Chemical perturbation of the lowermost stratosphere through exchange with the troposphere

Abstract

In the troposphere, anthropogenic emissions of nitrogen oxides, hydrocarbons and carbon monoxide cause large‐scale photochemical build up of ozone. In the stratosphere breakdown of anthropogenic halocarbons damages 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 HNO 3 , O 3 and CO over western Europe in July 1994 (5 flights of several hours during a 10‐day period', at approximately 1–2 km above the tropopause. The HNO 3 mixing ratios observed were highly variable (0.76–1.2 ppbv', while HNO 3 /O 3 ratios seem relatively high (5.2–7.0 10 −3 '. Moreover, several times we observed very high levels of pollutant CO (up to ∼0.5 ppmv' that did not originate from aircraft exhausts. Instead, we pose that it had mixed‐in from the troposphere. Cross‐tropopause mixing also helps explaining the variable HNO 3 and relatively high HNO 3 /O 3 ratios. These measurements suggest that relatively short‐lived surface emitted pollutants can reach the lowermost stratosphere. We expect that this contributes to O 3 formation.