SURFACE OZONE EXPOSURES MEASURED IN FINLAND

The occurrence of ozone concentrations and exposure indices related to the adverse effects of ozone upon vegetation are reported for four Fi nnish background stations. In Finland, ozone concentrations are often near the background tropospheric values of cn. 30 ppb. Very high conce ntrations are not observed. The maximum 1-h average in this data set w as 79 ppb. The exposure parameter, which accumulates growing season 1- h average concentrations above a 40 ppb base-line in daylight hours, g ave clearly different exposure sums for the stations. These values var ied between 4000 and 8500 ppb-h in the southern archipelago, 3000-6500 ppb-h in the southern coastal region, 2000-4000 ppb-h in central part s of the country, and 400-1000 ppb-h in the northern parts of the coun try. The date of the start of the vegetative season is important in hi gh northern latitudes, because the spring maximum of ozone concentrati ons is relatively intense compared to the summer maximum. In northern Scandinavia, ozone exposures are particularly sensitive to the date of the start of the growing season. The long daylight period in northern Scandinavia is less important in this respect, since during the growi ng season ozone concentrations are usually below 40 ppb during the mor ning and evening hours. A good correlation was found between growing s eason average concentrations of the sum of gaseous HNO3 and particulat e NO3-, and on ozone exposure index which accumulates concentrations a bove a 40 ppb base-line, confirming the anthropogenic origin of the el evated ozone exposures.