(298b) The Oxidation Of Fatty Acids In Pure And Internally Mixed Submicron Aerosols

We studied the OH oxidation of submicron aerosols consisting of a) pure palmitic acid (PA) and b) internal mixtures of PA and inorganic salts. Experiments were performed using a continuous flow photochemical reaction chamber coupled to a chemical ionization mass spectrometer (CIMS) system. The CIMS was fitted with a heated inlet for volatilization and detection of organics in the particle phase simultaneously with the gas phase. A DMA/CPC was used for determining aerosol size distributions. The OH oxidation of pure PA aerosols is efficient, with reactive uptake coefficient γ_OH ~ 1 calculated from the observed loss rate of PA in the particle phase. The OH oxidation of submonolayer coatings of PA on aqueous and effloresced salt particles is less efficient, with γ_OH ~ 0.1. Implications for surface vs. bulk processes will be discussed. We will also discuss the oxidation products observed for the different particle populations. In all cases, most products were observed to be in the gas phase at ambient temperature, consistent with recent suggestions that aerosol aging could be a source of VOCs. We conclude that the 1 e-fold lifetime of organic material in aerosol particles subject to OH oxidation can vary from hours to days under typical tropospheric conditions, depending on particle composition, morphology, and size. The barrier action of saturated organic films is expected to persist for days at the gas-aerosol interface.