LAUSR

The effect of foggy/moist conditions on the characteristics of secondary organic aerosol (SOA) was studied. Analysis of the high‐resolution time‐of‐flight aerosol mass spectrometer measurements identified two fresh biogenic SOA factors (BSOA: BSOA‐1 and BSOA‐2), hypothesized to be formed through the nighttime reaction of biogenic volatile organic compounds with nitrate radical. During foggy periods, the more oxidized oxygenated OA (MO‐OOA) was scavenged most efficiently (86% removal), followed by the less oxidized OOA (LO‐OOA) (55%), SO4 (53%), and BSOA (25%). Aerosol mass increased during fog events, and the post‐fog aerosol mass was higher (11%) than during the prefog period, with a reduction for MO‐OOA and an enhancement for LO‐OOA and BSOA. The high positive correlation observed between aerosol liquid water (ALW) and BSOA suggests the importance of aqueous‐phase processing on the BSOA formation, and an increase in log10(ALW) of 1 corresponded to an average increase of about 0.9 μg m−3 in BSOA.