What are secondary organic aerosols (SOA) and how are they formed?

• A. SOA are particles formed from oxidation of VOCs by OH radicals, leading to condensable products that grow into aerosols.
• B. SOA are directly emitted soot from combustion.
• C. SOA are inorganic salts from sea spray.
• D. SOA are gaseous pollutants.

Answer: (A)

SOA form when volatile organic compounds in the atmosphere are oxidized into products that have low enough volatility to condense onto existing particles or nucleate new aerosol particles. The key oxidant is the hydroxyl radical (OH) during the day, but ozone and nitrate radicals also contribute. This oxidation breaks VOCs into a range of products such as organic acids, carbonyls, and weigh- down products that don’t readily stay in the gas phase, so they partition into the particle phase and grow aerosols. The process depends on how volatile the products are, as well as conditions like temperature and the amount of other particles present, which can provide surfaces for condensation. In essence, SOA are not emitted as particles themselves; they are formed when VOCs oxidize and their oxidation products become condensable, adding to aerosol mass and influencing air quality and climate.