United States-Canada expedition, ABLE-3B, was conducted in the summer of 1990 to extend these measurements to peatland and boreal forest regions of northern Canada, location of the world's largest continuous wetland region.

Research results from the ABLE-3A expedition are only now being integrated to yield a basic assessment of the processes at work within the northern wetlands; a definitive report on these regions must in any case await the completion of ABLE-3B. However, some of the initial ABLE-3A findings merit particular attention:

• Methane flux from the Alaskan ecosystem. The ABLE-3A expedition combined flux measurements from ground-based enclosures, an instrumented tower, widely ranging aircraft surveys, and satellite observations of the various subregions to produce, for the first time, an integrated measurement of total methane emission from the Yukon-Kuskokwim Delta (50-60 million kg/yr.). The ABLE-3A results clearly confirmed that Alaskan wetlands are a significant source of methane.
• Sensitivity of Alaskan methane fluxes to temperature: a greenhouse feedback effect. Alaskan methane emission was found to be highly sensitive to soil-moisture variations, increasing by a factor of 100 along the moisture gradient from dry to wet tundra. The methane arises from biological decomposition of plant material in the thin, organic-rich "active" layer of soil above the permafrost substrate typical of Arctic lands. Any net surface-temperature rise would presumably trigger an increase in methane fluxes into the atmosphere--a striking and important example of a natural feedback effect that would amplify global warming.
• Arctic removal of oxides of nitrogen. Arctic tundra was found to remove important trace nitrogen compounds from the atmosphere with high efficiency through biological processes. This reduces nitrogen oxide concentrations in the lower and middle regions of the Arctic troposphere to exceedingly low levels and so prevents significant ozone formation.