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 log(10)(ALW) of 1 corresponded to an average increase of about 0.9 mu g m(-3)in BSOA. Plain Language Summary Ground-level aerosols impact human health, climate, and ecosystem health. Aerosols have complex chemical composition, and the chemistry of the carbonaceous fraction is a topic of great interest to atmospheric chemistry and air quality. Aerosol liquid water is an important component of these particles in humid environments, and it becomes the dominant component if the particle grows to become a fog or cloud droplet. In addition to influencing particle growth, the aerosol liquid water provides another medium in which chemical reactions can occur and produces species that remain in the condensed phase and thereby increase aerosol mass loading. This study investigated the evolution of aerosol mass and chemical composition in moist and foggy nighttime environments in a rural forested area of the Northeast U.S. Organic aerosols that were identified as biogenic secondary organic aerosols were observed to increase in mass concentration during foggy conditions, and in this environment, they increased linearly in mass as a function of the base 10 logarithm of aerosol liquid water over a wide range. The influence of aerosol liquid water on the chemical reactivity of aerosols could help to bridge the gap between modeled and observed secondary organic aerosol mass.