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Dust aerosols affect the radiative and energy balance at local and global scales by scattering and absorbing sunlight and infrared light. A previous study suggests that dust size distribution is one of the major sources of uncertainty in modeling the dust global distribution. Climate models overestimate the fine dust (≤5 µm) by an order of magnitude, while underestimates of the coarse dust (≥5 µm) range between 0.5 to 1.5 orders of magnitude compared with the global observations. Here we improved the simulated size distribution of dust aerosol using a sectional aerosol model (Community Aerosol and Radiation Model for Atmospheres) coupled with the Community Earth System Model (CESM1/CARMA). Simulated dust mass size distributions peak at around 2–3 µm in diameter and increase by 4 orders of magnitude from 0.1 to 2 µm. Our model demonstrates that North African, Middle Eastern, and Asian dust accounts for ∼ 59.7 %, 12.5 %, and 13.3 % of the global annual mean dust emissions, with the remaining 14.5 % originating from scattered smaller dust sources. The model dust vertical distributions are validated against the NASA Atmospheric Tomography (ATom) field campaign datasets. Both simulations and ATom in situ measurements during the ATom field campaign suggest that dust mass concentrations over the remote ocean drop by 2 to 3 orders of magnitude from the surface to the upper troposphere (200 hPa). Our model suggests that Asian dust contributes to more than 40 % of annual mean dust mass abundances in the global upper troposphere and lower stratosphere (UTLS). The model suggests that Asian dust dominates the dust mass budget in the UTLS of the Asian summer monsoon (ASM) region, with a relative contribution 1–2 orders of magnitude higher than the dust originating from the North African and Middle Eastern deserts.

Unidad de excelencia María de Maeztu CEX2019-000940-M Atmospheric soot loadings from nuclear weapon detonation would cause disruptions to the Earth's climate, limiting terrestrial and aquatic food production. Here, we use climate, crop and fishery models to estimate the impacts arising from six scenarios of stratospheric soot injection, predicting the total food calories available in each nation post-war after stored food is consumed. In quantifying impacts away from target areas, we demonstrate that soot injections larger than 5 Tg would lead to mass food shortages, and livestock and aquatic food production would be unable to compensate for reduced crop output, in almost all countries. Adaptation measures such as food waste reduction would have limited impact on increasing available calories. We estimate more than 2 billion people could die from nuclear war between India and Pakistan, and more than 5 billion could die from a war between the United States and Russia-underlining the importance of global cooperation in preventing nuclear war.

The direct effects of nuclear war would be horrific, with blast, fires, and radiation killing and injuring many people. But in 1983, United States and Soviet Union scientists showed that a nuclear war could also produce a nuclear winter, with catastrophic consequences for global food supplies for people far removed from the conflict. Smoke from fires ignited by nuclear weapons exploded on cities and industrial targets would block out sunlight, causing dark, cold, and dry surface conditions, producing a nuclear winter, with surface temperatures below freezing even in summer for years. Nuclear winter theory helped to end the nuclear arms race in the 1980s and to produce the Treaty on the Prohibition of Nuclear Weapons in 2017, which led to the 2017 Nobel Peace Prize. Because awareness of nuclear winter is now widespread, nuclear nations have so far not used nuclear weapons. But the mere existence of nuclear weapons means that they can be used, by unstable leaders or because of an accident, computer malfunction, sensor malfunction, human error, or terrorism. Because they cannot be used without the danger of escalation and a global humanitarian catastrophe, and because of recent threats to use them by Russia, it is even more urgent for scientists to broadly communicate their results and work for the elimination of nuclear weapons.

Dust aerosols affect the radiative and energy balance at local and global scales by scattering and absorbing sunlight and infrared light. Parameterizations of dust lifting, microphysics, as well as physical and radiative properties of dust in climate models are still subject to large uncertainty. Here we use a sectional aerosol model (CARMA) coupled with a climate model (CESM1) to investigate the global distribution of dust aerosols, with an emphasis on the vertical distribution of dust. Consistent with observations at locations remote from source regions, simulated dust mass size distributions peak at around 2–3 micrometres in diameter and increase by 4 orders of magnitude from 0.1 μm to 2 µm. The size distribution above 2 µm is highly variable depending on distance from the source, and subject to uncertainty due to possible size dependent changes in physical properties such as shape and density. Simulated annual mean dust mass concentrations are within one order of magnitude of those found by the surface measurement network around the globe. Simulated annual mean aerosol optical depths are ~10 % lower than AERONET observations near the dust source regions. Both simulations and in-situ measurements during the NASA ATom field campaign suggest that dust mass concentrations over the remote ocean drop by two to three orders of magnitude from the surface to the upper troposphere (200 hPa). The model suggests that Saharan, Middle Eastern, and Asian dust accounts for ~59.7 %, 12.5 %, and 13.3 % of the global annual mean dust emissions, with the remaining 14.5 % originating from scattered smaller dust sources. Although Saharan dust dominates global dust mass loading at the surface, the relative contribution of Asian dust increases with altitude and becomes dominant in the upper troposphere. The simulations show that Asian dust contributes ~60.9 % to the global and annual mean dust concentration between 266 hPa and 160 hPa. Asian dust is mostly lifted in the spring by mid-latitude frontal systems. However, deep convection during the Asian summer monsoon (ASM) favours the vertical transport of local dust to the upper atmosphere. Simulated dust accumulates in the ASM anticyclone and forms a local maximum; however, the simulated dust mass concentration is only ~0.04 % of the total aerosols in the Asian Tropopause Aerosol Layer (ATAL), which are dominated by organics, sulfates and nitrates.

Significance Nuclear conflict poses the chilling prospect of triggering abrupt global cooling, and consequently, severely reduced crop production. However, the impacts on marine fisheries are unknown. If agricultural yields fall on land, could we turn to the sea instead? Here, we show that agricultural losses could not be offset by the world’s fisheries, especially given widespread overfishing. Cold temperatures and reduced sunlight would decrease the growth of fish biomass, at worst as much as under unmitigated climate change. Although intensified postwar fishing could yield a small catch increase, dramatic declines would ensue due to overharvesting. However, effective prewar fisheries management would create a substantial buffer of fish in the ocean, greatly increasing the oceans’ potential contribution during a global food emergency., Nuclear war, beyond its devastating direct impacts, is expected to cause global climatic perturbations through injections of soot into the upper atmosphere. Reduced temperature and sunlight could drive unprecedented reductions in agricultural production, endangering global food security. However, the effects of nuclear war on marine wild-capture fisheries, which significantly contribute to the global animal protein and micronutrient supply, remain unexplored. We simulate the climatic effects of six war scenarios on fish biomass and catch globally, using a state-of-the-art Earth system model and global process-based fisheries model. We also simulate how either rapidly increased fish demand (driven by food shortages) or decreased ability to fish (due to infrastructure disruptions), would affect global catches, and test the benefits of strong prewar fisheries management. We find a decade-long negative climatic impact that intensifies with soot emissions, with global biomass and catch falling by up to 18 ± 3% and 29 ± 7% after a US–Russia war under business-as-usual fishing—similar in magnitude to the end-of-century declines under unmitigated global warming. When war occurs in an overfished state, increasing demand increases short-term (1 to 2 y) catch by at most ∼30% followed by precipitous declines of up to ∼70%, thus offsetting only a minor fraction of agricultural losses. However, effective prewar management that rebuilds fish biomass could ensure a short-term catch buffer large enough to replace ∼43 ± 35% of today’s global animal protein production. This buffering function in the event of a global food emergency adds to the many previously known economic and ecological benefits of effective and precautionary fisheries management.

In a nuclear war, bombs targeted on cities and industrial areas would start firestorms, injecting large amounts of soot into the upper atmosphere, which would spread globally and rapidly cool the planet. The soot loadings would cause decadal disruptions in Earth’s climate, which would impact food production systems on land and in the oceans. In 1980s, investigations of nuclear winter impacts on global agricultural production and food availability for 15 nations, but new information now allows us to update those estimates. Recently, several studies analyzed changes of major grain crops and marine wild-catch fisheries for different scenarios of regional nuclear war using sophisticated models. However, the impact on the total food supply available to humans is more complex. Here we show that considering all food sources and potential adaptation measures, such as using animal feed directly for humans, famine would result for most of Earth even from a war between India and Pakistan using less than 3% of the global nuclear arsenal. We look at the climate impacts from a range of scales from regional to global nuclear war, and estimate the total amount of food calories available in each nation, including crops, livestock, and fisheries, for each year following a nuclear holocaust. Our findings quantify the global indirect impacts of nuclear war away from target areas, and demonstrate the need to prevent any scale of nuclear war.

Solar-powered lofting makes stratospheric aerosol injection viable with the existing aircraft fleet., Many climate intervention (CI) methods have been proposed to offset greenhouse gas–induced global warming, but the practicalities regarding implementation have not received sufficient attention. Stratospheric aerosol injection (SAI) involves introducing large amounts of CI material well within the stratosphere to enhance the aerosol loading, thereby increasing reflection of solar radiation. We explore a delivery method termed solar-powered lofting (SPL) that uses solar energy to loft CI material injected at lower altitudes accessible by conventional aircraft. Particles that absorb solar radiation are dispersed with the CI material and heat the surrounding air. The heated air rises, carrying the CI material to the stratosphere. Global model simulations show that black carbon aerosol (10 microgram per cubic meter) is sufficient to quickly loft CI material well into the stratosphere. SPL could make SAI viable at present, is also more energy efficient, and disperses CI material faster than direct stratospheric injection.

The climate impacts of smoke from fires ignited by nuclear war would include global cooling and crop failure. Facing increased reliance on ocean-based food sources, it is critical to understand the physical and biological state of the post-war oceans. Here we use an Earth system model to simulate six nuclear war scenarios. We show that global cooling can generate a large, sustained response in the equatorial Pacific, resembling an El Nino but persisting for up to seven years. The El Nino following nuclear war, or Nuclear Nino, would be characterized by westerly trade wind anomalies and a shutdown of equatorial Pacific upwelling, caused primarily by cooling of the Maritime Continent and tropical Africa. Reduced incident sunlight and ocean circulation changes would cause a 40% reduction in equatorial Pacific phytoplankton productivity. These results indicate nuclear war could trigger extreme climate change and compromise food security beyond the impacts of crop failure. A nuclear conflict could lead to global cooling followed by a large and sustained El Nino-like response in the tropical Pacific region, according to an evaluation of six scenarios for nuclear conflict with an Earth system model.

Significance Impacts of global warming on agricultural productivity have been evaluated extensively. The implications of sudden cooling for global crop growth, however, are as yet little understood. While crop failures after historic volcanic eruptions are documented, a nuclear conflict can cause even more severe and longer-lasting climate anomalies. India and Pakistan are contributing to a de facto Asian arms race and the political instability in South Asia increasingly imposes a global threat. Based on comprehensive climate and crop model ensemble simulations, we provide critical quantitative information on how sudden cooling and perturbations of precipitation and solar radiation could disrupt food production and trade worldwide for about a decade—more than the impact from anthropogenic climate change by late century., A limited nuclear war between India and Pakistan could ignite fires large enough to emit more than 5 Tg of soot into the stratosphere. Climate model simulations have shown severe resulting climate perturbations with declines in global mean temperature by 1.8 °C and precipitation by 8%, for at least 5 y. Here we evaluate impacts for the global food system. Six harmonized state-of-the-art crop models show that global caloric production from maize, wheat, rice, and soybean falls by 13 (±1)%, 11 (±8)%, 3 (±5)%, and 17 (±2)% over 5 y. Total single-year losses of 12 (±4)% quadruple the largest observed historical anomaly and exceed impacts caused by historic droughts and volcanic eruptions. Colder temperatures drive losses more than changes in precipitation and solar radiation, leading to strongest impacts in temperate regions poleward of 30°N, including the United States, Europe, and China for 10 to 15 y. Integrated food trade network analyses show that domestic reserves and global trade can largely buffer the production anomaly in the first year. Persistent multiyear losses, however, would constrain domestic food availability and propagate to the Global South, especially to food-insecure countries. By year 5, maize and wheat availability would decrease by 13% globally and by more than 20% in 71 countries with a cumulative population of 1.3 billion people. In view of increasing instability in South Asia, this study shows that a regional conflict using

Large fires can inject smoke into the upper troposphere and lower stratosphere. Detailed fire simulations allow for assessment of how local weather interacts with these fires and affects smoke lofting. In this study, we employ the fire simulation package in the Weather Research and Forecasting model (WRF-Fire), Version 4.0.1, to explore how smoke lofting from a fire burning a homogeneous fuel bed changes with varying local winds, relative humidity, and atmospheric boundary-layer stability for two different-sized areal fires. We find that the presence of moisture has the greatest influence on the results by raising the altitude of lofting, while faster winds speeds dampen lofting and lower the injection height. Stably stratified conditions further inhibit plume propagation compared with neutrally stratified conditions.

Deep convective transport of gaseous precursors to ozone (O(3)) and aerosols to the upper troposphere is affected by liquid phase and mixed-phase scavenging, entrainment of free tropospheric air and aqueous chemistry. The contributions of these processes are examined using aircraft measurements obtained in storm inflow and outflow during the 2012 Deep Convective Clouds and Chemistry (DC3) experiment combined with high-resolution (dx ≤ 3 km) WRF-Chem simulations of a severe storm, an air mass storm, and a mesoscale convective system (MCS). The simulation results for the MCS suggest that formaldehyde (CH(2)O) is not retained in ice when cloud water freezes, in agreement with previous studies of the severe storm. By analyzing WRF-Chem trajectories, the effects of scavenging, entrainment, and aqueous chemistry on outflow mixing ratios of CH(2)O, methyl hydroperoxide (CH(3)OOH), and hydrogen peroxide (H(2)O(2)) are quantified. Liquid phase microphysical scavenging was the dominant process reducing CH(2)O and H(2)O(2) outflow mixing ratios in all three storms. Aqueous chemistry did not significantly affect outflow mixing ratios of all three species. In the severe storm and MCS, the higher than expected reductions in CH(3)OOH mixing ratios in the storm cores were primarily due to entrainment of low-background CH(3)OOH. In the air mass storm, lower CH(3)OOH and H(2)O(2) scavenging efficiencies (SEs) than in the MCS were partly due to entrainment of higher background CH(3)OOH and H(2)O(2). Overestimated rain and hail production in WRF-Chem reduces the confidence in ice retention fraction values determined for the peroxides and CH(2)O.

Of the nine countries known to have nuclear weapons, six are located in Asia and another, the United States, borders the Pacific Ocean. Russia and China were the first Asian nations with nuclear weapons, followed by Israel, India, Pakistan, and North Korea. Most of the world's nuclear powers are reducing their arsenals or maintaining them at historic levels, but several of those in Asia—India, Pakistan, and North Korea—continue to pursue relentless and expensive programs of nuclear weapons development and production. Hopefully, the nuclear agreement reached in July 2015 between Iran, the European Union, and the five permanent members of the United Nations Security Council will be a step toward eliminating nuclear weapons throughout Asia and the rest of the world. As we will discuss below, any country possessing a nuclear arsenal is on a path leading toward self-assured destruction, and is a threat to people everywhere on Earth.

The February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX data are publicly available online (at https://espoarchive.nasa.gov/).

Volcanic ash is often neglected in climate simulations because ash particles are assumed to have a short atmospheric lifetime, and to not participate in sulfur chemistry. After the Mt. Kelut eruption in 2014, stratospheric ash-rich aerosols were observed for months. Here we show that the persistence of super-micron ash is consistent with a density near 0.5 g cm−3, close to pumice. Ash-rich particles dominate the volcanic cloud optical properties for the first 60 days. We also find that the initial SO2 lifetime is determined by SO2 uptake on ash, rather than by reaction with OH as commonly assumed. About 43% more volcanic sulfur is removed from the stratosphere in 2 months with the SO2 heterogeneous chemistry on ash particles than without. This research suggests the need for re-evaluation of factors controlling SO2 lifetime in climate model simulations, and of the impact of volcanic ash on stratospheric chemistry and radiation., Volcanic ash is often neglected in climate simulations as it is assumed to have a short atmospheric lifetime. Here, the authors show a persistent super-micron ash layer after the Mt. Kelut eruption in 2014 that impacts the stratospheric sulfur burden and chemistry for over the first months after the eruption.

Severe global climate change and a record death toll could result from nuclear war between India and Pakistan in the next decade., Pakistan and India may have 400 to 500 nuclear weapons by 2025 with yields from tested 12- to 45-kt values to a few hundred kilotons. If India uses 100 strategic weapons to attack urban centers and Pakistan uses 150, fatalities could reach 50 to 125 million people, and nuclear-ignited fires could release 16 to 36 Tg of black carbon in smoke, depending on yield. The smoke will rise into the upper troposphere, be self-lofted into the stratosphere, and spread globally within weeks. Surface sunlight will decline by 20 to 35%, cooling the global surface by 2° to 5°C and reducing precipitation by 15 to 30%, with larger regional impacts. Recovery takes more than 10 years. Net primary productivity declines 15 to 30% on land and 5 to 15% in oceans threatening mass starvation and additional worldwide collateral fatalities.

Results from twenty‐two airborne field campaigns, including more than ten million high‐resolution particle images collected in cirrus formed in situ and in convective anvils, are interpreted in terms of particle shapes and their potential impact on radiative transfer. Emphasis is placed on characterizing ice particle shapes in Tropical Maritime and Mid‐Latitude Continental anvil cirrus, as well as in cirrus formed in situ in the upper troposphere, and subvisible cirrus in the upper tropical troposphere layer. There is a distinctive difference in cirrus ice particle shapes formed in situ compared to those in anvils that are generated in close proximity to convection. More than half the mass in cirrus formed in situ are rosette‐shapes (polycrystals and bullet rosettes). Cirrus formed from fresh convective anvils is mostly devoid of rosette‐shaped particles. However, small frozen drops may experience regrowth downwind of an aged anvil in a regime with RHice > ~ 120%, and then grow into rosette shapes. Identifiable particle shapes in Tropical Maritime anvils that have not been impacted by continental influences typically contain mostly single plate‐like and columnar crystals and aggregates. Mid‐Latitude Continental anvils contain single rimed particles, more and larger aggregates with riming, and chains of small ice particles when in a highly electrified environment. The particles in subvisible cirrus are < ~ 100 μm and quasi‐spherical with some plates and rare trigonal shapes. Percentages of particle shapes and power laws relating mean particle area and mass to dimension are provided to improve parameterization of remote retrievals and numerical simulations.

About 66 million years ago, an asteroid about 10 km in diameter struck the Yucatan Peninsula creating the Chicxulub crater. The crater has been dated and found to be coincident with the Cretaceous–Paleogene (K-Pg) mass extinction event, one of six great mass extinctions in the last 600 million years. This event precipitated one of the largest episodes of rapid climate change in Earth's history, yet no modern three-dimensional climate calculations have simulated the event. Similarly, while there is an ongoing effort to detect asteroids that might hit Earth and to develop methods to stop them, there have been no modern calculations of the sizes of asteroids whose impacts on land would cause devastating effects on Earth. Here, we provide the information needed to initialize such calculations for the K-Pg impactor and for a 1 km diameter impactor. There is considerable controversy about the details of the events that followed the Chicxulub impact. We proceed through the data record in the order of confidence that a climatically important material was present in the atmosphere. The climatic importance is roughly proportional to the optical depth of the material. Spherules with diameters of several hundred microns are found globally in an abundance that would have produced an atmospheric layer with an optical depth around 20, yet their large sizes would only allow them to stay airborne for a few days. They were likely important for triggering global wildfires. Soot, probably from global or near-global wildfires, is found globally in an abundance that would have produced an optical depth near 100, which would effectively prevent sunlight from reaching the surface. Nanometer-sized iron particles are also present globally. Theory suggests these particles might be remnants of the vaporized asteroid and target that initially remained as vapor rather than condensing on the hundred-micron spherules when they entered the atmosphere. If present in the greatest abundance allowed by theory, their optical depth would have exceeded 1000. Clastics may be present globally, but only the quartz fraction can be quantified since shock features can identify it. However, it is very difficult to determine the total abundance of clastics. We reconcile previous widely disparate estimates and suggest the clastics may have had an optical depth near 100. Sulfur is predicted to originate about equally from the impactor and from the Yucatan surface materials. By mass, sulfur is less than 10 % of the observed mass of the spheres and estimated mass of nanoparticles. Since the sulfur probably reacted on the surfaces of the soot, nanoparticles, clastics, and spheres, it is likely a minor component of the climate forcing; however, detailed studies of the conversion of sulfur gases to particles are needed to determine if sulfuric acid aerosols dominated in late stages of the evolution of the atmospheric debris. Numerous gases, including CO2, SO2 (or SO3), H2O, CO2, Cl, Br, and I, were likely injected into the upper atmosphere by the impact or the immediate effects of the impact such as fires across the planet. Their abundance might have increased relative to current ambient values by a significant fraction for CO2, and by factors of 100 to 1000 for the other gases. For the 1 km impactor, nanoparticles might have had an optical depth of 1.5 if the impact occurred on land. If the impactor struck a densely forested region, soot from the forest fires might have had an optical depth of 0.1. Only S and I would be expected to be perturbed significantly relative to ambient gas-phase values. One kilometer asteroids impacting the ocean may inject seawater into the stratosphere as well as halogens that are dissolved in the seawater. For each of the materials mentioned, we provide initial abundances and injection altitudes. For particles, we suggest initial size distributions and optical constants. We also suggest new observations that could be made to narrow the uncertainties about the particles and gases generated by large impacts.

Stratospheric aerosols cool the Earth by scattering sunlight. Although sulfuric acid dominates the stratospheric aerosol, this study finds that organic material in the lowermost stratosphere contributes 30-40% of the non-volcanic stratospheric aerosol optical depth (sAOD). Simulations indicate that non-volcanic sAOD has increased 77% since 1850. Stratospheric aerosol accounts for 21% of the total direct aerosol radiative forcing (which is negative) and 12% of the total AOD increase from organics and sulfate. There is a larger stratospheric influence on radiative forcing (i.e. 21%) relative to AOD (i.e. 12%) because an increase of tropospheric black carbon warms the planet while stratospheric aerosols (including black carbon) cool the planet. Radiative forcing from non-volcanic stratospheric aerosol mass of anthropogenic origin, including organics, has not been widely considered as a significant influence on the climate system.

The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field mission based at Ellington Field, Texas, during August and September 2013 employed the most comprehensive airborne payload to date to investigate atmospheric composition over North America. The NASA ER-2, DC-8, and SPEC Inc. Learjet flew 57 science flights from the surface to 20 km. The ER-2 employed seven remote sensing instruments as a satellite surrogate and eight in situ instruments. The DC-8 employed 23 in situ and five remote sensing instruments for radiation, chemistry, and microphysics. The Learjet used 11 instruments to explore cloud microphysics. SEAC4RS launched numerous balloons, augmented Aerosol RObotic NETwork, and collaborated with many existing ground measurement sites. Flights investigating convection included close coordination of all three aircraft. Coordinated DC-8 and ER-2 flights investigated the optical properties of aerosols, the influence of aerosols on clouds, and the performance of new instruments for satellite measurements of clouds and aerosols. ER-2 sorties sampled stratospheric injections of water vapor and other chemicals by local and distant convection. DC-8 flights studied seasonally evolving chemistry in the Southeastern U.S., atmospheric chemistry with lower emissions of NOx and SO2 than in previous decades, isoprene chemistry under high and low NOx conditions at different locations, organic aerosols, air pollution near Houston and in petroleum fields, smoke from wildfires in western forests and from agricultural fires in the Mississippi Valley, and the ways in which the chemistry in the boundary layer and the upper troposphere were influenced by vertical transport in convective clouds.

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., 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

International audience; We investigate the impact of the 2015 Mount Calbuco eruption and previous eruptions on stratospheric aerosols, polar stratospheric clouds, and ozone depletion using the Community Earth System Model coupled with the Community Aerosol and Radiation Model for Atmospheres compared with several satellite and balloon observations. The modeled volcanic sulfate aerosol size distribution agrees with the Light Optical Aerosol Counter observation at the Reunion Island site (21°S, 55°E) on 19 August at 20 km within estimated 0.1-to 1-μm radius error bars. Both the observed and simulated backscatter and extinction show that volcanic sulfate aerosol from the Mount Calbuco eruption was transported from midlatitude toward the Antarctic and slowly descended during transport. They also indicate that the SO 2 emission into the stratosphere from Mount Calbuco is 0.2-0.4 Tg. The modeled number density indicates that the volcanic sulfate aerosol from the Mount Calbuco eruption penetrated into the Antarctic polar vortex in May and thereafter and reduced the polar stratospheric clouds effective radius. In the simulations, the Antarctic stratosphere denitrified too early and by too much compared with Microwave Limb Sounder observations. Heterogeneous nucleation of nitric acid trihydrate or sophisticated gravity wave representations may be required to simulate nitric acid trihydrate particles with smaller falling velocity. The volcanic sulfate aerosol increases the ozone depletion in September especially around 100 hPa and 70°S, relative to a case without any volcanic eruptions. The simulated surface area density, earlier ozone loss, and larger area of the ozone hole are consistent with the presence of volcanic sulfate layers observed at 16 km as well as previous studies.

A sectional aerosol model (CARMA) has been developed and coupled with the Community Earth System Model (CESM1). Aerosol microphysics, radiative properties, and interactions with clouds are simulated in the size‐resolving model. The model described here uses 20 particle size bins for each aerosol component including freshly nucleated sulfate particles, as well as mixed particles containing sulfate, primary organics, black carbon, dust, and sea salt. The model also includes five types of bulk secondary organic aerosols with four volatility bins. The overall cost of CESM1‐CARMA is approximately ∼2.6 times as much computer time as the standard three‐mode aerosol model in CESM1 (CESM1‐MAM3) and twice as much computer time as the seven‐mode aerosol model in CESM1 (CESM1‐MAM7) using similar gas phase chemistry codes. Aerosol spatial‐temporal distributions are simulated and compared with a large set of observations from satellites, ground‐based measurements, and airborne field campaigns. Simulated annual average aerosol optical depths are lower than MODIS/MISR satellite observations and AERONET observations by ∼32%. This difference is within the uncertainty of the satellite observations. CESM1/CARMA reproduces sulfate aerosol mass within 8%, organic aerosol mass within 20%, and black carbon aerosol mass within 50% compared with a multiyear average of the IMPROVE/EPA data over United States, but differences vary considerably at individual locations. Other data sets show similar levels of comparison with model simulations. The model suggests that in addition to sulfate, organic aerosols also significantly contribute to aerosol mass in the tropical UTLS, which is consistent with limited data., Key Points: A sectional aerosol model has been developed and coupled with the community earth system modelModel performance is evaluated by comparing simulations with multiple data sets

Polar stratospheric clouds (PSCs) are critical elements of Arctic and Antarctic ozone depletion. We establish a PSC microphysics model using coupled chemistry, climate, and microphysics models driven by specific dynamics. We explore the microphysical formation and evolution of STS (Supercooled Ternary Solution) and NAT (Nitric Acid Trihydrate). Characteristics of STS particles dominated by thermodynamics compare well with observations. For example, the mass of STS is close to the thermodynamic equilibrium assumption when the particle surface area is >4 µm2/cm3. We derive a new nucleation rate equation for NAT based on observed denitrification in the 2010–2011 Arctic winter. The homogeneous nucleation scheme leads to supermicron NAT particles as observed. We also find that as the number density of NAT particles increases, the denitrification also increases. Simulations of the PSC lidar backscatter, denitrification, and gas phase species are generally within error bars of the observations. However, the simulations are very sensitive to temperature, which limits our ability to fully constrain some parameters (e.g., denitrification, ozone amount) based on observations.

To simulate polar stratospheric clouds (PSCs) during the Antarctic winter of 2010, we have developed a PSC model within the Community Earth System Model framework that includes detailed microphysics of sulfuric aerosols and three types of PSCs: supercooled ternary solution (STS), nitric acid trihydrate (NAT) and ice. Our model includes two major NAT formation mechanisms both of which are essential to reproduce the PSC and gas phase chemical features in the 2010 Antarctic winter. Homogeneous nucleation of NAT from STS produces NAT particles with sizes near 8 μm, which are important to properly simulate denitrification and the gas phase HNO3 observed by the Microwave Limb Sounder (MLS). Heterogeneous nucleation of NAT on ice particles or ice particles on NAT and subsequent evaporation of the ice produces NAT particles with sizes from sub-micrometers to a few micrometers. These particles account for the large backscattering ratio from NAT observed by the CALIPSO satellite, especially in the mid-winter season. Adding temperature fluctuations from gravity waves is important to produce larger number density and higher backscattering ratio from ice and NAT particles. However, our model needs a better representation of waves to improve the backscattering ratio and gas phase HNO3 compared with observations. Our model also includes homogeneous nucleation of ice from STS and heterogeneous nucleation of ice on NAT. The model reproduces the gas phase H2O during the winter within the uncertainty of the MLS observations.

An enhanced aerosol layer near the tropopause over Asia during the June-September period of the Asian summer monsoon (ASM) was recently identified using satellite observations. Its sources and climate impact are presently not well-characterized. To improve understanding of this phenomenon, we made in situ aerosol measurements during summer 2015 from Kunming, China, then followed with a modeling study to assess the global significance. The in situ measurements revealed a robust enhancement in aerosol concentration that extended up to 2 km above the tropopause. A climate model simulation demonstrates that the abundant anthropogenic aerosol precursor emissions from Asia coupled with rapid vertical transport associated with monsoon convection leads to significant particle formation in the upper troposphere within the ASM anticyclone. These particles subsequently spread throughout the entire Northern Hemispheric (NH) lower stratosphere and contribute significantly (∼15%) to the NH stratospheric column aerosol surface area on an annual basis. This contribution is comparable to that from the sum of small volcanic eruptions in the period between 2000 and 2015. Although the ASM contribution is smaller than that from tropical upwelling (∼35%), we find that this region is about three times as efficient per unit area and time in populating the NH stratosphere with aerosol. With a substantial amount of organic and sulfur emissions in Asia, the ASM anticyclone serves as an efficient smokestack venting aerosols to the upper troposphere and lower stratosphere. As economic growth continues in Asia, the relative importance of Asian emissions to stratospheric aerosol is likely to increase.