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Aqueous Phase Succinic Acid Photooxidation: Determination of Reaction Products, Hygroscopic Growth and Comparison to the Zsr Mixing Rule.
Atmospheric aerosol particles impact global climate indirectly by forming clouds that interact with solar radiation. The aerosol interaction with water is dependent on the particle composition and composition can change via reactions in the atmosphere. Dicarboxylic acids are a prevalent organic component of aerosol in our atmosphere and have the ability to readily react with both light and hydroxyl radicals. In this study, aqueous solutions of succinic acid (SA), a C4 dicarboxylic acid, and hydrogen peroxide (H2O2) were photolyzed resulting in the formation of hydroxyl radicals that in turn react with the succinic acid. Solutions of various concentrations of hydrogen peroxide to succinic acid (H2O2:SA) were measured from 0 to 120 min to study the change in particle composition and resulting hygroscopic growth (i.e., water uptake) of the products. The product composition was analyzed using gas chromatography coupled with either a flame ionization detector or mass spectrometry for concentration and composition information, respectively. Reaction products with carbon chains greater than succinic acid were formed at small H2O2:SA ratios (< 2:1 H2O2:SA) when C-C bond scission dominates over H abstraction while oxidation of succinic acid to smaller reaction products dominates at larger H2O2:SA ratios (> 5:1 H2O2:SA). The hygroscopic growth behavior of the reaction product mixtures was measured using a humidified tandem differential mobility analyzer (HTDMA). There was no change in hygroscopic growth when photolyzed from 15 to 120 minutes for a 1:1 H2O2:SA solution although the relative ratio of reaction products varies. There was an increase in hygroscopic growth (> 20%) with increased peroxide concentration when photolyzed for 30 minutes given the formation of much smaller, highly hygroscopic compounds for H2O2:SA ≥ 5:1. Growth factor data can be accurately represented using the Zdanovskii-Stokes-Robinson (ZSR) mixing rule when the majority of product mass balance is known through 1) identification of the majority of reaction products or 2) products are lost to the gas phase. These results indicate that aqueous oxidation of succinic acid could dramatically affect how succinic acid forms clouds and, in turn, affects climate.
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