Engineering topics

Toon, Owen B.

Publications and source records attributed to Toon, Owen B..

At least 73 records · Page 4

NASA NTRS1993

Airborne Arctic Stratospheric Expedition 2: An Overview

The sudden onset of ozone depletion in the antarctic vortex set a precedent for both the time scale and the severity of global change. The Airborne Antarctic Ozone Experiment (AAOE), staged from Punta Arenas, Chile, in 1987, established that CFCs, halons, and methyl bromide, the dominant sources of chlorine and bromine radicals in the stratosphere, control the rate of ozone destruction over the Antarctic; that the vortex is depleted in reactive nitrogen and water vapor; and that diabatic cooling during the antarctic winter leads to subsidence within the vortex core, importing air from higher altitudes and lower latitudes. This last conclusion is based on observed dramatic distortion in the tracer fields, most notably N2O. In 1989, the first Airborne Arctic Stratospheric Expedition (AASE-I), staged from Stavanger, Norway, and using the same aircraft employed for AAOE (the NASA ER-2 and the NASA DC-8), discovered that while NO(x) and to some degree NO(y) were perturbed within the arctic vortex, there was little evidence for desiccation. Under these (in contrast to the antarctic) marginally perturbed conditions, however, Cl0 was found to be dramatically enhanced such that a large fraction of the available (inorganic) chlorine resided in the form of Cl0 and its dimer ClOOCl. This leaves two abiding issues for the northern hemisphere and the mission of the second Airborne Arctic Stratospheric Expedition (AASE-II): (1) Will significant ozone erosion occur within the arctic vortex in the next ten years as chlorine loading in the stratosphere exceeds four parts per billion by volume? and (2) Which mechanisms are responsible for the observed ozone erosion poleward of 30 deg N in the winter/spring northern hemisphere reported in satellite observations?

Anderson, James G.↗

NASA NTRS1993

Ozone and aerosol changes during the 1991-1992 airborne Arctic stratospheric expedition

Stratospheric ozone and aerosol distributions were measured across the wintertime Arctic vortex from January to March 1992 with an airborne lidar system as part of the 1992 Airborne Arctic Stratospheric Expedition (AASE II). Aerosols from the Mount Pinatubo eruption were found outside and inside the vortex with distinctly different distributions that clearly identified the dynamics of the vortex. Changes in aerosols inside the vortex indicated advection of air from outside to inside the vortex below 16 kilometers. No polar stratospheric clouds were observed and no evidence was found for frozen volcanic aerosols inside the vortex. Between January and March, ozone depletion was observed inside the vortex from 14 to 20 kilometers with a maximum average loss of about 23 percent near 18 kilometers.

Browell, Edward V.↗

NASA NTRS1993

Numerical simulations of the formation and evolution of water ice clouds in the Martian atmosphere

A model of the formation, evolution, and description of Martian water ice clouds is developed which well reproduces the physical processes governing the microphysics of water ice cloud formation on Mars. The model is used to show that the cloud properties are most sensitive to the temperature profile, the number of days for which condensation previously occurred, the contact angle, and the presence of incoming meteoritic debris at the top of the atmosphere. The AM-PM differences in optical depths measured at the Viking Lander site were successfully simulated with the model, obtaining total column optical depths of ice of a few tenths in agreement with observations.

Michelangeli, Diane V.↗

NASA NTRS1993

Martian global dust storms - Zonally symmetric numerical simulations including size-dependent particle transport

A zonally symmetric primitive-equation grid-point model of the Martian atmosphere is coupled with an aerosol transport/microphysical model in order to numerically investigate the size-dependent transport of dust particles in the Martian atmosphere. The coupled model accounts for diabatic heating due to a radiatively active evolving dust field, but neglects feedbacks between atmosphere-surface interactions and surface dust lifting. The differing suspension lifetimes of dust particles of various sizes (radius = 1-80 microns), in conjunction with spatially varying atmospheric dynamics, result in latitudinal differences in several measurements of the column integrated particle concentration. This work indicates the importance of considering the full range of particle sizes (and shapes) of the suspended dust during Martian global dust storms and their impact upon the spatial extent and wavelength-dependent radiative influence of such storms.

Murphy, James R.↗

NASA NTRS1992

The potential effects of volcanic aerosols on cirrus cloud microphysics

The potential impact of volcanic aerosols on nucleation of ice crystals in upper tropospheric cirrus clouds is examined from a microphysical perspective. The sulfuric acid aerosols which form in the stratosphere are presumably transported into the troposphere by sedimentation and tropopause folding. The tropospheric volcanic aerosol size distribution is estimated from 10-micron lidar backscatter and in situ measurements. Microphysical simulations suggest that at temperatures below about -50 C the concentration of ice crystals which nucleate may be as much as a factor of 5 larger when volcanic aerosols are present. The simulations suggest that the presence of volcanic aerosols may increase the net radiative forcing (surface warming) of certain types of cirrus near the tropopause by as much as 8 W/sq m. Further observations are required to determine whether these effects actually occur, and their global impact.

Jensen, Eric J.↗

NASA NTRS1992

Titan's organic haze

The optical properties of the laboratory tholin is considered, and an attempt is made to critically evaluate the analog between laboratory tholin and Titan tholin. The processes that form the solid haze material from the gaseous atmosphere are considered. It is suggested that the N/C ratio may be a useful signature of the processes that determine the haze production rate.

Mckay, Christopher P.↗

NASA NTRS1991

Simulations of microphysical, radiative, and dynamical processes in a continental-scale forest fire smoke plume

The impact of a large forest fire smoke plume on atmospheric processes is studied through a numerical model of meteorology, aerosols, and radiative transfer. The simulated smoke optical depths at 0.63-micron wavelength are in agreement with analyses of satellite data and show values as high as 1.8. The smoke has an albedo of 35 percent, or more than double the clear-sky value, and cools the surface by as much as 5 K. An imaginary refractive index, n sub im, of 0.01 yields results which closely match the observed cooling, single scattering albedo, and the Angstrom wavelength exponent. An n exp im of 0.1, typical of smoke from urban fires, produces 9 K cooling. Coagulation causes the geometric mean radius by number to increase from the initial value of 0.08 micron to a final value of 0.15 micron, while the specific extinction and absorption increase by 40 and 25 percent, respectively.

Westphal, Douglas L.↗

NASA NTRS1991

Polar stratospheric clouds and the ozone hole

An account is given of physical processes governing the formation of stratospheric particles, in order to dramatize the interactions between polar stratospheric clouds and the Antarctic ozone-destruction mechanism. Attention is given to the successive stages of particle nucleation, condensation/evaporation and sedimentation/coagulation phenomena, and the ways in which polar stratospheric clouds are observed. Considerable evidence exists that polar stratospheric cloud particles are composed of nitric acid. The relatively small Arctic ozone hole depletion is due to the much smaller duration of Arctic stratospheric clouds.

Hamill, Patrick↗

NASA NTRS1991

Homogeneous freezing nucleation of stratospheric solution droplets

The classical theory of homogeneous nucleation was used to calculate the freezing rate of sulfuric acid solution aerosols under stratospheric conditions. The freezing of stratospheric aerosols would be important for the nucleation of nitric acid trihydrate particles in the Arctic and Antarctic stratospheres. In addition, the rate of heterogeneous chemical reactions on stratospheric aerosols may be very sensitive to their state. The calculations indicate that homogeneous freezing nucleation of pure water ice in the stratospheric solution droplets would occur at temperatures below about 192 K. However, the physical properties of H2SO4 solution at such low temperatures are not well known, and it is possible that sulfuric acid aerosols will freeze out at temperatures ranging from about 180 to 195 K. It is also shown that the temperature at which the aerosols freeze is nearly independent of their size.

Jensen, Eric J.↗

NASA NTRS1991

The short-term temperature response to smoke from oil fires

The short-term effect of smoke, similar to that being emitted from the Kuwaiti oil fields, on the ground temperature of the afffected region was investigated using a numerical model of atmospheric dynamics, aerosol microphysics, and radiative transfer similar to that used previosly by Westphal and Toon (1991). Results show that, after one diurnal cycle beneath smoke plumes of moderate visible absorption, ground cooling and ground warming were both possible; the response of the ground temperature and the boundary layer dynamics depended on the surface characteristics, with desert exhibiting the strongest cooling of all the land types used.

Westphal, Douglas L.↗

NASA NTRS1991

Application of physical adsorption thermodynamics to heterogeneous chemistry on polar stratospheric clouds

Laboratory isotherms for the binding of several nonheterogeneously active atmospheric gases and for HCl to water ice are translated into adsorptive equilibrium constants and surface enthalpies. Extrapolation to polar conditions through the Clausius Clapeyron relation yields coverage estimates below the percent level for N2, Ar, CO2, and CO, suggesting that the crystal faces of type II stratospheric cloud particles may be regarded as clean with respect to these species. For HCl, and perhaps HF and HNO3, estimates rise to several percent, and the adsorbed layer may offer acid or proton sources alternate to the bulk solid for heterogeneous reactions with stratospheric nitrates. Measurements are lacking for many key atmospheric molecules on water ice, and almost entirely for nitric acid trihydrate as substrate. Adsorptive equilibria enter into gas to particle mass flux descriptions, and the binding energy determines rates for desorption of, and encounter between, potential surface reactants.

Elliott, Scott↗

NASA NTRS1991

Making Mars habitable

The possibility is considered that the atmosphere and climate of Mars could be altered to allow terrestrial life forms, and possibly human beings, to survive on the surface. Production of CFCs or other greenhouse gases on Mars would warm the surface enough for the regolith and polar caps to release their CO2 and raise atmospheric pressure to 100 mbar. If a large regolith and polar CO2 reservoirs exist, the pressure would continue to rise on its own. If these are absent, additional CO2 would have to be released from carbonate minerals. At this point, perhaps between 100 and 100,000 yrs, Mars might be suitable for plants. If there is a mechanism for sequestering the reduced carbon, these plants could slowly transform the CO2 to produce an O2-rich atmosphere in perhaps 100,000 yrs. If sufficient N2 could be released from putative soil deposits and the CO2 level could be kept low enough, then a human-breathable atmosphere would be produced.

Mckay, Christopher P.↗

NASA NTRS1991

Polar stratospheric clouds and ozone depletion

A review is presented of investigations into the correlation between the depletion of ozone and the formation of polar stratospheric clouds (PSCs). Satellite measurements from Nimbus 7 showed that over the years the depletion from austral spring to austral spring has generally worsened. Approximately 70 percent of the ozone above Antarctica, which equals about 3 percent of the earth's ozone, is lost during September and October. Various hypotheses for ozone depletion are discussed including the theory suggesting that chlorine compounds might be responsible for the ozone hole, whereby chlorine enters the atmosphere as a component of chlorofluorocarbons produced by humans. The three types of PSCs, nitric acid trihydrate, slowly cooling water-ice, and rapidly cooling water-ice clouds act as important components of the Antarctic ozone depletion. It is indicated that destruction of the ozone will be more severe each year for the next few decades, leading to a doubling in area of the Antarctic ozone hole.

Toon, Owen B.↗

NASA NTRS1990

Thermal emission spectra of Mars (5.4-10.5 microns) - Evidence for sulfates, carbonates, and hydrates

Spectra of the Martian thermal emission in the 5.4-10.5 micron region are reported. Emission features at 7.8 and 9.7 microns are attributed to surface silicates, and an emission feature at 6.1 micron is attributed to a molecular water component of the surface material. An absorption band at 8.7 micron and a possible one at 9.8 microns is attributed to sulfate or bisulfate anions probably located at a distorted crystalline site, and an absorption band at 6.7 microns is attributed to carbonate or bicarbonate anions located in a distorted crystalline site. Spectral simulations indicate that the sulfate- and carbonate-bearing minerals are contained in the same particles of airborne dust as the dominant silicate minerals, that the dust optical depth is about 0.6 at a reference wavelength of 0.3 micron over the area of the observed spots, and that sulfates and carbonates constitute 10-15 percent and 1-3 percent by volume of the airborne dust, respectively.

Pollack, James B.↗

NASA NTRS1990

Numerical simulations of the decay of Martian global dust storms

The decay of Martian global (great) dust storms is investigated. One-dimensional (vertical, static atmosphere) and two-dimensional (latitude-height, steady state circulation) simulations carried out with an aerosol transport-microphysical model indicate that atmospheric motions play a significant role in the observed decay of global dust storms. Spacecraft observations (Mariner 9, Viking) of the 1971 and the two 1977 planet-encircling dust storms have provided suggestions about some characteristics of storm decay. Specifically, the dust particle size distribution is inferred to have remained essentially unchanged for particles with radii between 1 and 10 microns during decay of the 1971 storm, and surface visible opacity declined quasi-exponentially with time in northern midlatitudes during the decay of the two 1977 storms. The results from this investigation indicate that two- and three-dimensional dynamical processes play a significant role the observed decay features of Martian global dust storms. The most important processes are the lofting of dust by vertical motions in the dust source region of the Southern Hemisphere subtropics and a continuing advective resupply of atmospheric dust into the dust sink regions of the Northern Hemisphere. This work has implications for Viking data analyses and future Mars observer observations and requires that the particle size distribution be treated as a time and latitude dependent quantity.

Murphy, James R.↗

NASA NTRS1990

Preliminary simulations of the large-scale environment during the FIRE cirrus IFO

Large scale forcing (scales greater than 500 km) is the dominant factor in the generation, maintenance, and dissipation of cirrus cloud systems. However, the analyses of data acquired during the first Cirrus IFO have highlighted the importance of mesoscale processes (scales of 20 to 500 km) to the development of cirrus cloud systems. Unfortunately, Starr and Wylie found that the temporal and spatial resolution of the standard and supplemental rawinsonde data were insufficient to allow an explanation of all of the mesoscale cloud features that were present on the 27 to 28 Oct. 1986. It is described how dynamic initialization, or 4-D data assimilation (FDDA) can provide a method to address this problem. The first steps towards application of FDDA to FIRE are also described.

Westphal, Douglas L.↗

NASA NTRS1990

An analysis of lidar observations of polar stratospheric clouds

Lidar observations by Browell et al. (1990) are interpreted using single scattering calculations for nonspherical particles and aerosol microphysical calculations. Many of the lidar observations are consistent with particles containing 10 ppbv of condensed nitric acid vapor and an equivalent mass of water. The lidar observations of these Type 1 clouds identify two subtypes, whose properties are deduced. Type 1b particles are spherical, or nearly spherical, and typically have radii near 0.5 micron; Type 1a particles are not spherical, and have a spherical volume equivalent radius exceeding 1.0 micron. Several factors may cause variations in the size of the particles. The most significant factors are the cooling rate and the degree to which the air parcels cool below the condensation point. Specific examples in which cooling rate and cooling point may have led to variations in particle size are found in the Browell et al. (1990) data set. Condensation of 1 ppmm of water or less is quantitatively sufficient to account for the magnitude of the lidar backscatter observed from water ice clouds. The ice particles are not spherical in shape. The sizes of particles in water ice clouds cannot be determined because they are much larger than the wavelength of the lidar.

Toon, Owen B.↗

NASA NTRS1990

Incorporation of stratospheric acids into water ice

Hydrochloric and hydrofluoric acids are absorbed within the water ice lattice at mole fractions maximizing below 0.00001 and 0.0001 in a variety of solid impurity studies. The absorption mechanism may be substitutional or interstitial, leading in either case to a weak permeation of stratospheric ices by the acids at equilibrium. Impurities could also inhabit grain boundaries, and the acid content of atmospheric ice crystals will then depend on details of their surface and internal microstructures. Limited evidence indicates similar properties for the absorption of HNO3. Water ice lattices saturated with acid cannot be a significant local reservoir for HCl in the polar stratosphere.

Elliott, Scott↗