Radiative Effects of Aerosols in the Remote Marine Atmosphere (ACE-1)

A Field Experiment of the International Global Atmospheric Chemistry Project

The Problem....

Microscopic atmospheric aerosol particles can cool the Earth's surface by reflecting sunlight back into space. When present in large quantities, these particles form a visible haze. Many of the larger particles can form cloud droplets which also reflect incoming solar radiation. Although atmospheric aerosols are chemically complex and may be influenced strongly by local emissions, one consistent feature, world-wide, is a strong presence of sulfate. Before the industrial period these particles were formed from sulfur gases emitted from the ocean and volcanoes. Today, the major source of atmospheric sulfate particles is the sulfur dioxide emitted from fuel combustion by power plants and industry.

It is difficult to calculate the effect of aerosol particles on the Earth's climate because of a lack of globally distributed data and a clear understanding of the processes that link gas emissions with particle formation and growth. Present estimates of the cooling over industrial regions (eastern USA, Europe, eastern Asia) due to the sulfate haze alone is slightly greater on a regional basis than the warming due to greenhouse gases produced by human activities. However, the uncertainties in these estimates are very high. Estimates of the cooling which results from aerosols forming additional cloud droplets are even more uncertain.

The Plan...

The International Global Atmospheric Chemistry Program (IGAC) has planned a series of experiments to better understand the chemical, physical, and radiative properties of atmospheric aerosols and the processes which control these properties. The goals of these Aerosol Characterization Experiments (ACE) are to provide the necessary data to incorporate aerosol particles into global climate models and to reduce the overall uncertainty in calculating the climatic effect of aerosols.

The first experiment, ACE-1, took place in the minimally polluted marine atmosphere south of Australia. Non-sea-salt sulfate aerosol particles in the remote marine atmosphere are thought to have only one primary gaseous precursor, dimethlysulfide (DMS), thereby simplifying studies involving the formation and growth of the aerosol. This marine environment also afforded an opportunity to study the properties of natural aerosols and thus provides a background from which to compare and quantify any human induced changes.

The experiment involved scientists from 44 research institutions in 11 countries and included measurements from the NCAR C-130, the NOAA research vessel Discoverer , the Australian fisheries research vessel Southern Surveyor, and land-based stations at Cape Grim and Macquarie Island, Australia and Baring Head, New Zealand. Observations from these six platforms will be compared with simultaneous satellite measurements to test and improve models of sunlight reflection by atmospheric particles.

The Benefits...

The experiment will provide data on:

  1. the chemical composition, size distribution, and radiative and cloud nucleating properties of aerosols in the marine atmosphere,

  2. the amount of biogenic sulfur (DMS) released from the ocean to the atmosphere,

  3. the rates and efficiencies of sulfur gas oxidation in the marine atmosphere, and

  4. the rates and efficiencies of the processes controlling the formation, growth, distribution, and removal of particles in the marine atmosphere.

These data will be used to test regional and global aerosol climate models and thus produce more accurate assessments of the climatic effect of aerosol particles. The net result will be more accurate predictions of climate change. Since climate change may cause some regions to become more arid while others experience more frequent flooding, heavier snowfall or more severe storms, the ability to accurately predict regional climate changes will have a tremendous economic impact.

ACE-1 is the first in a series of experiments. The understanding gained in ACE-1 will be used to study progressively more complex environments. ACE-2 will extend these studies to the North Atlantic Ocean and focus on the anthropogenic aerosols from the European continent and desert dust form the African continent.

ACE-1 was conducted from 15 November to 14 December 1995 over the Southwest Pacific Ocean, South of Australia and involved the joint efforts of the International Global Atmospheric Chemistry Project's (IGAC) Aerosol Characterization and Process Studies (ACAPS) Activity and Marine Aerosol and Gas Exchange (MAGE) Activity.

Major funding for ACE-1 has been provided by:

The U.S. National Science Foundation (NSF), Atmospheric Chemistry Program

The U.S. National Oceanic and Atmospheric Administration (NOAA), Office of Global Programs

The U.S. National Aeronautical and Space Administration (NASA)

The Australian Commonwealth Scientific and Industrial Research Organization (CSIRO)

The Australian Bureau of Meteorology (BoM)

The Australian Antarctic Division (ANARE)

The New Zealand National Institute for Water and Atmospheric Research (NIWA)

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