Stable oxygen isotope constraints on nitrate formation in Beijing in springtime.

Rapid accumulation of aerosol nitrate (NO) contributes to haze pollution; however, studies quantifying NO formation mechanisms remain scarce. To explore aerosol nitrate formation pathways, total suspended particulate (TSP) samples were collected in Beijing during the spring of 2013, and the concentration of NO and δO- NO value were analyzed. The NO concentrations on polluted days (PD) were higher than those on non-polluted days (NPD). Furthermore, higher δO- NO values were observed on PD (86.8 ± 8.1‰) as compared with NPD (73.7 ± 11.0‰) suggest that more nitrate was produced by pathways with relative high δO-HNO values during PD. Based on the calculated δO-HNO values from different formation pathways and the observed δO- NO values, the possible fractional contributions of HNO formed via various pathways to TSP NO were estimated using the Bayesian isotope mixing model. The δO- NO constrained calculations suggest that the pathways of NO + HO/Cl, NO + VOCs, and ClNO + HO possibly contributed 53%-89% to nitrate production during PD. During NPD, the NO + OH pathway produced 37%-69% of the NO. Using the δO- NO value combined with the isotope mixing model is a promising approach for exploring NO formation pathways.