Fine particles in the global troposphere: A review

The available body of data on fine particles (< 1 μm radius) in the troposphere is reviewed in relation to our understanding of sources, sinks and transformation processes of atmospheric aerosols. This review yields the following results. There are no data available to characterize the free troposphere above the boundary layer. Most of the boundary layer data suffer from the lack of a well-defined upper size limit of the samplers at about 1 μm radius and concurrent results on total fine particle mass. The fine particle composition data are consistent with our understanding of natural and anthropogenic sources of trace substances, which lead to fine particles. The chemical composition is presented for the three aerosol types: urban, non-urban continental and remote regions. Special attention needs to be focused on fine particle interaction with clouds for three reasons. Fine particles control number- and surface-distribution of the condensed matter; the state of precondensation haze and clouds provide the most important processes of fine particle elimination from the atmosphere. Furthermore, climatic effects of fine particles as expressed by their interaction with atmospheric radiation is strongest during their passage through clouds. Very little is known about the processing of aerosols through clouds. Consequently, there are large uncertainties about their climatic effect and their lifetime in the atmosphere. Finally, no single type of secular trend in particle concentration is found on a global scale. Depending on which component and which geographical region is monitored, both decreasing and increasing concentrations have been measured. The differences are consistent with our understanding of aerosol properties and their gas phase precursors.