A New Pathway for Oxidation of Organics through Aqueous Nitrate Ion Photochemistry

Not much is known about the interactions of gas phase organic compounds with nitrate-containing particles in the presence of light. In this work, the photooxidation of alpha-pinene in the presence of deliquesced NaNO3 was studied, and for comparison, with increasing concentrations of NO2 in a 100 L Teflon chamber at 70-88 % R.H. and 296-304 K. The gas phase reaction products tentatively identified by proton transfer reaction mass spectrometry (PTR-MS) include formaldehyde, acetaldehyde, acetone, glyoxal, propanedial, 2,3-dioxobutanal, 3,5,6-trioxoheptanal, pinonaldehyde, pinene oxide, and 4-oxopinonaldehyde. Single particle mass spectrometry (SPLAT II) measured the effective density of newly formed SOA (1.25 g cm-3) and showed that such SOA consisted of a complex mixture of organics and organic nitrates, possibly including pinonic acid and trans- sobrerol. The loss of alpha-pinene per mass of SOA formed was significantly greater in the NaNO3 experiments than in NO2 experiments, indicating enhanced loss of alpha-pinene at the NaNO3 thin film during photolysis. Molecular dynamics simulations predict that alpha-pinene has a significant residence time and contact with nitrate at the surface of the deliquesced nitrate thin film. The combination of experiment and theory indicate that alpha-pinene is oxidized at the interface of the aqueous NaNO3 by O(3P) and OH generated in the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in some field studies, and it may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. Some results from a new, large (20 foot, 18 in. dia.) flow system designed to study nitrate aerosol interaction with organics during irradiation are also presented.