SEASONAL-VARIATION OF ATMOSPHERIC NONMETHANE HYDROCARBONS ON THE WESTERN COAST OF BRITTANY, FRANCE

Continuous measurements of NMHCs were made at the TOR station of Porspoder, located in western Brittany (France), on the Atlantic shore. Twelve samples per day have been performed since February 1992. Four-day spanned, three-dimensional back-trajectories reaching 913 mbar at Porspoder site at 0 and 12 h, have been used to differentiate the air parcels according to their origin and directions. The outcome of this analysis shows that such air parcels can fall within five classes: continental air masses, North Sea air masses, England air masses, stagnant oceanic air masses and oceanic air masses. The general trend of the seasonal variation shows a maximum concentration in winter and a minimum in summer, regardless of the air mass origin. The low concentrations are obtained when the site is influenced by oceanic air masses. With few exceptions, the composition of the four other classes are similar and largely have higher concentrations than the previous class. A variation of the winter-summer ratio, as a function of the rate constants of reaction of light hydrocarbons with hydroxyl radicals, shows that, in the case of continental air masses, hydroxyl chemistry is the main process involved which removes hydrocarbons in the troposphere, and in the case of oceanic air masses, a signature of previous continental passages was observed when air masses were sampled at Porspoder. Other oxidation pathways in addition to reaction with hydroxyl chemistry and NO3 radical are suspected for the branched-chain alkanes.