C2-C6 HYDROCARBON MEASUREMENTS AT 4 RURAL LOCATIONS ACROSS CANADA

Observations of low molecular weight hydrocarbons at four rural locati ons in Canada are reported. The measurements cover a period of one yea r (1991), and the seasonal trends are discussed. It is deduced that mo st variation is due to photochemically driven processes (OH chemistry) . Although at least two of the sites were well removed from large urba n source regions, the observations show a clear anthropogenic influenc e on the rural hydrocarbon levels at all sites. Air mass back-trajecto ries have been used to investigate the origin of the observed hydrocar bons. Weekday/weekend effects are distinguishable at two of the sites, and correlation with a limited set of CO observations at one of the s ites is found to be very good for those compounds that are known to or iginate from transportation related processes. Comparison of the data with published observations suggests that the background distribution of hydrocarbons over the North American continent is quite homogeneous . Isoprene is the only hydrocarbon of biogenic origin that was measure d in this study, and its importance relative to the other measured com pounds with respect to photochemical processes is indicated.