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The Effects of Anthropogenic Emissions on Cloud Condensation Nuclei and Droplet Formation

Creative Commons 'BY' version 4.0 license

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

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