Chemical aging of ambient organic aerosol from heterogeneous reaction with hydroxyl radicals

The chemical transformation of ambient particles due to heterogeneous oxidation was investigated. Ambient aerosol particles sampled in Toronto, Ontario during September and October of 2007 were exposed to OH radicals in a flow tube reactor to simulate relevant atmospheric OH exposures. An Aerodyne Aerosol Mass Spectrometer measured changes in chemical composition and degree of oxidation of reacted ambient particles. Mass spectral changes of ambient particles exposed to OH revealed a loss in the hydrocarbon‐like organic fraction along with an increase in the oxygenated organic fraction. Furthermore, a minor fraction of the organic mass was lost through volatilization at high OH exposure. These observations indicate that an atmospheric OH exposure time of no more than 4 days for a typical OH concentration of 2 × 10 6 cm −3 causes enhanced oxidation of the organic matter in ambient particles. Therefore, heterogeneous oxidation may be an important chemical transformation pathway for ambient particles during regional transport.