Reactivity of oleic acid in organic particles: changes in oxidant uptake and reaction stoichiometry with particle oxidation.

The heterogeneous reaction of ozone with oleic acid has been studied extensively as a simple model system for investigating the oxidation of organic compounds in atmospheric particles. In this work, we simultaneously quantify oleic acid and ozone decay during three stepwise oxidation events, allowing us to quantify reactivity of oleic acid throughout the oxidative lifetime of initially pure particles. Throughout their lifetime, uptake in these particles is driven by reaction, as evidenced by similar timescales for ozone and oleic acid decay. The oleic acid decay rate slows with increasing particle oxidation, most likely due to the continued dilution of the particles with oxidation products. However, the initial stoichiometry is as high as 3.75 oleic acid molecules destroyed per ozone molecule lost. This significantly exceeds the 2:1 ratio that can be explained by an initial ozonolysis reaction and known secondary chemistry between the Criegee intermediate and the organic acid moiety. It implies that there is additional, previously unrecognized secondary chemistry that likely involves the carbon backbone. Our understanding of reactivity, even in this simple system, remains incomplete.

Laboratory measurements of the heterogeneous oxidation of condensed-phase organic molecular makers for motor vehicle exhaust.

Triterpanoid hopanes and steranes are petroleum biomarkers used to apportion fine particulate matter to motor vehicle emissions. To investigate the chemical stability of these compounds, aerosolized motor oil was exposed to the hydroxyl radical (OH) in a smog chamber and the reaction rate constants of hopanes, steranes, and n-alkanes were measured. The experiments were conducted across a range of atmospheric conditions including low and high relative humidity (RH) and with mixtures of lubricating oil and secondary organic aerosol. Hopanes and steranes were found to react at atmospherically significant rates across the entire range of experimental conditions; they are estimated to have lifetimes on the order of several days at average summertime OH levels. The one experimental parameter that strongly influenced the effective rate constants was RH; oxidization of hopanes and steranes was about a factor of 4 slower at 75% RH than at 10% RH. Chemical mass balance (CMB) analysis was performed to illustrate the effects of oxidation on source apportionment estimates. As the extent of oxidation increases, traditional CMB analysis increasingly underestimates the contribution of gasoline vehicles butthe diesel estimates are largely unaffected. The results demonstrate that even modest levels of oxidation can alter policy-relevant conclusions about the total and relative contribution of gasoline and diesel vehicle emissions to ambient fine particle concentrations.

Laboratory measurements of the heterogeneous oxidation of condensed-phase organic molecular makers for meat cooking emissions.

Multiphase oxidation of trace organic constituents inside of complex atmospheric particles is not well understood. In this study, organic aerosol formed from flash-vaporized residual grease from meat cooking was exposed to atmospherically relevant ozone concentrations in a smog chamber for 4-6 h. Changes in particle composition were measured to determine reaction rates for important molecular markers used for source apportionment analysis (oleic acid, palmitoleic acid, and cholesterol). Results are also presented for palmitic and stearic acids and likely reaction products. To quantify oxidation rates over a range of atmospheric conditions, separate experiments were conducted at low and high relative humidity and using particles mixed with and without secondary organic aerosol. Although particle composition, relative humidity, and secondary organic aerosol all influence the reaction rates, the unsaturated compounds were rapidly oxidized in every experiment. At typical summertime conditions, palmitoleic acid, oleic acid and cholesterol are estimated to have a chemical lifetime of about one day. The experimentally determined reaction rates are used in conjunction with the chemical mass balance model to investigate the effects of aging on source apportionment estimates. The results highlight that assumptions regarding the photochemical stability of molecular markers can lead to substantial biases in predictions of receptor models.

Organic aerosol formation from photochemical oxidation of diesel exhaust in a smog chamber.

Diluted exhaust from a diesel engine was photo-oxidized in a smog chamber to investigate secondary organic aerosol (SOA) production. Photochemical aging rapidly produces significant SOA, almost doubling the organic aerosol contribution of primary emissions after several hours of processing at atmospherically relevant hydroxyl radical concentrations. Less than 10% of the SOA mass can be explained using a SOA model and the measured oxidation of known precursors such as light aromatics. However, the ultimate yield of SOA is uncertain because it is sensitive to treatment of particle and vapor losses to the chamber walls. Mass spectra from an aerosol mass spectrometer (AMS) reveal that the organic aerosol becomes progressively more oxidized throughout the experiments, consistent with sustained, multi-generational production. The data provide strong evidence that the oxidation of a wide array of precursors that are currently not accounted for in existing models contributes to ambient SOA formation.

Rethinking organic aerosols: semivolatile emissions and photochemical aging.

Most primary organic-particulate emissions are semivolatile; thus, they partially evaporate with atmospheric dilution, creating substantial amounts of low-volatility gas-phase material. Laboratory experiments show that photo-oxidation of diesel emissions rapidly generates organic aerosol, greatly exceeding the contribution from known secondary organic-aerosol precursors. We attribute this unexplained secondary organic-aerosol production to the oxidation of low-volatility gas-phase species. Accounting for partitioning and photochemical processing of primary emissions creates a more regionally distributed aerosol and brings model predictions into better agreement with observations. Controlling organic particulate-matter concentrations will require substantial changes in the approaches that are currently used to measure and regulate emissions.