An investigation into the traffic-related fraction of isoprene at an urban location

Abstract

Continuous hourly measurements of isoprene and 30 other hydrocarbons were performed at an urban centre site in Lille, France, from May 1997 to April 1999. Parallel mass emissions of the same hydrocarbons from in-service passenger vehicles were determined from measurements made on a chassis dynamometer using the European MVEG driving cycle. On the one hand, descriptive statistics and principal component analysis revealed the strong traffic origin of isoprene in winter months and its double biogenic and anthropogenic origin during the summer. On the other hand, the emission measurements of individual hydrocarbons in exhaust gases confirmed the presence of isoprene in petrol fuelled (with or without catalytic converters) and diesel car exhausts. Finally, the isoprene/acetylene ratios, both of them derived from ambient concentrations and emission factors, were compared. No statistically significant difference was found in winter, indicating the strict traffic origin of isoprene during that period. For the winter period, a simple regression analysis was performed on daily isoprene concentrations vs. those of acetylene and three other exhaust gases tracers—propene, ethylene and 1,3-butadiene. The established regression equations, together with the four tracer concentrations, were used to estimate the vehicle exhaust fractions of isoprene. From November to March, vehicle exhaust explained the totality of isoprene levels. While traffic remained the major source of isoprene with a contribution greater than 50% during the growing season, it still constituted a non-negligible source of isoprene in summer, anti-correlated to temperature and fluctuating between 10% and 50%. The application with 1,3-butadiene gives the greatest estimation of the anthropogenic fraction of isoprene. Other sources of 1,3-butadiene, acetylene, ethylene and propene were suspected in addition to their known traffic origin.