Aging of Secondary Organic Aerosols at Atmospherically Relevant Acidities

CYNTHIA WONG, Sergey Nizkorodov, University of California, Irvine

     Abstract Number: 86
     Working Group: Aerosol Chemistry

Abstract
The acidity of atmospheric aerosols spans a wide range, with the most acidic particles having negative pH values, which can promote acid-catalyzed reactions. Many reactions in the atmosphere are pH-sensitive including, hemiacetal and acetal formation, hydration of complex aldehydes, ketones and carbonyls, aldol condensation, esterification, and epoxide protonation. However, the impact of highly acidic conditions on aerosol chemical composition and aging processes remains uncertain. Our previous experiments investigated the aging of SOA in a bulk H₂SO₄ solution, whereas this work focuses on our ongoing work, wherein an aliquot of sulfuric acid is dispersed in a small amount of SOA to achieve a more relevant H₂SO₄:SOA ratio in order to elucidate mechanisms of acid-catalyzed aging of mixtures of SOA compounds. SOA samples were generated in a continuous flow reactor and collected onto quartz windows. A small aliquot of aqueous sulfuric acid, ranging in concentration, was added and pressed between the quartz window with SOA and a clean quartz window. The sample was then aged for 2 days and then extract using a 1:1 mixture of water and acetonitrile. Analysis of fresh and aged samples was conducted using ultra-liquid chromatography coupled with a photodiode array spectrophotometer and a high-resolution mass spectrometer to study the change in the chemical composition of SOA before and after aging. We found that SOA aged in highly acidic conditions resulted in a change in the chemical composition, including the formation of sulfur-containing compounds. The findings from this study will improve our understanding of SOA aging processes in the upper troposphere and lower stratosphere and highly polluted areas where there are significant contributions of sulfates and high acidity.