The seasonal variation of nonmethane hydrocarbons in the free troposphere over the North Atlantic Ocean: Possible evidence for extensive reaction of hydrocarbons with the nitrate radical

Detailed observations have been made of the seasonal variation in concentration of a wide range of nonmethane hydrocarbons over the North Atlantic Ocean in air with a predominantly polar maritime origin. The results bear out many of the findings of our previous studies Lightman et al ., 1990 with the observation of a concentration maximum in the winter and a minimum in the summer. The more recent results indicate that the amplitude of the seasonal cycle is remarkably constant. Also the total concentration of reactive carbon in the various forms of nonmethane hydrocarbons probably exceeds 20 ppbC. This constitutes a substantial reservoir of material which could take part in ozone production in the relatively remote troposphere, if sufficient nitrogen oxides are copresent. The hydrocarbon composition of air in winter is strongly dependent on its origin, with tropical air having a composition similar to polar air in the spring months. The relative magnitude of the seasonal cycles for some hydrocarbons is proportional to their rates of reaction with hydroxyl radicals. This is true for straight chain paraffins up to C 5 , acetylene and benzene, and it suggests that the removal of these molecules from the atmosphere occurs predominantly by reaction with hydroxyl radicals. For other molecules, particularly branched chain paraffins and substituted aromatic molecules, there is evidence that other removal processes are operating in competition with the hydroxyl radical, especially in winter. Arguments are advanced that in the case of the branched chain paraffins this may be caused by reaction with nitrate radicals.