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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 8 | Copyright
Atmos. Chem. Phys., 18, 5515-5528, 2018
https://doi.org/10.5194/acp-18-5515-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 Apr 2018

Research article | 23 Apr 2018

Isotopic constraints on heterogeneous sulfate production in Beijing haze

Pengzhen He1, Becky Alexander2, Lei Geng1, Xiyuan Chi1, Shidong Fan1, Haicong Zhan1, Hui Kang1, Guangjie Zheng3,a, Yafang Cheng3,4, Hang Su4,3, Cheng Liu1,5,6, and Zhouqing Xie1,5,6 Pengzhen He et al.
  • 1Anhui Province 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 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 323µg 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  ≥  75µg 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.

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We use observations of the oxygen isotopic composition of sulfate aerosol as a fingerprint to quantify various sulfate formation mechanisms during pollution events in Beijing, China. We found that heterogeneous reactions on aerosols dominated sulfate production in general; however, in-cloud reactions would dominate haze sulfate production when cloud liquid water content was high. The findings also suggest the heterogeneity of aerosol acidity should be parameterized in models.
We use observations of the oxygen isotopic composition of sulfate aerosol as a fingerprint to...
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