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

Continuous measurements of NMHCs were made at the TOR station of Porsp oder, located in western Brittany (France), on the Atlantic shore. Twe lve samples per day have been performed since February 1992. Four-day spanned, three-dimensional back-trajectories reaching 913 mbar at Pors poder site at 0 and 12 h, have been used to differentiate the air parc els according to their origin and directions. The outcome of this anal ysis shows that such air parcels can fall within five classes: contine ntal air masses, North Sea air masses, England air masses, stagnant oc eanic air masses and oceanic air masses. The general trend of the seas onal variation shows a maximum concentration in winter and a minimum i n summer, regardless of the air mass origin. The low concentrations ar e 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 variati on 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 pro cess involved which removes hydrocarbons in the troposphere, and in th e case of oceanic air masses, a signature of previous continental pass ages was observed when air masses were sampled at Porspoder. Other oxi dation pathways in addition to reaction with hydroxyl chemistry and NO 3 radical are suspected for the branched-chain alkanes.