AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Hygroscopicity, CCN Activity and Droplet Kinetics of Aged Vehicle Emissions
Emmanuel Fofie, Patrick Roth, Georgios Karavalakis, AKUA ASA-AWUKU, University of California, Riverside
Abstract Number: 88 Working Group: Aerosols, Clouds, and Climate
Abstract In this study, we evaluate the hygroscopicity and droplet kinetics of fresh and aged emissions from new generation gasoline direct injector engines retrofitted with a gasoline particulate filter (GPF). Furthermore, ageing and subsequent secondary aerosol formation is conducted in the presence of (NH4)2SO4-seeded and non-seeded experiments. We explore the impacts on measured and predicted hygroscopicity, CCN-activity, and droplet kinetics of secondary aerosol mixed with initially insoluble carbonaceous materials versus very soluble (NH4)2SO4 seed. The chemical composition and density of the secondary aerosol (SA) formed from aging is measured with an HR-TOF-AMS and a custom-built APM-SMPS system. The supersaturated and subsaturated hygroscopicity of the fresh and aged emission is measured with a DMT Streamwise Thermal Gradient CCN counter and a hygroscopicity tandem differential mobility analyzer (HTDMA), respectively. The measurements show that the fresh gasoline emissions is only slightly hygroscopic in both supersaturated and subsaturated environments, κCCN = 0.0014 and κHTDMA = 0.006, respectively. Photochemical aging and subsequent condensation of the secondary aerosol formed from the co-emitted gas phase precursors increases the hygroscopicity of gasoline emissions. Without the GPF, both subsaturated and supersaturated hygroscopicity increased from κCCN = 0.0014 to κCCN= 0.0014 and κHTDMA = 0.006 to κHTDMA = 0.006, respectively. When the engine was retrofitted with the GPF, the SA experiments were seeded with (NH4)2SO4. In these experiments the presence of the condensing SA depresses the hygroscopicity of the salt-SA mixture ( κCCN = 0.50 to κCCN = 0.4). The hygroscopicity was also depressed in the subsaturated regime ( κHTDMA = 0.18 to κHTDMA = 0.10 ) with time. These changes in the hygroscopicity with aging were additionally sensitive to aerosol dry size distribution. We also used threshold droplet growth analysis (TDGA) to evaluate the effects of the condensing SA on droplet kinetics. These results have important implications for the assessment of cloud-aerosol indirect effects sulfate seeded of black carbonaceous aerosol cores.