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 but the 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.