Owen B. Toon

Incorporation of mineralogical composition into models of the radiative properties of mineral aerosol from UV to IR wavelengths

We describe a technique to model the radiative properties of mineral aerosols which accounts for their composition. We compile a data set of refractive indices of major minerals and employ it, along with data on mineralogical composition of dust from various locations, to calculate spectral optical and radiative properties of mineral aerosol mixtures. Such radiative properties are needed for climate modeling and remote sensing applications. We consider external mixtures of individual minerals, as well as mixtures of aggregates. We demonstrate that an external mixture of individual minerals must contain unrealistically high amounts of hematite to have a single scattering albedo lower than 0.9 at 500 nm wavelength. In contrast, aggregation of hematite with quartz or clays can strongly enhance absorption by dust at solar wavelengths. We also simulate the daily mean net (solar + infrared) forcing by dust of varying compositions. We found that, for a given composition and under similar atmospheric conditions, a mixture of aggregates can cause the positive radiative forcing while a mixture of individual minerals gives the negative forcing.

Smoke and Dust Particles of Meteoric Origin in the Mesosphere and Stratosphere

Abstract A height profile of ablated mass from meteors is calculated, assuming an incoming mass of 10−16 g cm−2 s−1 (44 metric tons per day) and the velocity distribution of Southworth and Sekanina, which has a mean of 14.5 km s−1. The profile peaks at 84 km. The fluxes of micrometeorites and residual meteoroids are also calculated. The coagulation of the evaporated silicates into “smoke” particles is then followed by means of a model adapted from a previous study of the stratospheric sulfate layer. Numerous sensitivity tests are made. Features of the results are a sharp cutoff of the particle distribution above 90 km, and a surface area close to 10−9 cm2 cm−3 all the way from 30 to 85 km. Some confirmation is obtained from balloon studies of condensation nuclei, although the various measurements differ greatly. The optical scattering and extinction am shown to be undetectable. Several potential applications are suggested: nucleation of sulfate particles and noctilucent clouds, scavenging of metallic ions...

Nuclear Winter: Global Consequences of Multple Nuclear Explosions

The potential global atmospheric and climatic consequences of nuclear war are investigated using models previously developed to study the effects of volcanic eruptions. Although the results are necessarily imprecise due to wide range of possible scenaros and uncertainty in physical parameters, the most probable first-order effects are serious. Significant hemispherical attenuation of the solar radiation flux and subfreezing land temperatures may be caused by fine dust raised in high-yield nuclear surface bursts and by smoke from city and forest fires ignited by airbursts of all yields. For many simulated exchanges of several thousand megatons, in which dust and smoke are generated and encircle the earth within 1 to 2 weeks, average light levels can be reduced to a few percent of ambient and land temperatures can reach -15 � to -25 �C. The yield threshold for major optical and climatic consequences may be very low: only about 100 megatons detonated over major urban centers can create average hemispheric smoke optical depths greater than 2 for weeks and, even in summer, subfreezing land temperatures for months. In a 5000-megaton war, at northern mid-latitude sites remote from targets, radioactive fallout on time scales of days to weeks can lead to chronic mean doses of up to 50 rads from external whole-body gamma-ray exposure, with a likely equal or greater internal dose from biologically active radionuclides. Large horizontal and vertical temperature gradients caused by absorption of sunlight in smoke and dust clouds may greatly accelerate transport of particles and radioactivity from the Northern Hemisphere to the Southern Hemisphere. When combined with the prompt destruction from nuclear blast, fires, and fallout and the later enhancement of solar ultraviolet radiation due to ozone depletion, long-term exposure to cold, dark, and radioactivity could pose a serious threat to human survivors and to other species.

The impact of humidity above stratiform clouds on indirect aerosol climate forcing

Some of the global warming from anthropogenic greenhouse gases is offset by increased reflection of solar radiation by clouds with smaller droplets that form in air polluted with aerosol particles that serve as cloud condensation nuclei. The resulting cooling tendency, termed the indirect aerosol forcing, is thought to be comparable in magnitude to the forcing by anthropogenic CO2, but it is difficult to estimate because the physical processes that determine global aerosol and cloud populations are poorly understood. Smaller cloud droplets not only reflect sunlight more effectively, but also inhibit precipitation, which is expected to result in increased cloud water. Such an increase in cloud water would result in even more reflective clouds, further increasing the indirect forcing. Marine boundary-layer clouds polluted by aerosol particles, however, are not generally observed to hold more water. Here we simulate stratocumulus clouds with a fluid dynamics model that includes detailed treatments of cloud microphysics and radiative transfer. Our simulations show that the response of cloud water to suppression of precipitation from increased droplet concentrations is determined by a competition between moistening from decreased surface precipitation and drying from increased entrainment of overlying air. Only when the overlying air is humid or droplet concentrations are very low does sufficient precipitation reach the surface to allow cloud water to increase with droplet concentrations. Otherwise, the response of cloud water to aerosol-induced suppression of precipitation is dominated by enhanced entrainment of overlying dry air. In this scenario, cloud water is reduced as droplet concentrations increase, which diminishes the indirect climate forcing.

Algorithms for the calculation of scattering by stratified spheres

Efficient, numerically stable, methods for the calculation of light-scattering intensity functions for concentrically coated spheres are discussed. Earlier forms of these equations are subject to various numerical difficulties which give rise to significant errors, especially for thin absorbing shells. The present equations are accurate for all refractive indices, for large and small particles, and for cores with any relative size.

Absorption of visible radiation in atmosphere containing mixtures of absorbing and nonabsorbing particles

Recent measurements of the single-scattering albedo omega(0) of tropospheric aerosols indicate the presence of a strongly absorbing material which has tentatively been identified as graphitic carbon (soot). Theoretical calculations, based on several different models of the way in which soot might be mixed with other aerosol materials, show that a minimum of 20% soot by volume is necessary to achieve the observed urban value of omega(0) = 0.6. Rural values of the order of 0.8 can be accounted for with 1-5% soot by volume. These same values of omega can be produced by similar amounts of the iron oxide magnetite, which is shown to be virtually indistinguishable from soot by optical measurements performed on bulk samples. Calculations of phase functions for various mixtures of soot also indicate the difficulty of determining aerosol composition by optical scattering techniques. The climatic effects of these absorbing aeorosols are computed using a simple one-layer model, and the results suggest that heating rates in urban pollution layers may be of the order of 4 K/day.

Physical properties of the particles composing the Martian dust storm of 1971–1972

Infrared spectra obtained from the Mariner 9 spacecraft during the 1971–1972 dust storm are used to derive information on the composition and particle size distribution of the dust and to study the time evolution of the storm. The dust is not composed of pure granite, basalt, basaltic glass, obsidian, quartz, andesite, or montmorillonite. The infrared spectra suggest that the dust is a mixture of materials, dominated by igneous silicates with >62;60% SiO2, or weathering products such as clay minerals, but the dust could possibly have a significant component of lower SiO2 materials such as basalt. Substantial quantities of carbonates, nitrates, or carbon suboxide are excluded from the mixture. All infrared, visible, and ultraviolet data on the Martian surface composition seem consistent with a mixture of basalt and clay minerals or high SiO2 igneous rocks, with a surface patina of oxides of iron. For all candidate compositions, the data are best matched with a size distribution that approximates a differential power law function of slope −4. This size distribution is quite similar to terrestial size distributions in regions remote from sources of dust. The relative abundance of particles between 1- and 10-μm radius did not change during the Mariner 9 mission; thus suspended particles did not experience Stokes-Cunningham fallout but instead were supported by turbulence with an eddy diffusion coefficient, Ke ⋍ 7 × 106 cm2sec−1. The aerosol optical depth, standardized to 0.3-μm wavelength, varied from about 1.5 early in the mission to about 0.2 at Orbit 200.

Environmental perturbations caused by the impacts of asteroids and comets

We review the major impact-associated mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 104 Mt (i.e., impact frequencies less than one in 6 × 104 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 104 or 105 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 104–105 Mt (intervals up to 3 × 105 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 105 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 105 to 106 Mt (intervals up to 2 × 106 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild-fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock-heated air to destroy the ozone shield. Between 106 and 107 Mt (intervals up to 1.5 × 107 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km², and the resulting smoke would reduce light levels even further. At energies above 107 Mt, blast and earthquake damage reach the regional scale (106 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the worlds ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 109 Mt (>108 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous-Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.

The astronomical theory of climatic change on Mars

Abstract We examine the response of Martian climate to changes in solar energy deposition caused by variations of the Martian orbit and obliquity. We systematically investigate the seasonal cycles of carbon dioxide, water, and dust to provide a complete picture of the climate for various orbital configurations. We find that at low obliquity (15°) the atmospheric pressure will fall below 1 mbar; dust storms will cease; thick permanent CO2 caps will form; the regolith will release CO2; and H2O polar ice sheets will develop as the permafrost boundaries move poleward. At high obliquity (35°) the annual average polar temperature will increase by about 10°K, slightly desorbing the polar regolith and causing the atmospheric pressure to increase by not more than 10 to 20 mbar. Summer polar ground temperatures as high as 273°K will occur. Water ice caps will be unstable and may disappear as the equilibrium permafrost boundary moves equatorward. However, at high eccentricity, polar ice sheets will be favored at one pole over the other. At high obliquity dust storms may occur during summers in both hemispheres, independent of the eccentricity cycle. Eccentricity and longitude of perihelion are most significant at modest obliquity (25°). At high eccentricity and when the longitude of perihelion is close to the location of solstice hemispherical asymmetry in dust-storm generation and in polar ice extent and albedo will occur. The systematic examination of the relation of climate and planetary orbit provides a new theory for the formation of the polar laminae. The terraced structure of the polar laminae originates when eccentricity and/or obliquity variations begin to drive water ice off the dusty permanent H2O polar caps. Then a thin (meters) layer of consolidated dust forms on top of a dirty, slightly thicker (tens of meters) ice sheet and the composite is preserved as a layer of laminae composed predominately of water ice. Because of insolation variation on slopes, a series of poleward- and equatorward-facing scarps are formed where the edges of the laminae are exposed. Independently of orbital variations, these scarps propagate poleward both by erosion of the equatorward slopes and by deposition on the poleward slopes. Scarp propagation resurfaces and recycles the laminae forming the distinctive spiral bands of terraces observed and provides a supply of water to form new permanent ice caps. The polar laminae boundary marks the furthest eqautorward extension of the permanent H2O caps as the orbit varies. The polar debris boundary marks the furthest equatorward extension of the annual CO2 caps as the orbit varies. The Martian regolith is now a significant geochemical sink for carbon dioxide. CO2 has been irreversibly removed from the atmosphere by carbonate formation. CO2 has also benn removed by regolith adsorption. Polar temperature increases caused by orbital variations are not great enough

Optical properties of some terrestrial rocks and glasses.

Abstract The optical constants of five naturally occurring rocks have been determined in the spectral range between 0.2 and .50 μm. Between 0.2 and 5 μm, the real and imaginary parts of the index of refraction were found from a combination of reflectivity and transmission measurements by using Beers law and the Fresnel reflectivity equation. At wavelenths beyond 5 μm only reflectivity measurements could be made and both constants were found from an application of classical dispersion theory. These optical constants will be valuable for studies of the interaction of light with rock surfaces, atmospheric dust and interplanetary and interstellar dust grains.