Cholesterol Ozonolysis: Kinetics, Mechanism, and Oligomer Products

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Abstract
Fine particles of cholesterol were reacted with ozone under pseudo-first-order conditions in an aerosol bag reactor. Gas-phase ozone was monitored using an ozone meter. Particle size distribution functions were determined using a scanning mobility particle sizer, which selected particle sizes for introduction into a photoionization aerosol mass spectrometer (PIAMS). PIAMS was used to determine the concentration of cholesterol in the aerosol as a function of reaction time. Dilution corrected rate coefficients were used to calculate the reactive uptake coefficient for ozone onto cholesterol particles as (2.8 ± 0.4) × 10-6. Uptake was found to be independent of particle diameter for the sizes studied (100 and 200 nm), suggesting that the uptake is surface mediated. The reaction products were also collected on filters and analyzed by electrospray ionization (ESI) mass spectrometry with both direct infusion and liquid chromatography sample introduction. The main primary reaction products contained one, two, or three oxygens added to the cholesterol moiety. Secondary oligomeric products were also observed, consisting of covalently bound dimers and trimers. Tandem mass spectrometry was used to confirm the expected structures of these compounds. The dimers appear to be acyl hydroperoxides, consistent with a previously reported mechanism for the reaction in a nonparticipating solvent. Finally, the magnitude of the uptake coefficient confirms that cholesterol is suitable as a local source tracer for source apportionment of ambient organic aerosol.

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