Chlorine activation and enhanced ozone depletion induced by wildfire aerosol

Viscosity and Phase State of Wildfire Smoke Particles in the Stratosphere from Pyrocumulonimbus Events: An Initial Assessment

Understanding the viscosity and phase state of biomass-burning organic aerosol (BBOA) from wildfires and pyrocumulonimbus (pyroCb) events in the stratosphere is critical for predicting their role in stratospheric multiphase chemistry and ozone depletion. However, the viscosity and phase state of BBOA under stratospheric conditions, including interactions with sulfuric acid (H2SO4), remain largely unquantified. In this study, we combine laboratory data with a thermodynamic model to predict the viscosity and phase state of BBOA under stratospheric conditions. Our results suggest that BBOA with a H2SO4-to-BBOA mass ratio of 0.37─an estimated upper limit for pyroCb smoke in the lower stratosphere after two months of aging─is highly viscous and frequently exists in a glassy state. Even at a higher H2SO4-to-BBOA mass ratio of 0.79─an estimated upper limit after nine months of aging─BBOA can still transition to a glassy state under certain stratospheric conditions. In the glassy state, bulk reactions are suppressed, and multiphase chemistry may be limited to the particle surfaces. We also highlight key areas for future research needed to better constrain the viscosity and phase state of BBOA in the stratosphere and its subsequent impact on ozone.

Fingerprinting the recovery of Antarctic ozone

The Antarctic ozone 'hole' was discovered in 1985 (ref. 1) and man-made ozone-depleting substances (ODSs) are its primary cause2. Following reductions of ODSs under the Montreal Protocol3, signs of ozone recovery have been reported, based largely on observations and broad yet compelling model-data comparisons4. Although such approaches are highly valuable, they do not provide rigorous statistical detection of the temporal and spatial structure of Antarctic ozone recovery in the presence of internal climate variability. Here we apply pattern-based detection and attribution methods as used in climate-change studies5-11 to separate anthropogenically forced ozone responses from internal variability, relying on trend pattern information as a function of month and height. The analysis uses satellite observations together with single-model and multi-model ensemble simulations to identify and quantify the month-height Antarctic ozone recovery 'fingerprint'12. We demonstrate that the data and simulations show compelling agreement in the fingerprint pattern of the ozone response to decreasing ODSs since 2005. We also show that ODS forcing has enhanced ozone internal variability during the austral spring, influencing detection of forced responses and their time of emergence. Our results provide robust statistical and physical evidence that actions taken under the Montreal Protocol to reduce ODSs are indeed resulting in the beginning of Antarctic ozone recovery, defined as increases in ozone consistent with expected month-height patterns.

Ongoing large ozone depletion in the polar lower stratospheres: the role of increased water vapour

The very low temperatures of the polar lower stratosphere lead to the efficient seasonal depletion of ozone following the formation of polar stratospheric clouds (PSCs) and heterogeneous chlorine-activating reactions on their surfaces. The Montreal Protocol has controlled the production of major chlorine- (and bromine-) containing Ozone Depleting Substances (ODSs) and the stratospheric Cl and Br loadings have been slowly decreasing for over two decades. However, we are still experiencing very large (by some measures record) ozone depletion in the Antarctic and cold Arctic springs. There are a variety of factors involved but here we focus on the possible role of increased stratospheric water vapour, for example as occurred due to the eruption of the underwater volcano Hunga Tonga-Hunga Ha'apai in January 2022. We perform idealised TOMCAT three-dimensional chemical transport model experiments to investigate the impacts of a Hunga-like eruption being followed by conditions such as the very cold Arctic winter of 2019/2020; and contrast the impact of the cold Antarctic spring of 2020 with the previous warmer, more disturbed year of 2019. In the Antarctic, efficient dehydration by sedimenting ice PSCs limits the impact of a 1 ppmv increase in H2O to a maximum additional depletion of 16 Dobson Units (DU) in 2020 and 11 DU in 2019 at the vortex edge in late September. A 1 ppmv H2O increase in the cold Arctic vortex of 2019/2020 causes a maximum additional depletion of 16 DU at the vortex edge in mid March. The direct chemical impact of water vapour from a Hunga-like eruption on polar ozone is therefore modest in any given year, given natural variability. However, regular increased H2O injection or production from increased CH4 oxidation could represent an important factor in gradual long-terms trends.

Convective potential and fuel availability complement near-surface weather in regulating global wildfire activity

Wildfires are favored by hot, dry, windy, rainless conditions-this knowledge about fire weather informs both short-term forecast and long-term prediction of wildfire activity. Yet, wildfires rely on the availability of ignition and fuel, which are underrepresented in fire forecast and prediction practices. By analyzing satellite measurements and atmospheric reanalysis, here we show that near-surface weather only partially captures wildfire occurrence and intensity across the daily to seasonal timescales. Beyond near-surface weather, convection and fuel abundance play a complementary role in regulating burning processes. Specifically, enhanced atmospheric convection is identified for over 40% of the low-human-impact regions and 61% of global burnable areas during wildfire ignition and spreading periods. Meanwhile, 56% of shrublands and 54% of grasslands see higher fuel load with actual occurrence of fire. Our results highlight the role of convection and fuel in wildfire forecast, prompting a revisit of wildfire prediction under intertwined atmospheric and terrestrial changes.

The upper tropospheric-lower stratospheric ozone drives summer precipitation and wildfire changes in West Siberia

Siberian wildfires are pivotal in determining the carbon cycle and climate dynamics, exerting a profound impact on the ecosystems of the entire Arctic region. Over the past few decades, variations in summer precipitation in West Siberia have significantly influenced wildfire activity. This study analyzed precipitation trends in West Siberia from 1982 to 2021 using observations and transient simulations, uncovering a strong correlation between precipitation variability and ozone concentrations in the upper troposphere-lower stratosphere (UTLS). Heightened UTLS ozone levels warm the upper atmosphere over West Siberia during summer. This warming modifies the regional polar jet stream, intensifying its southern branch and weakening the northern one, leading to a southward shift in the jet stream. Consequently, cyclonic circulation anomalies emerge in the upper troposphere, characterized by a barotropic structure with unusual upward movements around 60°N. This upward motion triggers corresponding anomalies in zonal winds in the lower troposphere, fostering a low-pressure system at the surface. This atmospheric shift results in an influx of warm, moist air from the south and cold, dry air from the north into Siberia, enhancing cloud formation and precipitation. Notably, our analysis suggests that the rise in summer precipitation in West Siberia between 1993 and 2010 is linked to increased UTLS ozone concentrations during this period. Conversely, the decline in UTLS ozone since 2010 may increase the risk of wildfires by suppressing precipitation. Our findings underscore the pivotal role of stratospheric chemistry in shaping the regional climate and wildfire behavior.

Contrasting Responses of Smoke Dispersion and Fire Emissions to Aerosol-Radiation Interaction during the Largest Australian Wildfires in 2019-2020

The record-breaking 2019-2020 Australian wildfires have been primarily linked to climate change and its internal variability. However, the meteorological feedback mechanisms affecting smoke dispersion and wildfire emissions on a synoptic scale remain unclear. This study focused on the largest wildfires occurring between December 25, 2019 and January 10, 2020, under the enhanced subtropical high, when the double peak in wildfire evolution was favored by sustained low humidity and two synchronous increases in temperature and wind. Based on the coupled atmospheric chemical transport model, we revealed an abnormal downdraft and a lowered planetary boundary layer over southeastern Australia, caused by the radiative cooling effects (exceeding -100 W m-2 at surface) of carbonaceous aerosols (CAs) from wildfires. These changes hindered the smoke dispersion and increased the PM2.5 concentration by ∼27.8%. By contrast, the low-level anomalous cyclonic circulation induced by CAs brought more water vapor toward the fire zone. This, combined with surface cooling and low wind speeds, suppressed wildfire emissions, thereby reducing PM2.5 concentration by ∼11.6%. These findings highlight the critical role of aerosol-radiation interaction in wildfire behavior.

Astaxanthin attenuates UV-irradiation aging process via activating JNK-1/DAF-16 in Caenorhabditis elegans

Astaxanthin (AST) is a xanthophyll carotenoid with strong oxidation resistance, which can effectively scavenge various free radicals and protect organisms from oxidative damage. AST is also known to have prominent anti-aging effects, but the underlying mechanism of AST in anti-radiation aging is largely unknown. In this work, we applied ultraviolet (UV) irradiation to accelerate the aging of Caenorhabditis elegans (C. elegans) and treated the nematodes with AST to explore whether and how AST could attenuate the radiation-induced aging effect. Our results showed that AST improved the survival rate of C. elegans, reduced the aging biomarkers, and alleviated the mitochondrial dysfunction caused by the irradiation. Based on the transcriptome sequencing analysis, we identified that the key genes regulated by AST were involved in JNK-MAPK and DAF-16 longevity signaling pathways. Furthermore, we employed jnk-1 and daf-16 mutants and verified the role of the JNK-1/DAF-16 signaling pathway in the anti-aging effect. As such, this study has not only demonstrated that AST can resist the aging process caused by UV-irradiation but also revealed the anti-aging mechanism of AST through JNK-1/DAF-16 activation in C. elegans.

Advances and future directions of environmental risk research: A bibliometric review

Environmental risk is one of the world's most significant threats, projected to be the leading risk over the next decade. It has garnered global attention due to increasingly severe environmental issues, such as climate change and ecosystem degradation. Research and technology on environmental risks are gradually developing, and the scope of environmental risk study is also expanding. Here, we developed a tailored bibliometric method, incorporating co-occurrence network analysis, cluster analysis, trend factor analysis, patent primary path analysis, and patent map methods, to explore the status, hotspots, and trends of environment risk research over the past three decades. According to the bibliometric results, the publications and patents related to environmental risk have reached explosive growth since 2018. The primary topics in environmental risk research mainly involve (a) ecotoxicology risk of emerging contaminants (ECs), (b) environmental risk induced by climate change, (c) air pollution and health risk assessment, (d) soil contamination and risk prevention, and (e) environmental risk of heavy metal. Recently, the hotspots of this field have shifted into artificial intelligence (AI) based techniques and environmental risk of climate change and ECs. More research is needed to assess ecological and health risk of ECs, to formulize mitigation and adaptation strategies for climate change risks, and to develop AI-based environmental risk assessment and control technology. This study provides the first comprehensive overview of recent advances in environmental risk research, suggesting future research directions based on current understanding and limitations.

Pervasive fire danger continued under a negative emission scenario

Enhanced fire-prone weather under greenhouse gas warming can significantly affect local and global carbon budgets from increased fire occurrence, influencing carbon-climate feedbacks. However, the extent to which changes in fire-prone weather and associated carbon emissions can be mitigated by negative emissions remains uncertain. Here, we analyze fire weather responses in CO2 removal climate model experiments and estimate their potential carbon emissions based on an observational relationship between fire weather and fire-induced CO2 emissions. The results highlight that enhanced fire danger under global warming cannot be restored instantaneously by CO2 reduction, mainly due to atmospheric dryness maintained by climatic inertia. The exacerbated fire danger is projected to contribute to extra CO2 emissions in 68% of global regions due to the hysteresis of climate responses to CO2 levels. These findings highlight that even under global cooling from negative emissions, increased fire activity may reinforce the fire-carbon-climate feedback loop and result in further socio-economic damage.

Smoke-charged vortex doubles hemispheric aerosol in the middle stratosphere and buffers ozone depletion

Australian mega-wildfires in the summer of 2019-2020 injected smoke into the stratosphere, causing strong ozone depletion in the lower stratosphere. Here, we model the smoke plume and reproduce its unexpected trajectory toward the middle stratosphere at ~35-kilometer altitude. We show that a smoke-charged vortex (SCV) induced and maintained by absorbing aerosols played a key role in lofting pollutants from the lower stratosphere and nearly doubled the southern hemispheric aerosol burden in the middle stratosphere. The SCV caused a redistribution of stratospheric aerosols, which boosted heterogeneous chemistry in the middle stratosphere and enhanced ozone production, compensating for up to 70% of the ozone depletion in the lower stratosphere. As global warming continues, we expect a growing frequency and importance of SCVs in promoting the impacts of wildfires on stratospheric aerosols and chemistry.

Plastics in the environment in the context of UV radiation, climate change and the Montreal Protocol: UNEP Environmental Effects Assessment Panel, Update 2023

This Assessment Update by the Environmental Effects Assessment Panel (EEAP) of the United Nations Environment Programme (UNEP) considers the interactive effects of solar UV radiation, global warming, and other weathering factors on plastics. The Assessment illustrates the significance of solar UV radiation in decreasing the durability of plastic materials, degradation of plastic debris, formation of micro- and nanoplastic particles and accompanying leaching of potential toxic compounds. Micro- and nanoplastics have been found in all ecosystems, the atmosphere, and in humans. While the potential biological risks are not yet well-established, the widespread and increasing occurrence of plastic pollution is reason for continuing research and monitoring. Plastic debris persists after its intended life in soils, water bodies and the atmosphere as well as in living organisms. To counteract accumulation of plastics in the environment, the lifetime of novel plastics or plastic alternatives should better match the functional life of products, with eventual breakdown releasing harmless substances to the environment.

Fine particle pollution during megafires contains potentially toxic elements

Wildfires that raged across Australia during the 2019-2020 'Black Summer' produced an enormous quantity of particulate matter (PM) pollution, with plumes that cloaked many urban centres and ecosystems along the eastern seaboard. This has motivated a need to understand the magnitude and nature of PM exposure, so that its impact on both built and natural environments can be more accurately assessed. Here we present the potentially toxic fingerprint of PM captured by building heating, ventilation, and air conditioning filters in Sydney, Australia during the peak of the Wildfires, and from ambient urban emissions one year later (Reference period). Atmospheric PM and meteorological monitoring data were also assessed to determine the magnitude and source of high PM exposure. The wildfires were a major source of PM pollution in Sydney, exceeding the national standards on 19 % of days between November-February. Wildfire particles were finer and more spherical compared to Reference PM, with count median diameters of 892.1 ± 23.1 versus 1484.8 ± 96.7 nm (mean ± standard error). On an equal-mass basis, differences in potentially toxic elements were predominantly due to higher SO42--S (median 20.4 vs 4.7 mg g-1) and NO3--N (2.4 vs 1.2 mg g-1) in Wildfire PM, and higher PO43--P (10.4 vs 1.4 mg g-1) in Reference PM. Concentrations of remaining elements were similar or lower than Reference PM, except for enrichments to F-, Cl-, dissolved Mn, and particulate Mn, Co and Sb. Fractional solubilities of trace elements were similar or lower than Reference PM, except for enhanced Hg (12.1 vs 1.0 %) and greater variability in Cd, Hg and Mn solubility, which displayed upper quartiles exceeding that of Reference PM. These findings contribute to our understanding of human and ecosystem exposures to the toxic components of mixed smoke plumes, especially in regions downwind of the source.

Potential drivers of the recent large Antarctic ozone holes

The past three years (2020-2022) have witnessed the re-emergence of large, long-lived ozone holes over Antarctica. Understanding ozone variability remains of high importance due to the major role Antarctic stratospheric ozone plays in climate variability across the Southern Hemisphere. Climate change has already incited new sources of ozone depletion, and the atmospheric abundance of several chlorofluorocarbons has recently been on the rise. In this work, we take a comprehensive look at the monthly and daily ozone changes at different altitudes and latitudes within the Antarctic ozone hole. Following indications of early-spring recovery, the October middle stratosphere is dominated by continued, significant ozone reduction since 2004, amounting to 26% loss in the core of the ozone hole. We link the declines in mid-spring Antarctic ozone to dynamical changes in mesospheric descent within the polar vortex, highlighting the importance of continued monitoring of the state of the ozone layer.

Impact of the Hunga Tonga volcanic eruption on stratospheric composition

The explosive eruption of the Hunga Tonga-Hunga Ha'apai (HTHH) volcano on 15 January 2022 injected more water vapor into the stratosphere and to higher altitudes than ever observed in the satellite era. Here, the evolution of the stratospherically injected water vapor is examined as a function of latitude, altitude, and time in the year following the eruption (February to December 2022), and perturbations to stratospheric chemical composition resulting from the increased sulfate aerosols and water vapor are identified and analyzed. The average calculated mass distribution of elevated water vapor between hemispheres is approximately 78% Southern Hemisphere (SH) and 22% Northern Hemisphere in 2022. Significant changes in stratospheric composition following the HTHH eruption are identified using observations from the Aura Microwave Limb Sounder satellite instrument. The dominant features in the monthly mean vertical profiles averaged over 15° latitude ranges are decreases in O3 (-14%) and HCl (-22%) at SH midlatitudes and increases in ClO (>100%) and HNO3 (43%) in the tropics, with peak pressure-level perturbations listed. Anomalies in column ozone from 1.2-100 hPa due to the HTHH eruption include widespread O3 reductions in SH midlatitudes and O3 increases in the tropics, with peak anomalies in 15° latitude-binned, monthly averages of approximately -7% and +5%, respectively, occurring in austral spring. Using a 3-dimensional chemistry-climate-aerosol model and observational tracer correlations, changes in stratospheric composition are found to be due to both dynamical and chemical factors.

Taming the flame, from local to global extreme wildfires

Australia rethinks strategies after 2019 to 2020 bushfires.