The paper provides an informative and up-to-date overview of the progress made in understanding the evolution of the stratospheric ozone layer since the introduction of the Montreal Protocol to a broad and interested readership. It discusses new evidence that, despite the success of the Montreal Protocol, the stratospheric ozone layer will not necessarily return in the long term to the state it was in before the start of the release of CFCs (before 1970). New challenges arise, due to man-made climate change or natural extreme events like volcanic eruptions that have been shown to impact the ozone layer. The authors' appeal is therefore to continue to closely monitor the state of the stratospheric ozone layer, which is important for life, in the future.

Equilibrium climate sensitivity (ECS), with a specific definition, has been used as a convenient measure, encapsulated in a single number, of the response of the climate to increases in long-lived greenhouse gases. The authors recall some of the history of how ECS has been estimated, by models and observations, including paleoclimate data and note recent progress in reducing uncertainty in the value of ECS. However they also note that there are important aspects of future potential climate change that are not captured by the ECS measure and therefore that there will be limited usefulness in too strong a focus on reducing uncertainty in ECS alone.

The influence aerosols have on climate has long been recognised, but our ability to observe and quantify them on global scales came of age with the use of new satellite-based instruments and data products in recent decades. As instrumentation technologies improve alongside our abilities to handle large quantities of data and process using the latest data science approaches, the data products available to atmospheric science will only improve in decades to come. This opinion article tries to anticipate what tools and insights the community can look forward to in the near future.

Natural aerosols have long been understood to have a substantial effect on Earth’s climate, in particular because they define a natural baseline that affects the magnitude of anthropogenic aerosol radiative forcing. In this Opinion the authors highlight the very large uncertainties in the radiative effects of aerosols particularly from fires and dust, which they estimate to exceed the magnitude of anthropogenic aerosol forcing over the entire industrial period. Rather than just defining a pre-industrial baseline for aerosol, natural emissions have changed substantially during pre-industrial times and through the industrial period, making a very poorly defined contribution to climate change. Based on the estimated large uncertainties, the authors argue for greater research efforts to narrow them.

This article offers a wide ranging review of current understanding of the role various tropical cirrus cloud types play in the redistribution of water within the atmosphere and how they affect the changing Earth's energy balance by reflecting sunlight and preventing the escape of thermal energy to outer space. Improved understanding of these dynamics has been identified as critical for predicting whether such clouds may amplify or slow future global climate change. A clear exposition is provided of the various methods used to study tropical cirrus cloud characteristics and processes, including remote sensing, in situ measurements, modeling and laboratory work. Key questions include identifying how small-scale microphysical processes affect larger cloud structure, and how cirrus altitude and extent responds to changing radiation and thermodynamic profiles. A call is made not only for improved representations of cloud processes at finer scales, but also for a more holistic approach using a hierarchy of model detail. Such efforts would enable new knowledge obtained from studying even the smallest scales to be more readily placed within a broader context applicable to climate studies.

This personal tribute, authored by close colleagues and friends, is an homage to Paul Crutzen's profound and multifaceted impact in the field of atmospheric chemistry and physics. The article summarizes not only the exceptional breadth of research ranging from tropospheric ozone formation to stratospheric ozone loss, and coining the term ‘anthropocene’ but it also describes Paul Crutzen’s instrumental role in shaping the next generations of atmospheric scientists, many of whom now are leaders in the field. Furthermore, Crutzen’s legacy is manifested by his contributions to the creation of the journal Atmospheric Chemistry and Physics (ACP), that laid the foundation for EGU's pioneering open-access publication model, fostering public peer review and interactive scientific discourse.

For well over a century, many advances in tropospheric chemistry and physics have been driven by in situ observations, with measurements tending to be either specific measurements over multi-year periods, often as part of a network, or short term ‘intensive’ observations over of a more limited duration, typically 1-2 months. But in recent decades, many major developments in atmospheric science have been fueled through long-term comprehensive observations, and the Finnish SMEAR project is a major pioneering example of this. As part of the 20th anniversary Special Issue of Atmospheric Chemistry and Physics, this opinion piece reflects on the manifold and sustained contributions this has made to atmospheric science.

This provocative opinion piece examines the theoretical, numerical, and observational evidence in support of two highly cited proposed mechanisms for invigorating deep convective clouds through higher aerosol concentrations. Both start with high concentrations of water droplets. Through cold-phase invigoration, precipitation is reduced allowing for greater release of latent heat from freezing higher up in clouds. With warm-phase invigoration, increased latent heating occurs lower down due to accelerated liquid condensation. In both cases, the article persuasively argues from a variety of standpoints that the evidence to support the importance of the effects is weak, particularly once the full complexity of clouds and their interactions with their environment is fully taken into account. Concrete suggestions are made for improving definition, observations, and modeling of the problem, but also an admonishment that attention in the field might be better directed towards more fruitful aspects of the aerosol-cloud interaction problem.

Tropospheric ozone is an extremely important atmospheric compound as it oxidises, and thus removes, chemical compounds in the atmosphere. One important source in the troposphere are anthropogenic emissions. At elevated concentrations ozone presents a pollutant that is harmful to human health and ecosystems, and it acts as a short-lived greenhouse gas in the upper troposphere. Based on previously developed community models to describe the spatial and temporal variation of tropospheric ozone, this highly controversially discussed Opinion article proposes the establishment of a conceptual, simplified, but widely accepted tropospheric ozone model to facilitate communication between different communities concerned with different ozone impacts, and between ozone research and policy. This step could pave the way for a coordinated global action to improve air quality standards worldwide in an efficient and cost-effective manner.