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Discussion

Comparing the different scenarios used in the model runs, the inert tracer gives the best agreement with the observations. This is surprising, as several studies favour mesospheric loss to explain inconsistencies between different tracers (Strunk et al., 2000; Harnisch et al., 1998) or models (Hall and Waugh, 1998). The transport properties of the present KASIMA version seem to be not untypical for 3D models of the stratosphere. For example, the mean age distributions shown in Figure 2 compare well with the distributions shown by Waugh et al. (1997). Somewhat higher mean age values in the upper stratosphere may be explained by the higher upper boundary of the present model. A satisfactory agreement of the model is found for the latitudinal profiles of mean age at about 20 km derived from aircraft obserations made in the months of October/November (Waugh et al., 1997, Fig. 10). The mean age values also compare well with simulations made using isentropic models (A. Iwi, personal communication). On the other hand, our model yields generally higher mean age values than those models presented in the study of Hall et al. (1999) . Harnisch et al. (1999) derived mean age values from measurements of about 10 years at about 60 km height and about 7 years at about 30 - 40 km. This compound is believed to show a negligible loss even in the mesosphere. The stratospheric values are in accordance with our model, but the high mesospheric values are not reached in the model. Consequently, our simulation seems to yield too young an age for that height.

The observed age profiles shown in Fig 4 are characterized by a rapid increase of age up to a certain height. Here, we find a convincing agreement between the observations and the model up to about 22 km for most profiles, irrespective of the latitude and season. This may be compared with the study of Hall et al. (1999)(their Figure 5), in which most models a far too young compared to the observations. Above that height, a plateau in the age value is often indicated in the observations. Whereas this plateau seems to be smoothed in the simulations, the altitude level and the value of the plateau are reproduced for most tropical and midlatitude profiles. In the case of profile P5, it is difficult to explain the mean age observations of only 2.5 years at 35 km height by common transport processes. Furthermore, profile P9 is exceptional in its high age values for the whole altitude range. We therefore conclude that in cases where mesospheric loss is not expected to influence the profiles, the agreement between the model with an inert tracer and observations is satisfactory.

In the case of high latitude observations, the picture is quite different. As already mentioned, even the simulations for the inert tracer at and above 25 km, especially in 1997, give consistently higher age values than those observed. As the chemical effect would only lead to apparently older air masses, the reason for these discrepancies must lie in the transport properties of the model. Deficits of the model's overall meridional circulation or incorrect match of polar vortex air and midlatitude air masses in the comparisons, especially near the vortex edge, may cause these deviations. The variation of the profiles in the proximity of the observation, shown in Figure 4 for profiles P10, P11, and P13, would support the latter suggestion. For all polar winter profiles at maximum height levels, the inert tracer gives the best agreement, chemistry scenarios S3 and S5 are very difficult to reconcile with the observations.

Using loss rates and the corresponding mixing ratios in the model, atmospheric lifetime can be deduced for the different chemistry scenarios. As exhibits a strong trend, only the so-called instantaneous lifetime can be given:

where is the local number density of the trace gas and denotes the local loss frequency. The brackets mean annual average. The result is given in Table 3 as a mean for the years 1990 - 1998. A range of mean atmospheric lifetimes between 400 and 10000 years is deduced, with the expected trend of long lifetime for chemistry scenarios including all stabilizing mechanisms. The lower value found for scenario S1 is of the order of the value given by Morris et al. (1995) who find a lifetime of 800 years. Harnisch et al. (1999) conclude from their analysis, that must have a much higher atmospheric lifetime of about 5000 years; thus our findings support their conclusion. For the atmospheric lifetimes a strong semiannual oscillation (for example a 25% amplitude for scenario S4) was found, but no significant trend or other periodicity.

The low lifetime for scenario S8 compared with scenario S7 can be explained by the non negligable chemical loss in the stratosphere caused by cosmic ray ionisation. Obviously, stratospheric loss strongly influences atmospheric lifetime, but for profiles of mean age, the effect of stratospheric loss is small.