Scientists suspect that increasing numbers of small particles of sulfur compounds and other pollutants floating in the atmosphere may cancel or even reverse greenhouse warming in heavily industrialized regions. By reflecting sunlight back to space, these tiny airborne particles, or aerosols, can cool the earth beneath and possibly slow a rise in global average temperature. During ACE-1, scientists hope to learn more about background aerosols--the naturally occurring counterparts to these pollutants.
"This is the largest and most comprehensive experiment on natural background aerosols that we have ever done," states Huebert. Over 100 investigators from 57 institutions representing Australia, France, Germany, Italy, Japan, the Netherlands, New Zealand, Sweden, the United Kingdom, and the United States are participating in this major study of airborne particles. NSF is the primary sponsor of the U.S. component.
Besides Huebert, principal investigators are Tim Bates of NOAA's Pacific Marine Environment Laboratory and John Gras of Australia's Commonwealth Scientific and Industrial Research Organization. NCAR scientists Darrel Baumgardner and Greg Kok (Atmospheric Technology Division) and Fred Eisele (Atmospheric Chemistry Division) are among the many researchers involved. The UCAR Joint International Climate Projects/Planning Office and Office of Field Project Support are also heavily involved. Gus Emmanuel (JICP/PO) is operations center director, Dick Dirks (OFPS) is deputy director, Jim Moore (also of OPFS) is communications coordinator, and Gene Martin (JICP/PO) is logistics coordinator. OFPS will manage the project's data base.
"Existing theories suggest that it should be very hard to create new aerosols in the lower atmosphere, yet they keep showing up," explains Huebert. "We're deploying state-of-the-art instruments to the remote marine atmosphere for the first time to seek the source of these new aerosols." Among the high-tech instrumentation will be NCAR's dual-wavelength airborne lidar, which will map the vertical extent of aerosol layers in the atmosphere.
The fully equipped C-130, owned by NSF and operated by NCAR, will join NOAA's Seattle-based research vessel Discoverer . Australia will provide a second research ship, the Southern Surveyor.
Researchers aboard the C-130 spent as many flight hours taking measurements during the two-week trip from the north Alaska coast to south of New Zealand as they will during the operations in Tasmania. In Alaska, they began their research measurements with a flight toward the North Pole and back. While in Hawaii, the plane flew through the Kilauea volcano plume to study how its particles form and how much sunlight they reflect. After arrival at Hobart, a flight toward the South Pole will complete the study's nearly pole-to-pole measurements, giving the distribution of remote marine aerosols from 78íN to 70íS latitude. The target altitude for observations ranges between seven and eight kilometers on the journey and within the lowest one or two kilometers of the atmosphere in the area of operations.
ACE-1 is the first of the Aerosol Characterization Experiments, a series of international field programs to help scientists understand the chemical, physical, and optical properties of aerosols; how aerosols form and grow; and their effect on radiation and climate. The series is part of the International Global Atmospheric Chemistry Program.
Like carbon dioxide, sulfate aerosols are produced by human activity, mainly the burning of fossil fuels. They also exist naturally as sulfur emissions from living organisms and volcanoes. By scattering incoming solar energy back to space, both the natural and pollutant aerosols directly affect the amount of radiation entering the earth's atmosphere. They also serve as tiny sites for water vapor to condense on, allowing more small droplets to form within a cloud. This change in the droplets' size distribution makes the cloud more reflective, bouncing more solar radiation back to space and cooling the earth below.
In ACE-1, scientists will study the natural marine system distant from the Northern Hemisphere sulfate aerosols produced by human activity. ACE-2, scheduled for 1997, will focus on the polluted marine atmosphere near European industrialized areas. As scientists learn more about aerosols naturally occurring in the undisturbed atmosphere, they can better assess the growing influence of human-produced sulfate aerosols on climate.
"Until recently all climate models have supposed that the only human activity driving climate change was the production of carbon dioxide and other greenhouse gases," explains Tom Wigley, NCAR senior scientist and a lead author of this year's Intergovernmental Panel on Climate Change report. "We now believe that other factors, particularly sulfate aerosols, may be as important as greenhouse gases." --Anatta