The differing impact of air stagnation on summer ozone across Europe

Abstract

We have examined the joint impact of daily maximum temperature and air stagnation on observed maximum daily 8-h running average (MDA8) and hourly near-surface ozone (O3) over eight regions of Europe for the summer period 1998–2015. The percentage of stagnant area in a region is a better predictor of summer MDA8 O3 in central/southern Europe (correlation coefficient R = 0.50–0.70) than in the north (R = 0.06–0.39). The correlations of MDA8 O3 with temperature are higher than with stagnation for most regions, and the impact of stagnation on those correlations is region-dependent. MDA8 O3 mixing ratios consistently increase over central/southern Europe under stagnant conditions, but this has not been found for some temperature ranges in the north. Under non-stagnant situations and relatively high temperatures (20–25 °C), southerly advection often brings aged air masses from more polluted areas to the receptor regions in the north. Furthermore, the occurrence of stagnation tends to amplify the ozone diurnal cycles in the center/south of the continent, yielding lower nighttime and higher daytime mixing ratios than on non-stagnant days, again with a less clear effect in the north. Changes in planetary boundary layer height, accumulation of ozone and precursors, and subsequent photochemical production during stagnant days are the presumed underlying mechanisms for the amplification of the diurnal cycles. The results from this study prove that the effects of stagnation on summer ozone are regionally dependent across Europe. Consequently, climate model projections of increases in stagnation should not directly be translated into degraded air quality without a proper assessment of the regional impacts.