Convective redistribution of ozone and oxides of nitrogen in the troposphere over Europe in summer and fall

The fluxes of ozone and NOx out of the atmospheric boundary layer (ABL) over Europe are calculated in a mesoscale chemical transport model (MCT) and compared with the net chemical production or destruction of ozone and the emissions of precursors within the ABL for two 10 days' periods which had quite different synoptic situations and levels of photochemical activity (1-10 July 1991 (JUL91) and 26 October-4 November 1994 (ON94)). Over the European continent, about 8% of the NOx emissions were brought from the ABL to the free troposphere as NOx, while about 15% of the NOx emissions were brought to the free troposphere as NOy-NOx, i.e. as PAN or HNO3. The convection dominates over the synoptic scale vertical advection as a transport mechanism both for NOx and NOy out of the boundary layer in the summertime high pressure situation (JUL91), while in the fall situation (ON94) the convective part was calculated to be the smallest. NOx was almost completely transformed to NOy-NOx or removed within the ABL. Also for NOy the major part of the atmospheric cycle is confined to the ABL both for JUL91 and ON94. The vertical transport time out of the ABL is of the order of 100 h both for the total model domain and over the European continent. The net convective exchange of ozone from the ABL is not a dominant process for the amount of ozone in the ABL averaged over 10 days and the whole domain, but convection reduces the maximum ozone concentration in episodes significantly. The ozone producing efficiency of NOx is calculated to increase with height to typically 15-20 in the upper half of the troposphere from around 5 in the ABL, but in the middle free troposphere the concentration of NOx is often too low to cause net chemical formation of ozone there.