Assessment of vehicular and non-vehicular contributions to hydrocarbons using exclusive vehicular indicators

This study investigates suitable non-methane hydrocarbon (NMHC) species to be used as motor vehicle indicators, and examines their possible applications and limitations. Suitable indicators are employed to assess vehicular and non-vehicular contributions of individual NMHCs in a given environment. The concept involves using concentration ratios of individual NMHCs to the indicators characteristic of vehicular emissions to reveal the excess concentration contributed by non-vehicular sources. By comparing the correlation between NMHCs and methyl tert-butyl ether (MTBE), a proven good motor vehicle indicator, in metropolitan Kaohsiung, Taiwan known for its heavy traffic and variety of heavy industries, it was found that 2,2-dimethylbutane (22DMC4A), 3-methylpentane (3MC5A), methylcyclopentane (McC5A), 2-methylhexane (2MC6A) and 3-methylhexane (3MC6A) revealed good concentration correlation with MTBE. The result implies these species in Kaohsiung have the same source as MTBE originated primarily from motor vehicle sources consisting of tailpipe exhaust and gasoline evaporation. 22DMC4A, 3MC5A, McC5A, 2MC6A and 3MC6A are common components in nearly all gasoline evaporation and tailpipe exhaust world-wide, and more exclusive to motor vehicle emissions. Moreover, they can be easily analyzed by the popular gas chromatography/flame ionization detection (GC/FID) and, thus can be the potential indicators to be applied generally in many regions of the world. Possible chemical and physical removals affecting the NMHCs/indicator ratios in the atmosphere are discussed. The ethylbenzene/m,p-xylene ratios could be used as an effective indicator of photochemical removal degree to test and sift out the data affected by excessive photochemical reaction to enhance the accuracy of NMHC source apportionment.