Aerosols have a direct effect on climate, through reflection of solar radiation and indirectly as seeds for cloud formation; cloud condensation nuclei (CCN). There is however, a large uncertainty in our understanding of the contributions of aerosols to climate change. It is therefore imperative to explore the chemical and physical properties of aerosol that influence CCN activity and droplet kinetics. This dissertation investigates the effects of aerosol mixing states, chemical composition and photochemical aging on the CCN forming potential of ambient salts and combustion aerosol. Using customized instrumentation and algorithms, the CCN droplet kinetics of inorganic salts and photochemically aged combustion aerosol is also evaluated.

The droplet growth of inorganic and organic salts and the effects of cloud condensation nuclei concentrations on the final droplet sizes were explored with a modified, higher sensitivity optical particle counter (OPC). From the higher sensitivity OPC data, the final droplet diameters of CCN appear independent of aerosol hygroscopicity but strongly dependent on the CCN concentration. We also evaluated the effects of excess non-condensable gases on the CCN activation and droplet kinetics in a cloud condensation nuclei counter (CCNc). Excess, heavier gases increased mass transfer of water vapor and modified the supersaturation in the CCNc column.

The CCN activity and droplet kinetics of aged anthropogenic primary and biogenic aerosol was evaluated. CCN activity and droplet kinetics of α-pinene SOA formed in an atmospheric reactor and mixed with diesel or motor oil-fuel primary organic aerosol (POA) was characterized by the single parameter κ-hygroscopicity. Results showed that a similarity in the CCN activity of organic aerosol may indicate a propensity for mixing. An empirical model developed using unit mass resolution (UMR) aerosol mass spectrometer data captures the complex CCN activity of the mixed systems. The hygroscopicity and droplet kinetics of fresh and aged emissions from new generation gasoline direct injection engines retrofitted with a gasoline particulate filter (GPF) was also evaluated. Photochemical aging and subsequent condensation of the SOA formed from the co-emitted gas phase SOA precursors increased the hygroscopicity of gasoline emissions.

This body of work provides new insights into the current understanding of the effects of aerosol chemical composition, mixing state and hygroscopicity. The findings will help reduce the prevailing uncertainty in estimating anthropogenic radiative forcing

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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