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 HNO3, O3, 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 HNO3 mixing ratios observed were highly variable (0.76--1.2 ppbv), while HNO3/O3 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 HNO3 and relatively high HNO3/O3 ratios. These measurements suggest that relatively short-lived surface emitted pollutants can reach the lowermost stratosphere. We expect that this contributes to O3 formation.¿ 1997 American Geophysical Union