PhD Thesis Defence Presentations - Damianos Pavlidis

Organic aerosol (OA) is a major component of fine particulate matter, and a large fraction of it is secondary in origin, formed through the atmospheric oxidation of gas-phase precursors. Anthropogenic activities, and in particular motor vehicles, are an important source of these precursors, yet their contribution to secondary organic aerosol (SOA) formation remains poorly understood. This thesis addresses these knowledge gaps through a combination of laboratory studies and field measurements.        
       In the first stage of this work, a new methodology was developed to estimate the collection efficiency of the aerosol mass spectrometer, accounting for its dependence on particle composition and, consequently, particle size. The approach improves the accuracy of aerosol mass concentration estimates in experiments involving the formation of new organic particles and reduces uncertainties in the determination of the resulting SOA mass.        
       SOA formation from the oxidation of large anthropogenic volatile and intermediate-volatility organic compounds was then investigated. The studied cycloalkanes (C₁₁–C₁₆) exhibited higher SOA yields than the corresponding aromatic compounds (C₉–C₁₈), whereas the aromatic precursors produced more oxidized SOA. Parameterizations describing SOA formation based on the volatility of the oxidation products were also developed for implementation in chemical transport models.        
       Cold start emissions from approximately 21,000 passenger vehicles were subsequently studied in an underground parking garage under real-world conditions. The measurements were used to derive cold start emission factors and to chemically characterize both gas- and particle-phase pollutants. The emitted submicron particulate matter consisted primarily of OA (61%) and black carbon (39%), while volatile organic compounds were dominated by C₆–C₉ aromatics (64%). These measurements are used to improve European emission inventories.        
       Finally, SOA formation from the emissions of 14 vehicles representing different categories and emission-control technologies was investigated. The vehicles were tested individually on a dynamometer, and their exhaust was introduced into an atmospheric simulation chamber and an oxidation flow reactor. The two reactors provided similar estimates of SOA formation potential, although the flow reactor produced more oxidized aerosol because of its operating conditions. Motorcycles (pre-Euro–Euro 5) and tractors exhibited the highest SOA formation potentials, while the oxidation products of most vehicles were dominated by intermediate-volatility organic compounds. 
       Overall, the findings of this work improve our understanding of anthropogenic SOA formation and contribute to more accurate measurements, parameterizations, and representation of anthropogenic emissions and SOA in chemical transport models.