Aqueous-Phase Formation of Nitrogen-Containing Secondary Organic Aerosols in China Winter Haze Continuously Enhanced in the Past Decade

While efficient emission controls have significantly reduced PM_2.5 (particle with an aerodynamic diameter < 2.5 μm) levels in China in the past decades, concentrations of atmospheric secondary organic aerosol (SOA) in the country remain high, with formation mechanisms poorly understood. By combing an intensive field online measurement in the North China Plain (NCP) during winter 2023 with an offline analysis, we find that aqueous-phase-formed SOA (aqSOA) is the major component of organic aerosol (OA) during the haze periods in NCP, accounting for 54% of the total OA mass. Meanwhile, nitrogen-to-carbon ratio of OA linearly increased along with an increasing fraction of aqSOA (f_aqSOA) during the haze development, which is largely driven by aqueous-phase reactions of NH₃ with carbonyls. We also find that the role of aqSOA in haze formation in the NCP since 2013 has become increasingly important, along with an enhancement in N/C ratio of OA from 0.017 in 2013 to 0.028 in 2023, suggesting enhanced aqueous formation of nitrogen-containing SOA in the region. Since nitrogen-containing organics are mostly light-absorbing, such an enhanced formation of nitrogen-containing SOA may partly offset the aerosol absorption decline from black carbon, raising a new challenge for air quality and climate policy in China.