Fog Intensity and Droplet Size-Resolved Aqueous Secondary Organic Aerosol (aqSOA) Formation in Polluted Winter Fog: New Insights from a Combined Observational and Modeling Study

Abstract Fog is a chemically active aqueous medium in which dissolved organic gases and particles are converted into highly oxidized aqueous secondary organic aerosol (aqSOA). Its magnitude is difficult to constrain, and most published estimates rely on models. We analyzed size-resolved fogwater collected over two winters at Kanpur, India, by offline high-resolution aerosol mass spectrometry and apportioned by positive matrix factorization (PMF). Three highly oxidized factors (O/C 0.95–1.19) were summed as an aqSOA proxy. Combining this proxy with colocated liquid water content and water-soluble organic carbon yielded 3.1–8.4 μg m–3 aqSOA across seven events. The droplet aqSOA accounts for a median of about 7% of ambient SOA and 5% of ambient OA, reaching 13% of SOA in the densest fogs. Fine droplets carried the largest aqSOA share (43–84%), driven by higher concentrations of dissolved Fe and Cu and longer residence times. Ambient OA was less oxidized during dense than light fog (O/C 0.52 versus 0.61). Returning aqSOA after dissipation gave direct-radiative-effect increments of −0.048 W m–2 (dense) and −0.032 W m–2 (light). These size- and intensity-resolved observational constraints quantify fog–aerosol interactions and their air quality consequences in a heavily polluted region.