CHEMICAL PERTURBATION OF THE LOWERMOST STRATOSPHERE THROUGH EXCHANGE WITH THE TROPOSPHERE

In the troposphere, anthropogenic emissions of nitrogen oxides, hydroc arbons and carbon monoxide cause large-scale photochemical build up of ozone. In the stratosphere breakdown of anthropogenic halocarbons dam ages the ozone layer. In the extratropics a transition region between these air layers occurs, the lowermost stratosphere (below 12-14 km), in which about half the current subsonic air traffic takes place. Here , we report aircraft measurements of HNO3, O-3 and CO over western Eur ope in July 1994 (5 flights of several hours during a 10-day period), at approximately 1-2 km above the tropopause. The HNO3 mixing ratios o bserved were highly variable (0.76-1.2 ppbv), while HNO3/O-3 ratios se em relatively high (5.2-7.0 . 10(-3)). Moreover, several times we obse rved very high levels of pollutant CO (up to similar to 0.5 ppmv) that did not originate from aircraft exhausts. Instead, we pose that it ha d mixed-in from the troposphere. Cross-tropopause mixing also helps ex plaining the variable HNO3 and relatively high HNO3/O-3 ratios. These measurements suggest that relatively short-lived surface emitted pollu tants can reach the lowermost stratosphere. We expect that this contri butes to O-3 formation.