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. Stratosphere–troposphere exchange (STE) is an important source oftropospheric ozone, affecting all of atmospheric chemistry, climate, and air quality. The study of impacts needs STE fluxes to be resolved by latitude and month, and for this, we rely on global chemistry models, whose results diverge greatly. Overall, we lack guidance from model–measurement metrics that inform us about processes and patterns related to the STE flux of ozone (O3). In this work, we use modeled tracers (N2O and CFCl3), whose distributions and budgets can be constrained by satellite and surfaceobservations, allowing us to follow stratospheric signals across thetropopause. The satellite-derived photochemical loss of N2O on annualand quasi-biennial cycles can be matched by the models. The STE flux ofN2O-depleted air in our chemistry transport model drives surfacevariability that closely matches observed fluctuations on both annual andquasi-biennial cycles, confirming the modeled flux. The observed tracercorrelations between N2O and O3 in the lowermost stratosphereprovide a hemispheric scaling of the N2O STE flux to that ofO3. For N2O and CFCl3, we model greater southern hemisphericSTE fluxes, a result supported by some metrics, but counter to the prevailing theory of wave-driven stratospheric circulation. The STE flux of O3, however, is predominantly northern hemispheric, but evidence shows that this is caused by the Antarctic ozone hole reducing southern hemispheric O3 STE by 14 %. Our best estimate of the current STE O3 flux based on a range of constraints is 400 Tg(O3) yr−1, with a 1σ uncertainty of ±15 % and with a NH : SH ratio ranging from 50:50 to 60:40. We identify a range of observational metrics that can better constrain the modeled STE O3 flux in future assessments.

From the middle stratosphere to the surface, using nitrous oxide to constrain the stratosphere–troposphere exchange of ozone