The focus of my research is to apply stable isotopes as a new tool to understand free radical chemistry in the troposphere. Stable carbon isotope ratios are used as indirect tracers of the reaction of nonmethane hydrocarbons (NMHC) with OH radicals in ambient air from Toronto and Vancouver air-sheds, Canada. Compound-specific stable carbon isotope ratios of NMHC were determined in ambient air from urban, suburban, rural and source sites in the air-sheds using Gas Chromotograph-Isotope Ratio Mass Spectrometry (GC-IRMS). In the Greater Toronto Area, the average isotope ratio of all ambient measurements of NMHC, including halogenate NMHC was found to be -25.7 + 3.4 %o. Traffics related source sites in the Greater Toronto Area have an average isotope ratio of -25.7 + 3.5 %o, whereas the ratio for traffic related emissions in the Lower Fraser Valley are -25.9 + 4.2 %o. The extent of chemical processing due to OH radical reactions that the individual NMHC has experienced since emission is quantitatively determined. It is shown that in combination with concentration measurements, isotope ratio measurements are an extremely valuable new approach to study the spatial and temporal differences in chemical removal mechanisms, mixing and dilution processes.