C2---C6 hydrocarbon measurements at four rural locations across Canada

Observations of low molecular weight hydrocarbons at four rural locations in Canada are reported. The measurements cover a period of one year (1991), and the seasonal trends are discussed. It is deduced that most variation is due to photochemically driven processes (OH chemistry). Although at least two of the sites were well removed from large urban source regions, the observations show a clear anthropogenic influence on the rural hydrocarbon levels at all sites. Air mass back-trajectories have been used to investigate the origin of the observed hydrocarbons. Weekday/weekend effects are distinguishable at two of the sites, and correlation with a limited set of CO observations at one of the sites is found to be very good for those compounds that are known to originate 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 measured in this study, and its importance relative to the other measured compounds with respect to photochemical processes is indicated.