Atmospheric reactions are strongly affected by the number of suspended solid particles and their properties. The particles supply the surfaces on which reactions can occur thus acting as catalysts. They can also affect the absorption spectrum through the adsorption of gases (i.e., in the wavelength range of adsorbed radiation) and thus affect the intensities of radiation absorption and photochemical reactions. Moreover, solid particles can react with industrially emitted gases in common chemical reactions.

Combustion, volcanic eruptions, dust storms, and sea spray are a few processes that emit particles. Many par-ticulates in the air are metal compounds that can catalyze secondary reactions in the air or gas phase to produce aerosols as secondary products. Physical processes such as nucleation, condensation, absorption, adsorption, and coagulation are responsible for determining the physical properties (i.e., the number concentration, size distribution, optical properties, and settling properties) of the formed aerosols. Particles below 0.1 /, (known as Aitken nuclei), although not significant by gravity, are capable of serving as condensation nuclei for clouds and fog. Secondary effects are the results of gas-phase chemistry and photochemistry that form aerosols.

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