Potential Environmental Effects of Nuclear War (2025)
Chapter: 4 Plume Rise, Fate and Transport of Aerosols, and Gas-Phase Chemistry
4
Plume Rise, Fate and Transport of Aerosols, and Gas-Phase Chemistry
4.1 INTRODUCTION
This report explores factors contributing to the environmental effects and societal and economic consequences that would follow in the weeks to decades after a nuclear war, beginning with a description of a plausible set of scenarios for the employment of nuclear weapons (Chapter 2) and an exploration of the fire dynamics and emissions resulting from the nuclear detonations (Chapter 3). In this chapter, the discussion turns to the transport and fate of these emissions once aerosolized (Chapter 4), and sets up an examination, in subsequent chapters, of the effects on the climate and physical Earth system (Chapter 5); and the impacts on ecological and societal and economic systems (Chapters 6 and 7).
BOX 4-1
Key Terms
Aerosol: Any solid or liquid droplets suspended in the atmosphere.
Conflagration: Large and uncontrolled fire.
Firestorm: Intense conflagration that creates its own convective wind patterns, radially drawing in air near the surface, with strong updrafts above the fire.
Heterogeneous Chemistry: Ensemble of chemical processes involving aerosol phases (liquid and solid particles) in the atmosphere.
Particulate Matter (PM): Generic term to classify air pollutants comprising of suspended particles in air, varying in composition and size, resulting from various anthropogenic activities (El Morabet, 2018).
Polycyclic Aromatic Hydrocarbons (PAHs): Consisting of three or more fused aromatic (benzene) rings and produced as by-products of fossil fuel, diesel, fat, and biomass burning. Some PAHs have been identified as carcinogenic, mutagenic, and teratogenic. Three-ringed PAHs occur in the atmosphere predominantly in the vapor phase whereas four-ringed PAHs can occur in both the vapor and particle phase. Multiringed PAHs with five rings or more are mostly bound to particles and are considered to be very hazardous to human health (Schwela, 2014).
Pyrocumulonimbus (pyroCb): Extreme manifestation of a pyrocumulus cloud, generated by the heat of a wildfire, that often rises to the upper troposphere or lower stratosphere (WMO, 2017).
Smoke: The airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, and often used as an informal term for a fire-emitted aerosol (NFPA, 2021).
Soot1: Black particles of carbon produced in a flame (NFPA 2021).
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1 There is a diversity of technical terminology used to identify combustion products across multiple disciplines. The definitions used in this report are sufficiently broad to be applicable across different chapters, however, the committee acknowledged that practitioners in specific research communities may use more precise definitions. For example, in the field of soot formation, soot is, “carbonaceous particles formed during the incomplete combustion or pyrolysis of hydrocarbons, including incipient soot particles, mature soot particles, and all of the intermediate particulate stages between inception, maturity, and complete oxidation to gas-phase species” (Michelsen et al., 2020).
Stratosphere: Stable (stratified) layer of atmosphere extending from the tropopause upward to a height of about 50 km.
Tropopause: Natural limit between the troposphere (Greek tropos = turn; troposphere = turning or mixing sphere) and the stratosphere (stratified as opposed to mixed). The tropopause can exist anywhere between about 70 hPa (∼18 km) and 400 hPa (∼6 km), and it is therefore not convenient to use a constant pressure level to describe the tropopause.
Troposphere: Lowest layer of Earth’s atmosphere in direct contact with Earth’s surface. Most of the weather phenomena, systems, convection, turbulence, and clouds occur in this layer, although some may extend into the lower portion of the stratosphere, immediately above the troposphere. The height of the troposphere varies from about 7–8 km (5 mi) at the poles to about 16–18 km (10–11 mi) at the Equator (Reichle, 2023).
Tropospheric Ozone: Produced from the oxidation of hydrocarbons and carbon monoxide (CO) in the presence of nitrogen oxides (NOx ≡ NO + NO2) and sunlight. The net sign of ozone production from formation and loss reactions in the troposphere depends critically on the level of the precursor gas NOx (or more specifically, the level of NO), which acts as a catalyst in ozone chemistry (Ma et al., 2022).
4.1.1 Description of the Atmosphere
There are two key regions of the atmosphere relevant to the prediction of the environmental effects of the detonation of nuclear weapons; these are the troposphere and stratosphere, which are separated by the tropopause. The troposphere is the portion of the atmosphere adjacent to Earth’s surface and extending to the height of the tropopause at an altitude of 7 km at the poles and 20 km at the Equator. The overlying stratosphere contains the critical ozone layer, which protects Earth’s lifeforms from ionizing UV radiation, and extends from the tropopause to the stratopause approximately 50 km above Earth’s surface. The troposphere is further divided into a planetary boundary layer (PBL), which is adjacent to the surface and where wind is influenced by frictional interaction with vegetation, topography, and other surface features, and the overlying free troposphere. The PBL thickness is highly variable in space and time, but it rarely exceeds 1 km.
4.1.2 Troposphere Versus Stratosphere
For the purposes of understanding and quantifying the rise, fate, and transport of smoke, the key processes to consider are the physics governing vertical stratification, vertical transport and mixing across that stratification, lateral transport and mixing, and local sources and sinks of the smoke (i.e., the primary mechanisms for its creation in and removal from a given location). Each of these processes operates on characteristic lengths and timescales, which in combination, strongly affect the long-range transport and lifetime of smoke in Earth’s atmosphere. These processes and their characteristic length and timescales differ systematically between the troposphere and stratosphere. The reason the literature on EENW places such an emphasis on stratospheric injection of smoke is that the smoke can both last much longer (up to years) and spread much farther in the stratosphere, resulting in much longer and more widespread perturbations to the solar insolation at Earth’s surface.
4.1.2.1 Stratification
Atmospheric temperatures generally decrease with height in the troposphere due to adiabatic cooling and increase with height in the stratosphere due to radiative heating from the absorption of sunlight by ozone.
4.1.2.2 Vertical Mixing Within a Stratified Atmosphere
In the troposphere, air can become buoyant due to external heat sources such as fires or internal sources such as the condensation of water vapor to liquid or solid phases. This buoyant air can rise until its level of neutral buoyancy in the upper troposphere or lower stratosphere, thereby transporting aerosols it contains to the same upper altitudes on timescales of minutes to hours. In the stratosphere, the inverted temperature profile strongly inhibits vertical mixing.
4.1.2.3 Lateral Mixing
Lateral mixing in the troposphere occurs on length scales ranging from individual weather systems, which can mix air in all directions, to the permanent large-scale Hadley, Ferrel, and polar cells in each hemisphere with attendant subtropical and polar jets. These circulation systems can efficiently mix air laterally and vertically throughout the troposphere on timescales of a month or less. In the stratosphere, the lateral transport is dominated by the Brewer-Dobson circulation connecting upwelling air in the tropical troposphere to downwelling air in the extratropics and poles with overturning times of roughly 3 years.
4.1.2.4 Primary Sinks
In the troposphere, aerosols can be removed by either dry or wet deposition. Dry deposition involves the direct settling of particles on surfaces such as vegetation or buildings, primarily through gravitational settling or impaction. Wet deposition involves removal through precipitation, either directly via scavenging by precipitation (“washout”) or indirectly via incorporation into cloud droplets or particles, growth of those droplets via condensation and coalescence, and subsequent deposition of these droplets onto the surface (“rainout”). The lifetime of hygroscopic (wettable) aerosols in the troposphere is comparable to 1 week. In the stratosphere, the primary removal mechanism is slow gravitational settling or sedimentation. The lifetime of stratospheric aerosols observed and simulated for volcanic eruptions typically ranges from one to several years.
4.1.3 Chapter Context: Physical and Chemical Processes and Transport
Emissions from the nuclear denotation and subsequent fires and material entrained into the nuclear cloud are introduced into the troposphere, and possibly the stratosphere. The injection height of the emissions is critical to understanding the ensuing environmental impacts. Material that is transported to the stratosphere has a longer atmospheric lifetime, lengthening the timescale with which the material can affect the environment. Whether said material is injected or lofted into the stratosphere, and in what quantities, is highly dependent on the nature of the weapon employment scenario and target characteristics that contribute to fire emissions (e.g., fire area and conditions, fuel loading). Plume rise, from the fire source to the altitude of neutral buoyancy, is discussed in Section 4.2.1 and the evolution of those emitted aerosols and gases is discussed in Section 4.2.2. Aerosols are removed from the atmosphere, primarily through sedimentation and wet removal mechanisms, the subject of Section 4.2.3. Removal can occur during plume rise or much later during atmospheric transport. Uncertainty, gaps in data and committee recommendations are addressed in Sections 4.2.4 and 4.2.5.
Material from the nuclear detonations and fires will either be deposited or remain in the atmosphere, both fates causing environmental effects. Tropospheric behavior and effects are addressed in Section 4.3, and stratospheric behavior and impacts are addressed in Section 4.4. Natural analogs within the environment, such as volcanic and wildfire emissions, are included in the latter section. A more complete treatment of local and regional effects of fires at the wildland–urban interface (WUI), including plume evolution and exposure pathways, for which there may be similarities, can be found in the National Academies report on The Chemistry of Fires at the Wildland-Urban Interface (NASEM, 2022).
SOURCE: Kremser et al., 2016.
4.2 PLUME DYNAMICS AND TRANSPORT MODELING
4.2.1 Plume-Rise Dynamics
Mass fires that result from a nuclear detonation can be a conflagration, defined as having a moving fire front that can be wind-driven, or a firestorm, which results when fires merge to form a single convective column. In the prior case, the fire can be sustained if there is fuel available to the moving fire front. In the latter case, surface winds are directed inward at the fire front toward the convective column, preventing spread and resulting in a stationary fire front. Generally, there is complete consumption of combustible fuel in a firestorm and the duration is limited by the availability of fuel.
The height of the smoke plume from these fires plays a critical role in determining the potential for these aerosols to produce climate effects. If the smoke is injected into the lower or mid-troposphere, it is generally removed from the atmosphere quickly through wet scavenging processes. Fires with greater intensity can result in plumes that reach the upper troposphere or even inject smoke directly into the stratosphere, transporting large amounts of aerosol within the thermally buoyant plumes. A portion of this smoke can be removed in the troposphere through wet removal processes, scavenged by condensing and precipitating water within the buoyant plume that is formed as the air parcels rise and cool. Smoke that reaches the upper troposphere or stratosphere can self-loft to higher altitudes through heat absorption of solar radiation. Tropopause folding events provide an additional mechanism for
transporting smoke across the tropopause. Once smoke is transported into the stratosphere, the atmospheric lifetime of the smoke can greatly increase because wet removal processes are minimal and continued self-lofting counteracts sedimentation processes.
Smoke from high-intensity wildland fires has been observed to reach the upper troposphere and lower stratosphere in pyrocumulonimbus (pyroCb) clouds, lofting large quantities of aerosols (see Box 4-2 for a case study from the 2019–2020 Australian wildfires). Katich et al. (2023) found that pyroCb clouds are responsible for up to 25% of the BCand organic aerosols found in the lower stratosphere. Smoke from these recent Australian wildfires was observed to self-loft from 14 to 30 km in height between January and March 2020 (Hirsch and Koren, 2021; Kablick et al., 2020; Khaykin et al., 2020; Ohneiser et al., 2020; 2022), demonstrating the self-lofting vertical transport mechanism for wildfire smoke plumes.
Plume or cloud height observations from mass fires during wartime are sparse. During World War II, incendiary bombing resulted in a firestorm in Hamburg, Germany on July 27, 1943 with an estimated plume height of 9–12 km (Manins, 1985). The fire plume approximately 3 hours after the August 6, 1945 detonation of an atomic bomb in Hiroshima is pictured in Figure 4-2 (Broad, 2016), although height estimates are uncertain. The subsequent atomic bomb detonation in Nagasaki, resulted in a smaller fire with an area approximately one-fourth of the fire area in Hiroshima, with reasons for this difference hypothesized to be limited fuel availability, the presence of fire breaks, and reduced fire starts due to topographic shielding of thermal radiation (Glasstone and Dolan, 1977). This counterexample to the Hiroshima firestorm demonstrates the importance of fuel availability and other emplacement conditions near ground zero.
SOURCE: Photo by U.S. Army, Courtesy of Hiroshima Peace Memorial Museum.
Plume height is highly sensitive to both the characteristics of the fire, such as the intensity, size, and shape, and to the ambient atmospheric state including humidity, wind speed and shear, and stability. Many modeling studies have investigated plume height from fires. During the 1980s, modeling studies of fire plumes generally used two-dimensional grids and often included assumptions of an axisymmetric fire
plume (Heikes et al., 1990; Penner et al., 1986; Small and Heikes, 1988), although some three-dimensional studies were also completed (Cotton, 1985; Penner et al., 1986). The focus of these studies was largely on relating fire size and heat flux to plume height and understanding the role of cloud microphysics. Recent plume-rise studies have allowed for turbulence-resolving large-eddy simulations on three-dimensional grids. These simulations also include parameterized atmospheric physics, such as cloud microphysics, which have demonstrated that atmospheric moisture plays a large role in plume-rise and stabilization height, as latent heating from condensing water vapor can increase plume height by several kilometers (Redfern et al., 2021; Tarshish and Romps, 2022; Wagman et al., 2020). Studies have also shown that plume heights increase with increasing fire intensity or size (Badlan et al., 2021; Freitas et al., 2007; Penner et al., 1986), area fires have higher plume heights than line fires with the same area owing to reduced entrainment of ambient air into the convective core (Badlan et al., 2021), increased wind speeds and shear enhance entrainment and decrease plume height (Freitas et al., 2010; Redfern et al., 2021; Wagman et al., 2020), and atmospheric stability of the boundary layer plays a limited role in plume height (Redfern et al. 2021). Figure 4-3 shows an example of plume height while varying the ambient atmospheric moisture content (mass mixing ration of water vapor qv) in the left panel, wind speed in the middle panel, and fuel loading (and therefore fire heat flux) in the right panel. Each profile indicates the vertical distribution of smoke within a plume. In the first panel, the vertical distribution of smoke in a climatological atmosphere is shown in black. The red profiles show that the plume height is greatly reduced if the atmosphere is completely dry or if the moisture content is reduced by half. The middle panel indicates that plume height will increase with decreasing wind speed. The right panel illustrates that plume heights increase with increasing fuel loads for the fire. In all panels, the climatological tropopause height is indicated by the horizontal dashed line.
SOURCE: Wagman et al., 2020.
The presence of a smoke plume can modify atmospheric dynamics and processes, for example by blocking sunlight and cooling the atmosphere below or by lofting moist air so that the moisture precipitates, which can in turn affect plume height and aerosol transport. Regardless, most studies have treated smoke as a passive tracer, neglecting coupled atmosphere–chemistry and aerosol–cloud interactions. Recent efforts have been made in this area; for example, Kochanski et al. (2019) coupled the fire spread and chemistry models within the WRF model, representing fire emissions as SO2, PM2.5, PM10, and organic and black carbon.
Modeling using coupled atmospheric-fire models is still an emerging area of research, with modeling capabilities for urban fires more limited than those for wildland fires. Use of a fire-spread model can yield important insights into plume-rise dynamics by quantifying time-varying fire spread and fluxes (e.g., heat, moisture, emissions) for the specified fuel type and density, topography, and atmospheric conditions, and by taking combustion chemistry into account. The WRF atmospheric model has been coupled to fire behavior models and used to simulate wildland fires (Coen et al., 2013). Reisner et al. (2018) used the coupled atmosphere-fire dynamics model HIGRAD-FIRETEC to simulate both fire spread and plume rise after a nuclear denotation in an urban environment.
The magnitude of climate impacts due to fire emissions is highly dependent on the mass of particulate matter (PM) reaching the stratosphere, which in turn is critically dependent on plume height. While intense fires can directly inject smoke into the stratosphere, smoke in the upper tropopause can self-loft to the stratosphere or be transported by tropopause folding. Wagman et al. (2020) showed negligible climate impacts from lower-intensity fires with plume heights between 2 and 4 km, with increasingly greater climate impacts from higher-intensity fires depositing an increasingly larger smoke mass in the upper troposphere. Lee et al. (2023) performed global simulations of smoke from wildfires using the Energy Exascale Earth System Model and demonstrated that modeling of stratospheric smoke transport, lofting, and lifetime was most sensitive to the initial injection height of the smoke source.
4.2.2 Aerosol and Gas Transport and Evolution
4.2.2.1 Aerosol Transport and Evolution
The height reached by aerosols determines their lifetime, and hence, climate and chemistry effects. Within the troposphere, removal occurs rapidly, through wet and dry deposition. Within the stratosphere, aerosols will have a longer lifetime on the order of one to several years, depending on altitude and latitude of injection or lofting (Waugh and Hall, 2002). Many recent studies (Waugh and Hall, 2002) have evaluated the lifetime (or age) of air in the stratosphere using tracers including sulfur hexafluoride (SF6) and nitrous oxide (N2O) and have compared the findings to results from models. Stratospheric ages are typically in the range of 3-5 years, depending on height, season, and latitude. Lofting can transport smoke plumes to higher altitudes, where ages are longer (see e.g., Dietmüller et al. 2018). The stratospheric model that has been used in several nuclear war studies is the Whole Atmosphere Community Climate Model (WACCM). The version of that model evaluated by Dietmüller et al. (2018) and Minganti et al. (2020) displayed shorter ages of air in many regions other than those inferred from observations, implying that it would be conservative insofar as smoke plume residence times are concerned.
Satellite-era studies demonstrate the importance of self-lofting of aerosols through absorption of radiation and hence heating. BC is a very strong absorber of incoming sunlight and subject to rapid lofting (Bardeen et al., 2021). However, smoke is often composed not just of BC but also organic carbon (OC) that consists of compounds such as levoglucosan, furans, and alcohols. Wildfire data demonstrates that smoke composition changes as the particles age after initial release, with a higher fraction of oxidized material such as organic acids and alcohols being observed with time and disappearance of compounds such as levoglucosan (see references in Solomon et al. [2023]). Recent major wildfires have been observed by satellite, allowing categorization of their composition (using infrared absorption spectroscopy of the aerosols themselves, see Boone et al. [2020] referenced in Solomon et al. [2023]) and
lofting rates. Yu et al. (2021) showed that the observed lofting of Australian wildfire aerosols in 2020 was well reproduced with smoke aerosols containing about 2.5% BC (with the rest being OC). The BC to OC ratio in smoke is thus a key parameter in determining the degree of self-lofting of aerosols from the surface fires (e.g., wildland and/or urban). Importantly, some nuclear war simulations (e.g., Bardeen et al., 2021) have assumed that the stratospheric smoke was pure BC, which would be subject to extremely rapid lofting and able to reach much higher altitudes and hence produce longer stratospheric chemistry and surface climate impacts. The BC content of the smoke produced by nuclear wars is not well established. These studies underscore that understanding the composition of smoke, particularly its BC and OC content, is a key (and uncertain) parameter in evaluating the timescale and extent of the impacts of nuclear war.
4.2.2.2 Gas-Phase and Heterogeneous Chemistry
Earth’s atmosphere is a highly oxidizing environment, in both the troposphere and stratosphere. Smoke from fires in urban and surrounding regions can be expected to produce some hydrocarbon compounds. These will oxidize in the gas phase through reactions with the hydroxyl radical (OH) and (in some cases) ozone. The smoke product species can be taken up on particles and can undergo oxidation in the liquid phase as well. Aged smoke aerosols are composed of carbonaceous compounds including aldehydes, alcohols, and organic acids. The transformation of gas-phase carbonaceous compounds produced from fires into particulate matter gives rise to organic aerosols in large amounts in fire smoke. Particularly if there are also soot occlusions within the particles, the organic particles strongly absorb radiation and can thereby loft to higher altitudes. Wildfire smoke plumes have been observed to rise from the troposphere to the stratosphere, where they cause strong heating and may also provide sites for heterogeneous stratospheric chemical reactions.
Some studies have examined how gas-phase stratospheric ozone chemistry may be affected by stratospheric heating due to the presence of particles that absorb radiation. Here the report briefly outlines the primary mechanisms that can be important.
Stratospheric ozone chemistry is sensitive to temperature, with many key gas-phase chemical reactions proceeding more rapidly at warmer temperatures that make reactive molecular collisions more effective. For example, the following reaction sequence of ozone destruction is an efficient one in the upper stratosphere:
| O3 + hv → O + O2, | Equation 4-1a | |
| O + O3 → 2O2, | Equation 4-1b | |
| Net: 2O3 + hv → 3O2, | Equation 4-1 |
Because the reaction (Equation 4-1b) has a substantial energy of activation (a barrier to reaction), it proceeds more rapidly and destroys more ozone in warmer temperatures. Similarly, temperature-sensitive reactions with a positive energy of activation are involved in the reactive nitrogen (NOx) and chlorine (ClOx) catalytic cycles of ozone destruction. Some studies suggest that even a limited regional nuclear exchange could inject enough soot to greatly heat the stratosphere (up to 80 K at some altitudes) over much of the stratosphere and thereby lead to substantial ozone losses (Bardeen et al., 2021, and references therein) through this process. Stratospheric heating would cause reduced ozone losses in the Antarctic and Arctic but would be expected to increase ozone loss at lower latitudes, where most of Earth’s ozone layer and life on the planet surface that is sensitive to ozone losses are found.
While the effect of heating is expected to dominate ozone losses following a nuclear war based on most recently published literature, other chemical processes have also been highlighted, including, for example, increases in reactive nitrogen produced directly by hot nuclear blasts that may reach stratospheric levels. It is well known that nitrogen and oxygen produce NO in high-temperature combustion, also called thermal NOx. NOx destroys ozone through, for example, the following catalytic cycle:
| NO + O3 → NO2 + O2, | Equation 4-2a | |
| O3 + hv → O + O2, | Equation 4-2b | |
| O + NO2 → NO + O2, | Equation 4-2c | |
| Net: 2O3 + hv → 3O2, | Equation 4-2 |
Mills et al. (2008, 2014) also highlighted the potential role of smoke aerosols produced in nuclear exchanges in heating the tropical tropopause region, which is sometimes considered the “cold trap” that limits transfer of water from the troposphere to the stratosphere. Increases in stratospheric water vapor act to cool the stratosphere but warm the troposphere and surface climate (Solomon et al., 2010). Available studies that include this effect in a fully interactive Earth system model are currently lacking.
Heterogeneous reactions can also be important. Volcanic eruptions can inject SO2 directly into the stratosphere, where it oxidizes and can greatly enhance the surface areas of sulfate/water particles there. Following volcanic eruptions, the nitrogen oxide reservoir species N2O5 can react with water on the surfaces of particles, forming HNO3 and drawing down NO and NO2. This in turn affects reactive chlorine and reactive hydrogen species through their interactions with NO, NO2, and HNO3, and the net effect in the modern atmosphere is mid- and high-latitude ozone loss (Portmann et al., 1996; Solomon et al., 1996). Indeed, substantial ozone losses were observed following the eruption of Mount Pinatubo in 1991, ascribed to this chemistry. In addition, atmospheric chlorine normally tied up in inert reservoirs (HCl, ClONO2) can be activated to form reactive species, such as Cl, ClO, etc., via heterogeneous chlorine reactions (see e.g., Solomon, 1999) such as
| HCl(het) + ClONO2 → Cl2 + HNO3(het), followed by | Equation 4-3a | |
| Cl2 + hv → 2Cl, | Equation 4-3b |
Recent evidence has yielded the surprising result that such processes are likely to occur on organic aerosols as well as sulfate aerosols, and could add to the gas-phase chemical losses discussed above. In particular, recent studies of the major Australian wildfires of 2020 suggest that smoke can perturb stratospheric chemistry and produce significant ozone losses (Bernath et al., 2022; Santee et al., 2022; Solomon et al., 2023). Although currently available nuclear war studies have included gas-phase chemistry and some potential smoke reactions, they have not yet included heterogeneous chlorine chemistry. Up-to-date modeling including all known processes is therefore unavailable.
4.2.3 Removal of Atmospheric Aerosol
Aerosol particles released into the atmosphere either directly by the nuclear detonation and entrained debris or by subsequent fires will evolve during transport due to microphysical processes. Some particles will be removed from the atmosphere due to forces by temperature gradients (thermophoretic), density differential with the ambient air (aerodynamic), Brownian and turbulent eddy diffusion, gravity, and wet scavenging (e.g., cloud and precipitation). These aerosol processes are particle-size dependent. The environmental conditions between the troposphere and stratosphere also determine which processes are important versus negligible when modeling aerosol transport and transformation. For instance, the stratosphere is significantly drier than troposphere. Wet scavenging is therefore negligible in the stratosphere, but is an important removal mechanism for tropospheric aerosols.
The general mathematical model describing the aerosol change is aerosol dynamics (Gelbard and Seinfeld, 1979; Hubbard et al., 2019; Jacobson and Turco, 1995) and is shown as follows:
| Equation 4-4 |
BOX 4-2
The 2019–2020 Australian Wildfire Event as a Test Case for the Efficiency of Stratospheric Aerosol Injection
For the same nuclear weapons exchange scenario, one of the primary factors in climate impacts estimated by various studies is differences in the magnitude of the particulate matter mass that reaches the stratosphere (Reisner et al., 2019; Robock et al., 2019; Toon et al., 2019).
The delivery of aerosol to the stratosphere depends on direct injection from the initial explosions along with the total amount of aerosol produced by subsequent surface fires and the efficiency by which these emissions are entrained in pyrocumulonimbus activity and lifted into the stratosphere. This latter efficiency is partly determined by the spatial structure and intensity of surface fires, including the magnitude and duration of the surface heating, which, in turn, influences plume dynamics and surface entrainment (Wagman et al., 2020). Atmospheric processes, including atmospheric stability, horizontal wind shear, precipitation scavenging within plumes, radiative lofting of the particles, aerosol detrainment within the troposphere, and the height of the tropopause, can also be important in regulating this efficiency. Modeling interactions among these processes is challenging because of the different temporal and spatial scales needed to simulate fire spread, plume dynamics, and global atmospheric circulation. The 2019-2020 wildfires in Australia provide an empirical test case for exploring how the combined set of processes influences this efficiency. Specifically, independent estimates of surface fire emissions and stratospheric aerosol mass have been developed, each drawing upon a different set of satellite observations.
Surface emissions from the Australian wildfires were estimated to range between 113 and 236 Tg C in one study using Tropospheric Monitoring Instrument (TROPOMI) satellite observations of CO and Orbiting Carbon Observatory-2 observations of CO2 (Byrne et al., 2021). In another study, fire emissions were estimated to be between 140 and 236 Tg C from an analysis using TROPOMI CO and a different atmospheric model (van der Velde et al., 2021). For illustration, here a central estimate of 175 ± 50 Tg C is used. To convert to aerosol mass, it is assumed 50 ± 2% of combusted dry biomass is carbon, and an emission factor for particulate matter less than 2.5 microns (PM2.5) of 18.5 ± 14.4 g per kg of dry matter is used (Andreae, 2019). This PM2.5 emission factor is derived from 29 studies sampling fires in temperate forest ecosystems. Combining these different estimates and propagating the uncertainties in a Monte Carlo simulation (assuming they each have a Gaussian error distribution and excluding negative emission factor values) a surface aerosol mass produced by the fires of 7.5 ± 5.0 Tg is calculated.
Estimates of stratospheric aerosol mass injected by the Australian wildfires are more variable and depend on the analytical approach. Khaykin et al. (2020) derived an estimate of 0.4 ± 0.2 Tg using aerosol extinction profiles from the Ozone Mapping and Profiler Suite Limb Profiler. Using aerosol optical depth observations from the Moderate Resolution Imaging Spectroradiometer, Hirsch and Koren (2021) derived an estimate of 2.1 ± 1.0 Tg. A third study adjusted aerosol mass within the Community Earth System Model (CESM) coupled to the Community Aerosol and Radiation Model for the Atmosphere until model estimates matched aerosol extinction coefficients from Stratospheric Aerosol and Gas Experiment III instrument on the International Space Station. This optimization yielded a stratospheric aerosol mass of about 0.9 Tg (Yu et al., 2021). Taking the average of these estimates, a stratospheric aerosol mass of about 1.1 ± 0.9 Tg is obtained.
In the final step, in a Monte Carlo simulation the stratospheric injection efficiency is computed from the ratio of the stratospheric and surface aerosol distributions, excluding negative values. This simple approach yields a mean efficiency of 0.28 ± 0.27. Thus, with high levels of uncertainty, about a quarter of the aerosol emissions produced by the Australian fires were lofted into the stratosphere. In interpreting this number, it is essential to note that not all of the Australian wildfires in the regional complex generated pyrocumulonimbus activity. This is also expected when fires started in a nuclear weapons exchange occur in areas where fuel levels are lower, such as in suburban areas, or where building collapse driven by the weapon’s shock wave limits the rate of fuel consumption (Reisner et al., 2018; Wagman et al., 2020). For the Australian wildfire complex, many of the most prominent aerosol plumes occurred between December 31, 2019 and 7 January 2020 (Khaykin et al., 2020). Confining the analysis to this shorter period, if flux estimates were available, would likely increase the estimate of the stratosphere injection efficiency. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar observations also provide evidence for lower-altitude plumes that did not reach the stratospheric and for elevated aerosols above convective centers downwind of the fires, highlighting the importance of aerosol mixing processes not directly connected to stratospheric injection from pyrocumulonimbus activity (Hirsch and Koren, 2021).
The general aerosol dynamics equation for a spatially homogeneous aerosol with a constant material density describes the aerosol number concentration (n) as a function of time (t) for given particle volume (v). The source term (Ψ) accounts for the generation of particles through homogeneous and heterogeneous nucleation, as well as other processes. The second term on the right-hand side of Equation 4-4 represents particle growth or particle shrinkage due to condensation and evaporation. ℜ is a removal term that can be used to represent processes like gravitational sedimentation or thermophoretic losses or other loss processes described previously. The integral terms on the right-hand side of Equation 4-4 represent coagulation which is a dominant process when the number concentration is high. In the simulation of coagulation, there are interactions between all particle sizes denoted v and u governed by the coagulation kernel β. Sedimentation and wet scavenging processes are the two major processes that remove aerosol particles from the atmosphere. The former removal process is particle-size dependent and can occur in both the troposphere and the stratosphere; the latter is particle-composition dependent, particularly on the hygroscopicity of the aerosol particles. Since the stratosphere is dry, wet scavenging processes are not important for particle removal in this region of the atmosphere. Though the processes discussed apply to other types of aerosols (e.g., minerals, oxides, and biogenic aerosols), presumably with different efficiencies, this section will address removal of two types of aerosols: debris resulting from detonation of nuclear bombs and BC particles from fires.
4.2.3.1 Sedimentation
Aerosol particles produced by and lofted into atmosphere by a nuclear detonation will be subject to sedimentation caused by gravitational forces causing particles to fall from the atmosphere in relation to the surrounding air because of their greater density. Air density is 1.3×103 g/cm3 (at 273 K and 101.3 kPa) while a BC particle is 1.1 ± 0.6 g/cm3 (Ren et al., 2023) and a NaCl particle is 2.16 g/cm3, for example. The density of particles is three orders of magnitude larger than that of the air.
Previous research shows that sedimentation of aerosol particles is negligible for particles smaller than 1 micron (µm) in the troposphere and lower stratosphere (Twomey, 1977). This is because vertical transport of aerosol particles caused by turbulent eddy diffusion and convection in these size ranges is much higher than the rate of sedimentation. This rapidly becomes the case for the population of aerosols remaining in the stratosphere after injection, accounting for their long stratospheric residence times and impacts. For larger particles, sedimentation enhances the rate of removal by bringing aerosol particles to lower altitudes or to the surface.
Table 4-1 shows the sedimentation velocity of spherical aerosol particles estimated using Stokes law, i.e.,
| Equation 4-5 |
where Vs is the gravitational or sedimentation velocity, Dp is the particle diameter assuming it is a sphere, ρp is the particle material density, Cc is the Cunningham slip correction factor, μ is the viscosity of air, and χ is the dynamic shape factor for nonspherical particles, while g is the gravitational constant of Earth.
4.2.3.1.1 Sedimentation of bomb debris and entrained materials
Sedimentation of fallout particles has been studied since the beginning of nuclear tests in the 1950s, primarily because of human health concerns. This removal process is modeled for nuclear debris and fallout as following Stokes law, described previously. Particles of diameter greater than 30 µm are removed from the air in a few hours after the detonation, and those smaller than 10 µm could remain in the atmosphere after 24 hours. Particle size distributions for airbursts range from diameters of approximately 0.01 to 20 µm, and surface bursts range from diameters of submicrons to millimeters (see Chapter 2 for further discussion). For objects such as rocks or dirt that are much larger than 100 µm (0.1
mm), they will not remain suspended in the air for long time periods, so while they would be considered in fallout calculations, they are not important in evaluation of the climatic impacts.
The relaxation time (RT) defines the time required for a particle to adjust or “relax” its velocity to a new condition of force. As shown in Table 4-1, the RT for submicron particles is in microseconds or shorter, indicating that the particles would equilibrate nearly instantly (as in synchronizing transport) with ambient air once they are released into it. If they are released in the upper troposphere and lower stratosphere, submicron particles would not settle down to the surface. In that case, their removal from the atmosphere would rely on other mechanisms. The particles larger than 10 µm in Table 4-1 have sufficient settling velocities and can settle an appreciable vertical distance in 24 hours, for example. Therefore, it is expected that particles greater than roughly 10 µm produced or entrained by a nuclear detonation would not remain in the air after 24 hours.
| Dp, µm | Vts, m/s | RT, s | D in 24 hour, m |
|---|---|---|---|
| 0.01 | 6.90E-08 | 7.10E-09 | 0.01 |
| 0.05 | 3.80E-07 | 3.80E-08 | 0.03 |
| 0.1 | 8.80E-07 | 9.00E-08 | 0.08 |
| 0.5 | 1.00E-05 | 1.00E-06 | 1 |
| 1 | 3.50E-05 | 3.50E-06 | 3 |
| 5 | 7.80E-04 | 7.90E-05 | 67 |
| 10 | 3.10E-03 | 3.10E-04 | 268 |
| 50 | 7.53E-02 | 7.70E-03 | 6,506 |
| 100 | 2.50E-01 | 3.10E-02 | 21,600 |
SOURCE: Hinds, 1999.
4.2.3.1.2 Sedimentation of soot
The material properties of soot particles differ from bomb debris and entrained debris, and significant differences in the sedimentation rate of these types of particles could exist. Differences in the effective density and morphology of the particles that affect the shape factor and gravitational pull will be the main causes of the discrepancy for the two aerosols of the same particle size.
The conventional view is that soot particles are formed by the cluster-dilute aggregation mechanism in wildfires and emitted as aggregates with a fractal dimension ~ 1.8 and mobility diameter ≤1 µm, and aerodynamic diameter ≤300 nm, but there were also reports that soot aggregates have a fractal dimension ~2.6, mobility diameter >1 µm, and aerodynamic diameter ≤300 nm indicating that the formation mechanism and structure of soot particles generated by wildfires is an active research area (Chakrabarty et al., 2014). Johnson et al. (1996) showed that settling velocities of fractal aggregates were generally much higher than those predicted using Stokes law (see Eq. 4-5) for a solid sphere.
The effective density of smoke particles is known to be lower than the material density of graphitic carbon (2.2 g/cm3 at 20°C), because smoke particles tend to form complex morphology such as those shown in Figure 4-4. Scanning electron microscope images by Chakrabarty et al. (2014) showed examples of the complex morphology of soot particles collected in the plume of wildfires. Restructuring of carbonaceous aerosol material by coagulation and/or aggregation are the two known mechanisms for the formation of complex morphology. Morphology affects the dynamic shape factor χ and thereby impacts the sedimentation.
SOURCE: Charkrabarty et al., 2014.
In addition to the morphological variation, chemical composition is another factor that may contribute to the uncertainty of effective aerosol density for soot particles. Chemical composition of soot particles produced by a fire is complicated, in part because a mixture of fuel of differing organic composition (e.g., pine trees versus oak) is involved and because fire temperatures vary, with hot fires likely to produce more soot. Semivolatile organic compounds that were also produced by the fire could condense on the soot surfaces as the cool ambient air was entrained into a fire plume. Organic coating could also increase the water-uptake ability of soot particles thereby growing the size of the soot particles and increasing its sedimentation removal potential. Once the soot particles are coated with hydrophilic organic, they could also become hygroscopic and be removed by the wet processes such as cloud and precipitation scavenging, discussed in the following sections.
4.2.3.2 Wet Removal
This section will address a major removal mechanism of aerosol from the atmosphere involving water, called wet removal, or deposition. Wet removal of gases and aerosols from the atmosphere is primarily accomplished within the troposphere by clouds and precipitation (e.g., rain and snow) and is not expected in the stratosphere where it is relatively dry compared to the troposphere. Clouds play a major role in Earth’s radiation budget and are key to delivering rain or snow to the surface in the hydrological cycle (Seinfeld et al., 1998). In addition to their role as a medium for aqueous-phase reactions, clouds also affect atmospheric vertical transport in updrafts and downdrafts influencing the vertical redistribution of gases and aerosol particles, but cloud feedbacks currently exhibit the largest uncertainty in accurate modeling of Earth’s climate or weather (IPCC, 2023).
4.2.3.2.1 Wet removal of soot
Wet deposition was estimated to account for 60–85% of total deposition (dry + wet) (Barrett et al., 2019) and represents the major pathway for removal of atmospheric black carbon (Kondo et al., 2016; Taylor et al., 2020). Liu et al. (2010) showed that 65% of black carbons in the atmosphere at Jungfraujoch, Switzerland was removed by precipitation. Dry and wet deposition are the only removal pathways known for atmospheric black carbons (Begam et al., 2016; Bibi et al., 2017) from the atmosphere, and though there have been model simulation studies of BC particles produced by massive wildfires attributed to nuclear detonations (Dubey et al., 2023; Mills et al., 2008; Reisner et al., 2018; Toon et al., 2008), experimental data of wet removal of the aerosol black carbon population generated by nuclear detonations do not exist.
Figure 4-5 depicts complex aerosol processes within a mixed-phase cloud (Yang, et al., 2019), where reactions between aerosol particles and water can occur in the cloud or below. Aerosol particles are captured by water droplets in a process called scavenging, which includes impaction scavenging (labeled 1 in Figure 4-5) and nucleation scavenging (labeled 2 in Figure 4-5). Impact scavenging occurs when aerosols collide with water droplets as they fall through the air (Liu et al., 2013; Ohata et al., 2016) or in a convective cloud environment where aerosol particles are mixed with water droplets in a turbulent flow. Studies on impaction scavenging of black carbon by snowfall are rare and results in the literature are highly variable (e.g., (Hegg et al., 2011; Wang et al., 2014)). Nucleation scavenging is the process where aerosols act as cloud condensation nuclei (CCN) to seed the growth of cloud droplets (Ching et al., 2018; Ohata et al., 2016; Seinfeld et al., 1998), and are thereby removed from the atmosphere as the cloud droplet precipitates. Ohata et al. (2016) demonstrated that activation of aerosols to cloud droplets predominantly controls the wet removal efficiency of particles ranging from 50 nm to 1 µm. Studies showed that deep convection could be an effective in-cloud removal of aerosols (Yu et al., 2019), but recently available literature suggests that ice nucleation processes induced by carbonaceous particles are not well understood (Alsante and Cheng, 2024).
SOURCE: Adapted from Yang, et al., 2019.
Hoose et al. (2008), Yang et al. (2019) reported the mass scavenging efficiencies of black carbons, in cloud or below cloud, are highly dependent on the particle properties (size and chemical compositions) and meteorological conditions (cloud water content, temperature, etc.). More specifically, meteorological factors that influence wet removal depend on the intensity, frequency, and duration of rainfall, the mixing process of rainwater and particles, size distribution of particles and rain droplets, and droplet falling velocity. Physicochemical properties, with less well-characterized effects on wet removal mechanisms, include size distribution of particles and surface properties such as wettability and pore distribution (Gao et al., 2022; Alsante and Cheng, 2024).
Recent research shows a wide range in optical properties of soot produced by different detonated explosives (Aiken et al. (2022) and a recent National Academies report (2022) discusses how black carbon produced by WUI fires could have different physicochemical properties leading to significantly different fate and transport behavior of the aerosol in the atmosphere, and thus removal efficiency. Taken together, these results indicate the importance of considering the range of particle properties that may exist given the source of combustion.
Also impacting wet removal, once soot particles produced by fire are released into the troposphere or stratosphere, as discussed in Section 4.2.2, their physicochemical properties will be modified significantly by environmental processes. Freshly emitted black carbon particles are hydrophobic and not readily available to act as CCN (Ching et al., 2018; Liu et al., 2013; Weingartner et al., 1997). During aging, several atmospheric processing mechanisms could cause black carbon particles to evolve from hydrophobic to hydrophilic materials, for example, coalescence with hygroscopic particles, coagulation with more soluble particles, and coating of oxidized organic species on the particles (Vignati et al., 2010). Cruz and Pandis (1997) showed that the ability of black carbon aerosols to act as CCN increase during atmospheric transport and processing such as mixing with organic and sulfate species. Coating by organic compounds and ammonium sulfate on soot could shift the black carbon aerosol from hydrophobic into hydrophilic affinity, also enhancing water uptake, which promotes their removal by wet processes in the atmosphere (Zhang et al., 2008). Shan et al. (2021) showed that a 20% increase in black carbon’s hygroscopicity, due to primary organic carbon, significantly enhanced wet removal of the particle. Perring et al. (2017) showed BC-specific hygroscopicity (κ) increased from a value of 0.0 to 0.09 with an e-folding time of 29 hours, clearly indicating the wildfire-produced BC could be effectively wet-removed. The variation of κ was largely due to the organic and ammonium sulfate coating thickness on the BC, which affects water uptake by BC-containing aerosols and promotes their removal from the atmosphere (troposphere) by wet processes (cloud and precipitation).
4.2.3.2.2 Wet removal of bomb debris and entrained material
Although wet removal processes of atmospheric carbonaceous particles have begun to be elucidated, there is little known about the removal on nuclear bomb detonation-derived particles or debris. With data from wet deposition studies of atmospheric particles discussed above, it is expected that nuclear detonation-derived particles would behave similarly and be removed from the atmosphere accordingly. There are a few studies supporting this conjecture, using data on strontium-90 (Sr-90) as a marker for characterizing atmospheric deposition of fallout debris from nuclear tests because of its solubility in water and interest in the toxicity and human exposure (Eisenbud, 1956; Glasstone and Dolan, 1977; Beck, 2010). As shown in Figure 4-6, the left panel shows the activity of Sr-90 in the stratosphere decreasing overtime as nuclear testing activities subsided, eventually returning to pre-test levels. The right panel in Figure 4-6 showed an increase in Sr-90 activity at the surface over time, peaking in the 70s and decreasing afterwards due to radioactive decay and atmospheric wet removal. The limited data suggests that aerosol particles consisting of nuclear detonation-derived debris could also be removed effectively by wet deposition processes. The details of such wet removal remain unknown.
SOURCE: Glasstone and Dolan, 1977.
Wet deposition is understood to be the primary mechanism for removal of aerosol particles including radioactive fallout, minerals, oxides, organic and or black carbons from the atmosphere after a nuclear explosion, but the details of these mechanisms, particularly in the mixed-phase cloud processes, remained elusive and require further study for future development of an accurate predictive model. Experimental data are critical for the model development and validation.
4.2.4 Key Uncertainties and Data Gaps
Current atmospheric models are capable of simulating fire plume-rise and stabilization height when adequate resolution and appropriate ambient atmospheric conditions are used with atmospheric physics parameterizations, such as those for cloud microphysics. The largest uncertainties surround the time-varying parameters of the fire, such as the fire size and geometry and fluxes of heat, moisture, and emissions. Models of fire dynamics can be used to constrain these parameters; however, major uncertainties exist surrounding their input data, especially for fires in the built environment. Without an improved understanding of these time-evolving fire parameters, it is difficult to predict plume height. However, recent wildfires have demonstrated the ability for intense fires to inject material within the upper troposphere and lower stratosphere. Inputs of NO and water vapor are important for ozone depletion and potentially also the climate response but are poorly characterized. Related weapon-employment-scenario-specific uncertainties that likely affect the fire and plume rise include weather and seasonality.
There are also large uncertainties regarding the aerosol properties from fire emissions and their evolution during plume rise and subsequent atmospheric transport. Composition of the OC (alcohols, acids, etc.) affects optical and chemical properties and is uncertain. These uncertainties impact accurate determination of fate and transport in the atmosphere and environmental consequences. Relevant uncertainties include:
- Effective density of soot;
- Morphology of soot (and therefore the dynamic shape factor);
- Mass loading of BC/OC ratios of soot and mixed particles;
- Type of organic compounds in the coating that affect the chemistry, hygroscopicity and heterogeneous density of aerosols; and
- Particle size distribution of soot particles.
Processes of wet removal of atmospheric particles, produced either directly from the nuclear detonation or by fire triggered by nuclear detonation, remain to be a major unknown. The major uncertainties of wet removal processes are as follows:
- Morphology of soot (and therefore the dynamic shape factor),
- Mass loading of BC/OC ratios of soot,
- Type of organic compounds in the coating that affect the hygroscopicity of soot and chemical processes that are important for ozone depletion,
- Particle size distribution of soot particles, and
- Vertical distribution of soot aerosol.
4.3 TROPOSPHERIC BEHAVIOR AND IMPACTS
4.3.1 State of Knowledge
Even when soot is not injected into the stratosphere and rather is constrained within the troposphere, research on wildfire has illustrated the importance of regional impacts of smoke. There is a wealth of research investigating the chemical and physical impacts of wildland fire plumes in the troposphere and their downwind impacts. Wildfires can contribute significantly to particulate pollution that ultimately impacts air quality and increases exposures that cause negative effects on public health (Jaffe et al., 2020; Sanderfoot et al., 2022). The composition of particles in wildfire smoke may vary; particles can be in the form of tar balls (Sparks and Wagner, 2021), or contain metals, and other components that have impacts on their physical interactions in the atmosphere as well as the health of those exposed.
Emissions from wildfires include the oxides of nitrogen (NOx) and volatile organic compounds (VOCs) that contribute to the chemical formation of ozone. Wildland fire plumes can impact downwind tropospheric ozone formation (Jaffe et al., 2008; Wilkins et al., 2018) even far downwind. For example, wildfire plumes in Siberia in summer 2003 were shown to increase the background ozone in western North America by 5–9 ppbv (Jaffe et al., 2004). Further, the particles in wildfire plumes may alter the chemical and physical characteristics of the area, for example, affecting photolysis rates and surface temperature (e.g., Jiang et al., 2012).
Chapter 4 of The Chemistry of Fires at the Wildland–Urban Interface (NASEM, 2022) summarizes the air quality and tropospheric composition impacts from WUI fires, highlighting both primary and secondary species with toxic potential that are either observed or anticipated to occur in WUI fires. More recent fires in urban areas and in the WUI have shown downwind enhancements of pollutants. Toxics, such as polycyclic aromatic hydrocarbons (PAHs), are semivolatile and can exist in the gas or particulate phase. These compounds can be carcinogenic and cause negative health outcomes for those exposed (Ghetu et al., 2022). Fires also emit dioxins and other toxics that have impacts on public and ecosystem health (Aurell and Gullett, 2013; Paul et al., 2023).
The lifetime of atmospheric particles in the troposphere ranges from a few days to a few weeks, depending on the location in the troposphere, interactions with clouds, and the impacts of dry and wet removal processes (Seinfeld, 2003). Gases are more variable; for example, the lifetime of methane (CH4) ranges from 7 to 12 years and is in part controlled by the oxidation capacity of the atmosphere, whereas compounds such as pinene (produced, for example, from pine trees), last only a few hours.
Stratospheric impacts of fire plumes may influence tropospheric chemistry and composition. Aerosols and thus radiation may impact the oxidation capacity of the troposphere, thus changing the lifetimes of key pollutants. For example, Madronich et al. (2023) report that OH concentrations in the troposphere can be increased due to the depletion of stratospheric ozone (which could be a result of emissions injected into the stratosphere; see Section 4.4).
4.3.2 Key Uncertainties and Data Gaps
The quantity and speciation of the aerosol and gas emissions from a nuclear blast and subsequent fires are highly uncertain. The identification and quantification of these emissions, as well as where they are injected into the atmosphere, are required to evaluate the tropospheric impacts. Potential impacts include air quality degradation through an increase in tropospheric ozone and PM concentrations. Toxics, such as PAHs and dioxins, may also be emitted and produce negative effects from public and ecosystem exposures downwind.
Tropospheric chemistry may also be affected by the changes in radiation due to aerosol loadings in the troposphere and stratosphere. This may include changes in the photolysis rates, and the concentrations of OH radicals. The results may include increases in the lifetimes of organic compounds (from CH4 to VOCs), and changes in the formation and concentrations of secondary aerosol formation and tropospheric ozone.
Key uncertainties in understanding the tropospheric impacts include
- the amount and chemical composition of emissions from the blast and resulting fires;
- the injection height of the plume (and how much stays within the troposphere); and
- the transport, chemical transformations, and deposition of the pollutants.
The uncertainties of this section can be tied back to the impacts on the stratosphere (Section 4.3.3), the emissions (Section 3.4), and the injection of the pollutants into the atmosphere (Section 4.2).
4.4 STRATOSPHERIC BEHAVIOR AND IMPACTS
4.4.1 Natural Analogs and Relevance for Nuclear Exchange Scenarios
Insofar as the stratospheric behavior of nuclear exchanges is concerned, there are two important natural analogs: volcanoes and wildfires. Here this report deals first with volcanic material and then discusses how wildfire aerosols affect our understanding.
While many volcanoes only put material into the troposphere (sometimes referred to as ‘smokers’ such as Kilauea), it is explosive (pyroclastic) volcanic eruptions that can inject particles and gaseous sulfur dioxide into the stratosphere. This sulfur dioxide oxidizes, and is taken up in stratospheric aerosols, enhancing their size and optical depth, sometimes leading to dramatic sunsets and milky skies and reflecting incoming solar energy out to space, thereby impacting surface climate.
Numerous volcanic events have occurred in the satellite era and have clearly demonstrated how increased loadings of stratospheric sulfate not only from large but also relatively small eruptions cool the climate (Santer et al., 2014; Solomon et al., 2011) and destroy stratospheric ozone through heterogeneous chemistry (e.g., Solomon et al., 1994). Further, paleoclimatic studies have also bolstered confidence in the role and quantification of the relationship between stratospheric aerosol loadings and climate changes through time (e.g., Hegerl et al., 2003; Jansen et al., 2007). Eruptions that reach the stratosphere at low latitudes are particularly long-lived because the particles are swept up in the stratospheric overturning circulation and can remain at stratospheric heights for years. Indeed, even particles produced by less-explosive tropical eruptions or surface tropical pollution can reach the stratosphere to a limited extent, due to lofting in for example the monsoon, followed by slow upwelling in the upper troposphere. The residence time of volcanic material from recent tropical volcanic eruptions that reach the stratosphere has also been well studied, and an e-folding timescale of about 2 years has been established for this type of particle (Deshler, 2008). Note, however, that one e-folding is generally not enough to empty the stratosphere completely. Following a large eruption that enhances stratospheric aerosol mass by, for example, a factor of 10 or more (as in the 1991 Pinatubo eruption, see Deshler, 2008), several e-foldings are required. The same would be expected for other aerosols and provides a baseline for calculating climate and other impacts from tropical events. At higher latitudes, the particles are injected into the
descending branch of the stratospheric circulation and can generally be swept out more rapidly, depending on the specific latitude and time of year of the eruption. Material from a nuclear cloud or fire plume would be expected to display a qualitatively similar latitude dependence insofar as its residence time is concerned. These particles may, however, differ importantly from volcanoes in that they would contain substances other than sulfate, which would affect their quantitative stratospheric residence time and therefore their stratospheric impacts.
Recent major wildfires have led to formation of pyrocumulonimbus towers that are capable of injecting significant amounts of wildfire aerosols composed of organic material or smoke into the stratosphere; its brown or black appearance signals the presence of black and organic carbon with the capability to strongly absorb sunlight (rather than largely reflect, as in the volcanic case above). Observations show that aged (after a few days) wildfire particles mainly contain oxidized organic substances such as organic alcohols and acids, along with soot. These can absorb sunlight strongly enough to produce lofting, as observed following the 2020 Australian fire event, extending the lifetime of the aerosols in the stratosphere and changing their chemistry. Some Australian fire particles self-lofted to the remarkable height of 35 km (Khaykin et al., 2020). Modeling studies after the 2020 Australian event suggested that a composition of 2% BC could best explain the observed average lofting following this event (Yu et al., 2022). Because emissions from fires after a nuclear detonation also involve burning of organic substances with emissions of black and organic carbon, they can likely remain in the troposphere and stratosphere longer than would otherwise be expected. However, it is likely that the composition of emissions after a nuclear detonation may well differ from those after a wildfire and is unknown, implying a major uncertainty.
In summary, it is well established that increases in stratospheric aerosol abundances sufficient to influence atmospheric optical depth can both cool the climate and destroy stratospheric ozone for at least several years (particularly for tropical eruptions), as demonstrated by significant volcanic events that have been documented during the satellite era. These provide a baseline for calculating impacts that can be expected from emissions following nuclear detonations that yield comparable stratospheric optical depths. Estimation of the amount of material that reaches stratospheric heights following any nuclear blast and its latitude of injection is key. Recent wildfire events, particularly the 2020 Australian wildfire event and to a lesser extent the smaller 2017 Pacific Northwest Event have also illuminated how fire smoke particles containing organic material and BC alter the picture in important ways. Smoke particles have different radiative properties from volcanic particles. Smoke from wildfires can absorb ambient radiation and loft, increasing its stratospheric lifetime and hence extending related impacts such as cooling the climate. Wildfire particles also likely destroy more ozone at mid-latitudes than a comparable amount of volcanic material (Solomon et al., 2023); insofar as fires after a nuclear detonation also contain organics similar effects may be expected but the composition of the emissions is a major uncertainty for estimates of the impacts of nuclear war.
BOX 4-3
Solar Geoengineering
It may be tempting to compare the ability of particles driven into the stratospheres by fires following nuclear detonation with “stratospheric” aerosol injection (SAI; a subset of proposed solar geoengineering techniques) to reduce global mean temperatures. Both have need for model evaluation, highlighted for SAI in the 2021 National Academies report on Solar Geoengineering.
SAI as generally proposed is however more comparable to volcanoes, with the intention of using sulfates to increase the scattering of sunlight, which would have different physicochemical impact. As such, SAI is not a useful analog for the injection of carbonaceous stratospheric particles from fires following a nuclear detonation and insights for EENW researchers from SAI research are limited.
4.4.2 Key Uncertainties and Data Gaps
The amount of total smoke and NOx directly injected is expected to depend on the weapon yield and height of burst; however, subsequent transport upward to the stratosphere is sensitive to uncertainties in calculated particle scavenging, self-lofting, convective processes, and large-scale circulation.
The amount of smoke produced from fires in urban areas differs among published studies, depending on assumptions about available burnable material and fire characteristics. The BC/OC ratio in smoke produced from nuclear weapons-induced fires is also highly uncertain. This strongly affects radiative properties of the particles and their subsequent lofting, which is key to the stratospheric amounts and timescale of stratospheric perturbations. Pure soot (BC), for example, strongly absorbs solar radiation and would be lofted far more rapidly and to greater heights than particles that contain low amounts of BC, implying much longer stratospheric residence times and larger impacts on climate and other variables; this is therefore a key factor.
4.5 SUMMARY
Weapon exchanges that result in many large fires may produce global and long-lasting distributions of particulates in the stratosphere, resulting in larger-scale environmental impacts stemming from changes in Earth’s radiation balance. Weapon employment scenarios that result in fewer large fires or less-intense fires may still produce regional-scale atmospheric perturbations with environmental and societal and economic effects. Fuel loading—which impacts heat flux at the surface and fire area—is a major driver of the resulting plume height, material, and chemistry, for which there is not currently enough information to fully characterize, as discussed in Chapter 3.
Among other uncertainties discussed in this chapter, variability in fuel loading and materials means that it is difficult to predict whether a fire will develop such that it injects soot in the upper troposphere or stratosphere. Reasonable assumptions when modeling the particulate behavior can result in a wide range of outcomes for the same scenario, particularly given the complexity associated with other factors such as seasonality, latitude, weather, and topography. The residence time and pathways of lofted emission particulates depend on the physicochemical properties of the aerosols, atmospheric chemistry, and atmospheric circulation. In turn, these features determine the impact on the physical earth system, for example, the radiation balance and hydrologic cycle, from which broader effects stem, discussed in subsequent chapters.
4.6 FINDINGS AND RECOMMENDATIONS
FINDING 4-1: Accurate predictions of plume height are critical for understanding the environmental effects of smoke. While atmospheric models for predicting plume height are largely capable, the existing models of fire ignition and spread are often inadequate (see finding 3-3) for correctly modeling fire fluxes important for predicting plume height, especially in intense wildland or urban fires as may occur upon the detonation of a nuclear weapon.
RECOMMENDATION 4-1: EENW researchers should further develop fire ignition and spread models, coupled with atmospheric models capable of resolving plume rise, and should validate predictions of plume-rise and stabilization height, especially for the unique fuel loadings in nuclear detonation scenarios.
FINDING 4-2: The effects of atmospheric moisture and cloud microphysics are essential inclusions in simulations of plume rise or determinations of smoke injection height following a nuclear detonation. Most modeling capabilities for plume rise lack the ability to efficiently model chemistry, aerosols, aerosol–cloud interactions, and dry and wet removal mechanisms, as their use is computationally prohibitive and observations to validate the modeling are sparse. This
leads to large uncertainties in estimating the injection of smoke plume materials and their properties (i.e., vertical distribution, mass, size, hygroscopicity, etc.) and removal processes.
Furthermore, a lack of observational data on wet removal processes (e.g., organic coatings, hygroscopicity, and ice nucleation, for modeling aerosol-cloud interaction) and dry deposition (e.g., density and morphology for computing gravitational and turbulent eddy-driven sedimentation) of carbonaceous particles potentially led to uncertainty and difficulty in validating model results for aerosols generated from nuclear detonation-triggered fires.
RECOMMENDATION 4-2: EENW researchers should include ambient atmospheric conditions and the effects of atmospheric physics in studies of plume rise and height.
High-resolution (plume-resolving) atmospheric models incorporating tropospheric gas and aerosol chemistry and aerosol–cloud interactions should be further developed and used to improve the understanding of the physiochemical processes and the fate and transport of smoke during plume rise.
RECOMMENDATION 4-3: EENW researchers should exploit observations of pyrocumulonimbus from wildfires to test and improve their models. Observational data from wildfires should be collected in situ and from satellites and used for systematic testing of these models.
FINDING 4-3: There is a lack of studies on the tropospheric impacts from emissions due to a nuclear blast, exclusive of radioactivity, for example, particulate matter, reactive gases, heavy metals, and toxic organic compounds produced by fire. Further, the interaction between stratospheric and tropospheric processes has not been fully investigated to elucidate the impacts on the troposphere. It is therefore similarly unclear what the relative importance of a nuclear blast on tropospheric composition and air quality impacts would be compared to radioactive fallout.
RECOMMENDATION 4-4: EENW researchers should include full tropospheric chemistry and use of tracers to investigate the changes in pollutant concentrations and the impacts of deposited toxins downwind and in different environmental media.
RECOMMENDATION 4-5: EENW researchers should use fully coupled models of the troposphere and stratosphere with full chemistry and radiative transfer to investigate the impact of increased stratospheric aerosol loadings and other chemistry resulting from a nuclear blast on tropospheric and stratospheric chemistry. These simulations should include the full suite of additional compounds injected into the stratosphere and troposphere (water, carbon monoxide, nitrogen compounds, etc., in addition to aerosols) to better understand atmospheric chemistry impacts and effects on circulation. Consideration of full stratospheric chemistry including the impact of organic particles generated by fire will also be important for an up-to-date understanding of the impacts on stratospheric ozone.
FINDING 4-4: Many studies have imposed material just below the tropopause which will rapidly rise into the stratosphere. More complex models have now attempted to model plume rise to the extent that they can, which is a more complete treatment but requires testing versus observations (e.g. of wildfire smoke plume rise).
RECOMMENDATION 4-6: EENW researchers should seek to better quantify and understand the range of possible climate responses to a broad set of possible detonation scenarios. Sensitivity studies should be performed covering a wide range of scenarios and parameters, rather than focusing on modeling specific detonation scenarios, as is common.
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