Struvite-Catalyzed Photochemical Reactions of Aqueous-Phase Guaiacyl Acetone and 3,4-Dimethoxybenzaldehyde

MARIA MISOVICH, Manoj Silva, Robert Blakeslee, Jonas Baltrusaitis, Alexander Laskin, Purdue University

     Abstract Number: 359
     Working Group: Aerosol Chemistry

Abstract
Struvite (NH₄MgPO₄•6H₂O) is a water insoluble mineral nanocrystal frequently found in fertilizers that can be transported to airborne particles by raindrop-induced aerosolization from agricultural soils exposed to irrigation and rain events. Guaiacyl acetone (GA, C₁₀H₁₂O₃), a prominent product of biomass burning emissions, partitions rapidly into the aqueous phase, suggesting that it is a relevant proxy compound to represent organic species in aqueous-phase atmospheric aerosol. When irradiated in the presence of triplet excited carbon (³C*), GA has been found to undergo rapid oxidation reactions to form monomeric and dimeric brown carbon products. This study investigates the effect of struvite on the photoreactions of aqueous GA in the presence of 3,4-dimethoxybenzaldehyde (DMB, C₉H₁₀O₃), a source of ³C* frequently found in biomass burning emissions. A solar simulator was used to mimic sunlight. GA and DMB were irradiated with and without struvite, and samples of the resulting reaction mixtures were collected at 0, 20, 60, and 120 minutes. Chemical components of the mixtures were analyzed using reversed phase liquid chromatography interfaced with a photodiode array detector and an ESI-HRMS Orbitrap mass analyzer. Our results indicate that introducing struvite into the system accelerates the formation of major chromophore products and catalyzes the formation of several minor unique chromophores not found in the struvite-free system. Struvite serves as a heterogenous catalyst, and it is also suspected to generate OH radicals in water. Top down mass absorption coefficient (MAC) plots indicate that total absorbance decreases with irradiation in all three systems, but that the rate is impacted by the presence of struvite. Mass spectra show that the products initially fall into classes of monomers and dimers, but that products outside of these classes formed after longer irradiation times. Elemental formulas were assigned for all prominent chromophores. Tentative structures and a mechanism were proposed and discussed.