Abstract.About 600 ozone sonde profiles and in situ ozone observations on 2300 aircraft flights were compared with several three-dimensional global chemistrytransport models, in the midlatitude lowermost stratosphere during 1996. The models use the same top boundary conditions and parameterized ozone chemistry.The comparisons show that the models using general circulation model (GCM) winds do not capture the seasonal ozone accumulation in the lowermost stratosphere. The models using winds from the European Centre for Medium-Range Weather Forecasts (ECWMF) overestimate ozone above this maximum, especially during spring. This overestimation increases with increasing latitude. Close to the tropopause. the best agreement between models and observations is found during •vinter and the worst occurs during summer, where most models underestimate the mean ozone concentration. This underestimation is partly caused by inaccurate description of the relatively small-scale transport processes, mainly associated with convective activity, and partly by incomplete ozone chemistry in the parameterized scheme. The models that use assimilated winds show significant differences in calculated ozone, despite their common source (ECWMF) to calculate the transport. We illustrate that the model performance significantly depends on the ECWMF processing method and that interpolation of wind data should be avoided. In addition, the results seem relatively insensitive to vertical resolution. To improve the model performance further in this region, the horizontal resolution should be higher than 2.5 ø .

Abstract. We report aircraft measurements from the Stratosphere-Troposphere Experiments by Aircraft Measurements (STREAM) II campaign, performed during February 1995 from Kiruna, northern Sweden, near 67øN latitude. We have measured trace species, e.g., 03, nitrogen compounds, HC1, hydrocarbons, CO, ice particles, and aerosols, to characterize the chemical conditions of the lowermost stratosphere at altitudes up to -12.5 km. From the observation of anomalously high CO/C2H6 ratios, caused by enhanced C2H 6 breakdown, we derive that heterogeneous chlorine activation has occurred. This has liberated a diurnal mean C1 concentration up to 1.0 x 104 atoms cm -3, which efficiently destroys ozone. We also infer that much of the C1 activation has taken place on ice crystals above the tropopause. The measured crystal number densities were typical of thin cirrus. Model calculations suggest that heterogeneous chlorine conversion on ice crystals, in addition to that on liquid aerosols, may contribute significantly to ozone destruction in the middle-and high-latitude lowermost stratosphere. [1996] observed strongly reduced NO/NOy ratios, attributed to nighttime heterogeneous conversion of NOx to HNO3. This significantly decreased the potential of these air masses to deactivate chlorine by C1ONO2 formation. This paper addresses the possible impact of heterogeneous chemistry on ice crystals in the lowermost stratosphere during late winter. We have performed aircraft measurements that provide strong indications that such particles occur in air masses that travel between middle latitudes and the Arctic at 10-12.5 km altitude. The associated heterogeneous processes contribute to ozone loss. Section 2 describes the materials and methods, i.e., the aircraft campaign, the instrumentation, and the model used to support analyses of the results. In section 3 we present empirical evidence of heterogeneous chlorine activation on ice particles on the basis of simultaneous measurements of carbon monoxide and ethane, the latter being selectively destroyed by chlorine atoms. We support these assertions by model calculations in section 4. Conclusions are presented in section 5. 8201

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Simultaneous in situ measurements of O3, HNO3, and N2O were performed in the Arctic (68°–74°N) lower stratosphere during February 1993 on board a Cessna Citation aircraft up to 12.5 km altitude, during the first Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) campaign. Strong variations in the concentrations, distributions, and ratios of these trace gases were found from the maximum altitude down to the tropopause. Close to the tropopause, vortex air was present with relatively low N2O concentrations. The observed N2O‐HNO3 relation agrees with earlier measurements of total nitrogen and N2O inside the vortex, suggesting subsidence of vortex air across the bottom of the vortex. This air also contained low O3 concentrations relative to N2O, indicating enhanced O3 loss by chemical reactions involving stratospheric particles. Based on trajectory calculations and assuming a potential temperature cooling rate of 0.6 K d−1, we estimate an O3 loss of 4–7 ppbv d−1 (0.9–1.2% d−1), in the Arctic lower stratosphere for the period January–February 1993. Air parcels originating from middle latitudes, containing relatively low O3 and N2O concentrations, may have originated from the vortex earlier in the winter. In addition, the results also show high HNO3 concentrations relative to O3 and N2O. Air parcels originating from high latitudes may have been enriched in HNO3 by sedimentation and evaporation of nitric acid containing particles, which would explain the relatively high HNO3 concentrations and HNO3/O3 ratios measured. Heterogeneous chemistry on sulfuric acid particles, probably enhanced in concentration by gravitational settling of the Pinatubo aerosol, is the most plausible explanation for the observed high HNO3 concentrations relative to N2O in air parcels originating from midlatitudes.

Abstract. Chlorine activation and ozone depletion in the Arctic winter stratosphere of 1996-1997 have been studied with a newly developed stratospheric chemistry-transport model (CTM). The chemistry scheme, using a Euler Backward Iterative approximation method, includes a comprehensive set of reactions on ternary aerosol and ice particles, which has been tested against a numerically exact solver. Tracer transports in the CTM are calculated from European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological analyses. Comparisons have been made with 03 and C10 measurements, and with ozone loss rates derived from observations during February and March 1997. C10 production and ozone depletion are somewhat underestimated by the model. Furthermore, uncertainties regarding the aerosol phase are tested. Assuming nitric acid trihydrate (NAT) particles to form at their melting point, while liquid aerosol is present simultaneously in the model, gives rise to the largest C10 production and the strongest ozone depletion. By correcting for an ECMWF temperature warm bias we obtain a similar large effect on calculated C10 production and ozone depletion for the 1996/1997 Arctic winter, whereas uncertainties in the chlorine abundance seem less important. An average warm bias of 1.3 K at polar stratospheric cloud temperatures on the 50 hPa model level reduces the calculated ozone depletion rates over February and March by 35%.Observations of C10 are reproduced when lower temperatures and maximum Cly abundance are assumed, but ozone depletion is slightly overestimated in that case.