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Quantification of SO_2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation

Hung, Hui-Ming and Hsu, Mu-Ni and Hoffmann, Michael R. (2018) Quantification of SO_2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation. Environmental Science and Technology, 52 (16). pp. 9079-9086. ISSN 0013-936X. https://resolver.caltech.edu/CaltechAUTHORS:20180724-153038359

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Abstract

Sulfate formation on the surface of aqueous microdroplets was investigated using a spray-chamber reactor coupled to an electrospray ionization mass spectrometer that was calibrated using Na_2SO_4(aq) as a function of pH. The observed formation of SO_3–•, SO_4–•, and HSO_4– at pH < 3.5 without the addition of other oxidants indicates that an efficient oxidation pathway takes place involving direct interfacial electron transfer from SO_2 to O_2 on the surface of aqueous microdroplets. Compared to the well-studied sulfate formation kinetics via oxidation by H2O2(aq), the interfacial SO_4^(2–) formation rate on the surface of microdroplets was estimated to be proportional to the collision frequency of SO2 with a pH-dependent efficiency factor of 5.6 × 10^(–5)[H+]^(3.7)/([H+]^(3.7)+10^(–13.5)). The rate via the acidic surface reactions is approximately 1–2 orders of magnitude higher than that by H_2O_2(aq) for a 1.0 ppbv concentration of H_2O_2(g) interacting with 50 μg/m^3 of aerosols. This finding highlights the relative importance of the interfacial SO_2 oxidation in the atmosphere. Chemical reactions on the aquated aerosol surfaces are overlooked in most atmospheric chemistry models. This interfacial reaction pathway may help to explain the observed rapid conversion of SO_2 to sulfate in mega-cities and nearby regions with high PM2.5 haze aerosol loadings.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/acs.est.8b01391DOIArticle
ORCID:
AuthorORCID
Hung, Hui-Ming0000-0002-6755-6359
Hoffmann, Michael R.0000-0001-6495-1946
Alternate Title:Quantification of SO2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation
Additional Information:© 2018 American Chemical Society. Received: March 14, 2018; Revised: July 5, 2018; Accepted: July 24, 2018; Published: July 24, 2018. Funding for this research was provided by the Ministry of Science and Technology in Taiwan (105-2111-M-002-001 and 106-2111-M-002-007). Additional support was provided by the US National Science Foundation (Grant Number: AGS 1744353). We also appreciate the instrument support provided by Dr. Chau-Chung Han from the Institute of Atomic and Molecular Sciences, Academia Sinica. Discussions with Prof. Becky Alexander from the University of Washington during a recent Gordon Research Conference and thermodynamic model discussions with Drs. Shaojie Song and Pengfei Liu of Harvard University, and the constructive comments from the anonymous reviewers are appreciated. The authors declare no competing financial interest.
Funders:
Funding AgencyGrant Number
Ministry of Science and Technology (Taipei)105-2111-M-002-001
Ministry of Science and Technology (Taipei)106-2111-M-002-007
NSFAGS-1744353
Issue or Number:16
Record Number:CaltechAUTHORS:20180724-153038359
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20180724-153038359
Official Citation:Quantification of SO2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation. Hui-Ming Hung, Mu-Ni Hsu, and Michael R. Hoffmann. Environmental Science & Technology 2018 52 (16), 9079-9086 DOI: 10.1021/acs.est.8b01391
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:88220
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:24 Jul 2018 22:48
Last Modified:03 Mar 2020 13:01

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