Three‐dimensional model study of the influence of stratosphere‐troposphere exchange 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 acid (HNO 3 ) on tropospheric chemistry. The stratospheric fluxes are parameterized as a tropospheric source of O 3 and HNO 3 . 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 includes no stratospheric fluxes of O 3 and HNO 3 . Stratosphere‐troposphere exchange (STE) increases the global average tropospheric ozone column by only 11.5%, increasing it in the Northern Hemisphere by 10.5% and in the Southern Hemisphere by 13%. STE shifts the springtime ozone maximum ∼1 month earlier in the Northern Hemisphere. The portion of the O 3 distribution 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 troposphere. The sensitivity of the tropospheric response to the spatiotemporal distribution of STE is also examined. On a hemispheric and annual scale the tropospheric composition is particularly sensitive to the temporal distribution of STE. The separate roles of stratospheric fluxes of O 3 and HNO 3 are also identified.