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The 2015 and 2020 ozone holes set record sizes in October-December. We show that these years, as well as other recent large ozone holes, still adhere to a fundamental recovery metric: the later onset of early spring ozone depletion as chlorine and bromine diminishes. This behavior is also captured in the Whole Atmosphere Chemistry Climate Model. We quantify observed recovery trends of the onset of the ozone hole and in the size of the September ozone hole, with good model agreement. A substantial reduction in ozone hole depth during September over the past decade is also seen. Our results indicate that, due to dynamical phenomena, it is likely that large ozone holes will continue to occur intermittently in October-December, but ozone recovery will still be detectable through the later onset, smaller, and less deep September ozone holes: metrics that are governed more by chemical processes.

The Los Chocoyos (14.6°N, 91.2°W) supereruption happened ?75,000 years ago in Guatemala and was one of the largest eruptions of the past 100,000 years. It emitted enormous amounts of sulfur, chlorine, and bromine, with multi-decadal consequences for the global climate and environment. Here, we simulate the impact of a Los Chocoyos-like eruption on the quasi-biennial oscillation (QBO), an oscillation of zonal winds in the tropical stratosphere, with a comprehensive aerosol chemistry Earth System Model. We find a ?10-year disruption of the QBO starting 4 months post eruption, with anomalous easterly winds lasting ?5 years, followed by westerlies, before returning to QBO conditions with a slightly prolonged periodicity. Volcanic aerosol heating and ozone depletion cooling leads to the QBO disruption and anomalous wind regimes through radiative changes and wave-mean flow interactions. Different model ensembles, volcanic forcing scenarios and results of a second model back up the robustness of our results.

We demonstrate that identification of stratospheric ozone changes attributable to ozone depleting substances and actions taken under the Montreal Protocol requires evaluation of confounding influences from volcanic eruptions. Using a state‐of‐the‐art chemistry‐climate model, we show that increased stratospheric aerosol loading from volcanic eruptions after 2004 impeded the rate of ozone recovery post‐2000. In contrast, eruptions increased ozone loss rates over the depletion era from 1980 to 1998. We also present calculations without any aerosol chemistry to isolate contributions from gas‐phase chemistry alone. This study reinforces the need for accurate information regarding stratospheric aerosol loading when modeling ozone changes, particularly for the challenging task of accurately identifying the early signs of ozone healing distinct from other sources of variability.

Stratospheric aerosols (SAs) are a variable component of the Earth's albedo that may be intentionally enhanced in the future to offset greenhouse gases (geoengineering). The role of tropospheric-sourced sulfur dioxide (SO2) in maintaining background SAs has been debated for decades without in-situ measurements of SO2 at the tropical tropopause to inform this issue. Here we clarify the role of SO2 in maintaining SAs by using new in-situ SO2 measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO2 mixing ratios to estimate the global flux of SO2 across the tropical tropopause. These analyses show that the tropopause background SO2 is about 5 times smaller than reported by the average satellite observations that have been used recently to test atmospheric models. This shifts the view of SO2 as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget.

Meteoric smoke has traditionally been understood as a passive tracer which follows the global mesospheric circulation. Smoke extinction measured by the Solar Occultation For Ice Experiment, however, shows that while this is true in the middle to upper mesosphere (pressure

[1] Observations suggest that the optical depth of the stratospheric aerosol layer between 20 and 30 km has increased 4–10% per year since 2000, which is significant for Earth's climate. Contributions to this increase both from moderate volcanic eruptions and from enhanced coal burning in Asia have been suggested. Current observations are insufficient to attribute the contribution of the different sources. Here we use a global climate model coupled to an aerosol microphysical model to partition the contribution of each. We employ model runs that include the increases in anthropogenic sulfur dioxide (SO2) over Asia and the moderate volcanic explosive injections of SO2 observed from 2000 to 2010. Comparison of the model results to observations reveals that moderate volcanic eruptions, rather than anthropogenic influences, are the primary source of the observed increases in stratospheric aerosol.

An upper limit for the gas phase absorption cross sections for H2SO4 over the wavelength range 195 to 330 nm has been determined. Absorption measurements were made using a diode array spectrometer and a 100 cm long heated absorption cell. Gas phase H2SO4 was generated in a gas flow by titrating (equilibrating) known amounts of SO3 via the reaction at elevated temperatures, ∼ 500 K. Upper limits to the H2SO4 absorption cross sections were obtained

Each spring a layer of small particles forms between 20 and 30 km altitude in the polar regions. We present the first self-consistent explanation of the observed CN layer from a 2D microphysical model of sulfate aerosol. Our theory relies on photolysis of H 2 SO 4 and SO 3 , consistent with recent laboratory measurements, to produce SO 2 in the upper stratosphere and mesosphere. An additional source of SO 2 may be required. Nucleation throughout the polar winter extends the top of the aerosol layer to higher altitudes, despite strong downward transport of ambient air. This may affect heterogeneous chemistry at the top of the aerosol layer in polar winter and spring.

[1] Recent optical observations of aerosols in the upper stratosphere and mesosphere show significant amounts of extinction at altitudes above about 40 km where the stratospheric sulfate aerosol layer ends. Recent modeling of this region reveals that meteoritic smoke settling from the mesosphere and its interaction with the upper part of the sulfate aerosol layer is the origin of the observed extinction. Extinction in this region has major implications for the interpretation and analysis of several kinds of aerosol data (satellite and lidar). We compare observations from the SAGE II satellite and from NOAA's lidar located at Mauna Loa, Hawaii to extinction profiles derived from the Whole Atmosphere Community Climate Model (WACCM) coupled with the Community Aerosol and Radiation Model for Atmospheres (CARMA). Our results show that a major source of extinction exists in the region above about 30 km that must be addressed by all remote sensing instruments that have traditionally used the stratosphere above about 30 km as an aerosol free region to estimate the molecular component of their total extinction. It is also shown that meteoritic smoke not only contributes to but also becomes the dominant source of aerosol extinction above 35 km and poleward of 30 degrees in latitude, as well as above 40 km in the tropics. After addressing the concerns described here, current and past observations of this region could be reanalyzed to further our understanding of meteoritic dust in the upper stratosphere.

Observations of the stratospheric NO2 column amount over the Colorado mountains during January, February, and March, 1992 are compared to concurrent lidar measurements. During this period, large aerosol amounts due to the volcanic material injected into the stratosphere by Mt. Pinatubo were observed over Colorado. The observed NO2 column displays a strong anti-correlation with increasing amounts of stratospheric aerosol near 25–30 km for relatively low aerosol loadings. At higher aerosol loadings, the NO2 response appears to saturate, displaying little further reduction. The reasons for the altitude dependence of this relationship are probed with a one-dimensional stratospheric model, and it is shown that such a saturation response is to be expected for conditions where the heterogeneous reaction between N2O5 and H2O dominates the NOx chemistry, as in Prather [1992]. The results therefore provide evidence that heterogeneous reactions influence the chemical composition of the mid-latitude stratosphere, and in the general manner predicted by theory.

[1] A new type of hydrocarbon rocket engine is expected to power a fleet of suborbital rockets for commercial and scientific purposes in coming decades. A global climate model predicts that emissions from a fleet of 1000 launches per year of suborbital rockets would create a persistent layer of black carbon particles in the northern stratosphere that could cause potentially significant changes in the global atmospheric circulation and distributions of ozone and temperature. Tropical stratospheric ozone abundances are predicted to change as much as 1%, while polar ozone changes by up to 6%. Polar surface temperatures change as much as one degree K regionally with significant impacts on polar sea ice fractions. After one decade of continuous launches, globally averaged radiative forcing from the black carbon would exceed the forcing from the emitted CO2 by a factor of about 105 and would be comparable to the radiative forcing estimated from current subsonic aviation.

The rate coefficients for the reactions of OH radical with CH3Br and CH2Br2 were measured as functions of temperature using the laser photolysis - laser induced fluorescence method. This data was incorporated into a semi-empirical model [Solomon et al., 1992] and a 2-D model to calculate the steady - state Ozone Depletion Potentials (ODP) and atmospheric lifetimes, τ, with greatly improved accuracy as compared to earlier studies. The calculated ODPs and τ are 0.65 and 1.7 years and 0.17 and 0.41 years for CH3Br and CH2Br2, respectively, using the semi-empirical model. These lifetimes agree well with those calculated using a 2-D model. This study better quantifies the ODPs and τ of these species which are needed inputs for discussion of possible regulation of human emissions currently under international considerations.

] Polar mesospheric cloud (PMC) observations haverevealed that interannual variability near the polar summermesopause can be forced by planetary wave activity in thewinter stratosphere. We use data from the Aeronomy of Icein the Mesosphere (AIM) satellite to investigate couplingbetween the Arctic winter stratosphere and PMC variabilityin the Antarctic summer of 2007–2008. We find a highcorrelation between zonal mean PMC frequency and Arcticwinter zonal mean winds from the Goddard EarthObserving System, as well as Microwave Limb Sounderzonal mean temperatures. The time lag between changes inthe winter stratosphere and the connected response in PMCsvaries from 2 to 8 days. We suggest that the differences inlag times are related to the evolution of cloud altitudesthroughout the season. The results here are the first to showevidence for intra-seasonal PMC variability forced by inter-hemispheric coupling.

[1] When Solar system passes through moderately dense interstellar clouds, Earth experiences a dramatic increase in the flux of the anomalous component of cosmic rays (ACRs) along with an increased flux of galactic cosmic rays. ACR flux across the Earth's orbit lasts as long as it takes to cross a moderately dense interstellar cloud, about 1 Myr years. A period of ∼1 Myr is long enough for Earth to experience at least one magnetic reversal allowing penetration of the cosmic rays deep into the atmosphere even at low latitudes. Such increased cosmic ray fluxes would enhance the abundance of stratospheric NOx ∼100 times between 20–40 km, which in turn would decrease the ozone column globally by at least 40% and in the polar regions up to 80%. Such ozone loss would last for the duration of the magnetic reversal and could trigger global extinctions.