SEASONAL CYCLE OF C-2-C-5 HYDROCARBONS OVER THE BALTIC SEA AND NORTHERN FINLAND

Measurements of C-2-C-5 hydrocarbons in ambient air from a marine and a mountain site in Scandinavia are presented in relation to observed o zone concentrations. The marine site (the island of Uto) is located in the Baltic Sea about 80 km southwest of mainland Finland, while the o ther site (Pallas) is 900 km to the north on a fell top in a sub-arcti c environment north of the Arctic Circle. The concentrations at both s tations show a seasonal cycle with a winter maximum which is typical o f arctic and mid-latitude areas. In spring, the concentrations decreas ed in steps when the air mass changed to a cleaner oceanic or arctic t ypes after a period with the prevailing continental air mass. The decr ease of concentrations in spring was further studied in terms of the c oncentration ratio between Uto and Pallas. For low and moderately reac tive alkanes the ratio decreased steadily from March to August, indica ting a long period of photochemical transformation from a winter conce ntration distribution to a summer one. However, for alkenes the concen trations at the northern site, compared to the more southerly one, wer e lower in spring and increased in summer. This suggests an emissions source that is active in summer in the vicinity of the northern terres trial site. Isoprene and 1-butene are only observed at the pristine no rthern site during the growing season. The local photochemical reactiv ity of the hydrocarbon mixture was estimated by calculating Propylene- Equivalent concentrations. In summer, isoprene, 1-butene, ethene, and propene comprise 50 and 80% of the total reactive mass at Uto and Pall as, respectively. The spring decrease of total reactive mass is much l ess compared to the total mass of light hydrocarbons. In fact, the tot al reactivity-scaled mass at the site north of the Arctic circle was r oughly the same in April and in July, the minimum being observed in Ma y before the beginning of the growing season. Logarithmic alkane ratio s were used to estimate photochemical age, assuming OH-initiated photo chemistry as the only sink process for these species. Based on the eth ane and propane concentrations, the calculations gave a seasonal cycle of photochemical activity with a concomitant increase with background ozone concentrations in spring. When the calculations were made using propane and n-butane concentrations, which have a shorter lifetime, t he resulting scale of photochemical histories was more regional. Ozone concentrations were higher during continental air masses compared to background ozone concentrations when the photochemical histories showe d a more aged hydrocarbon composition.