Cloud Forming Potential of Aerosol from Light-duty Gasoline Direct Injection Vehicles

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 explored in (NH 4 ) 2 SO 4 -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 (NH 4 ) 2 SO 4 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 (NH 4 ) 2 SO 4 . 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. 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 of salt-seeded and black carbonaceous aerosol cores. We the bin width The results from this study suggests that emissions from newer technology GDI engines form CCN active SA upon photochemical aging. The integration of gasoline particulate filters (which removes up to 90% of BC) will modify the hygroscopicity and CCN activity of the emissions based on the point of dispersion. In supersaturated urban environments where BC is abundant, the SA formed from GPF fitted GDI vehicles are more likely to condense onto BC and increase the hygroscopicity. In supersaturated rural regions, the SA will form mixtures (internal or external) with salts (nitrates, sulfates) and reduce the hygroscopicity of the new particle. In BC-core-shell mixtures, our results suggest that the hygroscopicity may be significantly inhibited by the less hygroscopic volume fraction. When the BC is removed however, hygroscopicity of the resulting internal mixtures are largely dependent on the most soluble, hygroscopic fraction (such as the cases where sulfates are dominant). It is therefore important to constrain the available seed chemical composition when modelling the effects of anthropogenic combustion vehicle emissions on cloud indirect effects. In the subsaturated environments however, the delinquency is only influenced by the SA coating making the point of emission dispersion unimportant to modeling cloud indirect effects.