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In contrast to the general stratospheric ozone recovery following international agreements, recent observations show an ongoing net ozone depletion in the tropical lower stratosphere (LS). This depletion is thought to be driven by dynamical transport accelerated by global warming, while chemical processes have been considered to be unimportant. Here we use a chemistry–climate model to demonstrate that halogenated ozone-depleting very short-lived substances (VSLS) chemistry may account for around a quarter of the observed tropical LS negative ozone trend in 1998–2018. VSLS sources include both natural and anthropogenic emissions. Future projections show the persistence of the currently unaccounted for contribution of VSLS to ozone loss throughout the twenty-first century in the tropical LS, the only region of the global stratosphere not projecting an ozone recovery by 2100. Our results show the need for mitigation strategies of anthropogenic VSLS emissions to preserve the present and future ozone layer in low latitudes.

The Montreal Protocol is successfully protecting the ozone layer. The main halogen gases responsible for stratospheric ozone depletion have been regulated under the Protocol, their combined atmospheric abundances are declining and ozone is increasing in some parts of the atmosphere1. Ozone depletion potentials2–4, relative measures of compounds’ abilities to deplete stratospheric ozone, have been a key regulatory component of the Protocol in successfully guiding the phasing out in the manufacture of the most highly depleting substances. However, this latest, recovery phase in monitoring the success of the Protocol calls for further metrics. The ‘delay in ozone return’ has been widely used to indicate the effect of different emissions or phase-down strategies, but we argue here that it can sometimes be ambiguous or even of no use. Instead, we propose the use of an integrated ozone depletion (IOD) metric to indicate the impact of any new emission. The IOD measures the time-integrated column ozone depletion and depends only on the emission strength and the whole atmosphere and stratospheric lifetimes of the species considered. It provides a useful complementary metric of the impact of specific emissions of an ozone depleting substance for both the scientific and policy communities.

The atmospheric concentration of CFC-11 (CCl3F) has declined in response to the phase-out of its production by the Montreal Protocol. Nevertheless, this atmospheric concentration decline suffered a slow-down around 2012 due to emissions from non-reported production. Since CFC-11 is one of the most important ozone-depleting chlorofluorocarbons (CFCs), its continuous monitoring is essential. We present the CFC-11 total column time series (2000–2020) retrieved in a consistent way from ground-based high-resolution solar absorption Fourier transform infrared (FTIR) spectra. These observations were recorded at two remote stations of the Network for the Detection of Atmospheric Composition Change (NDACC): the Jungfraujoch station (Northern Hemisphere) and the Lauder station (Southern Hemisphere). These time series are new. They were produced using improved line parameters and merged considering the instrument changes and setup modifications. Afterwards, they were compared with Cape Grim station in situsurface observations conducted within the Advanced Global Atmospheric Gases Experiment (AGAGE) network and with total column datasets calculated by the TOMCAT/SLIMCAT 3-D chemical transport model. Trend analyses were performed, using an advanced statistical tool, in order to identify the timing and magnitude of the trend change in both hemispheres. The observations are consistent with the model results and confirm the slowdown in the CFC-11 atmospheric concentration decay, since 2011 in the Northern Hemisphere, and since 2014 in the Southern Hemisphere.

The atmospheric concentration of trichlorofluoromethane (CFC-11) has been in decline since the production of ozone-depleting substances was phased out under the Montreal Protocol1,2. Since 2013, the concentration decline of CFC-11 slowed unexpectedly owing to increasing emissions, probably from unreported production, which, if sustained, would delay the recovery of the stratospheric ozone layer1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11–12. Here we report an accelerated decline in the global mean CFC-11 concentration during 2019 and 2020, derived from atmospheric concentration measurements at remote sites around the world. We find that global CFC-11 emissions decreased by 18 ± 6 gigagrams per year (26 ± 9 per cent; one standard deviation) from 2018 to 2019, to a 2019 value (52 ± 10 gigagrams per year) that is similar to the 2008−2012 mean. The decline in global emissions suggests a substantial decrease in unreported CFC-11 production. If the sharp decline in unexpected global emissions and unreported production is sustained, any associated future ozone depletion is likely to be limited, despite an increase in the CFC-11 bank (the amount of CFC-11 produced, but not yet emitted) by 90 to 725 gigagrams by the beginning of 2020.

Nitrous oxide (N2O), like carbon dioxide, is a long-lived greenhouse gas that accumulates in the atmosphere. Over the past 150 years, increasing atmospheric N2O concentrations have contributed to stratospheric ozone depletion1and climate change2, with the current rate of increase estimated at 2 per cent per decade. Existing national inventories do not provide a full picture of N2O emissions, owing to their omission of natural sources and limitations in methodology for attributing anthropogenic sources. Here we present a global N2O inventory that incorporates both natural and anthropogenic sources and accounts for the interaction between nitrogen additions and the biochemical processes that control N2O emissions. We use bottom-up (inventory, statistical extrapolation of flux measurements, process-based land and ocean modelling) and top-down (atmospheric inversion) approaches to provide a comprehensive quantification of global N2O sources and sinks resulting from 21 natural and human sectors between 1980 and 2016. Global N2O emissions were 17.0 (minimum–maximum estimates: 12.2–23.5) teragrams of nitrogen per year (bottom-up) and 16.9 (15.9–17.7) teragrams of nitrogen per year (top-down) between 2007 and 2016. Global human-induced emissions, which are dominated by nitrogen additions to croplands, increased by 30% over the past four decades to 7.3 (4.2–11.4) teragrams of nitrogen per year. This increase was mainly responsible for the growth in the atmospheric burden. Our findings point to growing N2O emissions in emerging economies—particularly Brazil, China and India. Analysis of process-based model estimates reveals an emerging N2O–climate feedback resulting from interactions between nitrogen additions and climate change. The recent growth in N2O emissions exceeds some of the highest projected emission scenarios3,4, underscoring the urgency to mitigate N2O emissions.

The January 2022 eruption of Hunga Tonga‐Hunga Ha'apai (HTHH) injected a huge amount (∼150 Tg) of water vapor (H2O) into the stratosphere, along with small amount of SO2. An off‐line 3‐D chemical transport model (CTM) successfully reproduces the spread of the injected H2O through October 2023 as observed by the Microwave Limb Sounder. Dehydration in the 2023 Antarctic polar vortex caused the first substantial (∼20 Tg) removal of HTHH H2O from the stratosphere. The CTM indicates that this process will dominate removal of HTHH H2O for the coming years, giving an overall e‐folding timescale of 4 years; around 25 Tg of the injected H2O is predicted to still remain in the stratosphere by 2030. Following relatively low Antarctic column ozone in midwinter 2023 due to transport effects, additional springtime depletion due to H2O‐related chemistry was small and maximized at the vortex edge (10 DU in column). Around 150 Tg (150 million tons) of water vapor was injected into the stratosphere during the eruption of Hunga Tonga‐Hunga Ha'apai. Water vapor is a greenhouse gas and this increase is expected to have a warming effect in the troposphere, as well causing perturbations in stratospheric chemistry and aerosols. We use an atmospheric model to study the residence time of this excess water vapor and its impact on the recent Antarctic ozone hole. The model performance is evaluated by comparison with satellite measurements. Wintertime dehydration in the Antarctic stratosphere in 2023 is found to be an important mechanism for removal of the volcanic water from the stratosphere. However, the overall removal rate is predicted to be slow; around 25 Tg (17%) is still present in 2030. The direct impact of the excess water vapor on ozone via chemical processes in the Antarctic ozone hole in 2023 is small. Antarctic dehydration is a major removal pathway of stratospheric H2O injected from Hunga Tonga‐Hunga Ha'apai (HTHH) eruptionHTHH H2O caused small (up to 10 DU) additional chemical ozone depletion in 2023 Antarctic springModel indicates e‐folding timescale of 4 years for removal of HTHH H2O from stratosphere Antarctic dehydration is a major removal pathway of stratospheric H2O injected from Hunga Tonga‐Hunga Ha'apai (HTHH) eruption HTHH H2O caused small (up to 10 DU) additional chemical ozone depletion in 2023 Antarctic spring Model indicates e‐folding timescale of 4 years for removal of HTHH H2O from stratosphere

The recovery of stratospheric ozone from past depletion is underway owing to the 1987 Montreal Protocol and its subsequent amendments, which have been effective in phasing out the production and consumption of the major ozone-depleting substances (ODSs). However, there is uncertainty about the future rate of recovery. This uncertainty relates partly to unexpected emissions of controlled anthropogenic ODSs such as CCl3F and slower-than-expected declines in atmospheric CCl4. A further uncertainty surrounds emissions of uncontrolled short-lived anthropogenic ODSs (such as CH2Cl2and CHCl3), which observations show have been increasing in the atmosphere through 2017, as well as potential emission increases in natural ODSs (such as CH3Cl and CH3Br) induced by climate change, changes in atmospheric concentrations of greenhouse gases N2O and CH4, and stratospheric geoengineering. These challenges could delay the return of stratospheric ozone levels to historical values, (for example, the abundance in 1980), by up to decades, depending on the future evolution of the emissions and other influencing factors. To mitigate the threats to future ozone recovery, it is crucial to ensure that the Montreal Protocol and its amendments continue to be implemented effectively in order to have firm control on future levels of ODSs. This action needs to be supported by an expansion of the geographic coverage of atmospheric observations of ODSs, by enhancing the ability of source attribution modelling, and by improving understanding of the interactions between climate change and ozone recovery. Recovery of the stratospheric ozone layer above Antarctica has not been straightforward, as a result of human activities and climate change. The recovery process might be delayed by up to decades if further mitigation actions are not taken.

In this work we investigate interannual variations in lower stratospheric ozone from 1984 to 2016 based on a satellite‐derived data set and simulations from a chemical transport model. An empirical orthogonal function (EOF) analysis of ozone variations between 2000 and 2016 indicates that the first, second, and third EOF modes are related to the quasi‐biennial oscillation (QBO), canonical El Niño–Southern Oscillation (ENSO), and ENSO Modoki events, respectively; these three leading EOFs capture nearly 80% of the variance. However, for the period 1984–2000, the first, second, and third modes are related to the QBO, ENSO Modoki, and canonical ENSO events, respectively. The explained variance of the second mode in relation to ENSO Modoki is nearly twice that of the third mode for canonical ENSO. Since the frequency of ENSO Modoki events was higher from 1984 to 2000 than after 2000, the Brewer‐Dobson circulation anomalies related to ENSO Modoki were stronger during 1984–2000, which caused ENSO Modoki events to have a greater effect on lower stratospheric ozone before 2000 than after. Ozone anomalies associated with QBO, ENSO Modoki, and canonical ENSO events are largely caused by dynamic processes, and the effect of chemical processes on ozone anomalies is opposite to that of dynamic processes. Ozone anomalies related to dynamic processes are 3–4 times greater than those related to chemical processes. The second and third EOFs of the interannual variations in lower stratospheric ozone during 2000‐2016 are related to canonical ENSO and ENSO Modoki, respectivelyFor the period 1984‐2000, the second EOF is related to ENSO Modoki and the third EOF to canonical ENSOThe higher frequency of occurrence of ENSO Modoki than canonical ENSO led to a stronger impact of ENSO Modoki on circulation and ozone anomalies between 1984 and 2000

Using Microwave Limb Sounder (MLS) satellite observations, ERA‐Interim reanalysis data, and a chemistry transport model simulation, we analyze and investigate the causes of the asymmetric hemispheric trends of N2O, CH4, and HCl in the stratosphere during the period 2004–2012. We find significant hemispheric asymmetries in the trends of these trace gases in the midlatitude middle and lower stratosphere. With regard to N2O and CH4, the enhanced downwelling branch of the residual circulation in the Northern Hemisphere (NH) middle and upper stratosphere transports more N2O/CH4‐poor air from the upper stratosphere to the lower stratosphere. The enhanced poleward meridional branch of the residual circulation in the Southern Hemisphere (SH) stratosphere brings more N2O/CH4‐rich air from lower to middle latitudes. These processes therefore contribute to the negative trends of N2O and CH4in the NH lower stratosphere and the positive trends in the SH middle stratosphere. A corresponding positive trend is found for HCl in the NH, where the deep branch of the residual circulation located in the middle and upper stratosphere strengthens, bringing more HCl‐rich air downward to the lower stratosphere, while the shallow branch of the residual circulation in the lower stratosphere weakens and leads to enhanced conversion of chlorine‐containing source gases of different lifetimes to HCl. A reversed picture emerges in the SH, where the deep branch of the residual circulation in the middle and upper stratosphere weakens, while the shallow branch in the lower stratosphere strengthens, resulting in less HCl there. In addition, the southward shift of the upwelling branch of the residual circulation in recent decades can partly explain trace gas trends above 20 hPa, while the eddy mixing has a small effect on the trends. Understanding these contributions from different processes to the hemispheric asymmetries in trends of these trace gases can help us to evaluate more accurately future changes in stratospheric composition. Significant hemispheric asymmetries exist in stratospheric trends of N2O, CH4, and HCl over the period 2004–2012 due to different trends of residual circulation in the Northern and Southern HemispheresThe trend asymmetries in N2O and CH4are opposite in sign to HCl, which is due to different stratospheric lifetimes and source gasesThe southward shift of the upwelling branch of the residual circulation and eddy mixing also contribute to the hemispheric asymmetries in trends of stratospheric trace gases

Chloroform (CHCl3), dichloromethane (CH2Cl2), perchloroethylene (C2Cl4), and 1,2‐dichloroethane (C2H4Cl2) are chlorinated Very Short‐Lived Substances (Cl‐VSLS) with a range of commercial/industrial applications. Recent studies highlight the increasing influence of Cl‐VSLS on the stratospheric chlorine budget and therefore their possible role in ozone depletion. Here we evaluate the ozone depletion potential (ODP) of these Cl‐VSLS using a three‐dimensional chemical transport model and investigate sensitivity to emission location/season. The seasonal dependence of the ODPs is small, but ODPs vary by a factor of 2–3 depending on the continent of emission: 0.0143–0.0264 (CHCl3), 0.0097–0.0208 (CH2Cl2), 0.0057–0.0198 (C2Cl4), and 0.0029–0.0119 (C2H4Cl2). Asian emissions produce the largest ODPs owing to proximity to the tropics and efficient troposphere‐to‐stratosphere transport of air originating from industrialized East Asia. The Cl‐VSLS ODPs are generally small, but the upper ends of the CHCl3and CH2Cl2ranges are comparable to the mean ODP of methyl chloride (0.02), a longer‐lived ozone‐depleting substance. Anthropogenic emissions of long‐lived chlorinated substances (e.g., chlorofluorocarbons) have led to global ozone layer depletion since the 1970s/1980s, including the Antarctic Ozone Hole phenomenon. The 1987 Montreal Protocol was enacted to ban production of major ozone‐depleting gases, and in consequence, there are signs that the ozone layer is recovering. However, emissions of so‐called very short‐lived substances, such as dichloromethane, have increased in recent years. Historically, these compounds have not been considered a major threat to stratospheric ozone, due to relatively short lifetimes, and they are not controlled by the Protocol. Given that production of these compounds is projected to increase, it is important to determine their ability to affect stratospheric ozone. We quantify the ozone depletion potential (ODP) of chloroform and perchloroethylene and, for the first time, dichloromethane and 1,2‐dichloroethane, the main chlorinated very short‐lived substances. We show that their ODPs vary depending on where the emission occurs. For example, the ODP from Asian dichloromethane emissions is up to a factor of two greater than that from European emissions. This reflects the relative efficiency of troposphere to stratosphere transport between different geographical areas; the transport of polluted boundary layer air from continental East Asia being one relatively efficient route. Ozone depletion potentials of very short‐lived substances (CHCl3, CH2Cl2, C2Cl4, and C2H4Cl2) were calculated using a chemical transport modelCalculated ozone depletion potentials vary by a factor of 2‐3 depending on emission location, larger ODPs for Asian emissionsEfficient transport of very short‐lived substances from continental East Asia to tropical lower stratosphere lead to larger Asian ODPs

Very short‐lived substances (VSLS), including dichloromethane (CH2Cl2), chloroform (CHCl3), perchloroethylene (C2Cl4), and 1,2‐dichloroethane (C2H4Cl2), are a stratospheric chlorine source and therefore contribute to ozone depletion. We quantify stratospheric chlorine trends from these VSLS (VSLCltot) using a chemical transport model and atmospheric measurements, including novel high‐altitude aircraft data from the NASA VIRGAS (2015) and POSIDON (2016) missions. We estimate VSLCltotincreased from 69 (±14) parts per trillion (ppt) Cl in 2000 to 111 (±22) ppt Cl in 2017, with >80% delivered to the stratosphere through source gas injection, and the remainder from product gases. The modeled evolution of chlorine source gas injection agrees well with historical aircraft data, which corroborate reported surface CH2Cl2increases since the mid‐2000s. The relative contribution of VSLS to total stratospheric chlorine increased from ~2% in 2000 to ~3.4% in 2017, reflecting both VSLS growth and decreases in long‐lived halocarbons. We derive a mean VSLCltotgrowth rate of 3.8 (±0.3) ppt Cl/year between 2004 and 2017, though year‐to‐year growth rates are variable and were small or negative in the period 2015–2017. Whether this is a transient effect, or longer‐term stabilization, requires monitoring. In the upper stratosphere, the modeled rate of HCl decline (2004–2017) is −5.2% per decade with VSLS included, in good agreement to ACE satellite data (−4.8% per decade), and 15% slower than a model simulation without VSLS. Thus, VSLS have offset a portion of stratospheric chlorine reductions since the mid‐2000s. It is well established that long‐lived halogen‐containing compounds of anthropogenic origin, such as chlorofluorocarbons, have led to depletion of the stratospheric ozone layer. As production of these compounds is now controlled by the Montreal Protocol, the atmospheric abundance of chlorine/bromine is in decline, and the ozone layer should “recover” in coming decades. Here we consider the contribution of Very Short‐Lived Substances to stratospheric chlorine. These compounds also have anthropogenic sources, though are much less efficient at destroying ozone compared to, for example, most chlorofluorocarbons (per molecule emitted) as they break down more readily close to Earth's surface. Using surface observations and atmospheric model simulations, we show that stratospheric chlorine from short‐lived substances has increased since the early 2000s. This increase is also apparent from airborne measurements of their atmospheric abundance over the same period. Using the model in conjunction with satellite estimates of stratospheric chlorine, we show rising levels of short‐lived substances may be causing upper stratospheric chlorine to decline at a slower rate relative to what would be expected in their absence. While this offset in the rate of chlorine decline is modest (15%), it is nonnegligible and should be considered in the analysis of stratospheric composition trends. Stratospheric chlorine from very short‐lived substances increased by 3.8 ppt/year over 2004–2017, with a growth slowdown in 2015–2017Chlorine from short‐lived substances improves model representation of upper stratospheric HCl trendsShort‐lived chlorine offsets the 2004–2017 rate of upper stratospheric HCl decline by 15%

Phosgene in the atmosphere is produced via the degradation of carbon tetrachloride, methyl chloroform, and a number of chlorine‐containing very short lived substances (VSLS). These VSLS are not regulated by the Montreal Protocol even though they contribute to stratospheric ozone depletion. While observations of VSLS can quantify direct stratospheric source gas injection, observations of phosgene in the upper troposphere/lower stratosphere can be used as a marker of product gas injection of chlorine‐containing VSLS. In this work we report upper troposphere/lower stratosphere measurements of phosgene made by the ACE‐FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument and compare with results from the TOMCAT/SLIMCAT three‐dimensional chemical transport model to constrain phosgene trends over the 2004–2016 period. The 13‐year ACE‐FTS time series provides the first observational evidence for an increase in chlorine product gas injection. In 2016, VSLS accounted for 27% of modeled stratospheric phosgene, up from 20% in the mid‐2000s. Atmospheric phosgene, a chlorine‐containing molecule, is produced via the degradation of the reasonably long lived species carbon tetrachloride and methyl chloroform, and a number of very short lived substances (VSLS), including dichloromethane, chloroform, and tetrachloroethene. Whereas the former are regulated by the Montreal Protocol because they contribute to stratospheric ozone depletion, these latter species are not. It is therefore important that we continue to monitor VSLS and the degradation products to determine how much additional chlorine, which catalyzes the destruction of ozone, is reaching the stratosphere. VSLS can either reach the stratosphere directly, via so‐called source gas injection, or degrade in the troposphere into products such as phosgene, which are then delivered into the stratosphere via so‐called product gas injection (PGI). Monitoring phosgene in the upper troposphere/lower stratosphere is therefore a marker for PGI due to chlorine‐containing VSLS. In this work we report measurements of phosgene in the upper troposphere/lower stratosphere made by the ACE‐FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument. The 13‐year ACE‐FTS time series provides the first observational evidence for an increase in chlorine PGI, which has been predicted by atmospheric models. A 13‐year global time series of phosgene in the upper troposphere/lower stratosphere is presented from ACE‐FTS dataThe observations provide evidence for an increase in phosgene in the upper troposphere and an overall decrease in the stratosphereThe increase in the upper troposphere is consistent with the increase in chlorine product gas injection predicted by a 3‐D model

Simulated stratospheric temperatures over the period 1979–2016 in models from the Chemistry‐Climate Model Initiative are compared with recently updated and extended satellite data sets. The multimodel mean global temperature trends over 1979–2005 are −0.88 ± 0.23, −0.70 ± 0.16, and −0.50 ± 0.12 K/decade for the Stratospheric Sounding Unit (SSU) channels 3 (~40–50 km), 2 (~35–45 km), and 1 (~25–35 km), respectively (with 95% confidence intervals). These are within the uncertainty bounds of the observed temperature trends from two reprocessed SSU data sets. In the lower stratosphere, the multimodel mean trend in global temperature for the Microwave Sounding Unit channel 4 (~13–22 km) is −0.25 ± 0.12 K/decade over 1979–2005, consistent with observed estimates from three versions of this satellite record. The models and an extended satellite data set comprised of SSU with the Advanced Microwave Sounding Unit‐A show weaker global stratospheric cooling over 1998–2016 compared to the period of intensive ozone depletion (1979–1997). This is due to the reduction in ozone‐induced cooling from the slowdown of ozone trends and the onset of ozone recovery since the late 1990s. In summary, the results show much better consistency between simulated and satellite‐observed stratospheric temperature trends than was reported by Thompson et al. (2012, https://doi.org/10.1038/nature11579) for the previous versions of the SSU record and chemistry‐climate models. The improved agreement mainly comes from updates to the satellite records; the range of stratospheric temperature trends over 1979–2005 simulated in Chemistry‐Climate Model Initiative models is comparable to the previous generation of chemistry‐climate models. A previous analysis by Thompson et al. (2012, https://doi.org/10.1038/nature11579) showed substantial differences between satellite‐observed and model‐simulated stratospheric cooling trends since the late 1970s. Here we compare recently revised and extended satellite temperature records with new simulations from 14 chemistry‐climate models. The results show much better agreement in the magnitude of stratospheric cooling over 1979–2005 between models and observations. This cooling was predominantly driven by increasing greenhouse gases and declining stratospheric ozone levels. An extended satellite temperature record and the chemistry‐climate models show weaker global stratospheric cooling over 1998–2016 compared to 1979–1997. This is due to the reduction in ozone‐induced cooling from the slowdown of ozone trends and the onset of ozone recovery since the late 1990s. There are larger differences in the latitudinal structure of past stratospheric temperature trends due to the effects of unforced atmospheric variability. In summary, the results show much better consistency between simulated and satellite‐observed stratospheric temperature trends than was reported by Thompson et al. (2012, https://doi.org/10.1038/nature11579) for the previous versions of the satellite record and last generation of chemistry‐climate models. The improved agreement mainly comes from updates to the satellite records, while the range of simulated trends is comparable to the previous generation of models. There is substantial improvement in the comparison between modeled and observed stratospheric temperature trends over the satellite eraObservations and models show weaker stratospheric cooling since ~1998 when ozone‐depleting substances have been declining in the atmosphereLarger differences exist between modeled and observed stratospheric temperature trends at high latitudes partly due to internal variability

We use height‐resolved and total column satellite observations and 3‐D chemical transport model simulations to study stratospheric ozone variations during 1998–2017 as ozone‐depleting substances decline. In 2017 extrapolar lower stratospheric ozone displayed a strong positive anomaly following much lower values in 2016. This points to large interannual variability rather than an ongoing downward trend, as reported recently by Ball et al. (2018, https://doi.org/10.5194/acp‐18‐1379‐2018). The observed ozone variations are well captured by the chemical transport model throughout the stratosphere and are largely driven by meteorology. Model sensitivity experiments show that the contribution of past trends in short‐lived chlorine species to the ozone changes is small. Similarly, the potential impact of modest trends in natural brominated short‐lived species is small. These results confirm the important role that atmospheric dynamics plays in controlling ozone in the extrapolar lower stratosphere on multiannual time scales and the continued importance of monitoring ozone profiles as the stratosphere changes. Emission of long‐lived chlorine and bromine‐containing ozone‐depleting substances has led to the depletion of the ozone layer, most notably the Antarctic ozone hole. Policy action through the Montreal Protocol has phased out the production of the major long‐lived ozone‐depleting substances. Consequently, stratospheric chlorine and bromine amounts are declining, and we expect the ozone layer to slowly recover. However, although the tropical lower stratosphere is not a region where large ozone loss has so‐far been observed, a recent study by Ball et al. (2018) suggested that ozone there is decreasing, in disagreement with models and expectations of ozone recovery. We use updated observations and an atmospheric model to investigate these issues. First, we use an additional year of observations which show that ozone values in the lower stratosphere increased in 2017, which is a consequence of variations in atmospheric dynamics. Second, our 3‐D model performs well in reproducing the observed ozone variations. Although the model is not perfect, the comparisons suggest that we do have a good understanding of the lower stratospheric ozone. Third, we quantify the role of short‐lived chlorine and bromine compounds, which are not controlled by the Montreal Protocol, on the recent ozone changes. The effect is small. Observations show that lower stratospheric ozone at extrapolar latitudes increased strongly in 2017 relative to a negative anomaly in 2016Model simulations reproduce the observed ozone variations well, and the main driver in the lower stratosphere is atmospheric dynamicsThe contribution of an observation‐based trend in short‐lived chlorine species to recent lower stratospheric ozone variations is small

We quantify the stratospheric injection of brominated very short‐lived substances (VSLS) based on aircraft observations acquired in winter 2014 above the Tropical Western Pacific during the CONvective TRansport of Active Species in the Tropics (CONTRAST) and the Airborne Tropical TRopopause EXperiment (ATTREX) campaigns. The overall contribution of VSLS to stratospheric bromine was determined to be 5.0 ± 2.1 ppt, in agreement with the 5 ± 3 ppt estimate provided in the 2014 World Meteorological Organization (WMO) Ozone Assessment report (WMO 2014), but with lower uncertainty. Measurements of organic bromine compounds, including VSLS, were analyzed using CFC‐11 as a reference stratospheric tracer. From this analysis, 2.9 ± 0.6 ppt of bromine enters the stratosphere via organic source gas injection of VSLS. This value is two times the mean bromine content of VSLS measured at the tropical tropopause, for regions outside of the Tropical Western Pacific, summarized in WMO 2014. A photochemical box model, constrained to CONTRAST observations, was used to estimate inorganic bromine from measurements of BrO collected by two instruments. The analysis indicates that 2.1 ± 2.1 ppt of bromine enters the stratosphere via inorganic product gas injection. We also examine the representation of brominated VSLS within 14 global models that participated in the Chemistry‐Climate Model Initiative. The representation of stratospheric bromine in these models generally lies within the range of our empirical estimate. Models that include explicit representations of VSLS compare better with bromine observations in the lower stratosphere than models that utilize longer‐lived chemicals as a surrogate for VSLS. Based on winter 2014 observations, very short‐lived bromocarbons produced by oceanic biology contribute 5 ± 2 ppt to stratospheric bromineOf the bromine from very short‐lived substances that reaches the stratosphere, 60% enters as organic species and 40% as inorganic speciesRepresentation of stratospheric bromine within global models is greatly improved upon consideration of very short‐lived bromocarbons

Substantial and prolonged enhancements in stratospheric water vapor (SWV) have occurred after large‐magnitude explosive tropical volcanic eruptions, with modified tropopause entry caused by aerosol‐absorptive heating. Here, we analyze the timing and longevity of heating‐driven post‐eruption SWV changes within CMIP6‐VolMIP short‐term climate‐response experiments with the UK Earth System Model (UKESM1). We find aerosol‐absorptive heating causes peak SWV increases of 17% (∼1 ppmv) and 10% (0.5 ppmv) at 100 and 50 hPa, at ∼18 and ∼23 months after a Pinatubo‐like eruption, respectively. We track the temperature response in the tropical lower stratosphere and identify the main SWV increase occurs only after the descending aerosol heating reaches the tropopause, suggesting a key role for aerosol microphysical processes (sedimentation rate). We explore how El Niño–Southern Oscillation variability modulates this effect. Post‐eruption SWV increases are ∼80% stronger for the La Nina phase compared to the ensemble mean. Tropical upwelling strongly mediates this effect. Strong volcanic eruptions, such as the 1991 eruption of Mt Pinatubo, inject a large amount of SO2directly into the stratosphere, thereby enhancing the stratospheric aerosol layer and causing a short‐term climatic perturbation. Another substantial part of the climatic influence is the change in stratospheric water vapor (SWV), which affects the chemical processes and the radiative budget of the atmosphere. Along with near‐instantaneous injection of water vapor into the stratosphere, volcanic eruptions can indirectly enhance the entry of water vapor into the stratosphere through aerosol‐induced tropopause heating. This work analyses Earth system model experiments designed to explore how volcanic impacts combine with internal climate variability. We find that peak SWV entry mixing ratios occur only within the second post‐eruption year, consistent with the substantially lagged timing of SWV increase seen in post‐Pinatubo satellite measurements. This analysis provides a new perspective on the temporal evolution of the observed post‐Pinatubo SWV increase and an improved quantification of its impacts. Aerosol‐induced absorptive‐heating increases stratospheric water vapor (SWV) by up to 17% at 18 months post‐eruption in a Pinatubo‐like experimentAnalyzing simulations by El Niño–Southern Oscillation (ENSO) variability show an 80% larger peak SWV increase occurs if an eruption is followed by a La Niña phaseThe timing of peak SWV increase occurs when volcanic‐aerosol‐induced heating reaches the tropopause, with ENSO modulation of tropical upwelling Aerosol‐induced absorptive‐heating increases stratospheric water vapor (SWV) by up to 17% at 18 months post‐eruption in a Pinatubo‐like experiment Analyzing simulations by El Niño–Southern Oscillation (ENSO) variability show an 80% larger peak SWV increase occurs if an eruption is followed by a La Niña phase The timing of peak SWV increase occurs when volcanic‐aerosol‐induced heating reaches the tropopause, with ENSO modulation of tropical upwelling

We present a comparison of atmospheric transport models that simulate carbonyl sulfide (COS). This is part II of the ongoing Atmospheric Transport Model Inter‐comparison Project (TransCom–COS). Differently from part I, we focus on seven model intercomparison by transporting two recent COS inversions of NOAA surface data within TM5‐4DVAR and LMDz models. The main goals of TransCom‐COS part II are (a) to compare the COS simulations using the two sets of optimized fluxes with simulations that use a control scenario (part I) and (b) to evaluate the simulated tropospheric COS abundance with aircraft‐based observations from various sources. The output of the seven transport models are grouped in terms of their vertical mixing strength: strong and weak mixing. The results indicate that all transport models capture the meridional distribution of COS at the surface well. Model simulations generally match the aircraft campaigns HIAPER Pole‐To‐Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom). Comparisons to HIPPO and ATom demonstrate a gap between observed and modeled COS over the Pacific Ocean at 0–40°N, indicating a potential missing source in the free troposphere. The effects of seasonal continental COS uptake by the biosphere, observed on HIPPO and ATom over oceans, is well reproduced by the simulations. We found that the strength of the vertical mixing within the column as represented in the various atmospheric transport models explains much of the model to model differences. We also found that weak‐mixing models transporting the optimized flux derived from the strong‐mixing TM5 model show a too strong seasonal cycle at high latitudes. Carbonyl sulfide (COS) is a significant sulfur‐containing trace gas in the atmosphere, which makes it important for studying climate change. One of the reasons it is worth investigating is because plants take up COS in a similar way as CO2during photosynthesis. However, the atmospheric sources and sinks of COS are not well understood. To address this knowledge gap, we evaluated the state‐of‐the‐art optimized surface COS fluxes from the inverse models TM5‐4DVAR and LMDz, and then seven atmospheric transport models were used to simulate COS mole fractions by transporting the optimized fluxes under the TransCom‐COS protocol. The results showed good agreement between the simulated COS and COS observations on independent platforms. The study also revealed that COS drawdown due to plant uptake can be observed over Pacific and Atlantic Oceans. However, discrepancies between the model simulations and observations were mainly found in free troposphere, emphasizing the need for further investigation into COS chemistry and model transport differences. These findings provide important reference for further investigation of COS global distribution and budget analysis. Simulations in seven models propagating optimized carbonyl sulfide (COS) fluxes derived from two inversions agree with independent observationsSimulated and observed COS drawdowns are captured in boundary layer over the Pacific and Atlantic Oceans due to plant uptake over landsWeak vertical mixing models using fluxes optimized from the fast‐mixing TM5 model overestimate the COS seasonal amplitude at high latitudes Simulations in seven models propagating optimized carbonyl sulfide (COS) fluxes derived from two inversions agree with independent observations Simulated and observed COS drawdowns are captured in boundary layer over the Pacific and Atlantic Oceans due to plant uptake over lands Weak vertical mixing models using fluxes optimized from the fast‐mixing TM5 model overestimate the COS seasonal amplitude at high latitudes

As a result of the 1987 Montreal Protocol and its amendments, the atmospheric loading of anthropogenic ozone-depleting substances is decreasing. Accordingly, the stratospheric ozone layer is expected to recover. However, short data records and atmospheric variability confound the search for early signs of recovery, and climate change is masking ozone recovery from ozone-depleting substances in some regions and will increasingly affect the extent of recovery. Here we discuss the nature and timescales of ozone recovery, and explore the extent to which it can be currently detected in different atmospheric regions.

We use the 3‐D Whole Atmospheric Community Climate Model to investigate stratospheric ozone depletion due to the launch of small satellites (e.g., CubeSats) with an iodine propulsion system. The model considers the injection of iodine from the satellites into the Earth's thermosphere and suggests a 4‐yr timescale for transport of the emissions down to the troposphere. The base case scenario is 40,000 small satellite launches per year into low orbit (100–600 km), which would inject 8 tons I yr−1above 120 km as I+ions and increase stratospheric inorganic iodine by ∼0.1 part per trillion (pptv). The model shows that this scenario produces a negligible impact on global stratospheric ozone (∼0.05 DU column depletion). In contrast, a 100‐fold increase in the launch rate, and therefore thermospheric iodine injection, is predicted to result in modeled ozone depletion of up to 14 DU (approximately 2%–7%) over the polar regions. Iodine has the potential to cause stratospheric ozone depletion. Small satellites (<10 kg) in low Earth orbit require electric propulsion to prolong their time in orbit, and there is strong interest in replacing the rare gas propellant (Xe or Kr) with I2. Here we estimate the potential impact of thermosphere iodine injection from such satellites on stratospheric ozone. Since the demand for small satellites could increase greatly in the future, it is important to understand the potential risks to ozone depletion if iodine propulsion systems are used. Assuming a scenario in which 40,000 small satellites are launched each year into relatively low orbits and each satellite is assumed to contain 200 g I2, this could inject 8 tons/year of iodine ions into the upper atmosphere (above 120 km). Using a 3‐D atmospheric chemistry‐climate model, we show that the perturbation to the total column ozone is very small, decreasing by 0.05 DU (<0.02%) globally and 0.2 DU (<0.1%) for September/October in the Antarctic polar region. However, a 100‐fold increase in the number of launches and therefore mass of iodine emitted into near‐Earth orbit is predicted to cause significant ozone depletion up to 14 DU (5.6%) in the Antarctic spring. The effect of injecting iodine into the atmosphere from I2‐propelled satellites has been modeled using an atmosphere chemistry‐climate modelInjection of 8 tons I yr−1, based on the expected launch rate, is predicted to cause negligible O3depletion (0.05 DU) globallyO3depletion increases near‐linearly with iodine mass injected, reaching up to 14 DU in the polar regions for 800 tons I yr−1 The effect of injecting iodine into the atmosphere from I2‐propelled satellites has been modeled using an atmosphere chemistry‐climate model Injection of 8 tons I yr−1, based on the expected launch rate, is predicted to cause negligible O3depletion (0.05 DU) globally O3depletion increases near‐linearly with iodine mass injected, reaching up to 14 DU in the polar regions for 800 tons I yr−1

We present a comparison of atmospheric transport model (ATM) simulations for carbonyl sulfide (COS), within the framework of the atmospheric tracer transport model intercomparison project “TransCom‐COS.” Seven ATMs participated in the experiment and provided simulations of COS mixing ratios over the years 2010–2018, using state‐of‐the‐art surface fluxes for various components of the COS budget: biospheric sink, oceanic source, sources from fire and industry. The main goal of TransCom‐COS is to investigate the impact of the transport uncertainty and emission distribution in simulating the spatio‐temporal variability of tropospheric COS mixing ratios. A control case with seasonal surface fluxes of COS was constructed. The results indicate that the COS mixing ratios are underestimated by at least 50 parts per trillion (ppt) in the tropics, pointing to a missing tropical source. In summer, the mixing ratios are overestimated by at least 50 ppt above 40°N, pointing to a likely missing sink in the high northern latitudes. Regarding the latitudinal profile, the model spread is greater than 60 ppt above 40°N in boreal summer. Regarding the seasonal amplitude, the model spread reaches 50 ppt at 6 out of 15 sites, compared to an observed seasonal amplitude of 100 ppt. All models simulated a too late minimum by at least 2–3 months at two high northern‐latitude sites, likely owing to errors in the seasonal cycle in the ocean emissions. This study highlighted the shortcomings in the COS global budget that need to be resolved before using COS as a photosynthesis tracer. In this study, we evaluate the state‐of‐the‐art fluxes for various components of the carbonyl sulfide (COS) budget: biospheric sink, oceanic source, sources from fire and industry. A control case with seasonal surface fluxes of COS was constructed. Seven atmospheric transport models provided simulations of COS mixing ratios. Then, the simulated mixing ratios were evaluated against atmospheric measurements at several surface sites. Results show that all models fail to capture the observed latitudinal distribution and that the model spread is small compared to the model‐observation mismatch. In summer, the overestimated mixing ratios above 40°N point to a likely missing sink in the high northern latitudes. The underestimated mixing ratios in the tropics point to a missing tropical source. This study highlighted the shortcomings in the COS global budget that need to be resolved before using COS as a photosynthesis tracer. The model‐observation mismatch suggests there is a missing source in the tropics and a missing sink in the high northern latitude in summerAt northern latitude sites, the model spread in seasonal amplitude reaches 50 ppt compared to a mean seasonal amplitude of about 100 pptThe diurnal rectifier effect is small, decreasing the seasonal amplitude by up to 20% at continental sites The model‐observation mismatch suggests there is a missing source in the tropics and a missing sink in the high northern latitude in summer At northern latitude sites, the model spread in seasonal amplitude reaches 50 ppt compared to a mean seasonal amplitude of about 100 ppt The diurnal rectifier effect is small, decreasing the seasonal amplitude by up to 20% at continental sites

The hydroxyl radical (OH) is the primary daytime oxidant in the troposphere and provides the main loss mechanism for many pollutants and greenhouse gases, including methane (CH4). Global mean tropospheric OH differs by as much as 80% among various global models, for reasons that are not well understood. We use neural networks (NNs), trained using archived output from eight chemical transport models (CTMs) that participated in the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols and Transport Model Intercomparison Project (POLMIP), to quantify the factors responsible for differences in tropospheric OH and resulting CH4lifetime (tCH4) between these models. Annual average tCH4, for loss by OH only, ranges from 8.0 to 11.6?years for the eight POLMIP CTMs. The factors driving these differences were quantified by inputting 3-D chemical fields from one CTM into the trained NN of another CTM. Across all CTMs, the largest mean differences in tCH4(?tCH4) result from variations in chemical mechanisms (?tCH4?=?0.46?years), the photolysis frequency (J) of O3???O(1D) (0.31?years), local O3(0.30?years), and CO (0.23?years). The ?tCH4due to CTM differences in NOx(NO?+?NO2) is relatively low (0.17?years), although large regional variation in OH between the CTMs is attributed to NOx. Differences in isoprene and J(NO2) have negligible overall effect on globally averaged tropospheric OH, although the extent of OH variations due to each factor depends on the model being examined. This study demonstrates that NNs can serve as a useful tool for quantifying why tropospheric OH varies between global models, provided that essential chemical fields are archived. Factors responsible for OH and CH4lifetime differences between eight models are quantified by using neural networksO3, the photolysis frequency (J) of O3to O(1D), CO, and chemical mechanism differences are main drivers of OH variationsH2O and NOxdifferences drive moderate OH variation on regional scale; isoprene and J(NO2) differences have small role in driving OH variations

The 2019/2020 Australian wildfires emitted large quantities of atmospheric pollutant gases and aerosols. Using state‐of‐the‐art near‐real‐time satellite measurements of tropospheric composition, we present an analysis of several emitted trace gases and their long‐range transport, and compare to the previous (2018/2019) fire season. Observations of carbon monoxide (CO) show that fire emissions were so intense that the distinct Australian fire plume managed to circumnavigate the Southern Hemisphere (SH) within a few weeks, with eastward propagation over the South Pacific, South America, the South Atlantic, Africa, and the Indian Ocean. Elevated atmospheric methane levels were also detected in January 2020 fire plumes over the Pacific, defined using CO as a plume tracer, even though sampling was restricted spatially by aerosols and clouds. Observations also show significant enhancements of methanol (CH3OH) from the fires, where CH3OH:CO enhancement ratios increased within the aged plume downwind over the South Pacific indicating secondary in‐plume CH3OH formation. The 2019/2020 Australian wildfires emitted large quantities of smoke and gaseous pollutants. Using satellite observations, we analyze the long‐range transport of several trace gases and compare with the previous (2018/2019) fire season. The 2019/20 fires were so intense that a distinct plume of carbon monoxide was observed to propagate around the entire Southern Hemisphere in just a few weeks. Enhancement of methane was also detected in the plume along with substantial quantities of methanol. The ratio between methanol and carbon monoxide increased over the Southern Pacific suggesting the secondary formation of methanol within the fire plume as it propagated eastwards. Satellite‐retrieved carbon monoxide (CO) plumes from the Australian fires circumvent the Southern HemisphereSatellite‐retrieved methanol (CH3OH) shows downwind enhancement of CH3OH:CO ratio suggesting in‐plume secondary CH3OH production as well as direct emissionSubstantial enhancements in methane (CH4) are detected from space Satellite‐retrieved carbon monoxide (CO) plumes from the Australian fires circumvent the Southern Hemisphere Satellite‐retrieved methanol (CH3OH) shows downwind enhancement of CH3OH:CO ratio suggesting in‐plume secondary CH3OH production as well as direct emission Substantial enhancements in methane (CH4) are detected from space

Polar stratospheric clouds (PSCs) play important roles in stratospheric ozone depletion during winter and spring at high latitudes (e.g., the Antarctic ozone hole). PSC particles provide sites for heterogeneous reactions that convert stable chlorine reservoir species to radicals that destroy ozone catalytically. PSCs also prolong ozone depletion by delaying chlorine deactivation through the removal of gas‐phase HNO3and H2O by sedimentation of large nitric acid trihydrate (NAT) and ice particles. Contemporary observations by the spaceborne instruments Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Microwave Limb Sounder (MLS), and Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) have provided an unprecedented polar vortex‐wide climatological view of PSC occurrence and composition in both hemispheres. These data have spurred advances in our understanding of PSC formation and related dynamical processes, especially the firm evidence of widespread heterogeneous nucleation of both NAT and ice PSC particles, perhaps on nuclei of meteoritic origin. Heterogeneous chlorine activation appears to be well understood. Reaction coefficients on/in liquid droplets have been measured accurately, and while uncertainties remain for reactions on solid NAT and ice particles, they are considered relatively unimportant since under most conditions chlorine activation occurs on/in liquid droplets. There have been notable advances in the ability of chemical transport and chemistry‐climate models to reproduce PSC temporal/spatial distributions and composition observed from space. Continued spaceborne PSC observations will facilitate further improvements in the representation of PSC processes in global models and enable more accurate projections of the evolution of polar ozone and the global ozone layer as climate changes. Polar stratospheric clouds (PSCs) occur during winter and early spring in the polar stratosphere, when temperatures are low enough to enable cloud formation despite the extremely dry conditions. Ground‐based PSC sightings date back to the late 19th century, but they were little more than a scientific curiosity until the discovery of the Antarctic ozone hole in 1985. Soon thereafter, it was shown that PSCs play a crucial role in converting stable halogen (mainly chlorine) species of anthropogenic origin into reactive gases that rapidly destroy ozone. Considerable progress was made over the next two decades in quantifying these processes through laboratory studies, field campaigns, and limited spaceborne observations. We are now reaping the benefits of new PSC observations over the entire polar regions from three complementary 21st century spaceborne instruments. This study reviews these instruments and highlights new findings on PSC occurrence and composition. These datasets have also triggered advances in understanding how PSCs form and the influence of atmospheric dynamics, as well as improvements in how detailed cloud processes are approximated in global models. This will ultimately lead to better predictions of how quickly the stratospheric ozone layer will recover from human influence as global climate changes in the future. We provide a new vortex‐wide climatology of polar stratospheric cloud occurrence and composition based on 21st century satellite dataWe review advances in understanding cloud formation, the role of dynamical processes, and heterogeneous chlorine activationWe highlight improvements in techniques for parameterizing polar stratospheric clouds and their effects in global models We provide a new vortex‐wide climatology of polar stratospheric cloud occurrence and composition based on 21st century satellite data We review advances in understanding cloud formation, the role of dynamical processes, and heterogeneous chlorine activation We highlight improvements in techniques for parameterizing polar stratospheric clouds and their effects in global models

In the winter and spring of 2019/2020, the unusually cold, strong, and stable polar vortex created favorable conditions for ozone depletion in the Arctic. Chemical ozone loss started earlier than in any previous year in the satellite era and continued until late March, resulting in the unprecedented reduction of the ozone column. The vortex was located above the Polar Environment Atmospheric Research Laboratory in Eureka, Canada (80°N, 86°W) from late February to the end of April, presenting an excellent opportunity to examine ozone loss from a single ground station. Measurements from a suite of instruments show that total column ozone was at an all‐time low in the 20‐year data set, 22–102 DU below previous records set in 2011. Ozone minima (<200 DU), enhanced OClO and BrO slant columns, and unusually low‐HCl, ClONO2, and HNO3columns were observed in March. Polar stratospheric clouds were present as late as 20 March, and ozonesondes show unprecedented depletion in the March and April profiles (to <0.2 ppmv). While both chemical and dynamical factors lead to reduced ozone when the vortex is cold, the contribution of chemical depletion (based on the variable correlation of ozone and temperature) was exceptional in spring 2020 when compared to typical Arctic winters. Mean chemical ozone loss over Eureka was estimated to be 111–126 DU (27%–31%) using April measurements and passive ozone from the SLIMCAT chemical transport model. While absolute ozone loss was generally smaller in 2020 than in 2011, percentage ozone loss was greater in 2020. While an ozone hole forms over Antarctica every year, the Arctic typically does not experience such dramatic ozone loss. The chlorine and bromine (halogen) reactions that destroy ozone require very low temperatures that are rarely observed in the Arctic stratosphere. The winter and spring of 2019/2020, however, was unusually cold in the Arctic, and consequently, a large amount of ozone was destroyed by halogen chemistry. To understand the behavior of ozone in spring 2020, we use measurements from the Polar Environment Atmospheric Research Laboratory in Eureka, Canada. Eureka (at 80°N) is one of the northernmost research stations in the world, and thus an ideal location to observe ozone loss. Spring 2020 ozone minima were lower than any in the 20‐year data set, and ozone destruction was ongoing until the end of March, which is rare in the Arctic. While ozone concentrations are largely determined by circulation patterns in the Arctic stratosphere, chemistry in spring 2020 was a much larger factor than usual. Halogen chemistry destroyed 27%–31% of the total ozone, compared to about 10% in a typical winter. The only year on record with comparable ozone loss is 2011, and a larger percentage of the ozone column was lost in 2020. Record low ozone columns (<200 DU) were observed over Eureka in spring 2020Limited dynamical resupply of ozone and chemical destruction both contributed to reduced ozone columnsMean chemical ozone loss of 111–126 DU (27%–31%) represents similar absolute loss and greater relative loss compared to that in spring 2011 Record low ozone columns (<200 DU) were observed over Eureka in spring 2020 Limited dynamical resupply of ozone and chemical destruction both contributed to reduced ozone columns Mean chemical ozone loss of 111–126 DU (27%–31%) represents similar absolute loss and greater relative loss compared to that in spring 2011

The phase out of anthropogenic ozone‐depleting substances such as chlorofluorocarbons under the terms of the Montreal Protocol led to the development and worldwide use of hydrofluorocarbons (HFCs) in refrigeration, air conditioning, and as blowing agents and propellants. Consequently, over recent years, the atmospheric abundances of HFCs have dramatically increased. HFCs are powerful greenhouse gases and are now controlled under the terms of the 2016 Kigali Amendment to the Montreal Protocol. HFC‐134a is currently the most abundant HFC in the atmosphere, breaking the 100 ppt barrier in 2018, and can be measured in the Earth's atmosphere by the satellite remote‐sensing instrument ACE‐FTS (Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer), which has been measuring since 2004. This work uses the ACE‐FTS v4.0 data product to investigate global distributions and trends of HFC‐134a. These measurements are compared with a simulation of SLIMCAT, a state‐of‐the‐art three‐dimensional chemical transport model, which is constrained by global surface HFC‐134a measurements. The agreement between observation and model is good, although in the tropical troposphere ACE‐FTS measurements are biased low by up to 10–15 ppt. The overall ACE‐FTS global trend of HFC‐134a for the altitude range 5.5–24.5 km and 2004–2018 time period is approximately linear with a value of 4.49 ± 0.02 ppt/year, slightly lower than the corresponding SLIMCAT trend of 4.66 ppt/year. Using a simple box model, we also estimate the annual global emissions and burdens of HFC‐134a from the model data, indicating that emissions of HFC‐134a have increased almost linearly, reaching 236 Gg by 2018. First comparison between long‐term satellite measurements (ACE‐FTS) of atmospheric HFC‐134a and a 3D chemical transport model (SLIMCAT)Observations agree well with SLIMCAT, although ACE‐FTS is biased low by up to 10–15 ppt in the troposphere between 30°S and 30°NGlobal trends are linear to a good approximation: 4.49 ppt/year for ACE‐FTS (2004–2018; 5.5–24.5 km) and 4.66 ppt/year for SLIMCAT First comparison between long‐term satellite measurements (ACE‐FTS) of atmospheric HFC‐134a and a 3D chemical transport model (SLIMCAT) Observations agree well with SLIMCAT, although ACE‐FTS is biased low by up to 10–15 ppt in the troposphere between 30°S and 30°N Global trends are linear to a good approximation: 4.49 ppt/year for ACE‐FTS (2004–2018; 5.5–24.5 km) and 4.66 ppt/year for SLIMCAT

Vegetation growth in northern high‐latitudes during springtime is strongly temperature limited, and thus anomalously warm springs are expected to result in an increased drawdown of carbon dioxide (CO2). However, a recent analysis of the relationship between spring temperature anomalies and atmospheric CO2anomalies at Point Barrow, Alaska, suggests that the link between spring carbon uptake by northern ecosystems and temperature anomalies has been weakening over recent decades due to a diminishing control of temperature on plant productivity. Upon further analysis, covering the 1982–2015 period, we found no significant change in the relationship between spring vegetation productivity derived from remote sensing data and air temperature. We showed that a reduction in spatial coherence of temperature anomalies, alongside a significant sensitivity to atmospheric transport, is likely responsible for the apparent weakening. Our results, therefore, suggest that spring temperature remains as an important control of northern high‐latitude CO2uptake. The timing of the vegetation growing season is strongly linked to the quantity of carbon dioxide (CO2) absorbed each year, however, it is unclear how this will evolve. The spring zero crossing (SZC) of atmospheric CO2is an indicator of how early the growing season starts. This is because the growing season is characterized by a significant photosynthetic drawdown of CO2by vegetation. This quantity has historically been strongly controlled by temperature in northern high‐latitude ecosystems. A recent study analyzed surface CO2data at a site in Alaska and suggested that this control has broken down. We show here, using satellite‐based vegetation data, that there has been no change in the control of temperature on vegetation productivity across the high northern latitudes. We then demonstrate that the temperature anomalies became more spatially variable across the northern high latitudes in recent decades and that variations in atmospheric circulation lead to different regions of influence over the Alaskan CO2site. These influences play a key role in the year‐to‐year variability of the SZC and its relationship with temperature. Our study demonstrates that the control of temperature on the year‐to‐year variations in the timing and magnitude of spring CO2uptake has remained strong. No significant change in the overall temperature sensitivity of high northern latitude spring greenness in the past 40 yearsDecreased spatial coherence of spring temperature anomalies has increased the influence of atmospheric transport on surface CO2dataTemperature sensitivity of spring carbon uptake remains strong when accounting for atmospheric transport No significant change in the overall temperature sensitivity of high northern latitude spring greenness in the past 40 years Decreased spatial coherence of spring temperature anomalies has increased the influence of atmospheric transport on surface CO2data Temperature sensitivity of spring carbon uptake remains strong when accounting for atmospheric transport

Satellite observations of relevant trace gases and meteorological data from ERA5 were used to describe the dynamics and chemistry of the spectacular Arctic 2019/20 winter/spring season. Exceptionally low total ozone values of slightly less than 220 DU were observed in mid‐March within an unusually large stratospheric polar vortex. Very high OClO and very low NO2column amounts observed by GOME‐2A were indicative of unusually large active chlorine levels and significant denitrification, which likely contributed to large chemical ozone loss. Using results from the TOMCAT chemical transport model (CTM) and ozone observations from S5P/TROPOMI, GOME‐2 (total column), SCIAMACHY, and OMPS‐LP (vertical profiles), chemical ozone loss was evaluated and compared with the previous record Arctic winter 2010/11. The polar‐vortex‐averaged total column ozone loss in 2019/20 reached 88 DU (23%) and 106 DU (28%) based upon observations and model, respectively, by the end of March, which was similar to that derived for 2010/11. The maximum ozone loss (∼80%) observed by OMPS‐LP was near the 450 K potential temperature level (∼18 km altitude). Because of the larger polar vortex area in March 2020 compared to March 2011 (about 25% at 450 K), ozone mass loss was larger in Arctic winter 2019/20. It is shown that Arctic cap temperatures were at a record 40‐year low in winter, suggesting that Arctic cold winters are getting colder. Tropical UTLS temperatures show an apparent upward trend during the last two decades that may indicate a weakening of the Brewer‐Dobson circulation and strengthening of the polar vortex. Very large OClO and very low NO2slant columns were observed by GOME‐2A during Arctic winter 2019/20Chemical total column ozone loss of 88 DU and 106 DU was derived from TROPOMI satellite observations and the chemical transport modelChemical ozone loss derived from OMPS‐LP satellite data reached 2.1 ppmv (80%) near the 450 K potential temperature level (∼18 km) Very large OClO and very low NO2slant columns were observed by GOME‐2A during Arctic winter 2019/20 Chemical total column ozone loss of 88 DU and 106 DU was derived from TROPOMI satellite observations and the chemical transport model Chemical ozone loss derived from OMPS‐LP satellite data reached 2.1 ppmv (80%) near the 450 K potential temperature level (∼18 km)

We use a three‐dimensional chemical transport model and satellite observations to investigate Arctic ozone depletion in winter/spring 2019/20 and compare with earlier years. Persistently, low temperatures caused extensive chlorine activation through to March. March‐mean polar‐cap‐mean modeled chemical column ozone loss reached 78 DU (local maximum loss of ∼108 DU in the vortex), similar to that in 2011. However, weak dynamical replenishment of only 59 DU from December to March was key to producing very low (<220 DU) column ozone values. The only other winter to exhibit such weak transport in the past 20 years was 2010/11, so this process is fundamental to causing such low ozone values. A model simulation with peak observed stratospheric total chlorine and bromine loading (from the mid‐1990s) shows that gradual recovery of the ozone layer over the past 2 decades ameliorated the polar cap ozone depletion in March 2020 by ∼20 DU. Ozone depletion in the polar stratosphere is caused by chlorine and bromine species which are activated by low temperatures. Chlorine and bromine are transported to the stratosphere following the surface emission of ozone‐depleting substances (ODSs). While springtime ozone depletion in the Antarctic is almost always large, it is much more variable in the Arctic due to warmer temperatures and more disturbed stratospheric dynamics. Using a three‐dimensional atmospheric chemical transport model and satellite observations, we show that the very low ozone columns observed in March 2020 were a consequence of large chemical destruction and weaker‐than‐normal replenishment by transport. These very low ozone levels are, by some measures, record values despite 2 decades of decreasing stratospheric chlorine and bromine through controls of the Montreal Protocol. Had the meteorology of 2019/20 occurred 2 decades ago, the ozone loss would have been notably larger. The Arctic stratospheric dynamics for 2019/20 are extreme relative to the past 2 decades but fit a compact relationship that links column ozone variations over Arctic and Antarctic winters. Large mean Arctic (>63°N) chemical ozone destruction in 2019/20 of 78 DU, similar to other extreme cold winters in the past 2 decadesAnomalously weak wintertime dynamical replenishment of only ∼60 DU contributed strongly to the very low observed ozone column in MarchOzone recovery caused 20 DU less mean Arctic ozone loss in March 2020 than would have occurred with stratospheric halogens at 1995 levels Large mean Arctic (>63°N) chemical ozone destruction in 2019/20 of 78 DU, similar to other extreme cold winters in the past 2 decades Anomalously weak wintertime dynamical replenishment of only ∼60 DU contributed strongly to the very low observed ozone column in March Ozone recovery caused 20 DU less mean Arctic ozone loss in March 2020 than would have occurred with stratospheric halogens at 1995 levels

Dichloromethane (CH2Cl2) and perchloroethylene (C2Cl4) are chlorinated very short lived substances (Cl‐VSLS) with anthropogenic sources. Recent studies highlight the increasing influence of such compounds, particularly CH2Cl2, on the stratospheric chlorine budget and therefore on ozone depletion. Here, a multiyear global‐scale synthesis inversion was performed to optimize CH2Cl2(2006–2017) and C2Cl4(2007–2017) emissions. The approach combines long‐term surface observations from global monitoring networks, output from a three‐dimensional chemical transport model (TOMCAT), and novel bottom‐up information on prior industry emissions. Our posterior results show an increase in global CH2Cl2emissions from 637 ± 36 Gg yr−1in 2006 to 1,171 ± 45 Gg yr−1in 2017, with Asian emissions accounting for 68% and 89% of these totals, respectively. In absolute terms, Asian CH2Cl2emissions increased annually by 51 Gg yr−1over the study period, while European and North American emissions declined, indicating a continental‐scale shift in emission distribution since the mid‐2000s. For C2Cl4, we estimate a decrease in global emissions from 141 ± 14 Gg yr−1in 2007 to 106 ± 12 Gg yr−1in 2017. The time‐varying posterior emissions offer significant improvements over the prior. Utilizing the posterior emissions leads to modeled tropospheric CH2Cl2and C2Cl4abundances and trends in good agreement to those observed (including independent observations to the inversion). A shorter C2Cl4lifetime, from including an uncertain Cl sink, leads to larger global C2Cl4emissions by a factor of ~1.5, which in some places improves model‐measurement agreement. The sensitivity of our findings to assumptions in the inversion procedure, including CH2Cl2oceanic emissions, is discussed. The 1987 Montreal Protocol banned production for dispersive uses of major ozone‐depleting gases, such as chlorofluorocarbons, due to their role in depletion of the stratospheric ozone layer. In consequence, the ozone layer is expected to recover in coming decades, as stratospheric chlorine from banned substances slowly declines. However, chlorinated very short lived substances (Cl‐VSLS), not controlled by the Montreal Protocol, represent a small, but growing, source of atmospheric chlorine that could potentially slow ozone recovery. It is thus important that the magnitude of emissions of these compounds, their spatial distribution, and changes with time are quantified. Here, we combined observations of Cl‐VSLS, prior estimates of their emissions, and a chemical transport model to produce an optimized set of emission estimates on a region‐by‐region basis between 2006 and 2017. We show that industrial emissions of dichloromethane, the most abundant Cl‐VSLS, increased by ~84% within this period, predominately due to an increase in Asian emissions, while European and North American emissions decreased. Over 2007–2017, emissions of perchloroethylene, a less abundant Cl‐VSLS, decreased, particularly in Europe and North America. We show that our new emission estimates lead to better agreement with observational data compared to previous estimates. Global CH2Cl2emissions increased by ~85% between 2006 and 2017, mostly due to increasing emissions from AsiaGlobal C2Cl4emissions decreased in the same period by ~25%, mainly due to reduced emissions from Europe and North AmericaPosterior CH2Cl2and C2Cl4emissions provide good agreement with surface and aircraft observational data

The decline in Amazonian deforestation rates and biomass burning activity (2001–2012) has been shown to reduce air pollutant emissions (e.g., aerosols) and improve regional air quality. However, in the Cerrado region (savannah grasslands in northeastern Brazil), satellite observations reveal increases in fire activity and tropospheric column nitrogen dioxide (an ozone precursor) during the burning season (August‐October, 2005–2016), which have partially offset these air quality benefits. Simulations from a 3‐D global chemistry transport model (CTM) capture this increase in NO2with a surface increase of ~1 ppbv per decade. As there are limited long‐term observational tropospheric ozone records, we utilize the well‐evaluated CTM to investigate changes in ozone. Here, the CTM suggests that Cerrado region surface ozone is increasing by ~10 ppbv per decade. If left unmitigated, these positive fire‐sourced ozone trends will substantially increase the regional health risks and impacts from expected future enhancements in South American biomass burning activity under climate change. Savannah‐type fires in the Cerrado, northeastern Brazil, have increased emissions of ozone precursor gases degrading air quality

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH3CCl3) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom‐up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long‐lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH‐SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long‐term trend and emissions derived from the measured hemispheric gradient, the combination of HFC‐32 (CH2F2), HFC‐134a (CH2FCF3, HFC‐152a (CH3CHF2), and HCFC‐22 (CHClF2), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF. The interhemispheric mixing ratio difference of a long‐lived gas can be used as a quantitative proxy to derive global emissionsThe combination of multiple species in the gradient trend‐based approach should reduce the uncertainties in global OH abundance estimateHFC‐32 and HFC‐134a, with the help of HFC‐152a and HCFC‐22, are likely the best CH3CCl3alternatives as atmospheric OH reference gases