Three-dimensional model study of the influence of stratosphere-troposphereexchange and its distribution on tropospheric chemistry

A three-dimensional global chemistry transport model is used to examine the impact of stratosphere to troposphere fluxes of ozone (O-3) and nitric aci d (HNO3) on tropospheric chemistry. The stratospheric fluxes are parameteri zed as a tropospheric source of O-3 and HNO3. The accuracy of the resulting model simulation is compared with measurements. The tropospheric impact of the stratospheric fluxes is examined through a parallel simulation that in cludes no stratospheric fluxes of O-3 and HNO3. Stratosphere-troposphere ex change (STE) increases the global average tropospheric ozone column by only 11.5%, increasing it in the Northern Hemisphere by 10.5% and in the Southe rn Hemisphere by 13%. STE shifts the springtime ozone maximum similar to 1 month earlier in the Northern Hemisphere. The portion of the O-3 distributi on of stratospheric origin in the troposphere increases with altitude, from a maximum of 10-20% in the lower troposphere to 40-50% in the upper tropos phere. The sensitivity of the tropospheric response to the spatiotemporal d istribution of STE is also examined. On a hemispheric and annual scale the tropospheric composition is particularly sensitive to the temporal distribu tion of STE. The separate roles of stratospheric fluxes of O-3 and HNO3 are also identified.