Aqueous production of secondary organic aerosol from fossil-fuel emissions in winter Beijing haze

Significance Secondary organic aerosol (SOA) from fossil-fuel combustion is a major component of Beijing winter haze pollution but its formation mechanisms are unclear. It is generally assumed to originate from atmospheric oxidation of volatile organic compounds (VOCs). However, we show here that fast aqueous-phase oxidation of directly emitted primary organic aerosol (POA) takes place at high relative humidity in Beijing winter haze and can...