Secondary organic aerosol

Secondary organic aerosol is particulate matter formed when gaseous organic compounds undergo atmospheric reactions and produce low-volatility products that condense or nucleate into particles.

Volatile organic compounds: Carbon-containing compounds that readily evaporate into the atmosphere under ordinary conditions. They supply many gaseous precursors that atmospheric reactions transform into secondary organic aerosol.
Biogenic volatile organic compounds: Volatile organic compounds emitted by living organisms, including plants and microbes. Plant emissions such as terpenes are major natural precursors of secondary organic aerosol.
Particulate matter: A mixture of solid particles and liquid droplets suspended in air. Secondary organic aerosol contributes to atmospheric particulate matter measured in air-quality monitoring.
Aerosol chamber: An enclosed experimental vessel used to study particle formation and atmospheric chemistry under controlled conditions. Chamber studies reveal formation pathways, but their results can depend on experimental conditions.
Atmospheric oxidation: Chemical reactions in air that add oxygen to compounds or remove electrons, often changing their volatility. Oxidants transform organic gases into products that can condense into particles.
Anthropogenic volatile organic compounds: Volatile organic compounds released by human activities, including fuel use, industry, and solvents. Human emissions provide precursors that form secondary organic aerosol in polluted air.
PM2.5: Airborne particles with aerodynamic diameters no greater than 2.5 micrometers. Much secondary organic aerosol occurs in the fine-particle size range covered by PM2.5.
Aerosol wall loss: The removal of suspended particles when they deposit on the walls of an experimental chamber. Wall losses complicate estimates of aerosol yields from chamber experiments.
Gas-particle partitioning: The distribution of a chemical compound between the gas phase and suspended particles. It determines how much oxidation product joins the aerosol rather than remaining airborne as gas.
Hydroxyl radical: A highly reactive atmospheric radical that initiates the oxidation of many trace gases, especially during daylight. Its reactions with organic vapors often start daytime aerosol-forming pathways.

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