Characterisation of lightly oxidised organic aerosol formed from the photochemical aging of diesel exhaust particles

Environmental context The effects of atmospheric fine particulate matter (aerosols) on climate and human health can be strongly influenced by the chemical transformations that the particles undergo in the atmosphere, but these ‘aging’ reactions are poorly understood. Here diesel exhaust particles are aged in the laboratory to better understand how they could evolve in the atmosphere, and subtle but unmistakable changes in their chemical composition are found. These results provide a more complete picture of the atmospheric evolution of aerosols for inclusion in atmospheric models. Abstract The oxidative aging of the semivolatile fraction of diesel exhaust aerosol is studied in order to better understand the influence of oxidation reactions on particle chemical composition. Exhaust is sampled from an idling diesel truck, sent through a denuder to remove gas-phase species and oxidised by hydroxyl (OH•) radicals in a flow reactor. OH• concentrations are chosen to approximately match the OH• exposures a particle would experience over its atmospheric lifetime. Evolving particle composition is monitored using aerosol mass spectrometry in two different modes, electron impact ionisation (EI) for the measurement of elemental ratios and vacuum ultraviolet (VUV) photoionisation for the measurement of molecular components. Changes to mass spectra in both modes indicate major changes to particle composition over the range of OH• exposures studied. The product aerosol is only lightly oxidised (O/C < 0.3), suggesting an intermediate oxidation state between primary organics and the highly oxidised organic aerosol observed in the atmosphere. These lightly oxidised organics appear to be composed of secondary organic aerosol from semivolatile species, as well as from heterogeneously oxidised particle-phase organics. Key chemical characteristics (elemental ratios, oxidation kinetics and mass spectrometric features) of the reaction system are examined in detail. Similarities between this laboratory-generated aerosol and ‘hydrocarbon-like organic aerosol’ (HOA) reported in ambient studies suggest that HOA might not be entirely primary in origin, as is commonly assumed, but rather might include a significant secondary component.