Atmospheric aerosol, as one of the most important components in the Earthatmosphere system, is defined as solid particles (i.e., mineral dust, smoke particles, microbes, and pollen) or liquid droplets suspended in air, with diameters in the range of about 0.001 to 100 μm. The particles induced by gasto- particle conversion processes, with diameters <0.01 μm, are known as ultrafine particles. Such small particles have been observed in bursts of very large numbers and can be converted to clusters by complex physical and chemical processes under favorable conditions. According to particle diameters, the commonly observed aerosol particles can be further classified into three categories: Aitken, accumulation, and coarse modes. The Aitken particles with diameters ranging from 0.01 to 0.1 μm act as nuclei for condensation of gaseous species under low vapor pressure, making them grow into larger particles. The accumulationmode particles generated from aggregation of smaller particles, condensation of gases, or re-evaporation of droplets have typical diameters between 0.1 and 1.0 μm. These particles usually comprise a substantial number of soluble inorganics such as ammonium, nitrate, and sulfate, as well as carbonaceous materials. The coarse mode corresponds to particles with diameters larger than 1.0 μm, and they are primarily produced by mechanical processes and introduced directly into the atmosphere from both natural and anthropogenic sources. The diameters of coarse-mode particles and cloud droplets have certain overlapping regions; however, the diameters of coarse-mode particles are usually smaller than those of cloud droplets, and the cloud droplets easily change into rain when their diameters are large enough under favorable conditions.
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