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Atmospheric aerosols (sub-micron and micron-sized particles suspended in air) originate both from natural and man-made sources. Due to their short lifetime and strong tropospheric interactions, their global concentrations and properties are poorly known. Aerosol particles affect atmospheric radiation and cloud microphysics, and are considered a major uncertainty in climate forcing. Saharan dust generated over Africa and transported over the Atlantic Ocean to America is an example of a natural source. It is hypothesized that the dust has a major role in fertilization of the Amazon forest. Also, oceanic phytoplankton generates dimethyl-sulfide gas that is a precursor of the oceanic sulfate aerosol, which is also a natural gas.
Man-made aerosol particles are produced in industrial and urban areas of the Northern Hemisphere by conversion of gases (e.g., SO2 and NOX) into liquids in chemical reactions, and through emission of graphitic carbon. This pollution spreads over the Norther Hemisphere and takes part in acidification of rain and reduction of visibility. Reflection of sunlight by the particles and their effect on cloud microphysics is purported to counteract a significant part of the expected greenhouse warming. Biomass burning in tropical regions, especially the Amazon rain forest, is used to clear land for agriculture. This generates a large fraction of the global flux of aerosol particles as well as trace gases such as CO2, CO, and CH4. Smoke particles have similar effects on visibility, clouds, precipitation, and radiation as industrial and urban aerosol. Branch research includes studies of the physical and optical properties of atmospheric aerosol particles using ground-based, airborne, and spaceborne instrumentation. An airborne experiment was conducted to measure the atmospheric impact of smoke generated from the oil fires in Kuwait. Field experiments have also been conducted in the Amazon Basin of Brazil and in a desert transition area in Israel. Development of radiative transfer models to treat the aerosol radiative effects and the inclusion of these effects in climate models are also an integral part of branch research activities. Tropospheric aerosols also obscure surface features that are observed by satellites. Methods are developed to correct satellite digital images in order to obtain clearer pictures of the surface, and estimates of aerosol properties are obtained from digital satellite data. Contact: Lorraine Remer
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