Source fingerprints for 18 nonmethane hydrocarbons (NMHCS) containing 2-8 carbon atoms are currently used to apportion NMHC sources by chemical mass balance (CMB) modeling approaches. The source profiles must be stable in the troposphere to obtain accurate CMB model predictions. The NMHC source apportionment compounds are susceptible to gas phase oxidation in the atmosphere by hydroxyl radical (OH). Expected tropospheric lifetimes for peak concentrations of OH in typical urban air range from an hour to over 100 hours. When OH oxidation of a source profile for vehicle tailpipe emissions is simulated, significant changes occur in the distribution of the source apportionment compounds after a reaction time of 2-4 hr. These calculations indicate that sampling strategies to define source-receptor relationships of NMHCs by CMB modeling techniques must be carefully designed to minimize the possibility of NMHC oxidation by OH.
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