Tropospheric ozone at the Swedish mountain site Areskutan: Budget and trends

Ozone measurements, performed since 1987, at the Swedish TOR/EUROTRAC station Areskutan (lat. 63.4 degrees N, long. 13.1 degrees E, 1250 m above sea level) are analyzed. The annual average ozone concentration at the site has increased by about 0.4 ppbv (similar to 1%) per year during the period 1987-1994. The corresponding trends for individual months show a decrease during April-September and an increase during the rest of the year. The ozone budget at Areskutan has been investigated using back trajectories of the air parcels, and the cosmogenic radionuclide Be-7 as a tracer of stratospheric air. From a simple diagnostic model, it is estimated that the contribution of stratospheric ozone to the concentrations measured at Areskutan is 5 ppbv (or 14% of the measured values) on average, reaching a maximum of 23 ppbv (50%), during the episodes of direct stratospheric influence. In spring, the stratospheric contribution to ozone budget at Areskutan is at its maximum, and approximately equal to the net photochemical ozone production in the air mass affecting the site, whereas in winter, it is compensated by ozone chemical sink during the transport of air masses from polluted European regions, to Scandinavia.