| Summary: | Aerosol particulate matter contributes significantly to the global climate directly, through scattering and absorption of solar radiation, and indirectly, through changing cloud reflectance and cloud cover persistence. This thesis presents the results of several studies on the hygroscopicity of marine aerosol and of anthropogenic aerosol. Analysis of a two-year long aerosol hygroscopicity dataset from the Mace Head atmospheric research station located near Carna, Co. Galway show that hygroscopic growth-factors (HGFs) were found to be associated with size distribution clusters, characteristic of five aerosol source categories: low sea-salt marine, high sea-salt marine, coastal-nucleation, open-ocean nucleation, and continental. Chemical composition of atmospheric aerosol particles under clean marine air conditions was found to contain the following constituents: 46% (30-54%) of partially-modified ammonium-sulphate particles, 23% (11-40%) of partially-modified sea-salt and the remaining 31% (25-35%) being attributed to two distinct aerosol organic species. On average, 30% of marine aerosol was found to be predominantly of organic origin and in the case of anthropogenically polluted aerosol, 60% of total aerosol mass was organic. Observation and analysis of open ocean new particle formation events revealed that, amongst potential gaseous precursors, nitrogenated compounds in the form of amines can stabilize clusters from evaporating, which effectively increases particle nucleation and growth, thus promoting open ocean new particle production. This work suggests that amides and organic nitrate may also play a key role in aerosol nucleation processes. Primary-produced sea-spray aerosol comprising sea salt when enriched with organic matter, may suppress light-scattering enhancement as a function of increasing relative humidity. Sea spray aerosol exhibits a dichotomous behaviour manifested in a low hygroscopic growth factor but with high cloud condensation nuclei (CCN) activation efficiency. Aerosol dominated by organics possesses a HGF value of 1.25 or less, while aerosol dominated by sulphate led to a HGF ~1.65. Sea salt dominated particles showed the highest values of HGF of more than 1.8. CCN activation was dependent on aerosol chemical composition, but did not follow a typical hygroscopicity dependence pattern - that is, particles with the lowest HGF were more readily activated than particles with the highest HGF values. This work also investigated properties and diurnal patterns of anthropogenically produced aerosol in one of the most polluted regions in Europe - river Po Valley in Italy. Large diurnal variations in the chemical composition were observed with massive peaks in nitrates, albeit without an increase in HGF, but also consistent with an increase in the hydrocarbon-like family (measured by the HR-Tof-AMS), suggesting a hydrophilic coating around the inorganic nitrate, thus suppressing water uptake. An unique opportunity of observing the Eyjafjallajökull volcano ash plume over Mace Head revealed a bimodal size distribution and an increased CCN activation efficiency which yielded information on ash cloud dispersion, as well as information on the impact of long range transport on volcanic ash physicochemical properties. These studies represents one of the first synergistic combinations of aerosol mass spectrometer chemical composition information with HGF information resulting in improved quantification of marine and urban aerosol influences on the direct aerosol radiative effect.
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