MODEL ANALYSIS OF STRATOSPHERE-TROPOSPHERE EXCHANGE OF OZONE AND ITS ROLE IN THE TROPOSPHERIC OZONE BUDGET

摘要

We have addressed several aspects of STE of ozone and the impact on tropospheric ozone levels. Using ozone observations in the upper troposphere and lower stratosphere from MOZAIC, we have examined the relation between ozone and PV in the lower stratosphere. A distinct seasonality in the ratio between ozone and PV is evident, with a maximum in spring and minimum in fall associated with the seasonality of downward transport in the meridional circulation and of the ozone concentrations in the lower stratosphere. The ozone-PV ratio is applied in our tropospheric chemistry-climate model to improve the boundary conditions for ozone above the tropopause, to improve the representativity of simulated ozone distributions near synoptic disturbances and realistically simulate cross-tropopause ozone transports. It is expected that the results will further improve when the model is applied in a finer horizontal and vertical resolution. In the literature a wide range of estimates regarding the influx of ozone from the stratosphere into the troposphere is presented, derived by many different methods. Some studies report an annual flux between 200-870 Tg O_3 yr~(-1) globally, whereas other studies report a flux between 500 and 1000 Tg O_3 yr~(-1) for the NH only. With our model we estimate for the NH a net downward flux from the stratosphere of 580 Tg O_3 yr~(-1), which is partly balanced by an upward flux of ozone from photochemical production in the troposphere of 210 Tg O_3 yr~(-1), yielding a net downward cross-tropopause ozone transport of 370 Tg O_3 yr~(-1). Globally, these values are 950, 370 and 580 Tg O_3 yr~(-1), respectively. (Note that the troposphere-to-stratosphere ozone flux in the extratropics has only a limited extent since this air does not penetrate the stratosphere above about 100 hPa, the "overworld"). It is evident that different definitions and assumptions may explain part of the range. For example, when upward transport of ozone across the tropopause is not, or only partly, accounted for, the estimated ozone STE flux represents mainly the downward flux and is likely to be in the high end of the range. This may be the case, for example, when global fluxes are extrapolated from the analysis of transports in a single mesoscale event. Also, it should be explicitly stated whether a study separates an upward flux of photochemically produced tropospheric ozone from the downward flux of stratospheric ozone. The results of this study indicate that chemical destruction in the troposphere, however important as a sink for O_3s, does not significantly influence the tropospheric budget of stratospheric ozone. Chemical destruction is only very efficient in a relatively small region well removed from the source region. Therefore, transport in the troposphere is an important factor, so that the representation of the large scale circulation, of synoptic scale events and the mixing of tropospheric and stratospheric air likely add to the range between estimates. Finally we examined the contribution of ozone from the stratosphere on surface ozone levels for pre-industrial, present-day and future atmospheres. Due to relatively low efficiency of photochemical ozone formation in the pre-industrial atmosphere, the seasonality of surface ozone was strongly linked to that of STE except in (sub-)tropical areas, with a clear maximum in winter. In the present-day atmosphere a build-up of anthropogenically emitted ozone precursors takes place at mid-high northern latitudes during winter, so that the winter maximum of ozone of stratospheric origin is followed by a peak in photochemical production in late spring, causing the ozone spring peak observed in NH remote locations. At present, ozone of stratospheric origin appears to contribute about 50% of total surface ozone in winter at mid-high latitudes, and only little during summer. Furthermore, our model simulations suggest that in the next few decades ozone concentrations will increase significantly