Isotope values of atmospheric halocarbons and hydrocarbons from Irish urban, rural, and marine locations

Stable carbon isotope ratios for 37 hydrocarbon, CFC and halocarbon compounds were determined over the course of 1 year (86 samples) from the urban Belfast environment, Northern Ireland (NI). A smaller number of samples were collected from rural locations at Crossgar and Hillsborough, NI, and one marine location at Mace Head, Republic of Ireland. Source δ13C “signatures” suggest that C5 alkanes and ≥C4 alkenes are most likely derived from vehicle emissions. C3–C5 hydrocarbons show significant enrichment of δ13C with iso‐alkanes < n‐alkanes < alkenes < alkynes. There is also significant enrichment of δ13C from propane to n‐butane to n‐pentane. There is no significant separation between n‐pentane, n‐hexane, n‐heptane or methyl‐butane, methyl pentane and methyl hexanes. Calculated hydroxyl reaction kinetic isotope effects and subsequent δ13C enrichment are insufficient to explain shifts in isotopic ratio relative to concentration for all compounds as is differences in origin of air mass. Very few compounds show significant diurnal shifts; however, alkanes and ≥C4 alkenes demonstrate consistent enrichment of isotopic ratios when summer samples are compared to winter samples. Benzene is the sole compound measured that appears to become more depleted in δ13C when summer samples are compared against winter samples. Urban air, sampled at Belfast, which has originated in Europe shows enriched values for hydrocarbons while air which has originated in the Arctic Ocean is most similar to marine air off the Atlantic, sampled at Mace Head, Ireland.