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 ozone concentrations. The marine site (the island of Uto) is located in the Baltic Sea about 80 km southwest of mainland Finland, while the other site (Pallas) is 900 km to the north on a fell top in a sub-arctic environment north of the Arctic Circle. The concentrations at both stations show a seasonal cycle with a winter maximum which is typical of arctic and mid-latitude areas. In spring, the concentrations decreased in steps when the air mass changed to a cleaner oceanic or arctic types after a period with the prevailing continental air mass. The decrease of concentrations in spring was further studied in terms of the concentration ratio between Uto and Pallas. For low and moderately reactive alkanes the ratio decreased steadily from March to August, indicating a long period of photochemical transformation from a winter concentration distribution to a summer one. However, for alkenes the concentrations at the northern site, compared to the more southerly one, were lower in spring and increased in summer. This suggests an emissions source that is active in summer in the vicinity of the northern terrestrial site. Isoprene and 1-butene are only observed at the pristine northern site during the growing season. The local photochemical reactivity 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 Pallas, respectively. The spring decrease of total reactive mass is much less compared to the total mass of light hydrocarbons. In fact, the total reactivity-scaled mass at the site north of the Arctic circle was roughly the same in April and in July, the minimum being observed in May before the beginning of the growing season. Logarithmic alkane ratios were used to estimate photochemical age, assuming OH-initiated photochemistry as the only sink process for these species. Based on the ethane 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, the 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 showed a more aged hydrocarbon composition.