Physicochemical properties and atmospheric ageing of soot - investigated though aerosol mass spectrometry

Sammanfattning: Aerosol particles contribute significantly to the global burden of disease, and remain the main source of uncertainty in assessments of human-induced climate change. The microphysical characterization of particles is necessary to understand the roles they play in the detrimental effects of air pollution on human health, and in the energy budget of our planet. Soot particles are especially relevant for climate and health, but the main component of soot – black carbon – is uniquely challenging to characterize. To meet this challenge, I have deployed a Soot-Particle Aerosol Mass Spectrometer (SP-AMS) to elucidate the physicochemical properties and atmospheric ageing processes of soot particles. The sources under investigation were appliances for residential heating, and light-duty vehicles. Wood combustion emissions of particulate polyaromatic hydrocarbons (PAHs) are strongly dependent on combustion intensity: intense combustion results in oxygen deficiency in the appliance, which favors PAH emission. While most (>90%) of the particulate PAH mass was C24 or smaller, the observed distributions continued up to C48, with exponentially decreasing abundances. Excessively intense combustion also results in elevated emissions of secondary organic aerosol (SOA) precursors and soot. Contrary to expectations, the absorption Ångström exponent of the soot emitted was found to be fairly insensitive to photochemical processing, and similar to that of diesel exhaust particles. Exhausts from gasoline powered light-duty vehicles were shown to readily form secondary organic aerosol (SOA). Roughly half of the SOA mass formed could be explained from reaction products of C6-C9 aromatics, which are known SOA precursors. The SOA had a similar elemental composition, volatility and density as the SOA formed by such precursors alone. The diesel exhaust particles were found to exhibit progressively enhanced hygroscopicity with photochemical processing due to the condensation of water-soluble material. The process of SOA condensation induced irreversible soot restructuring which was substantially accelerated by water uptake under humid (RH90) conditions. The diesel particles observed in urban air were similar to the particles investigated in the laboratory in terms of size, shape, and mass spectral signature. Pure carbon ions as well as ions containing carbon and oxygen were unambiguously assigned to the refractory soot cores, in both laboratory and ambient air. In roughly five hours, highly aspherical, lightly coated, and hydrophobic fresh diesel exhaust particles were found to have aged into near spherical, more coated, and by inference hygroscopic accumulation mode particles. The transformation occurred under cold and humid conditions, despite limited photochemistry. Coupled ammonium nitrate and liquid water condensation was identified as the main cause of the transformation. The processes controlling soot emission and transformation are elusive. Understanding has been impeded, despite considerable attention from the scientific community, by two inherent soot properties: stability and irregular shape. The same properties were exploited here, using state-of-the-art instrumentation to measure the physicochemical properties and atmospheric transformation of soot particles.