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Published October 6, 2020 | public
Journal Article Open

pH Dependence of the OH Reactivity of Organic Acids in the Aqueous Phase


Photochemical processing taking place in atmospheric aqueous phases serves as both a source and a sink of organic compounds. In aqueous environments, acid–base chemistry and, by extension, aqueous-phase pH, are an important yet often neglected factors to consider when investigating the kinetics of organic compounds. We have investigated the aqueous-phase OH-oxidation of pinic acid, cis-pinonic acid, limononic acid, and formic acid (FA) as a function of pH. We have also extended our studies to other organic acids (OAs) present in the water-soluble fraction of secondary organic aerosol (SOA) arising from the ozonolysis of α-pinene. Although all the OAs exhibited larger OH reactivities at pH 10, the pH dependence was dramatically different between FA, the smallest OA, and those that contained more than eight carbons. A kinetic box model was also employed to characterize our photoreactor and to provide confidence to our results. Our finding shows that the atmospheric lifetimes of small OAs (e.g., FA) are highly sensitive to cloud water pH. However, those of larger OAs and many other OAs in α-pinene SOA are affected to a much less extent. These results are of great importance for the simplification of cloud water chemistry models.

Additional Information

© 2020 American Chemical Society. Received: May 24, 2020; Revised: August 4, 2020; Accepted: September 2, 2020; Published: September 16, 2020. The authors thank Dr. Anne Myers and Canadian Ice Core Lab (CICL) for IC support, U of Alberta MS Facility for LC–MS support, Department of Chemistry Machine Shop and Glass Shop for flow tube support, Dr. Mike Serpe, Siyuan Guo, and Yingnan Zhang for the use of spectroradiometry, and Dr. Sarah Styler, Iris Chan, and Mario Schmidt for HPLC use. The authors also thank NSERC and U of Alberta for financial support. The authors declare no competing financial interest.

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Supplemental Material - es0c03331_si_001.pdf


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August 19, 2023
August 19, 2023