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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2017-977
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
06 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
Isotopic constraints on heterogeneous sulphate production in Beijing haze
Pengzhen He1, Becky Alexander2, Lei Geng1, Xiyuan Chi1, Shidong Fan1, Haicong Zhan1, Hui Kang1, Guanjie Zheng3,a, Yafang Cheng3,4, Hang Su4,3, Cheng Liu1,5,6, and Zhouqing Xie1,5,6 1Anhui P rovince Key Laboratory of Polar Environment and Global Change, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China
2Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
3Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz 55128, Germany
4Jinan University, Institute for Environment and Climate Research, Guangzhou, Guangdong 511443, China
5Key Lab of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China
6Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, Fujian 361021, China
aNow at: Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, NY 11973, USA
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 the first 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 12h-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 trends were observed in November 2014 to January 2015. Heterogeneous sulfate production rate (Phet) on aerosols was estimated to enhance with PM2.5 levels, generally dominating sulfate formation during haze days when cloud liquid water content (LWC) was low. When LWC was high, however, in-cloud reactions would dominate haze sulfate formation with a fractional contribution up to 68 %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggest 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 on acidic microdroplets via a radical chain mechanism, contributed the remain 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 on acidic microdroplets. Our calculation suggests 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 on acidic microdroplets can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our results also illustrate the utility of Δ17O(SO42−) for quantifying sulfate formation pathways and its inclusion in models may improve our understanding of rapid sulfate formation during haze events.

Citation: He, P., Alexander, B., Geng, L., Chi, X., Fan, S., Zhan, H., Kang, H., Zheng, G., Cheng, Y., Su, H., Liu, C., and Xie, Z.: Isotopic constraints on heterogeneous sulphate production in Beijing haze, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-977, in review, 2017.
Pengzhen He et al.
Pengzhen He et al.
Pengzhen He et al.

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