Formation mechanisms of atmospheric nitrate and sulfate during the winter haze pollution periods in Beijing: gas-phase, heterogeneous and aqueous-phase chemistry
Formation mechanisms of atmospheric nitrate and sulfate during the winter haze pollution periods in Beijing: gas-phase, heterogeneous and aqueous-phase chemistry
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北京冬季雾霾污染期间大气硝酸盐和硫酸盐的形成机制：气相、非均相和水相化学
DOI：
10.5194/acp-20-4153-2020
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发表时间：
2020-04-07
影响因子：
6.3
通讯作者：
Sun, Xu
中科院分区：
文献类型：
--
作者：
Liu, Pengfei;Ye, Can;Sun, Xu
中国的大部分地区目前正在经历严重的雾霾事件，冬季PM2.5浓度急剧升高。硝酸盐和硫酸盐是PM2.5的主要成分，但它们通过NO2和SO2氧化的形成仍然没有得到全面的了解，特别是在不同污染或大气相对湿度（RH）条件下。为了阐明硝酸盐和硫酸盐在不同污染情况下的形成途径，2016年冬季在北京连续采集了PM2. 5的小时样品。在采样期间合理识别了3个严重污染案例，并发现在相对较高的RH条件下，硝酸盐和硫酸盐的次生形成对大气PM2. 5的贡献占主导地位。在RH >= 60%条件下，夜间NO3-、NO3-/（NO3- + NO2）和[NO2]（2）x [O-3]之间存在显著相关性，表明N2 O 5与气溶胶液态水的非均相水解是夜间NO3-形成的主要原因。在30%< RH <60%的条件下，NO2和[HONO] x [DR] x [NO2]在白天的变化趋势比与[沙尘] x [NO2]的变化趋势更一致，这为NO2与OH的气相反应在中等RH条件下硝酸盐的日生成中起着关键作用提供了有力的证据。在RH ≥ 60%的条件下，SO_2水相氧化对SO_4 ~（2-）的形成有明显的贡献，SO_4 ~（2-）/（SO_4 ~（2-）+SO_2）在一整天内的平均值都很高。基于本研究中测量的参数和已知的硫酸盐生成速率计算方法，发现H2 O2而不是NO2的氧化途径对硫酸盐的水相形成有很大贡献。
A vast area in China is currently going through severe haze episodes with drastically elevated concentrations of PM2.5 in winter. Nitrate and sulfate are the main constituents of PM2.5, but their formations via NO2 and SO2 oxidation are still not comprehensively understood, especially under different pollution or atmospheric relative humidity (RH) conditions. To elucidate formation pathways of nitrate and sulfate in different polluted cases, hourly samples of PM2.5 were collected continuously in Beijing during the wintertime of 2016. Three serious pollution cases were identified reasonably during the sampling period, and the secondary formations of nitrate and sulfate were found to make a dominant contribution to atmospheric PM2.5 under the relatively high RH condition. The significant correlation between NOR, NOR = NO3-/ (NO3- + NO2), and [NO2](2) x [O-3] during the nighttime under the RH >= 60 % condition indicated that the heterogeneous hydrolysis of N2O5 involving aerosol liquid water was responsible for the nocturnal formation of nitrate at the extremely high RH levels. The more often coincident trend of NOR and [HONO] x [DR] (direct radiation) x [NO2] compared to its occurrence with [Dust] x [NO2] during the daytime under the 30 % < RH < 60 % condition provided convincing evidence that the gas-phase reaction of NO2 with OH played a pivotal role in the diurnal formation of nitrate at moderate RH levels. The extremely high mean values of SOR, SOR = SO42-/ (SO42- +SO2), during the whole day under the RH >= 60 % condition could be ascribed to the evident contribution of SO2 aqueous-phase oxidation to the formation of sulfate during the severe pollution episodes. Based on the parameters measured in this study and the known sulfate production rate calculation method, the oxidation pathway of H2O2 rather than NO2 was found to contribute greatly to the aqueous-phase formation of sulfate.