This Tropospheric Ozone Assessment Report Phase-II (TOAR-II) study has assessed the current state of knowledge regarding ozone's important role as a short-lived climate forcer (SLCF), as well as the influence of climate change on global tropospheric ozone, relying on new studies published within the past five years. Models estimate that the tropospheric ozone burden has increased substantially since the pre-industrial era, supported by available measurements, contributing to a radiative climate forcing from total column ozone of 0.50 (0.25 to 0.75) W m-2 for 1750-2024, making ozone the third most important anthropogenic greenhouse gas (GHG). Since 1990, surface, airborne and satellite measurements, as well as models, have suggested a continued rise in the tropospheric burden, although ground-level ozone is decreasing in some regions. This is driven, in part, by the continued rise of methane, despite estimates that global anthropogenic NOx emissions may have peaked recently. Climate change influences ozone through a variety of physical, biological and chemical processes. Increasing temperatures are expected to reduce the global average surface ozone owing to increased water vapour, but increase ozone in polluted regions. Future surface and free tropospheric ozone concentrations will be determined by a complex interplay of emission changes, as well as by climate change and land-biosphere feedbacks. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

Stratosphere-troposphere exchange (STE) of ozone is a significant source of tropospheric ozone, influencing air quality, atmospheric chemistry and climate forcing. This assessment synthesizes recent advances in understanding the magnitude, mechanisms, variability and future evolution of ozone transport across the tropopause, updating earlier reviews using observations, reanalyses and global chemistry-climate models. Globally, monthly mean STE is driven by the large-scale circulation of the stratosphere, but on shorter time scales and synoptic scales episodic dynamical processes such as tropopause folds, cutoff lows and wave breaking are locally important. Based on a synthesis of published estimates for 2000-2020, the present-day global net STE of ozone is assessed to be 380 (350-450) Tg O3 yr-1. This net flux is composed of a strong downward transport from the stratosphere into the troposphere of approximately 540 Tg O3 yr-1 in the extratropics, partially offset by an upward transport of ozone from the troposphere into the stratosphere of about 190 Tg O3 yr-1 in the tropics. Around 60% of the downward extratropical transport occurs in the Northern Hemisphere. STE exhibits pronounced seasonality as well as interannual variability linked to climate modes including the El Niño-Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO). Analyses indicate a decline in STE of 2.7% decade-1 during the late twentieth century associated with ozone depletion, while future projections consistently project an increase in STE of around 4.7% decade-1 as global circulation responds to climate change and stratospheric ozone recovers. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

As part of the second Tropospheric Ozone Assessment Report (TOAR-II), we synthesize global observations and observationally constrained estimates of surface-level ozone (O3) with contemporary literature to characterize the global distribution and trends of long-term O3 exposure, focusing on two health-relevant metrics and health impacts attributable to long-term exposure. The highest 2016-2020 surface-level O3 concentrations are observed or estimated in South Asia, eastern China, and parts of the Middle East and North Africa. From 2000 to 2020, O3 declined across southeastern North America and parts of Eastern Europe, while the largest increases occurred in South Asia and Sub-Saharan Africa. Global premature mortality from chronic obstructive pulmonary disease (COPD) attributable to long-term O3 exposure nearly doubled between 2000 and 2020, reaching approximately 500 000 deaths annually. This overall increase obscures decreasing attributable deaths across North America and Europe and increasing deaths across large parts of the Global South. To spur high-quality, robust research on O3 health impacts, we summarize state-of-the-science practices for conducting health impact assessments (HIAs) and characterizing associated uncertainty. We also call for expanded in situ monitoring, improved health datasets and epidemiologic evidence, and sustained precursor emission reductions to clarify and limit the growing global health burden of O3 under climate change and demographic shifts. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

Satellite observations enable global assessments of ozone in the free troposphere, yet uncertainties in earlier records limited the first Tropospheric Ozone Assessment Report's (TOAR-I) confidence in regional distributions and, especially, decadal trends. Here, we leverage longer and improved satellite records, additional datasets and new harmonization approaches to provide a second, more confident, TOAR assessment of present-day and longer-term changes. We quantify the 2018-2022 climatology and seasonal cycle at 1∘×1∘ resolution and evaluate trends in a combined tropospheric column record since 2005. Summertime maxima exceed 50 DU over the Middle East, and annual-mean values exceed 40 DU from East Asia to the Mediterranean. The present-day global tropospheric ozone burden is 336±6 Tg. Decadal ozone changes are pronouncedly nonlinear: increases of 2-4.5% per decade during 2005-2019 occur over 20% of the globe (high confidence), most notably across Asia, followed by widespread declines after 2020, producing a net decrease since 2005 across 70% of the globe. Dynamical and episodic drivers, such as El Niño-Southern Oscillation, extreme wildfires, and the COVID-19 pandemic, strongly influenced variability, underscoring the sensitivity of tropospheric ozone to both natural and anthropogenic influences and the critical need for sustained, coordinated satellite monitoring. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

Oceanic and polar regions are critical for global atmospheric processes, but an evaluation of ozone distributions and trends in these regions has been missing. In this assessment, part of the TOAR-II project, we report lower atmosphere ozone concentrations over open oceans and polar regions: with data from ships, buoys, aircraft and ground sites from 1975 to 2023, this is the most comprehensive dataset yet. Ozone trends were determined using Linear Mixed-effects Modelling (LMM). In most regions, trends have been modest since the mid-1970s (<1 nmol mol-1 per decade), positive in the mid-latitudes and tropical regions (with medium-low certainty), negative in the North and Tropical Pacific (with low and high certainty, respectively), increasing in Antarctica and decreasing in the Arctic. Seasonal effects often influence the long-term trends in extra-tropical regions. Five global and one hemispheric models were used to aid the interpretation of the sparse observations. The seasonal cycles are reasonably well reproduced, although models disagree with each other and with the observations on the distribution of ozone, especially in the tropics and in the Arctic. Changes in methane, iodine, shipping emissions, nitrogen oxides emissions, and terrestrial and marine volatile organic compound are likely to be important drivers for ozone over the oceans, although specific attributions are uncertain owing to insufficient data. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

We use the TOAR-II database to explore vegetation risk from surface ozone when characterized by different concentration- and flux-based metrics for three crop species (wheat, rice and soybean) and three forest species (beech and a sensitive and tolerant tropical tree species). From analysis of up to 3363 sites of present-day (2016-2021) ozone levels, we find some differences between metrics with 64% and 90% of sites exceeding critical levels for irrigated wheat and 85% and 79% for beech for concentration- and flux-based metrics, respectively. Overall, the flux-based metric tends to suggest higher levels of risk. Metrics estimate similar global trends at up to 3242 sites for the period 2005 to 2021 with between 14% and 40% of sites showing statistically significant decreasing trends, which are most prevalent in North America (NAM), followed by Europe (EUR). We use the flux-based metric and literature-derived flux-response relationships to estimate average relative yield losses of 4.9% for irrigated wheat, and losses of relative gross annual increment of 14.3%, 19.8% and 5.9% for beech, sensitive and tolerant tropical tree species, respectively. Results for soybean and rice are inconclusive. Future research should extend the TOAR database in Asia (AS), Africa and Latin America and improve flux-based methodologies. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

This study presents the first regional assessment of tropospheric ozone over South America for the early twenty-first century. We use observations from 14 combined satellite records, two atmospheric reanalyses, and model simulations to examine the spatial distribution, source contributions and trends in ozone and its precursors. Satellites and reanalysis data reveal enhanced tropospheric column ozone over the mid-latitudes of the west coast and the subtropical east coast, with 2005-2021 median values (±2σ) ranging from 46 [±11] to 49 [±9] nmol mol-1. By contrast, the lowest ozone levels occur over the northwest (39 [±8]-42 [±8] nmol mol-1) and the southern cone (38 [±11]-40 [±7] nmol mol-1). Tropospheric column ozone trends (±2σ) differ across South America (2005-2021), reflecting substantial subregional and partial column variability. Satellite observations suggest an overall negative trend (low certainty), whereas reanalyses show positive trends with high certainty, ranging from +0.55 [±0.85] to +0.85 [±0.70] nmol mol-1 decade-1. Model simulations identify global background methane as the dominant contributor to the South American tropospheric ozone burden, accounting for 41% (17 of 42 nmol mol-1), followed by global anthropogenically emitted nitrogen oxides (31%). These findings improve our understanding of chemical processes and provide valuable inputs for assessing air quality and climate impacts. This article is part of the theme issue 'Tropospheric Ozone Assessment Report II'.

In this work, we investigate eruptive phenomena associated with X-ray flares on AB Doradus using observations from XMM-Newton. Our aim is to detect such events with the help of continuous X-ray absorption. The variation in the hydrogen column density (NH) is used as a primary diagnostic to detect such absorption or spectral hardening in the soft X-ray band below 1-2 keV. These variations may represent the signatures of the eruptive processes associated with flares, such as stellar prominences, failed eruptions or coronal mass ejections. We applied a time-resolved spectroscopy technique to track changes in the absorption parameter during flaring episodes. Our analysis shows statistically significant enhancements in NH, spanning from 0.3 to 3.4×1020 cm-2 during the flare and post-flare intervals. Out of six datasets, two sets containing multiple-overlapped flares show significant fluctuations and the remaining showed no such variations in NH. These variations indicate the presence of dynamic absorbing material along the line of sight and point to ongoing physical processes within the stellar corona. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.

Stellar flares are sudden magnetic eruptions that release intense energy and cause detectable variations in stellar light curves. These transient events offer windows into the magnetic field dynamics, plasma behaviour and reconnection processes in active stellar regions. Despite their importance, the diversity in flare morphology and its underlying physical origins remains poorly understood. To explore this variability, we analyse 2319 white-light flares observed by Kepler, extracting a comprehensive set of 22 time- and frequency-domain features from each event. Using unsupervised clustering, we identify two distinct flare populations that are not associated with global stellar parameters such as rotation period, temperature or mass. Instead, the separation is primarily driven by differences in frequency evolution, damping persistence and local magnetic activity (LMA). Our findings demonstrate that flare light-curve morphology can be quantitatively grouped, revealing internal dynamical contrasts without reliance on host star properties. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.

Slow magnetoacoustic waves have been observed in the solar corona for decades; yet, existing theories do not fully explain the wide range of observed decay lengths, and measurements from different instruments frequently disagree. We present two events with simultaneous Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) and Solar Orbiter/EUI-HRIEUV observations of slow waves in sunspot-anchored coronal fan feathers. In Event 1, the lines of sight (LoS) are separated by 16.2∘, while in Event 2, they are almost parallel. Using time-distance analysis, we identify waves with periods in the 3-minute band and quantify wave decay using the exponential folding length. For non-parallel LoS, the ratio of decay lengths measured by the two instruments, λratio=λAIA/λEUI, varies widely, with both λratio>1 and λratio<1 observed. However, when the LoS are almost parallel, the decay lengths agree with one another, and λratio≈1. These results show that the angle between the LoS and the wave propagation direction is key to determining the apparent decay of slow waves, a factor that has not yet been systematically accounted for. Since statistical studies of decay lengths constrain coronal energy-loss mechanisms, observational effects beyond simple projection must be corrected to reliably infer the underlying physical dissipation processes. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.

Recent multi-wavelength observations of solar flares have provided new constraints on the physical origin of quasi-periodic pulsations (QPPs). In an X-class flare, we detect a short-lived approximately 5-min QPP simultaneously in hard X-rays, extreme-ultraviolet (EUV) and soft X-ray emissions, exhibiting a clear phase-drifting behaviour with emitting plasma temperature. Based on phase-resolved timing analysis, it is found that (i) the QPPs in all diagnostics share nearly identical oscillation periods, (ii) a systematic temperature-dependent phase drifting is present, with the phase delay relative to the hard X-ray emission increases systematically from the hottest to cooler EUV channels, and (iii) the QPP persists for only a few cycles during the impulsive phase. These properties imply that periodic magnetic reconnection, possibly triggered by the leakage of 5-min oscillations from the lower atmosphere, modulates the non-thermal electrons responsible for the leading hard X-ray QPPs. Subsequently, plasma heating and cooling processes manifest sequentially across passbands with different temperature responses, resulting in the observed temperature-dependent phase drifting. These results provide novel observational evidence supporting the use of multi-temperature, multi-wavelength phase relationships to constrain the temporal evolution of flare energy release and the origins of QPPs. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.

Slow magneto-acoustic and entropy waves can serve as diagnostic tools not only for probing plasma parameters but also for investigating non-adiabatic processes. Under coronal conditions, these waves are strongly affected by both thermal conduction and thermal misbalance. In this work, we obtain an exact solution to the linear evolutionary equation describing wave dynamics in plasma under the assumption of an infinitely strong magnetic field. We show that two dispersion sources significantly expand the set of parametric regions with qualitatively similar behaviour of dispersion curves and increase the number of ways an arbitrary initial disturbance can develop. The resulting solution allows for monitoring the behaviour of individual harmonics of the eigenmodes for the plasma parameter of interest, thus expanding the possibilities for fitting observations and studying the behaviour of model signals. Using the obtained solution, we show that several thermal misbalance regimes enable quasi-periodic pulsation formation. We also demonstrate that additional information about non-adiabatic processes can be gained by considering the amplitude ratios of various plasma parameters and their phase shifts for arbitrary harmonic numbers. In particular, this may be of interest for plasma diagnostics using sloshing oscillations, which involve the simultaneous development of multiple oscillations. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.

We investigate the influence of nonlinear longitudinal disturbances, triggered by an initial field-aligned velocity flow, on the damping of standing kink oscillations in fine-structured, cool coronal loop strands under isobaric conditions. Using two-dimensional ideal magnetohydrodynamic (MHD) simulations, we model different realistic flow geometries, bounded, unbounded and external, and quantify their impact on wave excitation, damping and energy leakage. Our modelled loop strand incorporates strong density contrasts (d=5 and d=20), providing a configuration consistent with cool-loop observations from Hinode/SOT and Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). Our results show that while the nonlinear disturbances exert mild influence on the oscillation period, they substantially modify the damping time. Unbounded flows yield the strongest damping, reducing the damping time by up to approximately 25% compared to bounded flows, caused by enhanced wave-flow coupling and scattering. Longitudinally inhomogeneous flows further intensify the damping, with high-density strands exhibiting faster convergence towards the uniform-flow limit. In cases with supersonic internal flow, slow sausage-mode harmonics and weak slow shocks are additionally excited, indicating the generation of mixed-mode wave responses in flow-dominated loops. These findings demonstrate that realistic, spatially extended nonlinear disturbances play a significant role in the damping of kink oscillations and should be incorporated into forward modelling and coronal seismology diagnostics. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flare'.

We analyse quasi-periodic pulsations (QPPs) in recurrent coronal jets driven by periodic magnetic reconnection associated with successive flux emergence in fan-spine magnetic topologies. Using Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we investigate three long-lived recurring jets spanning quiet Sun to moderate-field-strength regions, each exhibiting recurrent eruptions linked to episodic reconnection. Wavelet analysis of multi-thermal AIA extreme UV (EUV) jet-base light curves detects QPPs with periods of 6-13 minutes, exceeding typical p-mode oscillation periods. Distance-time analysis reveals quasi-periodic propagating ridges, interpreted as recurrent field-aligned plasma ejections and morphological similarity to slow magnetoacoustic waves, which cannot be entirely excluded. However, the dominant photospheric unsigned flux periodicities of 10-32 minutes at the jet source regions favour the reconnection-driven interpretation. Differential emission measure (DEM) analysis confirms multi-thermal plasma with the hottest emission concentrated near the jet base, and the QPP periods fall well below both radiative and conductive cooling time scales, implying persistent localized heating within the fan-spine configuration. These results demonstrate that periodic reconnection in fan-spine topologies drives recurrent jet eruptions and contributes to localized coronal heating across quiet Sun, moderate-field-strength regions and active regions. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.

Quasi-periodic pulsations (QPPs) are oscillatory signatures commonly detected in the light curves of solar and stellar flares, offering valuable diagnostics of the underlying magnetic and plasma processes. This review compares the observational characteristics, detection methods and physical interpretations of QPPs in both solar and stellar contexts. Solar flare QPPs, extensively studied in X-rays and extreme-UV (EUV) bands using instruments such as Geostationary Operational Environmental Satellite (GOES), Spectrometer Telescope for Imaging X-rays (STIX) on-board Solar Orbiter and Fermi, display typical periods of tens of seconds and show correlations with flare duration and magnetic loop length. Stellar QPPs, observed in X-rays and white light by missions such as Kepler, Transiting Exoplanet Survey Satellite (TESS) and XMM-Newton, exhibit much longer periods-ranging from minutes to hours-consistent with larger-scale magnetic structures in more active stars. Despite differences in scale and observing band, statistical and comparative studies reveal common scaling relations and damping behaviours, suggesting that both solar and stellar QPPs are manifestations of the same fundamental mechanisms, probably magnetohydrodynamic (MHD) oscillations or oscillatory reconnection within flare loops. The comparison underscores a continuity between solar and stellar magnetic activity, linking the solar detailed physical processes to stellar-scale phenomena and providing constraints for future models and surveys. This article is part of the theme issue 'Transferable mechanisms in the oscillatory processes of solar and stellar flares'.