(7c) Summertime Evaporation of Aqueous Secondary Organic Aerosol (aqSOA) in the Eastern United States: Field Measurements and Modeling Predictions

Aerosol liquid water (ALW) can affect the quantity and chemical composition of organic aerosols; however, the interaction between ALW and ambient organic aerosol compounds is highly uncertain at present. To characterize the behavior of organic aerosols under conditions of drying, water-soluble organic carbon in the particle phase (WSOC_p) in PM_2.5 was measured during two consecutive summers in Baltimore, MD namely: 2015 and 2016. The WSOC_p measurements were alternated through an unperturbed ambient channel and through a ‘dried’ channel maintained at ~35% relative humidity (RH). Sample drying induced systematic evaporation of the WSOC_p during both summers. The quantity of evaporated WSOC_p was strongly related to RH levels, WSOC_p concentrations, isoprene emissions and NO_x/isoprene ratios. Significant differences in meteorological conditions (including RH), isoprene emissions, NO_x/isoprene ratios and overall WSOC_p concentrations were observed between the two summers. This led to major differences in the amount of evaporated WSOC_p during summer 2016 in comparison to summer 2015. The sensitivity of evaporated mass to each of the four aforementioned parameters was used to build multivariate linear and non-linear statistical models describing the effect of liquid water evaporation on ambient organic matter in the eastern United States. Summertime measurements in 2015 were used as the explanatory dataset while the forecasting dataset was implemented using measurements of summer 2016. A comparison of the measured and modeled data is presented based on the explanatory and forecasting powers of the models. In addition to their implications for secondary organic aerosols formed through aqueous pathways (aqSOA), results presented herein have close associations with a measurement approach that is widely applied in aerosol science with direct influence on human health.