Abstract. Upper tropospheric and lower stratospheric acetone measurements have been performed in summer and winter 1994 through 1996 at latitudes between 30øN and 75øN using ionmolecule reaction mass spectrometry. We observed very high acetone volume mixing ratios of up to 3000 pptv (parts per trillion by volume) in extended air masses and in summer when acetone destruction by photodissociation is fast. This indicates efficient transport of acetone and photochemical acetone precursors to the upper troposphere and efficient upper tropospheric formation of acetone products, especially HO x radicals and PAN. Our data indicate large HOx production from acetone which has important implications for other trace gases and aerosols.

Abstract. Concentrations of CH3CN and HCN have been measured in the lower stratosphere by aircraft-based ion-molecule reaction mass spectrometry. The mean HCN volume mixing ratio of 164 parts per trillion by volume (pptv) is consistent with previous infrared remote sensing data (160 pptv). The measured CH3CN volume mixing ratios markedly exceed most previous lower stratospheric values and reveal a significant decrease from 160 to 110 pptv between 1.0 and 4.2 km above the tropopause. This latter feature suggests a stratospheric CH3CN lifetime much shorter than thought previously, which can be explained by an additional stratospheric sink, maybe ioncatalyzed conversion of CH3CN to HCN. Our present data indicate a mean global tropospheric CH3CN source of 1.6x 10 •2 g CH3CN per year which is slightly larger than the CH3CN source inferred from most previous atmospheric measurements of CH3CN but is in agreement with estimations inferred from laboratory simulation experiments of biomass burning.

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 HNO3, 03 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 03 formation.

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Abstract. Simultaneous in situ measurements of NOy, HNO3, 03, N20, and CO have been performed in the lower stratosphere during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) II intensive winter campaign in February 1995 from Kiruna airport (northern Sweden) with a Cessna Citation II twinjet aircraft up to a maximum altitude of 12.8 km. The flights were coordinated with the Arctic Second European Stratospheric Arctic and Midlatitude Experiment (SESAME) winter campaign. Strongly elevated levels of total reactive nitrogen (NOy) and its most abundant contributing species, nitric acid (HNO3) , with mixing ratios up to 9 parts per billion by volume (ppbv), were observed during all flights at altitudes near 12 km. On average, the measured NOy concentrations exceed the expected levels by a factor of 2-3. Normal background NOy has been calculated from observed N20 mixing ratios using the NOy-N20 correlation reported for the undisturbed northern hemisphere. This indicates that subsidence of air in the vortex alone cannot explain these findings. We propose that the elevated NOy concentrations were caused by nitrification of the lower stratosphere associated with sedimentation and evaporation of polar stratospheric cloud particles that carry down HNO3 from higher altitudes, that is, from altitudes up to about 25 km. IntroductionThe process of denitrification is considered to play an important role in the large-scale ozone loss observed in the Antarctic stratosphere during late winter and early spring after the return of sunlight. During the cold polar night, heterogeneous nucleation of nitric acid and water vapor in the form of nitric acid trihydrate (NAT) leads to the formation of polar stratospheric clouds. These so called type 1 23,559