Title: Gas-Aerosol Model For Mechanism Analysis
Description: Atmospheric aerosols are a major contributor to the total energy balance of the Earth's atmosphere. The exact effect of these aerosols on global climate is not well understood, due to poorly-characterized compositional variation that takes place over a given aerosol's lifetime. Organic aerosol (OA) species are of particular interest, forming through a myriad of gas- and aerosol-phase mechanisms and contributing to aerosol light absorbance, cloud formation properties, and overall particle lifetime. As different organic species will affect physical properties in different ways, proper prediction of these compounds forming in the aerosol phase is necessary to estimate the net physical properties of aerosols, and subsequently their effects on overall global climate. Several previous models exist that attempt to predict organic components of aqueous-phase mass in aerosols, with varying degrees of scope of chemistry and range of applicability. Many of such simulations emphasize OA formation via oxidation of gas-phase organic species that results in low-volatility compounds that subsequently partition into aerosols.

Other models focus on aqueous-phase processing of semi-volatile and non-volatile water-soluble organic compounds (WSOC's) under cloud water conditions. However, aqueous reactions that occur in atmospheric, deliquesced salt aerosols have recently also been found to be potentially important additional pathway for the creation of additional aerosol-phase organic mass, contributing different products due to the significantly higher inorganic concentrations present under these conditions. It is desirable to incorporate these reactions into analogous predictive simulations, allowing for the chemistry taking place in small, deliquesced salt atmospheric aerosols to be more accurately represented. In this work, we discuss a new photochemical box model known as GAMMA, the Gas-Aerosol Model for Mechanism Analysis. GAMMA couples gas-phase organic chemistry with highly detailed aqueous-phase chemistry, yielding speciated predictions for dozens of secondary organic aqueous aerosol-phase compounds under various atmospheric and laboratory initial conditions.

From these studies, we find that isoprene-derived epoxides (IEPOX) and their substitution products are predicted to dominate aqueous-phase organic aerosol mass in conditions with low NOx in the atmosphere, representative of rural environments. The contribution of these epoxide species is expected to be high under acidic conditions, though our findings still estimate significant contribution to aqueous-phase organic mass under higher pH or under cloudwater conditions, when acidity is expected to be lower. Under high-NOx conditions typical of urban environments, glyoxal is seen to form the majority of evolved aqueous organic species, with organic acids comprising the bulk of the difference. We then implement a series of physical property modules, designed to predict changes in aerosol absorbance and surface tension due to bulk concentrations of evolved OA species. Preliminary results from these modules indicate that bulk solution effects of aqueous-phase carbonyl-containing volatile organic compounds (CVOCs) and organic acids are insufficient to significantly affect net aerosol surface tension under any condition tested, implying that observed deviations from pure inorganic aerosol surface tension will arise from surface-aerosol partitioning rather than bulk compositional effects.

Light absorption of aqueous aerosols is seen to be driven by dark glyoxal chemistry in deliquesced salt aerosols and organic acids in cloud droplets, though additional information about the absorbance properties of IEPOX and its derivatives is required to accurately predict the net absorbance of aerosols where these species dominate OA mass. The predictions as described by GAMMA are comparable to field observations, and give further credence to the significance of epoxide formation as a source of aqueous-phase organic aerosol mass. These results also suggest the relative importance of specific organic compounds in the aqueous phase of both deliquesced salt aerosols and cloud droplets in the atmosphere, which gives direction to the study of compounds whose impact on aerosol physical properties will matter the most. In turn, new kinetic and physical information can be directly applied into the groundwork laid here, allowing GAMMA to provide a continuously better understanding of the effect of organic material on aqueous aerosols and their implicit effect on the environment.