Strong reduction of sulfur emission from shipping since 2020 provides a rare opportunity to examine the response of climate system to anthropogenic forcing. Using a global climate model, this study estimates a global aerosol effective radiative forcing of 0.13 W m-2 from ship emission reduction. This emission reduction leads to a global mean warming of 0.04 K in 2020-2049 with larger warming at regional scales. The warming may not be evident at present day because of the climate variability, but can represent a significant fraction (17%) of the remaining warming to 1.5 K target.

The explosive Hunga Tonga eruption in 2022 had several characteristics that are unique in the recent observational record. Its effects on the atmosphere are of great scientific interest and have included rapidly propagating patterns of Lamb and gravity waves and long-lived anomalies in stratospheric water vapour. This modelling study demonstrates how, through a coupled sequence of chemical, radiative and dynamical mechanisms, the additional water vapour in the tropical upper stratosphere is likely to have had a significant effect on the wintertime circulation of the stratosphere and hence the troposphere in Northern Hemisphere mid- and high latitudes. These results on the fascinating Hunga Tonga event have wider relevance to perturbations of the climate system where the initial impact is stratospheric but important secondary effects are felt elsewhere, including at the surface.

Our understanding of the impact of aerosol particles on deep-convective clouds is still incomplete. Earlier research found increased lightning above shipping lanes and hypothesized that the ship emissions support the development of deep-convective cloud systems. The present study investigates the effect of a sevenfold reduction in sulphur content of shipping fuel, implemented following the International Marine Organisation 2020 regulation. A significant decrease in lightning activity is found over shipping lanes in combination with a reduction of both the concentration and the size of the emitted particles, providing further evidence of a strong connection between shipping missions, deep-convective cloud development, and lightning activity. This highlights the need to resolve the still unclear acting mechanisms.

Aerosols moderate climate change by modifying cloud properties to make them more reflective to sunlight, but the magnitude of the effect remains very uncertain, with important implications for future climate change. This study combines satellite observations and reanalysis data to calculate a much weaker radiative forcing due to aerosol-cloud interactions than previous studies. The implication is that aerosols may play a smaller role in climate change than has been widely supposed.

Measurement datasets of cloud condensation nuclei (CCN) are vital for our understanding of aerosol-cloud interaction and reliable climate modelling. This study analyses and compares the only two global CCN datasets derived from satellite and reanalysis data. These key datasets are found to disagree over pristine oceans in terms of their climatology as well as seasonal and annual variations. Given the importance of CCN as a fundamental property in climate model simulations, further research is needed to reconcile these differences and to produce an observation-based dataset that can be confidently used to evaluate our understanding.

Volcanic aerosol can persist in the stratosphere for many months and have a significant cooling effect on the troposphere. The effects of the eruption of Mount Pinatubo in 1991 are an important recent example. The time that aerosol remains in the stratosphere is determined by the combined effects of transport and sedimentation. This paper sets out a simple conceptual framework for capturing these effects and combines it with observations and modelling to give robust estimates of key timescales. This framework will be valuable in studies of future eruptions and in studies of artificial stratospheric aerosol injection as a possible approach to solar radiation modification.

Ship tracks are the phenomenon of lines of enhanced cloud reflectivity visible from space that are caused by the particulate emissions of shipping vessels. The emissions cause cloud droplets to become smaller, with a more contentious possibility that they increase cloud water by shutting down precipitation, both effects having a cooling effect on the local climate. This study applies a clever new methodology to show that past studies that measured an increase of cloud water were likely erroneous due to a sampling artifact: they assumed ship tracks were randomly oriented where in fact ship tracks align with the winds. This orientation bias was cleverly ascertained by "sailing" ships through the winds and clouds of a different year than the one in which the ship tracks were measured. A climate cooling effect from increased cloud water was still obtained, but one much smaller than described previously. The result implies that ships may not be as effective a geoengineering tool as has previously been assumed.

This paper provides a guideline The Role of AerChemMIP and will therefore be helpful both for future assessments and for the development of the MIPs in general. Rolf Müller

There has been much emphasis on the increased predictability of the extratropical tropospheric circulation after stratospheric sudden warmings and the potential value of this to weather forecasting. But it remains the case that the sudden warmings themselves, which are significantly (but not exclusively) driven by variabiliity in the troposphere, have limited predictability. This paper uses ensemble forecasts to identify tropospheric circulation features that, if poorly predicted in the period prior to a sudden warming, lead to a poor prediction of the warming itself and hence provides a potentially useful focus for future improvements to forecast models.

Noctilucent clouds form in the extremely cold temperatures in the high-latitude summer mesosphere (altitudes of 75-85km). Their formation requires the right combination of water vapour concentrations and temperatures. It has been speculated for example, that increasing frequency of occurrence of such clouds might result from increases in methane concentrations, with the methane being converted into water vapour in the upper stratosphere and mesosphere. This paper reports observations of noctilucent clouds, made using an automated lidar system in southern Argentina, at unexpectedly low latitudes compared to previous Southern Hemisphere observations. Possible explanations, including systematic moistening of the mesosphere by space traffic, are discussed.

The paper presents the first quantification of the freezing behaviour of siver iodide (AgI) containg seed particles derived from glaciogenic cloud seeding experiments in the real atmosphere. The study uses a unique approach by releasing the seeding particles from an unmanned airborne vehicle in supercooled low stratus clouds and measuring the formed ice crystals by a tethered balloon placed downwind. Based on their unique observation data the authors propose a novel freezing mechanism based on the hygroscopic growth of the particles in to solution droplets, subsequent freezing of droplets and further growth of ice crystals while ruling out competing freezing mechanisms like contact freezing.

Effusive, long-lasting volcanic eruptions impact climate through emission of gases and subsequent production of aerosols. Although previous studies have shown that the sulphate aerosol produced by these eruptions cools Earth's climate, this Earth system modelling study shows that high-latitude effusive eruptions can cause Arctic warming during the fall and wintertime. This warming effect is caused by the enhancement of downward longwave radiation from very optically thin clouds. The results have implications for our understanding of future Arctic climate change as well as any efforts to deliberately modify the climate through solar radiation management.

It is widely accepted that secondary organic aerosols formed from the atmospheric processing of biogenic and anthropogenic VOCs represent a major component of particulate matter globally, affecting air quality, meteorology and climate, but predictive skill in atmospheric models remains poor. It has previously been theorised that water vapour may assist organic molecules forming particulates by condensing together, but until now this process has not generally been implemented in models. This paper demonstrates that by including this process using a new efficient parameterisation in the popular GEOS-CHEM global chemistry model, the effects on predicted aerosol formation can be substantial, so this effect warrants consideration going forward.

The cause of fluctuations of the stratospheric quasi-biennial oscillation (QBO) period has been debated since its discovery. Explanations for variations in the QBO period are of great interest given the relevance of the QBO as one of the major modes of atmospheric interannual variability that affects not only the tropics, but also on the surface weather and climate in the extra-tropics. This study analyses the latest versions of atmospheric reanalyses, which resolve a part of the gravity wave spectrum, to show that the period fluctuations can be explained by seasonal variations in the forcing by small-scale (gravity) waves in the equatorial region. In doing so, the study improves our understanding of one of the fundamental modes of stratospheric variability and suggests that improvements in our ability to capture multi-scale interactions in the models can improve the accuracy of seasonal forecasts and the reliability of future climate projections.

An important parameter for calculation of the impact of cirrus clouds on climate is the asymmetry parameter. The asymmetry parameter captures the degree to which light is scattered by individual ice crystals towards the ground rather than reflected upwards to space. The measurements presented in this study show that, for a common type of ice crystal called bullet rosettes, cirrus clouds may be far more efficient at reflecting sunlight than is typically assumed in the radiative transfer models used to represent the role of clouds in climate.

The manuscript provides an opinion and perspectives on the extent to which we can use snapshots of aerosol and cloud data (e.g. from satellite measurements) to infer understanding about the processes in the system. It draws on existing work and adds some new work to demonstrate the applicability of ergodicity, space-time exchange, and the Deborah number to the problem. The manuscript is conceptual and original. It is our hope that this work will encourage the atmospheric sciences community to explore the value of these concepts more deeply.

While it has been established for decades that particulate matter, especially the PM2.5 fraction, can be quantitatively linked to a variety of negative health outcomes from both long- and short-term exposure, our ability to conclusively link this to specific particle components or types has proved stubbornly elusive. If particular sources or chemical components were found to be the driving factors, these could lead to more effective interventions and regulations. Because of recent advances in our observational capabilities and fundamental understanding of particle composition and toxicity, this opinion piece argues that we are at a pivotal moment in this ongoing area of study, and summarises some recent advances in this area.

This paper is written by experts in the field and addresses new tropospheric chemical mechanism developments. It shows how new analytical methods, instrumentation and cheminformatics methods have led to the identification of key processes and knowledge gaps in our understanding of chemical mechanisms, both in the gas phase and condensed phases. The opinion paper presents a perspective on these developments in the present and future, and outlines new directions. This opinion will be a very valuable contribution for the community and I am convinced it will be frequently read and cited.