Chemistry of Volatile Organic Compounds in the Atmosphere

Abstract

Volatile organic compounds (VOCs) in the atmosphere include saturated, unsaturated, aromatic, and a variety of other substituted hydrocarbons. They are emitted from anthropogenic and natural sources mainly as gaseous, often nonpolar, compounds of high vapor pressure. Photochemical oxidation reactions involving hydroxyl (OH) and nitrate (NO₃) radicals, but also ozone and in some cases chlorine atoms, transform these compounds into mainly polar, water-soluble compounds of low vapor pressure. These products are finally removed from the atmosphere by dry or wet deposition. At the very end of the reaction chains, the final products are water vapor and carbon dioxide. While most of the VOCs themselves, especially at the relatively low concentrations, are harmless, the products formed during the oxidation of VOCs in the atmosphere such as photooxidants like ozone or peroxyacetyl nitrate (PAN) have a significant impact on air quality and can be harmful to human health. VOCs also are a source of secondary organic aerosol (SOA), which influences the solar radiation budget and acts as cloud condensation nuclei. Through all these complex reactions, VOCs play an important role in atmospheric chemistry, air quality, and climate.