Evaluations of tropospheric aerosol properties simulated by the community earth system model with a sectional aerosol microphysics scheme

Predictive modeling of atmospheric nuclear fallout microphysics

The capability to predict size, composition, and transport of nuclear fallout enables public officials to determine immediate and prolonged guidance in the event of a nuclear incident. Predictive computer models of fallout can also provide useful insight for nuclear forensic response when detailed radiochemical processes can be reliably included. Current post-detonation nuclear fallout models prescribe particle size distributions empirically or semi-empirically, based on measurements across limited conditions pertaining to tests conducted primarily in Nevada and the Pacific. These empirical fallout relationships may be subject to large uncertainties in particle size and radionuclide activity distribution if used to extrapolate to other regions with different environmental conditions (e.g., urbanized areas). Replacing empirical relationships with physics-based microphysical process modeling can enable significant advances in the fidelity of predictive models simulating distributions of fallout across diverse environments. Particle microphysics describes the formation and evolution of fallout particles, as well as the interaction of radioactive material with entrained particles, which requires accounting for fundamental processes such as nucleation, condensation, and coagulation. The objective of this perspective article is to summarize computational techniques to simulate particle microphysical processes advancing the fidelity of predicting nuclear fallout. We review current empirical models for simulating post-detonation fallout and assess promising research directions moving towards physics-based predictive systems.

Chlorine activation and enhanced ozone depletion induced by wildfire aerosol

Remarkable perturbations in the stratospheric abundances of chlorine species and ozone were observed over Southern Hemisphere mid-latitudes following the 2020 Australian wildfires^1,2. These changes in atmospheric chemical composition suggest that wildfire aerosols affect stratospheric chlorine and ozone depletion chemistry. Here we propose that wildfire aerosol containing a mixture of oxidized organics and sulfate^3-7 increases hydrochloric acid solubility^8-11 and associated heterogeneous reaction rates, activating reactive chlorine species and enhancing ozone loss rates at relatively warm stratospheric temperatures. We test our hypothesis by comparing atmospheric observations to model simulations that include the proposed mechanism. Modelled changes in 2020 hydrochloric acid, chlorine nitrate and hypochlorous acid abundances are in good agreement with observations^1,2. Our results indicate that wildfire aerosol chemistry, although not accounting for the record duration of the 2020 Antarctic ozone hole, does yield an increase in its area and a 3-5% depletion of southern mid-latitude total column ozone. These findings increase concern^2,12,13 that more frequent and intense wildfires could delay ozone recovery in a warming world.

On the stratospheric chemistry of midlatitude wildfire smoke

Large wildfires have been observed to inject smoke into the stratosphere, raising questions about their potential to affect the stratospheric ozone layer that protects life on Earth from biologically damaging ultraviolet radiation. Multiple observations of aerosol and NO₂ concentrations from three independent satellite instruments are used here together with model calculations to identify decreases in stratospheric NO₂ concentrations following major Australian 2019 through 2020 wildfires. The data confirm that important chemistry did occur on the smoke particle surfaces. The observed behavior in NO₂ with increasing particle concentrations is a marker for surface chemistry that contributes to midlatitude ozone depletion. The results indicate that increasing wildfire activity in a warming world may slow the recovery of the ozone layer.

Massive Australian wildfires lofted smoke directly into the stratosphere in the austral summer of 2019/20. The smoke led to increases in optical extinction throughout the midlatitudes of the southern hemisphere that rivalled substantial volcanic perturbations. Previous studies have assumed that the smoke became coated with sulfuric acid and water and would deplete the ozone layer through heterogeneous chemistry on those surfaces, as is routinely observed following volcanic enhancements of the stratospheric sulfate layer. Here, observations of extinction and reactive nitrogen species from multiple independent satellites that sampled the smoke region are compared to one another and to model calculations. The data display a strong decrease in reactive nitrogen concentrations with increased aerosol extinction in the stratosphere, which is a known fingerprint for key heterogeneous chemistry on sulfate/H₂O particles (specifically the hydrolysis of N₂O₅ to form HNO₃). This chemical shift affects not only reactive nitrogen but also chlorine and reactive hydrogen species and is expected to cause midlatitude ozone layer depletion. Comparison of the model ozone to observations suggests that N₂O₅ hydrolysis contributed to reduced ozone, but additional chemical and/or dynamical processes are also important. These findings suggest that if wildfire smoke injection into the stratosphere increases sufficiently in frequency and magnitude as the world warms due to climate change, ozone recovery under the Montreal Protocol could be impeded, at least sporadically. Modeled austral midlatitude total ozone loss was about 1% in March 2020, which is significant compared to expected ozone recovery of about 1% per decade.

Toward practical stratospheric aerosol albedo modification: Solar-powered lofting

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.

Black carbon lofts wildfire smoke high into the stratosphere to form a persistent plume

In 2017, western Canadian wildfires injected smoke into the stratosphere that was detectable by satellites for more than 8 months. The smoke plume rose from 12 to 23 kilometers within 2 months owing to solar heating of black carbon, extending the lifetime and latitudinal spread. Comparisons of model simulations to the rate of observed lofting indicate that 2% of the smoke mass was black carbon. The observed smoke lifetime in the stratosphere was 40% shorter than calculated with a standard model that does not consider photochemical loss of organic carbon. Photochemistry is represented by using an empirical ozone-organics reaction probability that matches the observed smoke decay. The observed rapid plume rise, latitudinal spread, and photochemical reactions provide new insights into potential global climate impacts from nuclear war.

A large source of cloud condensation nuclei from new particle formation in the tropics

Cloud condensation nuclei (CCN) can affect cloud properties and therefore Earth's radiative balance^1-3. New particle formation (NPF) from condensable vapours in the free troposphere has been suggested to contribute to CCN, especially in remote, pristine atmospheric regions⁴, but direct evidence is sparse, and the magnitude of this contribution is uncertain^5-7. Here we use in situ aircraft measurements of vertical profiles of aerosol size distributions to present a global-scale survey of NPF occurrence. We observe intense NPF at high altitudes in tropical convective regions over both Pacific and Atlantic oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40 per cent of Earth's surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where CCN can affect cloud properties. Our findings suggest that the production of CCN as new particles descend towards the surface is not adequately captured in global models, which tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth of new particles to CCN sizes.

The impact of air pollutant deposition on solar energy system efficiency: An approach to estimate PV soiling effects with the Community Multiscale Air Quality (CMAQ) model

Deposition and accumulation of aerosol particles on photovoltaics (PV) panels, which is commonly referred to as "soiling of PV panels," impacts the performance of the PV energy system. It is desirable to estimate the soiling effect at different locations and times for modeling the PV system performance and devising cost-effective mitigation. This study presents an approach to estimate the soiling effect by utilizing particulate matter (PM) dry deposition estimates from air quality model simulations. The Community Multiscale Air Quality (CMAQ) modeling system used in this study was developed by the U.S. Environmental Protection Agency (U.S. EPA) for air quality assessments, rule-making, and research. Three deposition estimates based on different surface roughness length parameters assumed in CMAQ were used to illustrate the soling effect in different land-use types. The results were analyzed for three locations in the U.S. for year 2011. One urban and one suburban location in Colorado were selected because there have been field measurements of particle deposition on solar panels and analysis on the consequent soiling effect performed at these locations. The third location is a coastal city in Texas, the City of Brownsville. These three locations have distinct ambient environments. CMAQ underestimates particle deposition by 40% to 80% when compared to the field measurements at the two sites in Colorado due to the underestimations in both the ambient PM₁₀ concentration and deposition velocity. The estimated panel transmittance sensitivity due to the deposited particles is higher than the sensitivity obtained from the measurements in Colorado. The final soiling effect, which is transmittance loss, is estimated as 3.17 ± 4.20% for the Texas site, 0.45 ± 0.33%, and 0.31 ± 0.25% for the Colorado sites. Although the numbers are lower compared to the measurements in Colorado, the results are comparable with the soiling effects observed in U.S.

Efficient In-Cloud Removal of Aerosols by Deep Convection

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.

Black carbon radiative effects highly sensitive to emitted particle size when resolving mixing-state diversity

Post-industrial increases in atmospheric black carbon (BC) have a large but uncertain warming contribution to Earth's climate. Particle size and mixing state determine the solar absorption efficiency of BC and also strongly influence how effectively BC is removed, but they have large uncertainties. Here we use a multiple-mixing-state global aerosol microphysics model and show that the sensitivity (range) of present-day BC direct radiative effect, due to current uncertainties in emission size distributions, is amplified 5-7 times (0.18-0.42 W m-2) when the diversity in BC mixing state is sufficiently resolved. This amplification is caused by the lifetime, core absorption, and absorption enhancement effects of BC, whose variability is underestimated by 45-70% in a single-mixing-state model representation. We demonstrate that reducing uncertainties in emission size distributions and how they change in the future, while also resolving modeled BC mixing state diversity, is now essential when evaluating BC radiative effects and the effectiveness of BC mitigation on future temperature changes.

Efficient transport of tropospheric aerosol into the stratosphere via the Asian summer monsoon anticyclone

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.

Surface dimming by the 2013 Rim Fire simulated by a sectional aerosol model

The Rim Fire of 2013, the third largest area burned by fire recorded in California history, is simulated by a climate model coupled with a size-resolved aerosol model. Modeled aerosol mass, number, and particle size distribution are within variability of data obtained from multiple-airborne in situ measurements. Simulations suggest that Rim Fire smoke may block 4-6% of sunlight energy reaching the surface, with a dimming efficiency around 120-150 W m-2 per unit aerosol optical depth in the midvisible at 13:00-15:00 local time. Underestimation of simulated smoke single scattering albedo at midvisible by 0.04 suggests that the model overestimates either the particle size or the absorption due to black carbon. This study shows that exceptional events like the 2013 Rim Fire can be simulated by a climate model with 1° resolution with overall good skill, although that resolution is still not sufficient to resolve the smoke peak near the source region.

Contact efflorescence as a pathway for crystallization of atmospherically relevant particles

Inadequate knowledge of the phase state of atmospheric particles represents a source of uncertainty in global climate and air quality models. Hygroscopic aqueous inorganic particles are often assumed to remain liquid throughout their atmospheric lifetime or only (re)crystallize at low relative humidity (RH) due to the kinetic limitations of efflorescence (salt crystal nucleation and growth from an aqueous solution). Here we present experimental observations of a previously unexplored heterogeneous nucleation pathway that we have termed "contact efflorescence," which describes efflorescence initiated by an externally located solid particle coming into contact with the surface of a metastable aqueous microdroplet. This study demonstrates that upon a single collision, contact efflorescence is a pathway for crystallization of atmospherically relevant aqueous particles at high ambient RH (≤80%). Soluble inorganic crystalline particles were used as contact nuclei to induce efflorescence of aqueous ammonium sulfate [(NH4)2SO4], sodium chloride (NaCl), and ammonium nitrate (NH4NO3), with efflorescence being observed in several cases close to their deliquescence RH values (80%, 75%, and 62%, respectively). To our knowledge, these observations represent the highest reported efflorescence RH values for microdroplets of these salts. These results are particularly important for considering the phase state of NH4NO3, where the contact efflorescence RH (∼20-60%) is in stark contrast to the observation that NH4NO3 microdroplets do not homogeneously effloresce, even when exposed to extremely arid conditions (<1% RH). Considering the occurrence of particle collisions in the atmosphere (i.e., coagulation), these observations of contact efflorescence challenge many assumptions made about the phase state of inorganic aerosol.