10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Formation of Secondary Organic Aerosol (SOA) during Winter in the Eastern United States
MARWA EL-SAYED, Christopher Hennigan, University of Maryland, Baltimore County
Abstract Number: 581 Working Group: Aerosol Chemistry
Abstract The aim of this study is to characterize the formation of secondary organic aerosol (SOA) during the winter in Baltimore, MD through simultaneous measurements of water-soluble organic carbon in the gas (WSOCg) and particle (WSOCp) phases. Two main processes were responsible for the wintertime SOA formation, namely: (1) gas-phase and (2) aqueous processes (aqSOA). SOA formation through the gaseous phase was interpreted based on the relationship between the partitioning coefficient, Fp = WSOCp/(WSOCg + WSOCp), and organic carbon (OC) concentrations. An increase in Fp as a function of OC indicates that the partitioning was dependent on the amount of pre-existing aerosol suggesting SOA formation through traditional gas-phase partitioning. Such a relationship was observed during colder periods, only (< 0 oC). At night, an enhancement in Fp was observed with increasing relative humidity (RH) for temperatures between 0 and 10 oC, suggesting the occurrence of aqSOA formation in aerosol liquid water (ALW) solely in this temperature range. For the periods < 0 oC and > 10 oC, there was no relationship between Fp and the RH, implying that aqSOA was not substantially formed under such conditions. A climatology of ALW during the winter showed a striking temperature dependence, with low ALW levels when the temperature was below 0 oC. Using back trajectories, we show that this observation was likely due to air masses from Canada/arctic associated with the lowest winter temperatures. Aerosol potassium concentrations were inversely correlated with temperature during winter, implying their link to biomass burning due to residential heating. Conditions for aqSOA formation occur in the winter when both ALW and sufficient biomass burning emissions are present – this corresponds to wintertime temperature between 0 and 10 oC in Baltimore. The WSOCp measurement was alternated between ambient (WSOCp) and dry (WSOCp,dry) channels to directly probe the effect of ALW evaporation on the SOA. A comparison of the WSOCp and WSOCp,dry concentrations indicated a WSOCp,dry/WSOCp ratio of unity for the data in the range 0 – 10 oC, the conditions corresponding to aqSOA formation. This denotes that WSOCg compounds taken up in aerosol water, causing the enhancement in Fp, remained in the condensed phase upon drying due to their formation irreversibly. In addition, the WSOCp,dry/WSOCp ratios were nearly constant across the entire RH range supporting the irreversible formation of aqSOA in the winter.