Tropospheric ozone at high latitudes in clean and polluted air masses, a climatological study

Abstract

Several years of continuous measurements of surface ozone at Norwegian monitoring sites are studied in a climatological way. The monitoring sites are at rural locations extending from 58°N, a few hundred kilometers from the European continent and into the Arctic at 79°N. The ozone observations are sorted into classes of integrated NO, emissions along 96 h back trajectories. The average seasonal cycles of ozone are estimated for each class separately. The differences indicate the change from the background air due to anthropogenic emissions. The average seasonal cycle of ozone in the cleanest air masses showed a maximum in spring and a minimum during summer and autumn at all sites, but the spring maximum was more pronounced at the southernmost locations. Polluted air masses showed an ozone deficit during winter and a surplus during summer. The deviation from the background was clearly linked to the integrated NO(x) emission along the trajectories. In summer the calculations indicate that the number of ozone molecules formed per NO(x) molecule drops with increasing emissions. The average seasonal cycle of ozone at Birkenes for different transport sectors indicate that the most pronounced ozone formation takes place in air masses from E-Europe/Russia.