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Massive Australian wildfires lofted smoke directly into the stratosphere in the austral summer of 2019/20. The smoke led to increases in optical extinction throughout the midlatitudes of the southern hemisphere that rivalled substantial volcanic perturbations. Previous studies have assumed that the smoke became coated with sulfuric acid and water and would deplete the ozone layer through heterogeneous chemistry on those surfaces, as is routinely observed following volcanic enhancements of the stratospheric sulfate layer. Here, observations of extinction and reactive nitrogen species from multiple independent satellites that sampled the smoke region are compared to one another and to model calculations. The data display a strong decrease in reactive nitrogen concentrations with increased aerosol extinction in the stratosphere, which is a known fingerprint for key heterogeneous chemistry on sulfate/H 2 O particles (specifically the hydrolysis of N 2 O 5 to form HNO 3 ). This chemical shift affects not only reactive nitrogen but also chlorine and reactive hydrogen species and is expected to cause midlatitude ozone layer depletion. Comparison of the model ozone to observations suggests that N 2 O 5 hydrolysis contributed to reduced ozone, but additional chemical and/or dynamical processes are also important. These findings suggest that if wildfire smoke injection into the stratosphere increases sufficiently in frequency and magnitude as the world warms due to climate change, ozone recovery under the Montreal Protocol could be impeded, at least sporadically. Modeled austral midlatitude total ozone loss was about 1% in March 2020, which is significant compared to expected ozone recovery of about 1% per decade. 

The inorganic chlorine (Cl_y) and odd nitrogen (NO_y) chemical families influence stratospheric O₃. In January 2020 Australian wildfires injected record‐breaking amounts of smoke into the southern stratosphere. Within 1–2 months ground‐based and satellite observations showed Cl_yand NO_ywere repartitioned. By May, lower stratospheric HCl columns declined by ∼30% and ClONO₂columns increased by 40%–50%. The Cl_yperturbations began and ended near the equinoxes, increased poleward, and peaked at the winter solstice. NO₂decreased from February to April, consistent with sulfate aerosol reactions, but returned to typical values by June ‐ months before the Cl_yrecovery. Transport tracers show that dynamics not chemistry explains most of the observed O₃decrease after April, with no significant transport earlier. Simulations assuming wildfire smoke behaves identically to sulfate aerosols couldn't reproduce observed Cl_ychanges, suggesting they have different composition and chemistry. This undermines our ability to predict ozone in a changing climate.

 

Summertime surface‐level ozone (O₃) is known to vary with temperature, but the relative roles of different processes responsible for causing the O₃‐temperature relationship are not well quantified. In this study we use simulations of NASA's Global Modeling Initiative chemical transport model to isolate and assess the relative impact of atmospheric transport, chemistry, and emissions on large‐scale O₃variability, events, and and the covariance of O₃with temperature. Using observations from the Clean Air Status and Trends Network in the contiguous United States, we show that the Global Modeling Initiative chemical transport model reproduces the spatiotemporal variability of O₃and its relationship with temperature during the summer. We use the change in O₃given a change in temperature (dO₃/dT) along with other metrics to understand differences between our simulations. In regions with moderate to strong positive correlations between temperature and O₃such as the northeast, Great Lakes, and Great Plains, temperature's association with transport yields a majority of the total O₃‐temperature relationship (∼60%), while temperature‐dependent chemistry and anthropogenic NO emissions play smaller roles (∼30% and ∼10%, respectively). There are regions, however, with insignificant correlations between temperature and O₃, and our findings suggest that transport is still an important driver of O₃variability in these regions, albeit not correlated with temperature. Transport is not directly dependent on temperature but rather is linked through an indirect association, and it is therefore important to understand the exact mechanisms that link transport to O₃and how these mechanisms will change in a warming world.