From AMS Glossary
Group of organic compounds containing the carboxylic group -C(O)OH, formed by the oxidation of aldehydes.
Acids such as formic, acetic, and pyruvic acids are often found in air masses containing the oxidation products of organic matter. Other multifunctional acids such as oxalic and malonic acid are often found in liquid droplets. The mechanism of their formation is not fully understood at the moment. The organic acids are relatively weak and do not contribute much to the acidity of the atmospheric aerosol.