Editor's choice 

Bromine chemistry in polar regions is important for the composition of the atmosphere as well as climate. Reactive bromine strongly affects the oxidation capacity and the local ozone budget, and through the export to lower latitudes, it affects the ozone budget and the atmosphere's radiative properties outside polar regions. The newly identified bromine reservoir changes our understanding of the chemical budgets of polar halogens which will have implications for the ozone and mercury removal cycles.

The Royal Society report on the Krakatoa eruption included some marvelous paintings of extraordinary and highly unusual green sunsets. This article provides a novel explanation employing a detailed physical model that emphasizes the necessity for large sizes and amounts of aerosols.

The field of climate prediction has been bedeviled by the problem of how to represent the enormous complexity of clouds. The usual strategy is to peform deterministic simulations with advanced cloud models. The study outlined here concentrates on a statistical approach that is arguably better suited to determining the mean climatological state. The presented observations from a wide range of satellite platforms show that a power-law well describes frequencies of occurence of cloud sizes across a very wide range of scales, and that the exponent is robust to local climatological characteristics as surface temperature, aerosol loading, Coriolis forces, or dominant cloud type. Instead, the distribution of cloud sizes emerge simply from a competition for energy and air that occurs due to small-scale cloud mixing processes at cloud edge.

Black carbon is a key source of uncertainty in regional climate predictions through aerosol-radiation interactions, cloud modifications and enhanced snow melt, and the arctic is particularly sensitive to these effects. Understanding the influence of continental emissions on arctic aerosols is crucial in earth system science, and this influence can be expected to evolve with changes to the atmospheric circulation in response to climate change. This paper uses a machine learning approach to study the factors controlling observations of black carbon in the arctic and quantitatively links these to meteorological processes and trends. This phenomenological assessment will facilitate predictions in the long range transport of black carbon transport under various climate change scenarios.

This paper presents a timely and topical research. It directly compares the marine cloud brightening (MCB) with the stratospheric aerosol injection (SAI) effects by utilizing the fully coupled atmosphere-ocean simulations. The study also reveals some new side effects by the MCB geoengineering, such as the locking of the climate into a permanent La Nina state and an increase in sea-level over the south Pacific Ocean. Those effects should be taken into account when developing geoengineering plans.

Giant CCN have long been recognised as highly important in warm marine clouds, as while these are low in number, they often dictate precipitation rates and thus many climate-important properties such as cloud optical thickness and lifetime. However, measuring these particles is remains challenging on a technical level and many models of their production are poorly constrained. This paper presents the results using a new methodology and goes on to explore the role of coastlines in enhancing wave breaking and thus giant CCN production.

Stratospheric ozone is important for the energy budget of the planetary atmosphere and for protecting life on Earth from harmful solar UV radiation. An unexpected decline in ozone in the extratropical lower stratosphere over the past two decades is therefore worrying and contrary to the Montreal Protocol's goal of ozone recovery. The study by Dube et al. adds important information to the ongoing discussion of this trend, which is being investigated internationally in activities such as SPARC/LOTUS, SPARC/OCTAVE-UTLS or the WMO ozone assessments. By their approach to use N2O as a tracer of stratospheric transport, the authors are able to separate ozone decrease due to circulation changes from a decrease caused by other, not further detailed reasons. They find that the latitudinal and altitudinal region where the ozone decrease cannot be explained by circulation changes is restricted to altitudes below 30hPa in the Northern hemisphere. They suggest a possible cause for the remaining ozone loss, namely a change in the tropopause altitude, thus narrowing successfully the search for processes leading to the observed ozone loss.

One of the largest sources of uncertainty in the overall anthropogenic forcing of climate is still the aerosol impact on liquid clouds. Disentangling the various aerosol-cloud interactions helps to improve estimates of the magnitude of global warming in the future. The current study provides the most rigorous method to date in assessing the aerosol radiative effects from satellite observations across the global ocean. The aerosol responses are decomposed into the Twomey effect (cooling due to an increase in cloud-droplet number concentration), and the adjustments of the cloud liquid water path and cloud fraction (often analysed separately) at a near-global scale. The total effective radiative forcing of liquid clouds since 1850 has been found to be negative, with the cloud adjustments larger than the Twomey effect, which was previously thought to be larger.

I agree with the handling editor that this paper shows an important aspect of heterogeneous ice nucleation in water under capillary tension. The results can be of general interest to a broad geoscience community.

The Amazon's role in the tropical and global carbon cycle is highly significant. Usually considered as the "lung of the planet", it is mandatory to monitor if this role is kept, or if the large rainforest even turns into a source of carbon dioxide. The study by Basso et al. finds that during the analysed years, from 2010 to 2018, the Amazon is a small net source of carbon to the atmosphere. They find that fire is the primary driver of the Amazonian source, while drought years intensify the carbon emissions. The study also examined the contributions of different regions to the Amazonian carbon budget and found that emissions in the eastern Amazon were greater than those in the western region, primarily due to fires. These findings are of high relevance - and concern - to the larger geosciences community and indicate how important it is to stop slash-and-burn in the large rainforests.

The international Montreal Protocol was signed in 1987 in order to protect the atmospheric ozone layer by phasing out the production of halogenated hydrocarbons that deplete stratospheric ozone. The protocol was successfully implemented and, over the years, amendments and adjustments of the protocol were essential to its success. Ultimately, the protocol has resulted in a reduced halogen loading of the atmosphere since the mid-1990s. Trifluoromethane (HFC-23) is one of the substances regulated by the Montreal protocol since the Kigali amendment in 2016. HFC-23 does not deplete stratospheric ozone but is a very potent greenhouse gas. Commitments were made to reduce emissions of HFC-23 during the production of HCFC-22 as part of agreements in the protocol. However, the data presented and analysed in this paper indicate that in China more than the agreed amount of HFC-23 has been emitted since 2015, resulting either from unsuccessful factory-level HFC-23 abatement and/or inaccurate quantification of emission reductions. The analysis provides valuable data of atmospheric HFC-23. The study is also a good example of how compliance with the Montreal Protocol can be monitored.

Methane is a potent greenhouse gas with a global warming potential much greater than carbon dioxide. In order to understand and limit its global warming effect, it is important to have have observation systems to estimate its anthropogenic emissions. This paper analyses in a novel way TROPOMI satellite observations to track methane emissions from the largest oil production basin in the United States over a 2-year period. The analysis shows that emission variability and trends are driven by multiple factors, among which new well development and natural gas spot price are the most significant ones. The work is an excellent demonstration of the potential of satellite observations for near-real-time monitoring of methane emissions. It opens a broad range of applications, helping science and policy to understand and mitigate climate change.

Large volcanic eruptions can have significant effects on the atmosphere and on climate. Comparing the observed effects against model predictions is also a very valuable test of our scientific understanding of how the atmosphere works. This paper describes a model investigation of a hypothetical eruption, about twice the size of that of Pinatubo in 1991, and is unusual that it focuses on the response in the mesosphere -- the part of the atmosphere in the 50-80km altitude range. The eruption deposits aerosol in the tropics in the 20-25km altitude range, in the lower stratosphere and well below the mesosphere. The effect is felt in the mesosphere through dynamical coupling communicated through small-scale gravity waves, which transport momentum and as a result can drive changes in winds and temperatures. The predictions made in the paper will be helpful in future in interpreting changes in the mesosphere observed following volcanic eruptions.

Polar Stratospheric Clouds (PSCs) catalyze ozone depletion at high latitudes with important impacts on energetic, biologically effective UV radiation. The present study is a unique experiment into PSC particle optical properties and formation performed in a large, coolable cloud chamber. Robert Wagner and his colleagues find evidence that a nitric acid dihydrate can be efficiently crystallized in PSCs through heterogeneous nucleation, for example by mineral dust or micrometeorites. It has a "beta-polymorph" crystalline structure - with distinct optical properties - clearly separable from an alpha-polymorph that forms at low temperatures through homogeneous nucleation. Interestingly, electron microscopy, infrared spectroscopy, X-ray diffraction and calculations all point to the same conclusion. The results have important implications for our understanding and the detection of PSCs.

After a period of near-zero growth of atmospheric methane, a major greenhouse gas, its growth rates have increased, with values in 2020 and 2021 exceeding all prior values since the beginning of systematic measurements in 1983. This recent acceleration, particularly during the covid-19 years 2020 and 2021, raises the question whether the methane increase was due to stronger sources or reduced sinks in the troposphere. A reduction of the tropospheric abundance of the hydroxyl radical (OH), the reaction of which is the main tropospheric methane sink, could be plausibly explained by global-scale reductions in nitrogen oxides due to pandemic-related industry shutdowns. Using an inversion scheme, Feng et al. demonstrate that such a reduction only accounts for about 34% in 2020 and 10% in 2021 of the observed methane rise. Instead, the authors attribute increased methane emissions to hydrological anomalies and microbial sources over the tropics, i.e., Eastern Africa and tropical South America, and temperate North America. The study demonstrates the importance of simultaneously accounting for changes in methane emissions and sinks for an improved quantitative understanding of the evolution of the methane concentrations to assess the role of greenhouse gases in climate change and tropospheric composition.

In the last decade it was discovered that autoxidation of monoterpenes produces highly oxidised organic molecules (HOM) in the atmosphere. These have low volatility and produce secondary organic aerosols that are relevant to climate and human health. Autoxidation involves organic peroxy radicals which undergo one or more intramolecular H-shifts with subsequent O2 addition leading to the formation of HOMs. The experimental and theoretical elucidation of the mechanism is challenging due to the large number and isomerism of possible intermediates and their numerous reaction pathways. In the present study, selective isotope labeling was combined with high-resolution mass spectrometry to greatly enhance the possibilities to identify relevant reaction pathways. Selective isotopic labeling of organic molecules is a powerful method for studying reaction mechanisms, and it is currently underutilized in the community. This is a great example of using this method, hopefully paving the way to more studies of this sort to come.

This paper investigates the influence of internationally-transported pollution on China, with a specific highlight on the formation of secondary PM2.5 in the form of nitrate. While sources from within China have traditionally been of most interest for domestic air quality policy, these have diminished over recent years, so sources from outside China may become more significant. The topic of transboundary exchange of air pollution has long been studied in other parts of the world, in particular among CLRTAP signatory countries in North America and Europe, but East Asian transboundary pollution represents a different challenge, in part owing to differences in geography and emissions, but also compared to Europe in particular, the transportation scales are much larger. This work not only quantifies the impacts of long distance pollution on Chinese air quality, but also highlights the complex chemical interactions between the local and transboundary pollutants. Papers such as this will likely influence the debate regarding international controls of air pollutants.

Wildfires enhance surface PM2.5 concentration and thus adversely affect air quality and human health. Based on online-coupled fire-climate-ecosystem model simulations, this paper projects a nearly 2-fold increase in wildfire-induced summer-mean surface PM2.5 by the mid-21st century over North America. The projected enhancement is substantial even in the less-densely forested eastern US - as already manifested in the poor air quality as observed in June 2023 due to Canadian wildfires -, which is attributed to the transport of smoke from North America as well as positive climatic feedback of smoke on PM2.5. Thus, future regulatory controls on PM2.5 in North America, particularly in the eastern US where anthropogenic emissions are falling, should consider the effect of future increases in wildfires.

Stratospheric aerosol injection (SAI) is often discussed in the media and in policy circles as a possible action to limit future increase in global temperatures. Indeed it has been demonstrated in model simulations that in principle injection could be 'controlled', using model information, to meet specific targets on the temperature increase and its spatial distribution. This paper shows that the simulated climate response to SAI is strongly model-dependent, reflecting fundamental uncertainties in model representation of key processes. In particular this means that the SAI determined by the control algorithms as those required to achieve temperature targets different significantly from one model to another. Specific mechanisms, in particular the difference in rapid response in clouds and in precipitation to an imposed radiative perturbation and the ensuing ocean circulation response, are identified that contribute to the strong differences in model response to SAI. There is also a strong sensitivity to the pre-existing sulphate distribution which will be determined by future anthropogenic emissions. The authors note that these inter-model differences are unlikely to be resolved quickly and that controlled SAI, to achieve specific temperature goals and with well-quantified risks of unexpected consequences, is likely to remain out of reach for many years.

This paper is a significant advancement in the understanding of the chemistry of dimethyl sulfide (DMS) and its oxidation products in the atmosphere. DMS is mainly emitted by marine phytoplankton and forms a relevant natural source of non-sea salt sulfate aerosols which plays an important role in global aerosol climate effects. However, the chemistry by which DMS oxidizes to form sulfate aerosols is highly complex. This work represents a next important step in the understanding of chemical processes within the reaction of OH with DMS. The results of this study allow to include a better, because less simplified, inclusion of DMS chemistry in large-scale models which can reduce errors in predicted aerosol radiative effects in past, present and future atmospheres.

Methane is a potent greenhouse gas and an important sink of atmospheric OH radicals, which determine the global atmospheric oxidation capacity. CH4 has increased since pre-industrial times until the year 2000, after which its global concentration has remained relatively stable. Since 2007, it is rapidly increasing again for reasons that are not well understood. The current paper analyses source specific methane emissions that are likely responsible for the recent increase. Global CH4 sources are determined using variational inversion based on measurements of methane and its isotope signatures (δ13C). This latter provide better constraints on the contributions of microbial and fossil fuel CH4 sources than the concentrations alone. The analysis points to a more strongly increasing contribution of microbial sources since 2007 than predicted by previous assessments (Global Carbon Project). These findings have the potential to lead to a major reassessment of the global methane budget.

This article describes the effect of the recent (January 2022) Hunga Tonga-Hunga Ha’apai volcanic eruption on the stratosphere. The eruption was highly energetic and as a result erupted material reached altitudes of around 30km. Such eruptions, with the 1991 Pinatubo eruption being a noteworthy example, often have a significant effect on tropospheric weather and climate, through the radiative effects of the volcanic aerosol, which may remain in the stratosphere for 2 or 3 years or more. In the 30 years since the Pinatubo eruption observations of the stratosphere, primarily from satellites, have improved enormously and in this Letter the authors provide a detailed description of the evolution of volcanic aerosol and of other chemical species injected by the eruption over a 6-month period following the eruption. The authors show that one important effect of the eruption was to inject a large quantity of water vapour into the stratosphere and suggest that the largest impact of the eruption on tropospheric weather and climate will be via the radiative effect of this water vapour, rather than of the injected aerosol. The initial detailed picture of the impact of the Hunga Tonga-Hunga Ha’apai eruption on the stratosphere provided in this Letter will stimulate further study of this remarkable natural event, which provides a rare opportunity to test our scientific understanding.

Emissions from conventional aircraft contribute to climate change by forming contrails and by increasing atmospheric CO2 concentrations. To fly faster and reduce the climate impact, super- or hypersonic aircraft fuelled by liquid hydrogen or natural gas are being considered. Hypersonic aircraft would fly at more than Mach 4 in the stratosphere, up to 35 km altitude, where the peak of the ozone layer resides. The paper by Pletzer et al. presents a thorough study of the chemical and radiative impacts of such high-speed aircraft using two chemistry-climate models. The study shows that hypersonic aircraft fuelled by liquid hydrogen and cruising at such altitudes would contribute to a significant global warming although they do not emit CO2. The main radiative effect comes from additional water vapour, with only a small effect from depletion of the ozone layer. Importantly, the authors discovered that although water vapour is destroyed in the stratosphere, perturbation of local photochemistry also creates water vapour. The authors estimate that the mean surface temperature change caused by a hypersonic transport fleet would be roughly 8-20 times larger than for a subsonic reference aircraft with the same transport volume. This comprehensive study provides convincing calculations for a large climatic effect of any future hydrogen-fuelled hypersonic aircraft fleet.

Large quantities of seasonal smoke are produced by agricultural burning in Southern Africa between the months of June and October. Between 2016 and 2018A a series of large field campaigns, ORACLES, CLARIFY, and LASIC, targeted study of the atmospheric impacts of these plumes. This study synthesizes these measurements with numerical simulations to investigate how biomass burning plumes blown westward affect a well-known transition from solid stratocumulus to broken cumulus over the Atlantic Ocean. The dynamics of the cloud transition are complex, and there are many possible ways that aerosols and clouds can interact. This study is particularly notable for considering not just how smoke particles directly modify the microphysical properties of clouds through interactions in the atmospheric boundary layer, but also how they impact clouds indirectly by absorbing solar radiation above the cloud deck. A surprising finding is that microphysical interactions have only a minimal impact on cloud transitions. Instead, the breakup of stratocumulus clouds decks is substantially slowed by the absorption of sunlight by smoke plumes above clouds and its subsequent impact on the vertical temperature and moisture profile of the atmosphere.

Banks of ozone-depleting halogenated carbon compounds continue to be sources of emissions. Their contribution to current and future emissions, however, is highly uncertain, and this uncertainty obscures ongoing emissions attribution and undermines international efforts to evaluate global compliance with the Montreal Protocol. The paper by Lickley et al. presents a convincing and thorough method to assess the banks of several halocarbons and their emissions based on known atmospheric lifetimes and constrained by observed atmospheric concentrations. The authors demonstrate that the banks of the halocarbons under assessment are very likely larger than previous international assessments suggest, and that total production has been very likely higher than reported. These are most important results that will find their way in the next WMO ozone assessment and have to be considered in assessments of global warming.

Methane is a greenhouse gas that significantly contributes to global warming. Its sources are not well constrained as many point sources are missing in emission inventories that are built based on bottom-up approaches. Emissions include sources caused by human activities (oil/gas, lifestock) but also natural ones, e.g. wetlands. The current paper fills this gap by comprehensively reviewing the capabilities of current and forthcoming satellites as powerful top-down tools to observe atmospheric methane and quantify emissions. Their most important application is to quantify anthropogenic methane sources , where there is substantial interest in identifying hot spots to reduce emissions, closing the methane budget, and to ensure compliance with international climate agreements. This paper is of broad interest for the geoscience community, as it not only presents an overview of the existing discrepancies in the atmospheric methane budget and emissions but also addresses the difficulties in defining its emission inventories on various spatial scales.

Tropical convective clouds distribute water vapour across the tropical tropopause layer (TTL), which affects the radiative budget in both the troposphere below and the stratosphere above. Water isotopes can be used as tracers to conclude on the history of water vapour in these altitudes, i.e. whether it went through phase change processes and was part of ice or liquid clouds before. de Vries and coworkers used an innovative modelling scheme capable of simulating convection together with the phase-change processes producing the isotopic signatures to simulate tropical ice clouds and compare their simulations to respective observations. By this, they were able to disentangle processes of convective updraft versus cirrus cloud formation in tropical cloud systems and demonstrate how the isotopic fractionation of water vapour does help to distinguish respective processes. Over all, this paper is innovative and a considerable step forward in studies of convection and transport of water vapour into the upper tropospher/lower startosphere (UTLS).

Krüger et al. present observations of the effect of the COVID-19 lockdowns on black carbon emissions using a combination of airborne in situ observations and models. The impact of lockdowns on emissions from transport is by now well established, however the majority of studies to date have focused on NO2 as this is a regulated pollutant and high resolution observations are available in new satellite data products. This work instead uses a state-of-the-art airborne facility (DLR HALO) to report data on black carbon, which has established impacts both as a airborne pollutant damaging to human health, and as a highly potent short-lived climate forcing agent. Data from during the European lockdown period is compared with the prior EMeRGe campaign (https://acp.copernicus.org/articles/special_issue1074.html). The authors found a decrease in BC concentrations of 48% over Western and Southern Europe, of which 7% was likely due to meteorological differences during the two campaigns and 3-9% was due to a long-term downward trend, leading them to conclude that the lockdowns were overall responsible for a reduction in ambient concentrations of 32-38%. By seizing this unique opportunity, this study offers a new and unparalleled insight into the current nature of black carbon emissions from Europe. It also demonstrates how airborne in situ measurements and models can be used in combination to study the emission and transport of pollutants in the troposphere.

This paper by Drews et al. reports model simulations of the effect of the 11-year solar cycle on the atmospheric circulation and hence on year-to-year variations in weather patterns, . The physics of the effect of the solar cycle is complex, but one important mechanism is believed to be via the variation in short-wave radiation, which perturbs the ozone distribution in the upper stratosphere. The key development in this study is a good model representation of chemistry, radiation and dynamics and their interactions to enable the dynamical feedback processes, which  potentially communicate the direct physical effects of the solar cycle to the lower part of the atmosphere, to be adequately simulated. An important aspect of the paper is that the  authors exploit an ensemble of simulations that make it possible to distinguish a signal due to solar-cycle effects from natural weather variability. The results convincingly show a solar cycle effect, over the North Atlantic in particular, where variations in the circulation have important implications for the weather experienced in Europe. This is particularly the case in the current period (since 1950 or so) when the 11-year solar variation is strong relative to the entire historical record (starting in about 1850).  The results of this paper suggest that including solar cycle effects in models used for decadal climate predictions can provide worthwhile improvements in the skill of such predictions.

Wildfires have attracted increasing attention in recent years because of their effects on local air quality, as well as on regional and global climate. Ohneiser et al. documents with impressive accuracy the appearance of wildfire plumes in the stratosphere over Australia in 2020/2021, showing strong influence on stratospheric ozone. We recommend readers to read this paper together with Solomon et al. ("On the stratospheric chemistry of midlatitude wildfire smoke", PNAS 2022,https://doi.org/10.1073/pnas.2117325119). Both studies highlight the importance of wildfires for stratospheric chemistry and the recovery of the ozone layer in a warming climate.

Marine stratocumulus clouds are highly abundant, and they exert a significant cooling effect on the Earth's atmosphere. Improved understanding of their evolution and abundance has been targeted as critically important for constraining future climates. Droplets in these clouds, if sufficiently large and abundant, can collide to be converted into drizzle drops, acting as a sink for cloud moisture thereby limiting stratocumulus cloud lifetimes. Currently, the primary tool for ground-based detection of the presence of drizzle in stratocumulus is Doppler radar, although the longer wavelengths of the electromagnetic pulses generally limit drizzle detection to larger drops. The current study presents an innovative approach that extends radar detection to fine drizzle by using a machine-learning algorithm trained with aircraft in-situ measurements, identifying skewness in the Doppler radar signal as important for discriminating fine drizzle presence. Based on this method, the authors conclude that drizzle is far more common in marine stratocumulus clouds than previously thought, a result that represents a potentially major advance in our understanding of marine stratocumulus properties. Moreover, the study demonstrates the great potential of machine-learning for extending the capabilities of well-established atmospheric measurement techniques.