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Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Water is a key component of atmospheric aerosol, affecting many aerosol processes including gas/particle partitioning of semi-volatile compounds. Water related to secondary organic aerosol (SOAW) is often neglected in atmospheric chemical transport models and is not considered in gas-to-particle partitioning calculations for inorganic species. We use a new inorganic aerosol thermodynamics model, ISORROPIA-lite, which considers the effects of SOAW, to perform chemical transport model simulations for a year over the continental United States to quantify its effects on aerosol mass concentration and composition. SOAW can increase average fine aerosol water levels up to a factor of two when secondary organic aerosol (SOA) is a major PM₁ component. This is often the case in the south-eastern U.S where SOA concentrations are higher. Although the annual average impact of this added water on total dry PM₁ concentrations due to increased partitioning of nitrate and ammonium is small (up to 0.1 &mu;g m^&minus;3), total dry PM₁ increases of up to 2 &mu;g m^&minus;3 (with nitrate levels increases up to 200 %) can occur when RH levels and PM₁ concentrations are high.