From biogenic emissions to cloud condensation nuclei

The role of atmospheric particles in cloud formation currently poses a major uncertainty in estimations of the net aerosol impact on climate. Assessments of these impacts could be considerably improved through a better understanding of the effects of clouds on the radiative budget and the hydrological cycle. The physical and chemical mechanisms that govern cloud droplet formation are, however, complex, and detailed knowledge of the potential of atmospheric particles to act as cloud condensation nuclei (CCN) is necessary. A major source of atmospheric particulate matter is the oxidation of emissions from vegetation. The oxidation products, which are less volatile than their parent hydrocarbons, can undergo gas-to-particle conversion, resulting in secondary organic aerosol (SOA) formation and growth. The studies in this thesis simulate this process in an environmental chamber, which is commonly used to provide controlled surrogate atmospheres in which the formation and growth of SOA particles can occur through the oxidation reactions of primary gaseous precursors. The CCN activities of particles formed in the chamber from the oxidation of two biogenic precursors, α-pinene and isoprene, are reported in this thesis. These precursors are selected for their atmospheric abundance and their measured particle mass yields. The atmospheric relevance of chamber simulations is developed with each successive study through the introduction of low hydrocarbon levels, NON, and photochemistry. Finally, results are compared with those measured above the Amazon rainforest, where biogenic emissions are high. A two-component (organic-sulfate) Köhler model based on the laboratory CCN observations is developed throughout this thesis. Improvements in KOhler model analyses not only provide insights into the physicochemical parameters of SOA particles, but may also lead to accurate and computationally tractable parameterizations that can be implemented in cloud modules of global climate models. One of the key findings in this thesis is the successful prediction of CCN activity using a single set of physicochemical parameters and the assumption of full solubility for the organic component. This suggests that computations of cloud droplet formation may be considerably simplified for mixed organic-sulfate particles formed from biogenic precursors, at least under the range of conditions studied in this thesis.