Effects of Organic Matrices on Nucleophilic Aqueous Aerosol Chemistry: Yields and Mechanistic Insight for Brown Carbon Formation from Glyoxal and Ammonia

Climate forcing effects of tropospheric aerosols are determined by their relative abilities to absorb and scatter light and also their effects on cloud properties and lifetime, which may lead to net warming or cooling of the atmosphere. Many details of the formation and lifecycle of organic content that contributes to aerosol light absorption, termed brown carbon (BrC), remain uncertain after extensive study. The reactive species that form BrC make up only a small fraction of total aerosol organic content, and the overwhelming remainder of organic content could greatly influence the rate of BrC formation. One significant route to BrC formation is the reaction of water-soluble carbonyl species to form larger conjugated and/or aromatic compounds, such as the reaction of di-aldehydes (e.g., glyoxal) with reduced nitrogen species (ammonia or amines) to form imidazole derivatives. In this study, we work to further address the complexity of atmospheric aerosols by adding a matrix of organic content to the glyoxal-ammonium system. We find that the addition of a broad range of organic species (alcohols, ketones, organic acids, etc.) to BrC-forming reaction mixtures can both increase and decrease the rate of BrC formation. The rate of BrC formation is shown to vary by more than an order of magnitude depending on the composition of the organic matrix present. UV-vis kinetic measurements and HPLC-ToF-MS product analysis of aqueous solutions with qualities similar to atmospheric aerosols reveal the specific steps in BrC formation affected by the presence of organic content. Our method of systematically adding complexity also proves a useful tool for mechanistic evaluation, and we provide evidence that the proposed cis-di-imine mechanism for the reaction of ammonia and glyoxal is unlikely, with amine species reacting instead.