National Center for Sustainable Transportation

In this study, the authors 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 explored in (NH4)2SO4-seeded and non-seeded experiments. The authors 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 are only slightly hygroscopic in both supersaturated and subsaturated environments. 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. When the engine was retrofitted with the GPF, the secondary aerosol (SA) experiments were seeded with (NH4)2SO4. In these experiments the presence of the condensing SA depresses the hygroscopicity of the salt-secondary aerosol mixture. The hygroscopicity was also depressed in the subsaturated regime with time. These changes in the hygroscopicity with aging were additionally sensitive to aerosol dry size distribution. The authors 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 of salt-seeded and black carbonaceous aerosol cores. The authors concluded that in the new generation GDI vehicles the point of aerosol emissions will have significant influence on the impacts of the secondary and primary aerosols on climate.

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