Abstract Biomass burning (BB) is a major global sources of ozone (O 3 ) precursors, including nitrogen oxides (NO x ), carbon monoxide (CO), and volatile organic compounds (VOCs). Here, we combine Community Earth System Model 1 with airborne measurements from the NASA Atmospheric Tomography mission conducted during 2016–2018 to assess the global impact of aged BB emissions on O 3 from the surface to the lower stratosphere. The model captures the broad spatial distribution of BB‐related tracers and O 3 in the Atlantic, but it significantly underestimates BB‐related aerosol concentrations by an order of magnitude above 3 km in the tropical and Northern Hemispheric Pacific, leading to an underestimation of BB‐induced O 3 concentrations. In the upper troposphere and lower stratosphere over the tropical Atlantic, BB‐induced O 3 enhancements are most pronounced during August to November, with peak values reaching up to 30 ppbv in October. These enhancements are primarily driven by the oxidation of BB‐emitted VOCs and CO, followed by the photolysis of the NO 2 . The Asian summer monsoon circulation plays a crucial role in lofting these precursors into higher altitudes, where NO x is converted into its reservoir form, peroxyacetyl nitrate. These findings demonstrate BB's extensive vertical and horizontal influence on atmospheric O 3 , underscoring the importance of accurately representing BB processes in climate and air quality models to better assess their impacts on global air pollution and climate change.

Stratospheric water vapour (SWV) is a key greenhouse gas that influences both global climate and stratospheric ozone chemistry1-4. Its abundance is strongly modulated by natural climate variability1,5-8. Volcanic eruptions have long been expected to humidify the stratosphere via tropopause warming9,10, but observational confirmation has been lacking. Here we provide observational evidence that moderate volcanic eruptions and extreme wildfires since 2005 have systematically increased SWV. Both contribute through aerosol-induced tropopause warming; however, extreme wildfires reveal an additional self-lofting pathway that transports water vapour into the stratosphere. Complementary analysis of satellite observations and climate model simulations reveals an SWV enhancement of about 0.1 ppmv at 83 hPa, accumulating 76-203 million tons of water vapour during 2005-2021. This contribution explains 36 ± 7% of the observed SWV trend over this period, comparable to that from the global surface temperature increase. SWV changes induced by the surface temperature trend, moderate volcanic eruptions and extreme wildfire events have together effectively offset the sudden 10% SWV decrease observed around 2000. Episodic aerosol perturbations from moderate volcanic eruptions and extreme wildfires therefore emerge as a previously overlooked driver of SWV variability. Future projections of stratospheric composition, radiative forcing and ozone recovery should account for these aerosol-mediated processes, especially as extreme fires intensify in a warming world.

Stratospheric aerosol injection (SAI) programs are generally understood to be technologically straightforward and inexpensive relative to other climate interventions or remedies. This gives rise to the sensible question of whether uninvolved states are at risk of having their climates covertly manipulated without their knowledge by other actors. This article seeks to contextualize and ameliorate that fear. We first survey the wide range of SAI experiments and deployments that are theorized to clarify the deployed mass requirements necessary to create discernable impacts in uninvolved states. We then explore two methods that could be reliably used by civilian uninvolved parties to detect such deployments well before they reached the scale required to induce climatic impacts. These involve detecting the plumes of sulfate precursor gases shortly after their injection using existing satellite instruments capable of monitoring point source emissions and identifying the aircraft fleets and support operations that would be required to transport and release those precursors high in the atmosphere. While small process experiments with negligible surface impacts could easily be conducted covertly, we demonstrate that deployments of the scale required to create climatic impacts would be discernible by uninvolved parties well before any climatic impacts would occur.

We present simulations from two Earth system models in which liquefied natural gas (LNG) rocket fuel burning injects gaseous and particulate material into the stratosphere during liftoff. We considered two heavy rocket launch scenarios representative for the year 2050, one with 1000 launches per year, a scenario equivalent with present-day emissions using kerosene propellant, and another with 10 times more. Injected species that include gaseous carbon monoxide, nitrogen oxides, and water vapor showed marginal or insignificant changes in atmospheric composition, while particulate black carbon was found to be the most influential of all injected material, via feedbacks that involve tropopause and stratospheric warming, followed by a moistening of the stratosphere via tropospheric water vapor intrusions. The fact that the LNG is still a concern via aerosol pollution in our simulations, despite burning more efficiently than conventional fuels, underscores the need to better understand the future climate impacts of space flight on atmospheric composition and climate.

The Asian Summer Monsoon (ASM), as well as tropical convection, transport aerosols and their precursors from the boundary layer to the upper troposphere and lower stratosphere (UTLS). We utilize the Community Aerosol and Radiation Model for Atmospheres (CARMA) coupled with the Community Earth System Model (CESM) to simulate all major tropospheric aerosols including sulfate, organics, ammonium, nitrate, sea salt, and dust. We evaluate the model during the ASM season by comparing the simulated aerosol microphysical properties, such as particle compositions, size distribution, and particle volume with MIPAS satellite and in-situ data from three field campaigns. We find nitrate, organics, and sulfate contribute significantly to the UTLS optical properties between 0-45˚N, 0-180˚E. The major source of nitrate is the ammonium nitrate formed locally and nitric acid condensation near the cold tropopause. Including nitrate in the model doubles the surface area density between 0-45˚N, 0-180˚E, which alter the chlorine partitioning at the UTLS region.

The recent surge in rocket launch rates, including the proposal of large low earth orbit satellite constellations (LLC’s) has renewed interest into how space traffic may impact Earth’s climate. In the future. The current annual mass flux from satellites vaporized in Earth’s middle atmosphere each year is ~0.4 Gg, well below the ~20 Gg/year natural mass emissions from meteor ablation. However, it is predicted that if all proposed LLC’s are implemented, the total number of satellites in low earth orbit (LEO) will balloon from ~5,000 to over 60,000 units. The corresponding annual emissions from satellite re-entry is also expected to increase and approach 10 Gg/yr. Although little is currently known about the composition of aerosols released during satellite ablation, we assume a significant portion of the aerosol population is metallic aluminum that will convert to aluminum oxide (Al2O3). Here we present results from a study which focuses on the radiative impacts and atmospheric transport of hypothetical Al2O3 emissions from satellite re-entry. The WACCM6 global model coupled with the CARMA sectional model was run with a 10 Gg/year mass flux of alumina aerosol between 60 km and 70 km. We evaluate how aerosol size and latitude of emission may impact the overall transport, atmospheric burden, and radiative impacts from satellite re-entry.

Stratospheric aerosol injection (SAI) using sulfur has been proposed to cool the planet by reflecting sunlight back to space. A commonly proposed SAI, with sulfur dioxide injection rate of 10 Tg yr-1 at 25 km, accumulates aerosols in the tropical lower stratosphere, causing a 6 K warming of the tropical lower stratosphere that impact the entry value of stratospheric water vapor and jet positions. This approach could also delay October Antarctic total column ozone (TCO) recovery to 1980s values by 25-55 years. We propose a novel SAI approach of injecting sulfur at 50 km (SAI50) that substantially reduces these negative impacts. In SAI50, the mean meridional overturning circulation near the stratopause rapidly transports gaseous SO2 to mid-high latitudes, preventing sulfate aerosol accumulation in the tropical lower stratosphere. This approach reduces tropical stratospheric warming to 3 K and shortens the Antarctic ozone recovery delay to 5 years. Furthermore, SAI50 demonstrates greater cooling efficiency, enhancing global and polar surface cooling by 22 % and 40 % respectively. Consequently, SAI50 preserves 20 % more Arctic September sea ice compared to lower-altitude SAI. These findings suggest that SAI50 could offer a more effective and less disruptive approach to climate intervention.

We present simulations of an Earth system model in which launch vehicles inject gaseous and particulate combustion products into the stratosphere. We considered two plausible scenarios representative for the year 2050, one with 1000 launches per year and another with 10 times more, all of which assuming heavy lift (80 tons to low Earth orbit; LEO) launches using methane as a proxy for liquefied natural gas (LNG) fuel. An industry-standard plume flowfield model was used to predict emission of gaseous species including carbon monoxide, nitrogen oxides, and water vapor. Black carbon was introduced assuming vacuum emission equal 25% of equivalent kerosene engine. The gas emissions showed marginal or insignificant changes in atmospheric composition, while the black carbon emission was found to be the most influential component. Feedbacks involve tropopause and stratospheric warming, followed by a moistening of the stratosphere via tropospheric water vapor intrusions, together with a reduction in global albedo. The fact that LNG-fueled rockets could be a concern from aerosol emissions in our simulations, despite burning more efficiently than conventional fuels, underscores the need to better understand actual black carbon emissions from rocket engines in order to accurately predict the global impacts of launch vehicles on future climate.

Abstract Approximately 150 Tg of water vapor and 0.42 Tg of sulfur dioxide were injected directly into the stratosphere by the January 2022 Hunga volcanic eruption, which represents the largest water vapor injection in the satellite era. A comparison of numerical simulations to balloon‐borne and satellite observations of the water‐rich plume suggests that particle coagulation contributed to the Hunga aerosol's effective dry radius increase from 0.2 μm in February to around 0.4 μm in March. Our model suggests that the stratospheric aerosol effective radius is persistently perturbed for years by moderate and large‐magnitude volcanic events, whereas extreme wildfire events show limited impact on the stratospheric background particle size. Our analysis further suggests that both the particle optical efficiency and the aerosols' stratospheric lifetime explain Hunga's unusually large aerosol optical depth per unit of the SO2 injection, as compared with the Pinatubo eruption.

The Asian Summer Monsoon (ASM) convection transports aerosols and their precursors from the boundary layer to the upper troposphere and lower stratosphere (UTLS). This process forms an annually recurring aerosol layer near the tropopause. Recent observations have revealed a distinct property of the aerosol layer over the ASM region, it is nitrate-rich. We present a newly implemented aerosol formation algorithm that enhances the representation of nitrate aerosol in the Community Aerosol and Radiation Model for Atmospheres (CARMA) coupled with the Community Earth System Model (CESM). The simulated aerosol chemical composition, as well as vertical distributions of aerosol size and mass, are evaluated using in situ and remote sensing observations. The simulated concentrations (ammonium, nitrate, and sulfate) and size distributions are generally within the error bars of data. We find nitrate, organics, and sulfate contribute significantly to the UTLS aerosol concentration between 15 degrees-45 degrees N and 0 degrees-160 degrees E. The two key formation mechanisms of nitrate-containing aerosols in the ATAL are ammonium neutralization to form ammonium nitrate in regions where convection is active, and condensation of nitric acid in regions of cold temperature. Furthermore, including nitrate formation in the model doubles the surface area density in the tropical tropopause region between 15 degrees-45 degrees N and 0 degrees-160 degrees E, which alters the chlorine partitioning and subsequently impacts the rate of ozone depletion. The Asian Summer Monsoon can efficiently transport pollutants into the upper troposphere and form a layer of aerosols called the Asian Tropopause Aerosol Layer (ATAL). This research investigates the two key formation mechanisms for nitrate aerosol in the ATAL: one is due to cold temperature, and one is due to ammonium neutralization. The simulations are validated against in situ measurements. It also found that O3 chemistry inside ATAL is influenced by nitrate aerosol. A new nitrate aerosol formation algorithm is implemented for the community model CARMA to represent the Asian Tropopause Aerosol Layer The simulated aerosol size distributions and compositions are within error bars of observations Including the new nitrate formation scheme doubles the simulated surface area, changing the chlorine partitioning and ozone depletion

We introduce a climate intervention strategy focused on decreasing water vapor (WV) concentrations near the tropopause and in the stratosphere to increase outbound longwave radiation. The mechanism is the targeted injection of ice-nucleating particles (INP) in air supersaturated with respect to ice at high altitudes in the tropical entryway to the stratosphere. Ice formation in this region is a critical control of stratospheric WV. Recent airborne in situ data indicate that targeting only a small fraction of air parcels in the region would be sufficient to achieve substantial removal of water. This "intentional stratospheric dehydration" (ISD) strategy would not counteract a large fraction of the forcing from carbon dioxide but may contribute to a portfolio of climate interventions by acting with different time and length scales of impact and risk than other interventions that are already under consideration. We outline the idea, its plausibility, technical hurdles, and side effects to be considered.

Abstract Aerosol particles play a critical role in the tropical tropopause layer (TTL) through cloud formation and heterogeneous chemistry, influencing the radiative and chemical balance of the stratosphere. However, aerosol measurements in the TTL are sparse, resulting in poor knowledge of aerosol abundance and distribution in this important region. Here, we present in situ aircraft measurements over the western tropical Pacific, revealing a persistent and altitude‐dependent enhancement of aerosol mass in the TTL compared to the convectively influenced troposphere below. Notably, our data demonstrate a striking positive correlation between aerosol mass and ozone. Model simulations suggest that organic materials constitute a substantial fraction of the total aerosol mass within the TTL. We further derived an empirical parameterization of TTL aerosol mass as a function of ozone based on their linear relationship. This framework holds potential for estimating the TTL aerosol abundance but requires further validation and refinement through future measurements.

We performed multiple thirty year simulations to investigate the potential atmospheric transport and radiative impacts from metallic aerosol emissions that could be released following the projected increase in satellite reentry frequency due to large Low Earth Orbit constellation deployments. Aluminum oxide was chosen as the likely primary emission from satellite reentry. The alumina was assumed to be emitted uniformly between 60 km and 70 km. Various emission patterns based on latitude and aerosol size were investigated. We find that aluminum oxide accumulates between 10 km and 30 km altitude and at high latitudes in both hemispheres. The time it takes for satellite reentry material to reach the poles equals roughly 5 to 24 months depending on the latitude of emissions and aerosol size. The radiative impact of the reentry aluminum oxide accumulation is small, on the order of several mW/m2. The associated stratospheric temperature perturbations are also small. Two emission scenarios produce statistically significant stratospheric temperature anomalies of a degree Kelvin or less over the southern hemisphere at high latitudes.

With surface temperatures already reaching unprecedented highs, resulting in significant adverse consequences for societies and ecosystems, there are increasing calls to expand research into climate interventions, including Stratospheric Aerosol Intervention (SAI). However, research and dissemination are currently fragmented and would benefit from a comprehensive international assessment of the current state of knowledge regarding impacts, risks, and recommendations for future SAI research directions. The goals of a scientific assessment would be to describe the current state of SAI research and evaluate proposed scenario-strategy combinations through well-designed evaluation guidelines. The suggested iterative approach would integrate natural and social science considerations to guide future research toward more plausible scenarios and strategy development to reduce uncertainties and minimize the risks of SAI. Here, we outline multidisciplinary research criteria to guide the assessment process and provide an overview of the benefits and risks of proposed SAI applications. We group these criteria into three categories: (1) technical and design requirements, (2) response and impacts, and (3) societal considerations. Including all three categories in a comprehensive assessment of potential SAI applications outlined here promotes enhanced interdisciplinary and international collaborations, intentionally engaging the underrepresented Global South. The assessment structure further promotes the need for recurring reports every few years with globally representative participation and could also be applicable to other Solar Radiation Modification methods or combined approaches. Such assessments are necessary to align research with considerations for decision-makers and the public on the feasibility of SAI in reducing the impacts of climate change and its potential societal and ecological trade-offs.

The Hunga Tonga-Hunga Ha'apai (HT-HH) volcanic eruptions on January 13 and 15, 2022, produced a plume with the highest signal in stratospheric aerosol optical depth observed since the eruption of Mt. Pinatubo in 1991. Suites of balloon -borne instruments on a series of launches from Reunion Island intercepted the HT-HH plume between 7 and 10 d of the eruptions, yielding observations of the aerosol number and size distribution and sulfur dioxide (SO2) and water vapor (H2O) concentrations. The measurements reveal an unexpected abundance of large particles in the plume, constrain the total sulfur injected to approximately 0.2 Tg, provide information on the altitude of the injection, and indicate that the formation of sulfuric acid aerosol was complete within 3 wk. Large H2O enhancements contributed as much as -30% to ambient aerosol surface area and likely accelerated SO2 oxidation and aerosol formation rates in the plume to approximately three times faster than under normal stratospheric conditions.

The eruption of the Hunga Tonga-Hunga Ha'apai volcano on 15 January 2022 offered a good opportunity to explore the early impacts of tropical volcanic eruptions on stratospheric composition. Balloon-borne observations near Réunion Island revealed the unprecedented amount of water vapor injected by the volcano. The enhanced stratospheric humidity, radiative cooling, and expanded aerosol surface area in the volcanic plume created the ideal conditions for swift ozone depletion of 5% in the tropical stratosphere in just 1 week. The decrease in hydrogen chloride by 0.4 parts per million by volume (ppbv) and the increase in chlorine monoxide by 0.4 ppbv provided compelling evidence for chlorine activation within the volcanic plume. This study enhances our understanding of the effect of this unusual volcanic eruption on stratospheric chemistry and provides insights into possible chemistry changes that may occur in a changing climate.

Satellite data records of stratospheric water vapour have been compared to balloon-borne frost point hygrometer (FP) profiles that are coincident in space and time. The satellite data records of 15 different instruments cover water vapour data available from January 2000 through December 2016. The hygrometer data are from 27 stations all over the world in the same period. For the comparison, real or constructed averaging kernels have been applied to the hygrometer profiles to adjust them to the measurement characteristics of the satellite instruments. For bias evaluation, we have compared satellite profiles averaged over the available temporal coverage to the means of coincident FP profiles for individual stations. For drift determinations, we analysed time series of relative differences between spatiotemporally coincident satellite and hygrometer profiles at individual stations. In a synopsis we have also calculated the mean biases and drifts (and their respective uncertainties) for each satellite record over all applicable hygrometer stations in three altitude ranges (10–30 hPa, 30–100 hPa, and 100 hPa to tropopause). Most of the satellite data have biases <10 % and average drifts <1 % yr−1 in at least one of the respective altitude ranges. Virtually all biases are significant in the sense that their uncertainty range in terms of twice the standard error of the mean does not include zero. Statistically significant drifts (95 % confidence) are detected for 35 % of the ≈ 1200 time series of relative differences between satellites and hygrometers.