Tropospheric sulfate aerosol formation via ion-ion recombination.

We propose a source of aerosols in the lower atmosphere associated with the creation, growth, and recombination of ubiquitous cosmogenically generated ions. This particle source should be favorable in the relatively clean, stable marine boundary layer, providing a uniform, continuous fine particle generator in the presence of dimethylsulfide emissions. Through this mechanism, new sulfate aerosols can be formed at sulfuric acid vapor partial pressures well below the supersaturations required for homogeneous binary nucleation of sulfuric acid/water solutions, which is consistent with numerous observations of new particle formation under sub-saturated conditions. The evolving aerosols in turn control the acid vapor concentration and thus modulate the sizes of the precursor ions and the rate of new particle formation. A simple model representing this nonlinear coupled system predicts that the physical and chemical processes connecting ions, vapors, and aerosols effectively constrain the particle population to a relatively narrow range of values. This self-limiting behavior may explain in part the apparent stability of the marine sulfate aerosol, with mean concentrations of the order of several hundred per cubic centimeter.

Stratospheric Chlorine Injection by Volcanic Eruptions: HCI Scavenging and Implications for Ozone.

Because the output of volatile chlorine during a major volcanic event can greatly exceed the annual anthropogenic emissions of chlorine to the atmosphere, the fate of volcanic chlorine must be known. Although numerous observations have shown that volcanoes do not significantly contribute to the stratospheric chlorine burden, no quantitative explanation has been published. Hydrogen chloride (HCI) scavenging processes during the early phases of a volcanic eruption are discussed. A plume dynamics and thermodynamics model is used to show that HCI removal in condensed supercooled water can reduce HCI vapor concentrations by up to four orders of magnitude, preventing substantial stratospheric chlorine injection.

Enhancements in biologically effective ultraviolet radiation following volcanic eruptions.

Aerosols injected into the stratosphere by large volcanic eruptions may induce ozone destruction through processes including heterogeneous chemical reactions. The effect of ozone reductions on surface ultraviolet irradiation is not obvious, however, because aerosols also increase the reflection of sunlight. Here we use a radiative transfer model to estimate the changes in biologically effective ultraviolet radiation (UV-BE) at the Earth's surface produced by the El Chichón (1982) and Mount Pinatubo (1991) eruptions. We find that in both cases surface UV-BE intensity can increase because the effect of ozone depletion outweighs the increased scattering.

A physical model of Titan's aerosols.

Microphysical simulations of Titan's stratospheric haze show that aerosol microphysics is linked to organized dynamical processes. The detached haze layer may be a manifestation of 1 cm sec-1 vertical velocities at altitudes above 300 km. The hemispherical asymmetry in the visible albedo may be caused by 0.05 cm sec-1 vertical velocities at altitudes of 150 to 200 km, we predict contrast reversal beyond 0.6 micrometer. Tomasko and Smith's (1982, Icarus 51, 65-95) model, in which a layer of large particles above 220 km altitude is responsible for the high forward scattering observed by Rages and Pollack (1983, Icarus 55, 50-62), is a natural outcome of the detached haze layer being produced by rising motions if aerosol mass production occurs primarily below the detached haze layer. The aerosol's electrical charge is critical for the particle size and optical depth of the haze. The geometric albedo, particularly in the ultraviolet and near infrared, requires that the particle size be near 0.15 micrometer down to altitudes below 100 km, which is consistent with polarization observations (Tomasko and Smith 1982, West and Smith 1991, Icarus 90, 330-333). Above about 400 km and below about 150 km Yung et al.'s (1984, Astrophys. J. Suppl. Ser. 55, 465-506) diffusion coefficients are too small. Dynamical processes control the haze particles below about 150 km. The relatively large eddy diffusion coefficients in the lower stratosphere result in a vertically extensive region with nonuniform mixing ratios of condensable gases, so that most hydrocarbons may condense very near the tropopause rather than tens of kilometers above it. The optical depths of hydrocarbon clouds are probably less than one, requiring that abundant gases such as ethane condense on a subset of the haze particles to create relatively large, rapidly removed particles. The wavelength dependence of the optical radius is calculated for use in analyzing observations of the geometric albedo. The lower atmosphere and surface should be visible outside of regions of methane absorption in the near infrared. Limb scans at 2.0 micrometers wavelength should be possible down to about 75 km altitude.

Reduced antarctic ozone depletions in a model with hydrocarbon injections.

Motivated by increased losses of Antarctic stratospheric ozone and by improved understanding of the mechanism, a concept is suggested for action to arrest this ozone loss: injecting the alkanes ethane or propane (E or P) into the Antarctic stratosphere. A numerical model of chemical processes was used to explore the concept. The model results suggest that annual injections of about 50,000 tons of E or P could suppress ozone loss, but there are some scenarios where smaller E or P injections could increase ozone depletion. Further, key uncertainties must be resolved, induding initial concentrations of nitrogen-oxide species in austral spring, and several poorly defined physical and chemical processes must be quantifed. There would also be major difficulties in delivering and distributing the needed alkanes.

Climate and smoke: an appraisal of nuclear winter.

The latest understanding of nuclear winter is reviewed. Considerable progress has been made in quantifying the production and injection of soot by large-scale fires, the regional and global atmospheric dispersion of the soot, and the resulting physical, environmental, and climatic perturbations. New information has been obtained from laboratory studies, field experiments, and numerical modeling on a variety of scales (plume, mesoscale, and global). For the most likely soot injections from a full-scale nuclear exchange, three-dimensional climate simulations yield midsummer land temperature decreases that average 10 degrees to 20 degrees C in northern mid-latitudes, with local cooling as large as 35 degrees C, and subfreezing summer temperatures in some regions. Anomalous atmospheric circulations caused by solar heating of soot is found to stabilize the upper atmosphere against overturning, thus increasing the soot lifetime, and to accelerate interhemispheric transport, leading to persistent effects in the Southern Hemisphere. Serious new environmental problems associated with soot injection have been identified, including disruption of monsoon precipitation and severe depletion of the stratospheric ozone layer in the Northern Hemisphere. The basic physics of nuclear winter has been reaffirmed through several authoritative international technical assessments and numerous individual scientific investigations. Remaining areas of uncertainty and research priorities are discussed in view of the latest findings.

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 degrees to -25 degrees 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.

Environmental effects of an impact-generated dust cloud: implications for the cretaceous-tertiary extinctions.

A model of the evolution and radiative effects of a debris cloud from a hypothesized impact event at the Cretaceous-Tertiary boundary suggests that the cloud could have reduced the amount of light at the earth's surface below that required for photosynthesis for several months and, for a somewhat shorter interval, even below that needed for many animals to see. For 6 months to 1 year, the surface would cool; the oceans would cool only a few degrees Celsius at most, but the continents might cool a maximum of 40 Kelvin. Extinctions in the ocean may have been caused primarily by the temporary cessation of photosynthesis, but those on land may have been primarily induced by a combination of lowered temperatures and reduced light.

Tunguska meteor fall of 1908: effects on stratospheric ozone.

In 1908, when the giant Tunguska meteor disintegrated in the earth's atmosphere over Siberia, it may have generated as much as 30 million metric tons of nitric oxide (NO) in the stratosphere and mesosphere. The photochemical aftereffects of the event have been simulated using a comprehensive model of atmospheric trace composition. Calculations indicate that up to 45 percent of the ozone in the Northern Hemisphere may have been depleted by Tunguska's nitric oxide cloud early in 1909 and large ozone reductions may have persisted until 1912. Measurements of atmospheric transparentiy by the Smithsonian Astrophysical Observatory for the years 1909 to 1911 show evidence of a steady ozone recovery from unusually low levels in early 1909, implying a total ozone deficit of 30 +/- 15 percent. The coincidence in time between the observed ozone recovery and the Tunguska meteor fall indicates that the event may provide a test of current ozone depletion theories.