Investigation on VOC Emissions from Automobile Sources by Means of Online Mass Spectrometry

This study reviews recent research on volatile organic compound (VOC) emissions from motorized vehicle sources by means of online mass spectrometry. Chemical ionization is a powerful tool that usually permits soft ionization of chemical species and it allows the time-resolved measurement of multiple VOCs, even in complex samples where many kinds of VOCs coexist. The vehicular exhaust gasses are investigated using H₃O⁺, NO⁺, Hg⁺, and CH₃C(O)O⁻ as a reagent ion in online chemical ionization mass spectrometry. The proton transfer using H₃O⁺ as a reagent ion was used for the detection of nitro-organic compounds such as nitromethane and nitrophenol. The time-resolved measurement of the nitro-organic compounds in the laboratory experiments with a chassis dynamometer system revealed their emission properties, such as the dependence of the emissions as a function of vehicular velocity and acceleration/deceleration, as well as the effect of various types of exhaust gas treatment. The data regarding the nitromethane and nitrophenol emissions obtained in the field measurements were consistent with the results of the laboratory experiments done with a chassis dynamometer system. In the experiments involving evaporative emissions from gasoline-powered cars, NO⁺ was used as a reagent ion. Online measurements showed that the adsorption of hydrocarbons in a sealed housing evaporative determination unit could result in emissions being underestimated, if the concentrations are monitored only before and after a diurnal breathing loss test. The composition analysis gave an estimated ozone formation potential (OFP) approximately 20 % higher for breakthrough emissions and refueling emissions than for the gasoline that was tested, but the OFP for the permeation emissions was almost the same as the OFP for the test fuel.