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 explain the observed SOA. This could explain why SOA has decreased in response to POA emission controls even as VOC emissions have stayed flat. The mechanism may involve ring-breaking oxidation and functionalization of polycyclic aromatic hydrocarbons.