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Diesel engines are known to emit high number concentrations of nanoparticles (diameter < 50 nm), but the physical and chemical mechanisms by which they form are not understood. Information on chemical composition is lacking because the small size, low mass concentration, and potential for contamination of samples obtained by standard techniques make nanoparticles difficult to analyze. A nano-differential mobility analyzer was used to size-select nanoparticles (mass median diameter approximately 25-60 nm) from diesel engine exhaust for subsequent chemical analysis by thermal desorption particle beam mass spectrometry. Mass spectra were used to identify and quantify nanoparticle components, and compound molecular weights and vapor pressures were estimated from calibrated desorption temperatures. Branched alkanes and alkyl-substituted cycloalkanes from unburned fuel and/or lubricating oil appear to contribute most of the diesel nanoparticle mass. The volatility of the organic fraction of the aerosol increases as the engine load decreases and as particle size increases. Sulfuric acid was also detected at estimated concentrations of a few percent of the total nanoparticle mass. The results are consistent with a mechanism of nanoparticle formation involving nucleation of sulfuric acid and water, followed by particle growth by condensation of organic species.

The chemistry of secondary organic aerosol formation from reactions of 1-tetradecene and O3 in dry air in the presence of excess alcohols and carboxylic acids was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer. Temperature-programmed thermal desorption of collected aerosol shows that in each reaction two major aerosol products are formed. The more volatile compounds in each pair of products are α-alkoxytridecyl or α-acyloxytridecyl hydroperoxides, which were identified by comparison of mass spectra with those of standard compounds generated by the corresponding liquid-phase ozonolysis reactions. The formation of organic hydroperoxides in the gas and liquid phases is consistent with a mechanism involving reactions of the alcohols and carboxylic acids with stabilized Criegee biradicals generated in the alkene−O3 reaction. The less volatile compounds are α-alkoxy-α‘-hydroxyditridecyl or α-acyloxy-α‘-hydroxyditridecyl peroxides (peroxyhemiacetals) formed by reactions of the hydroperoxides with tridecanal, which is generated along with formaldehyde during biradical formation. The vapor pressures of these compounds estimated from their desorption temperatures are ∼10-7−10-14 Torr. These types of reactions could play a role in atmospheric aerosol nucleation and growth and provide a mechanism for creating fine-particle organic peroxides, which are currently of interest because of their potential for adversely impacting human health.

No abstract

Previous studies in our laboratory have shown that the major aerosol products formed in gas-phase ozonolysis of 1-tetradecene in the presence of excess alcohols, carboxylic acids, water vapor, and aldehydes are R-alkoxytridecyl, R-acyloxytridecyl, R-hydroxytridecyl hydroperoxides, and secondary ozonides. These low volatility compounds are formed from reactions of C13 stabilized Criegee intermediates with the acidic compounds and aldehydes. To obtain a more quantitative understanding of the chemical mechanism and determine the importance of such reactions in the atmosphere, relative rate constant measurements were made using real-time quantitative thermal desorption particle beam mass spectrometry (TDPBMS) for aerosol analysis. The rates of reaction of methanol, 2-propanol, formic acid, water, and formaldehyde with C13 stabilized Criegee intermediates were measured relative to heptanoic acid. The results show that the reactivity of the studied compounds varies over 4 orders of magnitude and increases in the order: water , methanol < 2-propanol , formaldehyde < formic acid < heptanoic acid. The rate constants depend primarily on the nature of the functional group and correlate well with the gas-phase acidities, indicating a relatively polar transition state for the reaction. The relative rate constants depend only slightly on the size of the reactant species and are similar to those measured previously for a C1 stabilized Criegee intermediate. The atmospheric implications of these results are also discussed.