Quantifying the formation pathways of nitrate in size-segregated aerosols during winter haze pollution

Abstract

Nitrate (NO3−) is an important acidic component in haze pollution. The NO3− formation pathways for bulk aerosols have been studied thoroughly, leaving the size-segregated NO3− formation mechanisms underexplored. By coupling nitrate dual isotopes with aerosol chemical compositions, we studied size-segregated aerosols collected during winter haze pollution events at Chengdu, China. Different formation mechanisms for the size-segregated NO3− were analyzed. For fine particles (0.43–2.1 μm), NO3− was dominantly formed under NH4+ enriched conditions, during which the SIAR (stable isotope analysis in R) model suggested that N2O5 hydrolysis, NO3 + VOCs (volatile organic compounds) and NO2 + OH homogeneous reactions contributed 38–42 %, 32–38 %, and 22–30 % of NO3− production, respectively. For coarse particles (>3.3 μm), majority of NO3− was produced through the reaction of gas-phase HNO3 with CaCO3 (63–71 %). This work revealed differential formation pathways of size-segregated NO3− and provided quantitative insights on information on NO3− formation during the haze pollution.