Evolution of Aerosol Under Moist and Fog Conditions in a Rural Forest Environment: Insights From High-Resolution Aerosol Mass Spectrometry

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.