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Aerosol from the Hunga Tonga‐Hunga Ha'apai (HT‐HH) volcanic eruption (20.6°S) in January 2022 were not incorporated into the austral polar vortex until the following year, March 2023. Within the polar vortex in situ profiles of aerosol size spectra were completed in the austral autumns of 2019 and 2023, from McMurdo Station, Antarctica (78˚S), 30 months prior to and 15 months after the HT‐HH eruption. The measurements indicate that the HT‐HH impact on aerosol size was primarily confined to particles with diameters >0.5 μm leading to differences in aerosol mass, surface area, and extinction from factors of 2–4 at the volcanic layer's peak below 20 km, increasing to ratios of 5–10 above 20 km. Effective radius, with radiative and microphysical implications, increased from ∼0.2 to ∼0.3 μm. An Earth system model with a modal aerosol package compares favorably with the in situ measurements of the HT‐HH aerosol impact. Plain Language Summary: In austal autumn of 2019 and 2023 balloon‐borne measurements of aerosol size distributions were completed above McMurdo Station, Antarctica (78˚S, 167˚E). Size spectra were measured for particles between 0.01 and 16 μm from the surface to over 30 km. Between these two sets of measurements the Hunga Tonga‐Hunga Ha'apai (HT‐HH) volcano (20.6°S) started an eruptive phase culminating with a massive underwater explosion on 15 January 2022. Due to the timing of the eruption, the HT‐HH volcanic aerosol were not incorporated into the austral polar vortex until 2023, which formed about a month before the 2023 aerosol measurements were completed. The 2019 measurements were in a volcanically quiescent period and capture the background stratospheric aerosol. The two sets of measurements are used to characterize the impact of HT‐HH on stratospheric aerosol within the austral polar vortex, particularly on quantities important for assessing the volcanic impact on climate and heterogeneous chemistry: aerosol mass, surface area, and light extinction. Such quantities were observed to increase by factors of 2–4 below, and 5–10 above, 20 km after the eruption. An Earth System Model with an aerosol package compared favorably wih the measurements. Polar ozone loss in 2023 was, however, not impacted significantly. Key Points: Austral polar vortex aerosol concentrations from Hunga Tonga increased, for particles >0.5 μm, by factors of 2–5 below, >10 above, 18 kmConcomittant increases in SO4 mass, surface area, and near IR extinction were by factors of 2–4 below, and 5–10 above, 18 kmEffective radius, with radiative and microphysical implications, increased from ∼0.2 to ∼0.3 μm at the peak of the layer [ABSTRACT FROM AUTHOR]

The Chilean volcano Cerro Hudson erupted between August 8th and 15th, 1991, injecting between 1.7 and 2.9 Tg of SO2 into the upper troposphere and lower stratosphere. We simulate this injection using the Goddard Earth Observing System Earth system model with detailed sulfur chemistry and sectional aerosol microphysics, focusing on the resulting aerosols and their contribution to the 1991 Antarctic Austral Springtime ozone hole. The simulations show a column ozone deficit (12 DU) in the Southern Hemisphere vortex collar region. The majority of this effect is between 10 and 20 km and due to heterogeneous chemistry. The model shows a 26% decrease in ozone from background levels at these altitudes, compared with in‐situ observations of a 50% decrease. Above 20 km, the dynamical response to the eruption also causes lower ozone values, a novel modeling result. This experiment highlights potential interactions between proposed solar radiation management geoengineering aerosols and volcanic eruptions. Plain Language Summary: We simulated the August 1991 eruption of Chilean volcano Cerro Hudson. Cerro Hudson erupted 2 months after the June 15th eruption of Pinatubo which was one of the largest observed eruptions. The combination of these two eruptions impacted the atmosphere by injecting volcanic aerosol which are well represented by our model. In the case of Cerro Hudson, the simulations show that the volcanic plume may have impacted the 1991 Antarctic ozone hole, both by directly impacting the chemistry of ozone depletion and by altering the dynamics of the atmosphere, slowing down the normal breakdown of the ozone hole caused by changes in weather. The interaction between Cerro Hudson and Pinatubo shown here also reveal a need for more research in how potential goengineering scenarios would interact with volcanic eruptions. Key Points: The August 1991 eruption of Cerro Hudson is simulated using a model with detailed aerosol microphysics and stratospheric chemistryThe model shows skill in simulating the satellite and in‐situ observations of the volcanic plumeThese simulations present a prima facie case that this eruption contributed to the anomalously high 1991 Austral Springtime ozone loss [ABSTRACT FROM AUTHOR]

Tropospheric ozone is an air pollutant and a greenhouse gas whose anthropogenic production is limited principally by the supply of nitrogen oxides (NOx) from combustion. Tropospheric ozone in the northern hemisphere has been rising despite the flattening of NOx emissions in recent decades. Here we propose that this sustained increase could result from the photolysis of nitrate particles (pNO3−) to regenerate NOx. Including pNO3− photolysis in the GEOS‐Chem atmospheric chemistry model improves the consistency with ozone observations. Our simulations show that pNO3− concentrations have increased since the 1960s because of rising ammonia and falling SO2 emissions, augmenting the increase in ozone in the northern extratropics by about 50% to better match the observed ozone trend. pNO3− will likely continue to increase through 2050, which would drive a continued increase in ozone even as NOx emissions decrease. More work is needed to better understand the mechanism and rates of pNO3− photolysis. Plain Language Summary: In the troposphere, ozone is an air pollutant and a greenhouse gas. Tropospheric ozone forms from reactions involving carbon monoxide and volatile organic compounds in the presence of nitrogen oxides. Global emissions of nitrogen oxides have been leveling off in the past few decades, yet tropospheric ozone levels have kept on rising. We propose that this rise in ozone could be driven by a growing source of nitrogen oxides from the photolysis of nitrate particles, which have become more abundant due to falling sulfur dioxide and rising ammonia emissions. We find that including nitrate particle photolysis in an atmospheric chemistry model improves its consistency with the observed ozone distribution and trends. Our results point to the importance of considering nitrate particle photolysis for future projections of climate forcing from tropospheric ozone, and the need for further work to reduce the uncertainty in the mechanism and rates of the process. Key Points: Particulate nitrate photolysis improves the consistency of tropospheric ozone in the GEOS‐Chem model with observationsIncrease in particulate nitrate due to falling SO2 and rising NH3 emissions could augment the long‐term increase in tropospheric ozoneBetter characterization of the mechanism and rates of particulate nitrate photolysis is needed [ABSTRACT FROM AUTHOR]

Observed trends in tropospheric ozone, an important air pollutant and short‐lived climate forcer (SLCF), are estimated using available surface and ozonesonde profile data for 1993–2019, using a coherent methodology, and compared to modeled trends (1995–2015) from the Arctic Monitoring Assessment Program SLCF 2021 assessment. Increases in observed surface ozone at Arctic coastal sites, notably during winter, and concurrent decreasing trends in surface carbon monoxide, are generally captured by multi‐model median trends. Wintertime increases are also estimated in the free troposphere at most Arctic sites, with decreases during spring months. Winter trends tend to be overestimated by the multi‐model medians. Springtime surface ozone increases in northern coastal Alaska are not simulated while negative springtime trends in northern Scandinavia are not always reproduced. Possible reasons for observed changes and model performance are discussed including decreasing precursor emissions, changing ozone dry deposition, and variability in large‐scale meteorology. Plain Language Summary: The Arctic is warming much faster than the rest of the globe due to increases in carbon dioxide, and other trace constituents like ozone, also an air pollutant. However, improved understanding is needed about long‐term changes or trends in Arctic tropospheric ozone. A coherent methodology is used to identify trends in surface and regular profile measurements over the last 20–30 years, and results from six chemistry‐climate models. Increases in observed ozone are found at the surface and in the free troposphere during winter in the high Arctic. Paradoxically, decreases in nitrogen oxide emissions at mid‐latitudes appear to be leading to increases in ozone during winter, but associated increases in Arctic tropospheric ozone tend to be overestimated in the models. Increases are also found at the surface in northern Alaska during spring but not reproduced by the models. The causes are unknown but could be related to changes in local sources or sinks of Arctic ozone or in large‐scale weather patterns. Declining mid‐latitude emissions, or increased dry deposition to northern forests, may explain negative surface ozone trends over northern Scandinavia in spring that are not always captured by the models. Further work is needed to understand changes in Arctic tropospheric ozone. Key Points: Coherent ozone trend analysis methodology applied to multi‐decade, pan‐Arctic surface and ozonesonde datasets and multi‐model mediansIncreasing winter Arctic tropospheric ozone overestimated by models in the free troposphere, and spring surface changes not capturedSpring (summer) decreases (increases) in observed ozone throughout the troposphere, not always simulated by models [ABSTRACT FROM AUTHOR]

Year‐round precipitation in coastal East Antarctica and Antarctic Peninsula was used to investigate the seasonal patterns in sources of atmospheric perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$). Although featuring distinct climates, the two locations exhibit similar annual mean and seasonal cycles of ClO4− ${{\text{ClO}}_{4}}^{-}$ concentration, with higher values in autumn and lower concentrations in winter and spring. Tropospheric formation dominates atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ in spring and summer, which is influenced by both oxidants levels and environmental conditions (e.g., air humidity). Tropospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ production may also be promoted by elevated levels of oxidants brought by air mass from the interior Antarctic ice sheet in spring and summer. The autumn concentration maximum may originate from ClO4− ${{\text{ClO}}_{4}}^{-}$ produced in the stratosphere through reactions between reactive chlorine and ozone during spring and summer. In winter, the stratospheric input may contribute to ClO4− ${{\text{ClO}}_{4}}^{-}$ via polar stratospheric clouds sedimentation. Plain Language Summary: Perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$) is an inorganic anion with a persistent presence in the environment, where its exposure can pose a significant health risk to humans. Environmental ClO4− ${{\text{ClO}}_{4}}^{-}$ is derived from both man‐made and natural sources. Natural ClO4− ${{\text{ClO}}_{4}}^{-}$, widespread in the environment, is thought to be formed in the atmosphere. However, knowledge on the sources and, in particular, the formation mechanisms of natural ClO4− ${{\text{ClO}}_{4}}^{-}$ is quite limited. Antarctica, with negligible man‐made sources, is one of the best regions for investigations on natural ClO4− ${{\text{ClO}}_{4}}^{-}$. Year‐round precipitation samples were collected at China Zhongshan Station located in coastal East Antarctica and China Great Wall Station on King George Island, Antarctic Peninsula. Results show that atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ concentration presents obvious seasonal cycles, which are related to variations in sources. A significant amount of ClO4− ${{\text{ClO}}_{4}}^{-}$ is associated with tropospheric chemistry in spring and summer. Precursor levels, environmental conditions, and air mass from the interior Antarctica may influence tropospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ formation. The maximum concentration in autumn may originate from ClO4− ${{\text{ClO}}_{4}}^{-}$ formed in the stratosphere during spring and summer, and the stratospheric input may also contribute to atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ in winter. Key Points: Antarctic atmospheric perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$) concentration at different locations exhibits a clear seasonal cycleThe highest concentration in autumn is likely related to ClO4− ${{\text{ClO}}_{4}}^{-}$ produced in the stratosphere in spring and summerMajor source of ClO4− ${{\text{ClO}}_{4}}^{-}$ in spring and summer is tropospheric formation influenced by both oxidants and environmental conditions [ABSTRACT FROM AUTHOR]

The Pinatubo eruption in 1991 injected 10–20 Tg SO2 into the stratosphere, which formed sulfate aerosols through oxidation. Our modeling results show that volcanic heating significantly perturbs the heterogeneous and homogeneous chemistry including NOx and HOx catalytic cycles in the tropical stratosphere. The simulated tropical chemical ozone tendency is positive at 20 mb while negative at 10 mb in the tropics. The simulated ozone chemical tendency is of the same magnitude as the dynamical ozone tendency caused by the accelerated tropical upwelling, but with the opposite sign. Our study finds that the tropical ozone chemical tendency due to homogeneous chemistry becomes more important than heterogeneous chemistry 3 months after eruption. Sensitivity simulations further suggest that the tropical ozone tendency through heterogeneous chemistry is saturated when the injected amount exceeds 2 Tg. Plain Language Summary: In 1991, a large volcanic eruption injected 10–20 Tg SO2 into the stratosphere and perturbed the stratospheric chemistry and dynamics. In this study, we use a climate model to quantify the chemical and dynamical influence of the volcano on tropical stratospheric ozone. Model suggests that the ozone chemical tendency is positive around 28 km while negative around 35 km. Our study also suggests that both heterogeneous and homogeneous chemical reactions contribute to the ozone anomalies. With sensitivity studies, we show that the tropical ozone changes due to heterogeneous chemistry is saturated if the injected amount exceeds 2 Tg. Key Points: The simulated ozone tendency due to chemistry is of the same order of magnitude but of the opposite sign than that due to dynamicsThe chemistry‐driven change in the tropical ozone tendency is dominated by the gas‐phase rather than heterogeneous chemistryThe ozone tendency change due to heterogeneous chemistry is saturated when the injected SO2 amount exceeds 2 Tg [ABSTRACT FROM AUTHOR]

We combined observations and simulations to assess tropospheric ozone trends over Southeast Asia from 2005 to 2016. Multi‐platform observations showed that surface ozone had been increasing at rates of 0.7–1.2 ppb year−1 over the Peninsular Southeast Asia (PSEA) and 0.2–0.4 ppb year−1 over the Maritime Continents (MC); tropospheric ozone columns had been rising throughout Southeast Asia by 0.21–0.35 DU year−1. These observed ozone trends were better reproduced by simulations driven with satellite‐constrained NOx emissions, indicating that NOx emission growths may have been underestimated for the PSEA and overestimated for the MC in the Community Emissions Data System and the Global Fire Emissions Data set. The surface ozone increases over the PSEA were driven by rapidly growing local emissions, wherein fire emission growths may still be underestimated even with satellite constraints. We highlighted the need for better quantifying Southeast Asian emissions to benefit air quality management. Plain Language Summary: Tropospheric ozone at individual locations in Southeast Asia has reportedly been rising in recent decades, but the spatiotemporal characteristics and causes of such increases have not been assessed. We combined model simulations with surface, airborne, and satellite observations to assess the trends of surface and tropospheric ozone over Southeast Asia from 2005 to 2016. Surface and tropospheric ozone concentrations have risen throughout Southeast Asia, albeit with spatially and seasonally inhomogeneous trends. We showed that current emission inventories based on activity statistics may have underestimated the emission growths in the Peninsular Southeast Asia while overestimated the emission growths in the Maritime Continents. Emission inventories should be re‐evaluated to help guide air quality management in this region. Key Points: Surface and tropospheric ozone has risen significantly over Southeast Asia from 2005 to 2016NOx emission trends underestimated over Peninsular Southeast Asia but overestimated over Maritime Continents in inventoriesGrowing anthropogenic emissions drove large surface ozone increases over Peninsular Southeast Asia [ABSTRACT FROM AUTHOR]

Following the Hunga Tonga eruption (20.6°S, 175.4°W, mid‐January 2022), we present a balloon‐borne characterization of the stratospheric aerosol plume one week after its injection (on 23 and 26 January 2022, La Réunion island at 21.1°S, 55.3°E). Satellite observations show that flight (a) took place during the overpass of a denser plume of sulfate aerosols (SA) compared to a more diluted plume during flight. (b) Observations show that the sampled plumes (at around 22, 25 and 19 km altitude, respectively) consist exclusively of very small particles (with radius <1 µm). Particles with radii between 0.5 and 1.0 µm show optically transparent features pointing to predominant SA. Particles with radii below 0.5 µm are partly absorbing, which could point to small sulfate coated ash particles, a feature not identified with space‐borne observations. This shows that in situ observations are necessary to fully characterize the microphysical properties of the plumes tracked by space‐borne instruments. Plain Language Summary: The Hunga Tonga‐Hunga Ha'apai volcano (at 20.6°S, 175.4°W) erupted on 13 and 15 January 2022 with injection of gases and aerosols up to 55 km altitude. Here, we present a study based on in situ aerosol observations on weather balloons on La Réunion (21.1°S, 55.3°E) within the injected Hunga Tonga aerosol plume one week after the eruption (23 and 26 January 2022). With respective satellite observations, we show that the first measurement flight took place during the overpass of a denser aerosol plume compared to the second flight. We find that the plume exhibits only small particles <1 μm, mainly consisting of sulfate aerosols (for particles between 0.5 and 1 μm in size) and an absorbing component for very small particles (<0.5 μm), possibly pointing to small ash particles coated by sulfur. This letter "absorbing" feature is a unique contribution brought by in situ measurements that fills a gap left by space‐borne instruments. Key Points: Predominant particle size range of <1 μm within the stratospheric aerosol plume of the Hunga Tonga eruptionOptically absorbing particles within the plume for particles <0.5 μm point to fractured, very small ash particlesMostly optically semi‐transparent particles, for particle sizes between 0.5 and 1.0 μm result from small sulfur coated ash particles [ABSTRACT FROM AUTHOR]

The 2015 and 2020 ozone holes set record sizes in October–December. We show that these years, as well as other recent large ozone holes, still adhere to a fundamental recovery metric: the later onset of early spring ozone depletion as chlorine and bromine diminishes. This behavior is also captured in the Whole Atmosphere Chemistry Climate Model. We quantify observed recovery trends of the onset of the ozone hole and in the size of the September ozone hole, with good model agreement. A substantial reduction in ozone hole depth during September over the past decade is also seen. Our results indicate that, due to dynamical phenomena, it is likely that large ozone holes will continue to occur intermittently in October–December, but ozone recovery will still be detectable through the later onset, smaller, and less deep September ozone holes: metrics that are governed more by chemical processes. Plain Language Summary: The ozone hole that forms every spring in the Southern Hemisphere over Antarctica is expected to recover within the next 50 years. However, there have been several large ozone holes recently. In 2015, a record large volcanic eruption injected a large amount of sulfur into the upper atmosphere, which helps deplete ozone. In 2020, the Australian bushfires also injected a large amount of aerosol into the upper atmosphere, with a possible similar consequence. In this study, we show that by investigating a key time period in the early spring when the ozone hole is forming, the dynamical influence and the influence of volcanic and bushfire aerosols on ozone depletion over Antarctica is not as important and therefore ozone recovery can still be detected. Key Points: The seasonal onset of ozone depletion continues to slow down due to decreasing stratospheric chlorine and bromine concentrationsRecent large ozone holes of 2015, 2018, 2020, and 2021 display slower seasonal onset compared to 1999–2008 despite record sizes laterSeptember ozone hole size trends over 2000–2021 at different depth thresholds (such as 220, 175, 130 DU) indicate recovery [ABSTRACT FROM AUTHOR]

In the Antarctic ozone hole, ozone mixing ratios have been decreasing to extremely low values of 0.01–0.1 ppm in nearly all spring seasons since the late 1980s, corresponding to 95–99% local chemical loss. In contrast, Arctic ozone loss has been much more limited and mixing ratios have never before fallen below 0.5 ppm. In Arctic spring 2020, however, ozonesonde measurements in the most depleted parts of the polar vortex show a highly depleted layer, with ozone loss averaged over sondes peaking at 93% at 18 km. Typical minimum mixing ratios of 0.2 ppm were observed, with individual profiles showing values as low as 0.13 ppm (96% loss). The reason for the unprecedented chemical loss was an unusually strong, long‐lasting, and cold polar vortex, showing that for individual winters the effect of the slow decline of ozone‐depleting substances on ozone depletion may be counteracted by low temperatures. Plain Language Summary: The severe stratospheric chemical ozone loss in the Antarctic ozone hole and its impact on human health and climate have generated widespread public, political, and scientific interest. In contrast, Arctic stratospheric ozone reduction has been much more limited because of higher temperatures and higher transport variability in the Northern Hemisphere (lower temperatures lead to more chemical loss, and more transport can increase ozone values). In the Arctic spring 2020, however, observations of balloon sondes and satellites show that locally, absolute values of ozone (measured in mixing ratios, i.e., molecules of ozone per molecules of air) are significantly lower than in any previous year and are comparable to typical local values in the Antarctic ozone hole, albeit over a much narrower vertical layer. Locally, the chemical loss of ozone peaked at 93% in the Arctic spring of 2020, compared to values of 95–99% in the Antarctic in most winters since the late 1980s. The reason for the unprecedented loss was unusually cold and stable conditions in the Arctic stratosphere. Key Points: Local minimum ozone mixing ratios of 0.1–0.2 ppm observed by sondes in Arctic spring 2020 are significantly lower than in any previous yearLocal ozone loss (93%) and low mixing ratios are comparable to typical values in the Antarctic ozone hole (95–99%, 0.01–0.1 ppm)The reason for the unprecedented chemical loss was an unusually strong, long‐lasting, and record cold polar vortex [ABSTRACT FROM AUTHOR]

Using observations and model simulations (ESM4.1) during 1988–2018, we show large year‐to‐year variability in western U.S. PM2.5 pollution caused by regional and distant fires. Widespread wildfires, combined with stagnation, caused summer PM2.5 pollution in 2017 and 2018 to exceed 2 standard deviations over long‐term averages. ESM4.1 with a fire emission inventory constrained by satellite‐derived fire radiative energy and aerosol optical depth captures the observed surface PM2.5 means and extremes above the 35 μg/m3 U.S. air quality standard. However, aerosol emissions from the widely used Global Fire Emissions Database (GFED) must be increased by 5 times for ESM4.1 to match observations. On days when observed PM2.5 reached 35–175 μg/m3, wildfire emissions can explain 90% of total PM2.5, with smoke transported from Canada contributing 25–50% in northern states, according to model sensitivity simulations. Fire emission uncertainties pose challenges to accurately assessing the impacts of fire smoke on air quality, radiation, and climate. Plain Language Summary: Frequent and intense wildfires harm public health over the western United States. In order to understand how wildfires affect fine particulate air quality, we analyze surface and satellite measurements and computer model simulations of weather and atmospheric chemistry over the past 30 years. We show that widespread fires and regional transport of fire smoke are the main causes of year‐to‐year changes in summertime particle pollution measured at western U.S. surface sites. The U.S. Environmental Protection Agency defines daily particle concentration above 35 μg/m3 as unhealthy. In the summers of 2017–2018, record‐breaking wildfires, combined with stable weather conditions, resulted in daily particle concentration of 35 to 175 μg/m3 across western U.S. sites. These particle pollution extremes are twice as severe as long‐term average conditions. Wildfire emissions contributed 90% of particle levels on these periods. Notably, transport of fire smoke from southwestern Canada can explain 25% to 50% of particle pollution in northern states such as Washington. Our model successfully simulates these pollution extremes when applying a fire emission data set constrained by satellite observations of total particle abundances. Our results indicate fourfold to fivefold underestimates of particle emissions from the widely used Global Fire Emissions Database not constrained by satellite observations. Key Points: Large interannual variations of western U.S. fine particulate pollution in summer were driven by regional and distant firesWidespread wildfires and stagnation in 2017–2018 caused fine particulate extremes to exceed 2 standard deviations over long‐term averagesObservations and model analyses indicate fourfold to fivefold underestimate of aerosol emissions from the widely used Global Fire Emissions Database [ABSTRACT FROM AUTHOR]

Brown carbon (BrC) is an organic aerosol material that preferentially absorbs light of shorter wavelengths. Global‐scale radiative impacts of BrC have been difficult to assess due to the lack of BrC observational data. To address this, aerosol filters were continuously collected with near pole‐to‐pole latitudinal coverage over the Pacific and Atlantic basins in three seasons as part of the Atmospheric Tomography Mission. BrC chromophores in filter extracts were measured. We find that globally, BrC was highly spatially heterogeneous, mostly detected in air masses that had been transported from regions of extensive biomass burning. We calculate the average direct radiative effect due to BrC absorption accounted for approximately 7% to 48% of the top of the atmosphere clear‐sky instantaneous forcing by all absorbing carbonaceous aerosols in the remote atmosphere, indicating that BrC from biomass burning is an important component of the global radiative balance. Plain Language Summary: Combustion produces light‐absorbing aerosols that can affect the global radiation balance. Black carbon, which absorbs light over a broad wavelength range, has been extensively studied, but recent work shows that a significant component of the light‐absorbing aerosol is brown, absorbing mostly in the lower end of the visible and into the ultraviolet (UV). Incomplete combustion, such as in wild fires, is known to produce substantial levels of brown carbon. Here we report direct measurements of brown carbon determined from filter samples collected from aircraft flights that extended from pole to pole over three seasons. We observed brown carbon in aerosols that had been transported long distances from regions of wild fires at various locations across the globe. A radiative transfer model indicated that this brown carbon can substantially contribute to the overall radiative forcing by light‐absorbing aerosols. Key Points: Globally, biomass burning is a large source of light‐absorbing carbonaceous aerosol that directly affect the planetary radiation balanceTransported over long distances, brown carbon is a significant component of these aerosols, but its contribution was highly variableBrown carbon contributed up to 48% of average clear‐sky instantaneous forcing by light absorption by carbonaceous aerosols [ABSTRACT FROM AUTHOR]

An international effort to improve ozonesonde data quality and to reevaluate historical records has made significant improvements in the accuracy of global network data. However, between 2014 and 2016, ozonesonde total column ozone (TCO; O3) at 14 of 37 regularly reporting stations exhibited a sudden dropoff relative to satellite measurements. The ozonesonde TCO drop is 3–7% compared to satellite and ground‐based TCO, and 5–10% or more compared to satellite stratospheric O3 profiles, compromising the use of recent data for trends, although they remain reliable for other uses. Hardware changes in the ozonesonde instrument are likely a major factor in the O3 dropoff, but no single property of the ozonesonde explains the findings. The bias remains in recent data. Research to understand the dropoff is in progress; this letter is intended as a caution to users of the data. Our findings underscore the importance of regular ozonesonde data evaluation. Plain Language Summary: Balloon‐borne ozonesondes provide accurate measurements of atmospheric ozone (O3) from the surface to above 30 km with high vertical resolution. Dozens of global stations have regularly launched ozonesondes for decades, and they provide vital information for improving O3‐measuring satellite algorithms, tracking recovery of the stratospheric O3 layer, and our understanding of surface to lower stratospheric O3 changes in an evolving climate. We present the discovery of an apparent instrument artifact that has caused total column O3 measurements from about a third of global stations to drop by 3–7% starting in 2014–2016, limiting their suitability for calculating O3 trends. Work is underway to solve the problem, but the exact cause of the drop is still unknown. This letter serves as a caution to the community of ozonesonde data users. Key Points: We report a drop in ozonesonde total column O3 of 3–7% relative to independent measurements at a third of sites beginning around 2014Comparisons with satellite stratospheric O3 profiles show the artifact loss peaking at 5–10% or more in the middle and upper stratosphereChanges in the ozonesonde instrument are apparently associated with the dropoff, but no single factor appears to be the cause [ABSTRACT FROM AUTHOR]

Dry deposition (DD) is a major loss process for tropospheric ozone and some reactive nitrogen and carbon precursors. We investigate the response of summertime ozone and its production chemistry over the Southeast United States (USA) to variability in this sink. Turning off DD of oxidized nitrogen, ozone, or all species over the United States in the Geophysical Fluid Dynamics Laboratory AM3 model increases regional mean surface ozone by 5, 18, or 25 ppb, respectively. Additional sensitivity simulations demonstrate that, assuming linearity, surface ozone has a similar sensitivity to ozone DD as to NOx emissions. Trends in ozone production efficiency derived from observed relationships between ozone and precursor oxidation products may not solely reflect precursor emission changes if ozone DD varies (e.g., with meteorology). We conclude that DD variability merits consideration when interpreting observed ozone trends. Quantifying the impact of changes in sinks versus sources will require long‐term DD measurements across the region of interest. Plain Language Summary: Ozone is an air pollutant near Earth's surface and a greenhouse gas higher in the atmosphere. While many studies consider which ozone sources should be controlled to reduce air pollution, we focus here on a poorly understood removal pathway, "dry deposition" (DD), to the Earth's surface. We use a model to quantify how ozone responds to changes in the DD of ozone and related gases (oxidized nitrogen and carbon gases). We find that a 50% decrease in DD changes ozone concentrations in the Southeast United States by at least as much as recent changes in national emissions of nitrogen oxides (which form ozone in the presence of sunlight and reactive carbon gases). Our model suggests that DD variability, potentially driven by changes in meteorology or land use, could affect surface ozone trends. However, we lack the observations that would allow an assessment of how DD changes over space and time. Key Points: A 50% decrease in ozone dry deposition has twice the effect on surface ozone over the Southeast USA as a 25% increase in NOx emissionsTwo different metrics for ozone production efficiency show opposite‐signed responses to shutting off ozone dry depositionBounding the influence of ozone dry deposition on tropospheric composition requires long‐term, regional observations of its variability [ABSTRACT FROM AUTHOR]

Salt marsh assessments focus on vertical metrics such as accretion or lateral metrics such as open‐water conversion, without exploration of how the dimensions are related. We exploited a novel geospatial data set to explore how elevation is related to the unvegetated‐vegetated marsh ratio (UVVR), a lateral metric, across individual marsh "units" within four estuarine‐marsh systems. We find that elevation scales consistently with the UVVR across systems, with lower elevation units demonstrating more open‐water conversion and higher UVVRs. A normalized elevation‐UVVR relationship converges across systems near the system‐mean elevation and a UVVR of 0.1, a critical threshold identified by prior studies. This indicates that open‐water conversion becomes a dominant lateral instability process at a relatively conservative elevation threshold. We then integrate the UVVR and elevation to yield lifespan estimates, which demonstrate that higher elevation marshes are more resilient to internal deterioration, with an order‐of‐magnitude longer lifespan than predicted for lower elevation marshes. Plain Language Summary: Salt marshes are valuable ecosystems that change in response to sea‐level rise, sediment availability, storms, and other processes. Determining how salt marshes will change in the future is difficult. In this study we show that the elevation of a marsh relative to sea level is closely related to how much area it has been losing. The relationship between elevation and marsh area is the same across four large systems, and we can use these measurements to calculate how long a marsh will survive as sea‐level rises. We found that the lowest marshes have a shorter lifespan than higher marshes, and these estimates can be used to help prioritize investments. Key Points: Geospatial analysis reveals a consistent relationship between elevation and open‐water conversion in salt marshesThe unvegetated‐vegetated marsh ratio (UVVR) consistently indicates instability above a value of 0.1Marsh lifespan can be estimated by integrating elevation and the UVVR for current and potential rates of SLR [ABSTRACT FROM AUTHOR]

This study examines the variations of stratospheric temperature and ozone associated with the lifecycle of El Nino‐Southern Oscillation (ENSO). Significant stratospheric temperature and ozone anomalies are found not only during El Nino and La Nina, but also during the transition phases in between, and anomalies during the cold‐to‐warm transition generally mirror those during the warm‐to‐cold transition. Zonal mean temperature anomalies propagate slowly upward over northern midlatitudes but downward over the Arctic. The strongest ozone anomalies over Antarctica occur one year after El Nino and La Nina, which show a dipole structure between the upper stratosphere and the lower stratosphere. The ozone anomalies are associated with anomalous Brewer‐Dobson circulation and anomalous Eliassen‐Palm flux divergence. Key Points: Significant stratospheric temperature and ozone anomalies exist in the transition phases of El Nino‐Southern OscillationThe strongest ozone anomalies over Antarctica occur one year after El Nino and La Nina with anomalous Brewer‐Dobson circulation and wavesZonal mean temperature anomalies show slow upward propagation over northern midlatitudes but slow downward propagation over the Arctic [ABSTRACT FROM AUTHOR]

New proxy records from deep‐sea sediment cores from the northwestern continental margin of Western Australian reveal a 5.3 million year (Ma) history of aridity and tropical monsoon activity in northwestern Australia. Following the warm and dry early Pliocene (~5.3 Ma), the northwestern Australian continent experienced a gradual increase in humidity peaking at about 3.8 Ma with higher than present‐day rainfall. Between 3.8 and about 2.8 Ma, climate became progressively more arid with more rainfall variability. Coinciding with the onset of the Northern Hemisphere glaciations and the intensification of the Northern Hemisphere monsoon, aridity continued to increase overall from 2.8 Ma until today, with greater variance in precipitation and an increased frequency of large rainfall events. We associate the observed large‐scale fluctuations in Australian aridity with variations in Indian Ocean sea surface temperatures, which largely control the monsoonal precipitation in northwestern Australia. Plain Language Summary: Australia is the driest inhabited continent on the planet, with its moisture mostly sourced from the tropical monsoon in the north and the southern westerlies in the south. The continent has experienced large climate fluctuations in the geologic past, but long continuous records of paleoenvironmental changes are lacking, particularly prior to ~0.55 Ma. Here, we address this paucity by presenting a continuous and fluctuating record of continental aridity and monsoonal activity in northwestern Australia since the Pliocene (5.3 Ma). These records are based on bulk‐chemical X‐Ray Fluorescence scans and particle‐size distributions of the terrigenous fraction in two marine sediment cores from the NW Western Australian continental margin. A comparison with present‐day sources of windblown and fluvial sediments taken near the NW Western Australian coast corroborates our interpretation of the terrigenous fraction in the marine sediment cores. We show how the northwestern part of the Australian continent has experienced large climate fluctuations since 5.3 Ma, expressed by large aridity contrasts and great changes in monsoonal precipitation that are driven by Indian Ocean sea‐surface temperatures. Key Points: Continental margin sediments off northwestern Australia record large continental aridity shifts at 5.3, 3.8, 2.8, and 1.4 MaGrain size and chemistry of the terrigenous fraction of seafloor sediments and source areas on land allow a characterization of river mudMonsoonal activity responds to changes in Indian‐Ocean SSTs and drives river runoff in northwestern Australia [ABSTRACT FROM AUTHOR]

We report airborne measurements of acetaldehyde (CH3CHO) during the first and second deployments of the National Aeronautics and Space Administration Atmospheric Tomography Mission (ATom). The budget of CH3CHO is examined using the Community Atmospheric Model with chemistry (CAM‐chem), with a newly developed online air‐sea exchange module. The upper limit of the global ocean net emission of CH3CHO is estimated to be 34 Tg/a (42 Tg/a if considering bubble‐mediated transfer), and the ocean impacts on tropospheric CH3CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH3CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid is an ideal indicator of the rapid CH3CHO production in the remote troposphere. The higher‐than‐expected CH3CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry‐climate models. Plain Language Summary: The Earth's atmosphere and its ability to self‐regulate and cleanse itself is dependent on a complex interplay of trace chemical species, some of which are emitted from the biosphere, while others are from human activities or fires. One of these key species, acetaldehyde, was measured as part of the recent Atmospheric Tomography Mission, an aircraft (National Aeronautics and Space Administration DC‐8) experiment transecting the lengths of the Pacific and Atlantic Oceans during two seasons, measuring greenhouse gases and chemically reactive gases and particles. These measurements allow us to test our ability to model the chemical state of the atmosphere. The results indicate that the ocean is a large source of acetaldehyde and the analysis here suggests additional mechanisms that narrow the gap between observations and simulations but also reveal that an additional unexplained source or sources remain(s) in the remote free troposphere. It is critical to understand this missing carbon source because it has significant implications for understanding the cycle of oxidants which, in turn, provide for the means of removing (cleaning) trace gases including methane, an important greenhouse gas, from the atmosphere. Key Points: Observed acetaldehyde (CH3CHO) in the remote troposphere is compatible with observed peroxyacetic acid (PAA) within uncertaintiesWe speculate that there is a missing CH3CHO source in the remote atmosphere and observed organic aerosols cannot explain the observed CH3CHOThe ocean is a net source of CH3CHO; the ocean biogeochemistry control on the atmospheric chemistry warrants further investigation [ABSTRACT FROM AUTHOR]

Convective systems dominate the vertical transport of aerosols and trace gases. The most recent in situ aerosol measurements presented here show that the concentrations of primary aerosols including sea salt and black carbon drop by factors of 10 to 10,000 from the surface to the upper troposphere. In this study we show that the default convective transport scheme in the National Science Foundation/Department of Energy Community Earth System Model results in a high bias of 10–1,000 times the measured aerosol mass for black carbon and sea salt in the middle and upper troposphere. A modified transport scheme, which considers aerosol activation from entrained air above the cloud base and aerosol‐cloud interaction associated with convection, dramatically improves model agreement with in situ measurements suggesting that deep convection can efficiently remove primary aerosols. We suggest that models that fail to consider secondary activation may overestimate black carbon's radiative forcing by a factor of 2. Plain Language Summary: Convective systems dominate the vertical transport of aerosols and trace gases. The most recent global aerosol measurements show that the concentrations of primary aerosols including sea salt and black carbon drop by a factor of 10–10,000 from the surface to the upper troposphere. In this study, we show that a climate model overestimates black carbon and sea salt in the middle and upper troposphere by factors of 10–1,000 when compared with observations from a number of field campaigns. The default transport scheme in a climate model not only significantly overestimates black carbon and sea salt in the upper troposphere but also overestimates the total aerosol mass budget by a factor of 3 in the global middle and upper troposphere. We modify the convective transport scheme in the climate model by including aerosol activation and removal processes above the cloud base. The new convective transport scheme dramatically improves model performance on the global aerosol budget in the middle and upper troposphere. We show in this study that models fail to consider aerosol secondary activation from the entrained air may overestimate black carbon's radiative forcing by a factor of two. Key Points: Deep convection efficiently removes particles at and above the cloud baseThe default convective scheme in climate model overestimates middle and upper tropospheric aerosol (sea salt and black carbon) by a factor of 10–1,000The default convective transport scheme overestimates black carbon's radiative forcing by a factor of 2 [ABSTRACT FROM AUTHOR]

Abstract: Ozonesonde data from four sites are analyzed in relation to 191 solar proton events from 1989 to 2016. Analysis shows ozone depletion (~10–35 km altitude) commencing following the SPEs. Seasonally corrected ozone data demonstrate that depletions occur only in winter/early spring above sites where the northern hemisphere polar vortex (PV) can be present. A rapid reduction in stratospheric ozone is observed with the maximum decrease occurring ~10–20 days after solar proton events. Ozone levels remain depleted in excess of 30 days. No depletion is observed above sites completely outside the PV. No depletion is observed in relation to 191 random epochs at any site at any time of year. Results point to the role of indirect ozone destruction, most likely via the rapid descent of long‐lived NOx species in the PV during the polar winter. [ABSTRACT FROM AUTHOR]

Abstract: Attribution of Antarctic ozone recovery to the Montreal protocol requires evidence that (1) Antarctic chlorine levels are declining and (2) there is a reduction in ozone depletion in response to a chlorine decline. We use Aura Microwave Limb Sounder measurements of O3, HCl, and N2O to demonstrate that inorganic chlorine (Cly) from 2013 to 2016 was 223 ± 93 parts per trillion lower in the Antarctic lower stratosphere than from 2004 to 2007 and that column ozone depletion declined in response. The mean Cly decline rate, ~0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes. N2O measurements are crucial for identifying changes in stratospheric Cly loading independent of dynamical variability. From 2005 to 2016, the ozone depletion and Cly time series show matching periods of decline, stability, and increase. The observed sensitivity of O3 depletion to changing Cly agrees with the sensitivity simulated by the Global Modeling Initiative chemistry transport model integrated with Modern Era Retrospective Analysis for Research and Applications 2 meteorology. [ABSTRACT FROM AUTHOR]