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We used the seasonality of a combination of atmospheric trace gases andidealized tracers to examine stratosphere-to-troposphere transport and itsinfluence on tropospheric composition in the Arctic. Maximumstratosphere-to-troposphere transport of CFCs and O3 occurs in April asdriven by the Brewer-Dobson circulation. Stratosphere-troposphere exchange(STE) occurs predominantly between 40° N to 80° N with stratosphericinflux in the mid-latitudes (30–70° N) accounting for 67–81% of theair of stratospheric origin in the Northern Hemisphere extratropicaltroposphere. Transport from the lower stratosphere to the lower troposphere(LT) takes three months on average, one month to cross the tropopause, thesecond month to travel from the upper troposphere (UT) to the middletroposphere (MT), and the third month to reach the LT. During downwardtransport, the seasonality of a trace gas can be greatly impacted by wetremoval and chemistry. A comparison of idealized tracers with varyinglifetimes suggests that when initialized with the same concentrations andseasonal cycles at the tropopause, trace gases that have shorter lifetimesdisplay lower concentrations, smaller amplitudes, and earlier seasonalmaxima during transport to the LT. STE contributes to O3 in the Arctictroposphere directly from the transport of O3 and indirectly from thetransport of NOy. Direct transport of O3 from the stratosphereaccounts for 78% of O3 in the Arctic UT with maximum contributionsoccurring from March to May. The stratospheric contribution decreasessignificantly in the MT/LT (20–25% of total O3) and shows a veryweak March–April maximum. Our NOx budget analysis in the Arctic UTshows that during spring and summer, the stratospheric injection ofNOy-rich air increases NOx concentrations above the 20 pptvthreshold level, thereby shifting the Arctic UT from a regime of netphotochemical ozone loss to one of net production with rates as high as +16 ppbv/month

The governing processes and timescales of stratosphere-to-troposphere transport and its contribution to ozone in the Arctic troposphere