Abstract
Abstract. Stratosphere–troposphere exchange (STE) is an important source of tropospheric 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 surface observations, allowing us to follow stratospheric signals across the tropopause. The satellite-derived photochemical loss of N2O on annual and quasi-biennial cycles can be matched by the models. The STE flux of N2O-depleted air in our chemistry transport model drives surface variability that closely matches observed fluctuations on both annual and quasi-biennial cycles, confirming the modeled flux. The observed tracer correlations between N2O and O3 in the lowermost stratosphere provide a hemispheric scaling of the N2O STE flux to that of O3. For N2O and CFCl3, we model greater southern hemispheric STE 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.
Highlights
Introduction and backgroundThe influx of stratospheric ozone (O3) into the troposphere affects its distribution, variability, lifetime, and, its role in driving climate change and surface air pollution (Zeng et al, 2010; Hess et al, 2015; Williams et al, 2019)
In a previous work (Ruiz et al, 2021; hereafter R2021), we showed that historical simulations with three chemistry transport models (CTMs) were able to match the interannual surface variations observed in N2O
We find some evidence to support our model result that the stratosphere-to-troposphere exchange (STE) flux of depleted N2O air is greater in the Southern Hemisphere than in the Northern Hemisphere, altering the asymmetry in surface emissions in the source inversions (Nevison et al, 2007; Thompson et al, 2014)
Summary
Introduction and backgroundThe influx of stratospheric ozone (O3) into the troposphere affects its distribution, variability, lifetime, and, its role in driving climate change and surface air pollution (Zeng et al, 2010; Hess et al, 2015; Williams et al, 2019). The net stratosphere-to-troposphere exchange (STE) flux of O3 has a regular seasonal cycle in each hemisphere that is an important part of the tropospheric O3 budget (Stohl et al, 2003) Such fluxes are not directly observable, and we rely on observational estimates using trace gas ratios, in particular the O3 : N2O ratio in the lower stratosphere (Murphy and Fahey, 1994; McLinden et al, 2000), or dynamical calculations using measured/modeled winds and O3 abundances (Gettelman et al, 1997; Olsen et al, 2004; Yang et al, 2016). A similar case has been made for the radionuclide 7Be (Liu et al, 2016), but N2O has a wealth of model–observation metrics on hemispheric, seasonal, and interannual scales that constrain its STE flux very well (Prather et al, 2015; Ruiz et al, 2021)
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