Samsung Advanced Institute of Technology

 Secondary organic aerosol (SOA) produced by atmospheric oxidation of volatile organic compounds and directly-emitted primary organic aerosol (POA) is a major contributor to fine particulate matter (PM2.5) air pollution worldwide. Observations during winter haze pollution episodes in urban China show that most of this SOA originates from fossil fuel combustion but the mechanisms are not understood. Here we report field observations in a Beijing winter haze event of fast aqueous-phase conversion of fossil-fuel POA to SOA at high relative humidity, strongly correlated with sulfate formation. Analyses of aerosol mass spectra and elemental ratios indicate that ring-breaking oxidation of aromatic species with functionalization to carbonyls and carboxylic acids may serve as the dominant mechanism for this SOA formation. The resulting fossil fuel SOA is much less light-absorbing than the original POA, with implications for pollution-weather feedbacks and climate forcing.