Isotopic constraints on heterogeneous sulfate production in Beijing haze
Isotopic constraints on heterogeneous sulfate production in Beijing haze
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北京雾霾中异质硫酸盐生成的同位素限制
DOI：
10.5194/acp-18-5515-2018
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发表时间：
2018-04-23
影响因子：
6.3
通讯作者：
Xie, Zhouqing
中科院分区：
文献类型：
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作者：
He, Pengzhen;Alexander, Becky;Xie, Zhouqing
了解北京细颗粒物污染（以下简称雾霾）期间硫酸盐形成的机制，对于理解雾霾的快速演变以及制定具有成本效益的空气污染缓解策略至关重要。在此，我们展示了2014年10月至2015年1月在北京雾霾期间收集的PM2.5硫酸盐的氧 - 17超额（ΔO - 17(SO42 - )）的观测结果，以确定可能的硫酸盐形成途径。在整个采样期间，12小时平均PM2.5浓度范围为16至323μg/m³，平均值为（141 ± 88（1σ））μg/m³，其中SO42 - 占PM2.5质量的8 - 25%。观测到的ΔO - 17(SO42 - )在0.1%至1.6%之间变化，平均值为（0.9 ± 0.3）%。2014年10月，ΔO - 17(SO42 - )随PM2.5水平升高而增加，而2014年11月至2015年1月则观察到相反的趋势。我们的估算表明，由于云液态水含量相对较高，2014年10月案例II的污染天（PDs，PM2.5≥75μg/m³）中，云内反应在硫酸盐生成中占主导地位，其贡献率高达68%。在案例I和案例III - V的污染天期间，非均相硫酸盐生成（Phet）估计对总硫酸盐形成的贡献率为41 - 54%，平均值为（48 ± 5）%。对于二氧化硫非均相氧化的具体机制，化学反应动力学计算表明，气溶胶水中过氧化氢对S(IV)（= SO2·H2O + HSO3 - + SO32 - ）的氧化占P - het的5 - 13%。我们观测到的ΔO - 17(SO42 - )限制了其他机制的非均相硫酸盐生成的相对重要性。通过臭氧对S(IV)氧化产生的非均相硫酸盐生成估计平均占P - het的21 - 22%。导致ΔO - 17(SO42 - )为零的非均相硫酸盐生成途径，例如气溶胶水中二氧化氮对S(IV)的氧化和/或通过自由基链机制由氧气对S(IV)的氧化，占P - het剩余的66 - 73%。发现关于气溶胶热力学状态（稳定或亚稳定）的假设显著影响计算得到的气溶胶pH值（分别为7.6 ± 0.1或4.7 ± 1.1），从而影响通过二氧化氮和氧气对S(IV)氧化的非均相硫酸盐生成的相对重要性。我们基于当地大气条件的计算表明，在假设稳定状态下计算得到的高pH条件下的气溶胶中，通过二氧化氮氧化形成硫酸盐可能是主要途径，而如果气溶胶呈强酸性（pH值较低），则通过氧气对S(IV)的氧化可能是主要途径。
Abstract. Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present observations of the oxygen-17 excess of PM2.5 sulfate (Δ17O(SO42−)) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12-hourly averaged PM2.5 concentrations ranged from 16 to 323 µg m−3 with a mean of (141 ± 88 (1σ)) µg m−3, with SO42− representing 8–25 % of PM2.5 mass. The observed Δ17O(SO42−) varied from 0.1 to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰. Δ17O(SO42−) increased with PM2.5 levels in October 2014 while the opposite trend was observed from November 2014 to January 2015. Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM2.5 ≥ 75 µg m−3) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III–V, heterogeneous sulfate production (Phet) was estimated to contribute 41–54 % to total sulfate formation with a mean of (48 ± 5) %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggested S(IV) ( = SO2 ⚫ H2O + HSO3− + SO32−) oxidation by H2O2 in aerosol water accounted for 5–13 % of Phet. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ17O(SO42−). Heterogeneous sulfate production via S(IV) oxidation by O3 was estimated to contribute 21–22 % of Phet on average. Heterogeneous sulfate production pathways that result in zero-Δ17O(SO42−), such as S(IV) oxidation by NO2 in aerosol water and/or by O2 via a radical chain mechanism, contributed the remaining 66–73 % of Phet. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O2. Our local atmospheric conditions-based calculations suggest sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high-pH conditions calculated assuming stable state while S(IV) oxidation by O2 can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of Δ17O(SO42−) for quantifying sulfate formation pathways, but this estimate may be further improved with future regional modeling work.