Stable oxygen isotope constraints on nitrate formation in Beijing in springtime
2020
Luo, Li | Kao, ShuhJi | Wu, Yunfei | Zhang, Xiaoling | Lin, Hua | Zhang, Renjian | Xiao, HuaYun
Rapid accumulation of aerosol nitrate (NO3−) contributes to haze pollution; however, studies quantifying NO3− 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 NO3− and δ¹⁸O- NO3− value were analyzed. The NO3− concentrations on polluted days (PD) were higher than those on non-polluted days (NPD). Furthermore, higher δ¹⁸O- NO3− 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 δ¹⁸O–HNO₃ values during PD. Based on the calculated δ¹⁸O–HNO₃ values from different formation pathways and the observed δ¹⁸O- NO3− values, the possible fractional contributions of HNO₃ formed via various pathways to TSP NO3− were estimated using the Bayesian isotope mixing model. The δ¹⁸O- NO3− constrained calculations suggest that the pathways of N₂O₅ + H₂O/Cl⁻, NO₃ + VOCs, and ClNO₃ + H₂O possibly contributed 53%–89% to nitrate production during PD. During NPD, the NO₂ + OH pathway produced 37%–69% of the NO3−. Using the δ¹⁸O- NO3− value combined with the isotope mixing model is a promising approach for exploring NO3− formation pathways.
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