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pH-Dependence of the OH-Reactivity of Organic Acids in the Aqueous Phase
JESSICA LIMA AMORIM, Keifer Klimchuk, Florence Williams, Ran Zhao, University of Alberta
Abstract Number: 25
Working Group: Aerosol Chemistry
Abstract
Studies from the past decade have identified cloudwater as an important reaction media for many organic compounds in the atmosphere. In aqueous-phase environments, organic molecules are subjected to a wide range of conditions that can affect their chemical processing and lifetimes in the atmosphere. Of these conditions, acidity is particularly important, as acid-base chemistry is a unique nature of aqueous-phase processing. Yet, the impact of solution pH on the OH-reactivity of many organic acids important to SOA formation remains unknown. A major challenge in studying the aqueous-phase processing of SOA-relevant compounds is a lack of compounds commercially available, which is limited to a few monoterpene-derived compounds. The goal of this study is to improve our fundamental understanding of the pH dependence of the aqueous-phase oxidation of organic acids by hydroxyl radicals (OH). Using (-)ESI-LC/MS, we investigated the second-order reaction rate coefficients (kIIOH) of organic acids relevant to α-pinene SOA at uncontrolled pH, pH 2 and 10. We compared the kIIOH values of pinic acid (PA), cis-pinonic acid (CPA) and limononic acid (LA) - organic acids with large aliphatic chains (C9-10) - with formic acid (FA, C1). Here, PA was synthesized and its photochemistry was investigated for the first time. In addition to PA, CPA, LA and FA we extended our study to other organic acids present in the water-soluble fraction of α-pinene SOA. The results obtained by our study indicate that the kIIOH of formic acid, the smallest organic acid, is highly pH dependent, while PA and many other organic acids arising from α-pinene are not. These findings could help simplify model parameters needed for cloudwater chemistry models.