The formation of secondary organic aerosol (SOA) of the styrene-NO_x-air under irradiation at a room temperature of (301±2) K has been investigated in a smog chamber. The effects of high relative humidity (72%) on the chemical components, number concentrations and yield of SOA have been investigated, respectively. The gas-phase products were determined to be formaldehyde, benzaldehyde, formic acid, CO, etc. SOA was collected on the polytetrafluoroethylene (PTFE ) filter and analyzed with FTIR. The main products of SOA were found to be perbenzoyl nitrate, 2-nitrophenol, benzoic acid hydroxy-phenyl-methyl ester, peroxybenzoic acid and benzoic acid. It was found that at high relative humidity, gas-phase formic acid and benzoic acid are considerably increased, and the IR spectrum of SOA reveals a new absorption of benzoic acid, whereas the C＝O absorbance from benzoic acid hydroxy-phenyl-methyl ester is decreased. This confirms the pathway of the reaction between H₂O and Criegee biradical to produce benzoic acid hydroxy-phenyl-methyl ester. The SOA number concentration is decreased by about 36%～44% at high RH, whereas the SOA yield obtained from the mass collected by the PTFE membrane is increased by about 40% at high RH, compared with that at low RH, which demonstrates the increase in particle size at high RH.

Key words: secondary organic aerosol, photochemistry, smog chamber, styrene, relative humidity