Abstract
Abstract. We studied the OH oxidation of submicron aerosol particles consisting of pure palmitic acid (PA) or thin (near monolayer) coatings of PA on aqueous and effloresced inorganic salt particles. Experiments were performed as a function of particle size and OH exposure using a continuous-flow photochemical reaction chamber coupled to a chemical ionization mass spectrometer (CIMS) system, for detection of gas and particle-bound organics, and a DMA/CPC for monitoring particle size distributions. The loss rate of PA observed for pure PA aerosols and PA on crystalline NaCl aerosols indicates that the OH oxidation of PA at the gas-aerosol interface is efficient. The pure PA oxidation data are well represented by a model consisting of four main processes: 1) surface-only reactions between OH and palmitic acid, 2) secondary reactions between palmitic acid and OH oxidation products, 3) volatilization of condensed-phase mass, and 4) a surface renewal process. Using this model we infer a value of γOH between 0.8 and 1. The oxidation of palmitic acid in thin film coatings of salt particles is also efficient, though the inferred γOH is lower, ranging from ~0.3+0.1/−0.05) for coatings on solid NaCl and ~0.05 (±0.01) on aqueous NaCl particles. These results, together with simultaneous data on particle size change and volatilized oxidation products, provide support for the ideas that oxidative aging of aliphatic organic aerosol is a source of small oxidized volatile organic compounds (OVOCs), and that OH oxidation may initiate secondary condensed-phase reactions.
Highlights
Atmospheric aerosols impact Earth’s radiation balance both directly by scattering solar radiation and indirectly via cloud properties (Forster et al, 2007), and they influence atmospheric composition via heterogeneous and multiphase chemistry (Jacob, 2000; Liao and Seinfeld, 2005)
Taking all the results together, we feel it reasonable to conclude that particle mass loss by volatilization initially occurs with an effeciency similar to that observed by Molina, et al (2004) leading to an overall decrease in the size of pure palmitic acid particles upon OH exposure, but that the efficiency of volatilization per OH collision decreases exponentially with OH exposure such that this process would likely become unimportant in the atmosphere as an organic aerosol mass loss
We estimate that reaction with HO2 to form organic hydroperoxides (ROOH) chemistry may be occuring in our system to an extent that is greater than would occur in the atmosphere, and some fraction of the observed volatilization could be due to these unrealistic conditions which would further limit the actual importance of volatilization as an organic aerosol mass loss
Summary
Atmospheric aerosols impact Earth’s radiation balance both directly by scattering solar radiation and indirectly via cloud properties (Forster et al, 2007), and they influence atmospheric composition via heterogeneous and multiphase chemistry (Jacob, 2000; Liao and Seinfeld, 2005). Hearn et al (2007) used a relative rates technique to study Cl-initiated oxidation of liquid bis(2-ethylhexyl) sebacate (BES) particles and found that the reaction probability, γ Cl∼2 They concluded that a reaction probability exceeding unity implied the existence of radical-assisted secondary chemistry occuring in the particle bulk. By using palmitic acid in surface films and pure particles we tried to span the regimes examined in separate studies using SAMs or single component particles The results from both sets of our experiments, while showing some differences, are consistent with efficient oxidation occurring at the particle surface, and provide evidence for secondary condensed-phase chemistry, especially in the pure particle case. The data are represented well by a model that assumes palmitic acid reacts with OH and oxidation products, these reactions occur only at the particle surface, a process exists (i.e. volatilization) to renew the particle surface with palmitic acid from the bulk, and particle mass loss due to volatilization occurs at a rate proportional to the palmitic acid loss rate
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
