Does gas-phase sulfur dioxide remove films of atmosphere-extracted organic material from the aqueous aerosol air–water interface?†

Abstract

The reaction of gas-phase SO₂ with unsaturated carbon–carbon double bonds forms organosulfates at the surface of the aerosol. Previous studies have focused on the 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 if a product film is formed. 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 polyaromatic hydrocarbons (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 but there was no significant loss of material. In addition, the gas-phase ozone mixing ratios in the neutron blockhouse, which have historically been of some concern for reactions with organics, were found to be of the order 15 ppb, 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.