Does Gas-Phase Sulfur Dioxide Remove Films of Atmosphere-Extracted Organic Material from the Aqueous Aerosol Air-Water Interface?

The reaction of gas-phase SO2 with unsaturated carbon-carbon double bonds forms organosulfates. Previous works have focused on reaction products and not the fate of organic films in the atmosphere. Neutron reflectometry was used to study the interaction of gas-phase SO2 at the air-water interface with organic material extracted from atmospheric particulate matter and pure proxy chemicals to determine whether the reaction of organic films with SO2 removes the film and is there a product film. Films formed from atmospheric aerosol collected in urban and woodland environments typically produced a layer of approximately 0.6 nm thickness, whereas a thick (>40 nm) film was formed by the woodsmoke sample. Fitting of this thicker woodsmoke film suggested a three-layered structure at the interface that has been interpreted to be consistent with a surfactant-rich layer next to the air-water interface, a mid-layer rich in PAH and topped with a more aliphatic region. The multilayer structure of atmospheric extracted material at the air-water interface is potentially an exciting result that requires further study. Gas-Phase SO2 was confirmed to react with pure insoluble surfactant molecules at the air-water interface that contained carbon-carbon double bonds (oleic acid) and did not react with a similar saturated surfactant (stearic acid). No reaction was observed during the interaction of SO2 and atmospheric material extracted from urban and woodland environments, and no material appeared to be removed from the interface; however, films made from woodsmoke-extracted material did appear to be altered by SO2 suggesting a reaction with Poly Aromatic Hydrocarbons (PAHs) but no material was lost from the interface. In addition, the gas-phase ozone levels in the neutron blockhouse, which have historically been of some concern for reactions with organics, were found to be of the order 15ppb, with no evidence of additional production in the neutron beam-path. Owing to a lack of substantial removal of material from real atmospheric extracted films, SO2 is not considered atmospherically significant for the removal of organic films from the air-water interface