Long term hourly measurement of 62 non-methane hydrocarbons in an urban area: Main results and contribution of non-traffic sources

We present one of the largest and most complete databases of atmospheric non-methane hydrocarbons in the Iberian Peninsula, not only for its temporal resolution and seasonal extent, but for the number of species routinely measured. Results were obtained using an automatic gas chromatograph, during 1998–2001 (April–October) and 2004 (February–July), optimized for systematic, unattended measurement of 62 C2–C10 non-methane hydrocarbons (NMHC) every hour, 24 times a day, with detection limits below 0.1 ppbv.Annual average concentrations for most of the NMHC show very little variations along the years. Benzene is an exception; during year 2000, its concentration on ambient air decreased down to a half, after the enforcement of environmental regulations reducing the benzene content on fuels.Significant correlations found among various hydrocarbons and the observed daily cycle indicate that traffic is their principal source. However, some compounds like acetylene and benzene, commonly used as tracers of vehicular emissions, show in our case other sources, during working days for acetylene, and continuous for benzene. This is now relevant, especially when considering that such measuring systems are now starting to be deployed around many cities, all over the world, because in urban areas where similar influences by industrial or commercial sources are to be expected, ratios like toluene/benzene or ethene/acetylene, cannot be used to characterize traffic polluted air masses.Isoprene has been found to have mostly an anthropogenic origin during winter, and a dual anthropogenic–biogenic origin during summer, when natural isoprene amounts 55% of the total. A method for the estimation of the anthropogenic fraction of isoprene in urban areas, based on a correlation with 1-butene, is also proposed.