Cholesterol Ozonolysis:  Kinetics, Mechanism, and Oligomer Products

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) x 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.

Online investigations on ozonation products of pyrene and benz[a]anthracene particles with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer

“…(Poschl, 2002(Poschl, , 2003Schauer et al, 2003) The single photon vacuum ultraviolet photoionization is the latest technique applied by aerosol mass spectrometers, which affords the mass spectra of organics with less fragments or even fragments free (Woods et al, 2001;Oktem et al, 2004;Mysak et al, 2005). With this technique, it is possible to online analyze the reaction products of organic aerosols (Smith et al, 2002;Dreyfus et al, 2005). Recently, Gloaguen et al investigated the ozonation of anthracene coated on sodium chloride particles with a vacuum ultraviolet photoionization aerosol mass spectrometer.…”

Studying Interfacial Reactions of Cholesterol Sulfate in an Unsaturated Phosphatidylglycerol Layer with Ozone Using Field Induced Droplet Ionization Mass Spectrometry

“…The products at m/z 481, 497, and 513 correspond to singly (+16), doubly (+32), and triply (+48) oxygenated CholSO 4 , respectively. Based on the previous studies of ozonolysis of Chol [25][26][27][28][29], singly, doubly, and triply oxygenated products are suggested as epoxide (product I, m/z 481), dicarbonyl (product II, m/z 497), and carbonyl-acid (product III, m/z 513), respectively. Alternatively, the vinyl hydroperoxide may be generated for product III [26,30].…”

“…Based on the previous studies of ozonolysis of Chol [25][26][27][28][29], singly, doubly, and triply oxygenated products are suggested as epoxide (product I, m/z 481), dicarbonyl (product II, m/z 497), and carbonyl-acid (product III, m/z 513), respectively. Alternatively, the vinyl hydroperoxide may be generated for product III [26,30]. In addition, what we assume to be hydroxyhydroperoxide (HHP), methoxyhydroperoxide (MHP) products are observed at m/z 531 and 545, respectively [29,31].…”

“…R is H for water and CH 3 for methanol. Proposed structures of products I, II, and III are adopted from Reference [25][26][27][28][29]. Proposed structures of products IV and V are adopted from Reference [29,31] aerosol reported bound multimeric products, which are formed through aggregated gas phase clusters [26].…”

Rapid Sampling of Individual Organic Aerosol Species in Ambient Air with the Photoionization Aerosol Mass Spectrometer

“…Particles are delivered directly to the source region, so no sample work-up is required. Previously, PIAMS has been used to sample common organic emissions (Oktem et al 2004), characterize soot particles in a flame (Oktem et al 2005), monitor the rate of cholesterol ozonolysis (Dreyfus et al 2005), and characterize SOA formation from monoterpene ozonolysis (Heaton et al 2007;Tolocka et al 2006).…”