Engineering topics

Toon, Owen B.

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

At least 55 records · Page 3

NASA NTRS1997

A New Parameterization of H2SO4/H2O Aerosol Composition: Atmospheric Implications

Recent results from a thermodynamic model of aqueous sulfuric acid are used to derive a new parameterization for the variation of sulfuric acid aerosol composition with temperature and relative humidity. This formulation is valid for relative humidities above 1 % in the temperature range of 185 to 260 K. An expression for calculating the vapor pressure of supercooled liquid water, consistent with the sulfuric acid model, is also presented. We show that the Steele and Hamill [1981] formulation underestimates the water partial pressure over aqueous H2SOI solutions by up to 12% at low temperatures. This difference results in a corresponding underestimate of the H2SO4 concentration in the aerosol by about 6 % of the weight percent at approximately 190 K. In addition, the relation commonly used for estimating the vapor pressure of H2O over supercooled liquid water differs by up to 10 % from our derived expression. The combined error can result in a 20 % underestimation of water activity over a H2SO4 solution droplet in the stratosphere, which has implications for the parameterization of heterogeneous reaction rates in stratospheric sulfuric acid aerosols. The influence of aerosol composition on the rate of homogeneous ice nucleation from a H2SO4 solution droplet is also discussed. This parameterization can also be used for homogeneous gas phase nucleation calculations of H2SO4 solution droplets under various environmental conditions such as in aircraft exhaust or in volcanic plumes.

Tabazadeh, Azadeh↗

NASA NTRS1997

Environmental Perturbations Caused by the Impacts of Asteroids and Comets

We review the major mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary following an asteroid impact. We then discuss how the proposed extinction may relate to the impact of asteroids or comets in general. We discuss the limitations of these mechanisms in terms of the spatial scale that may be affected, and the time scale over which the effects may last. Our goal is to provide relatively simple prescriptions for evaluating the importance of colliding objects having a range of energies and compositions. We also identify the many uncertainties concerning the environmental effects of impacts. We conclude that, for impact energies below about 10(exp 4) Mts (megatons of TNT equivalent) - i.e., impact frequencies less than in 6 x 10(exp 4) yr, 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 10(exp 4) or 10(exp 5) km (exp 2), which corresponds to the area damaged in many natural disasters of recent history. However, tsunami could be more damaging, flooding a kilometer of coastal plane over entire ocean basins. In the energy range of 10(exp 4) to 10 (exp 5) Mts (intervals up to 3 x 10(exp 5) yr; comets and asteroids with sizes up to 800 m and 1.5 km, respectively) water vapor injections and ozone loss become significant on the global scale. In the submicrometer dust injection fraction from the pulverized target material is much higher than is presently thought to be most likely, then dust injection could be important in this energy range.

Toon, Owen B.↗

NASA NTRS1997

A 2D Microphysical Analysis of Aerosol Nucleation in the Polar Winter Stratosphere: Implications for H2SO4 Photolysis and Nucleation Mechanisms

Each spring a layer of small particles forms between 20 and 30 km in the polar regions. Results are presented from a 2D microphysical model of sulfate aerosol, which provide the first self-consistent explanation of the observed "CN layer." Photochemical conversion of sulfuric acid to SO2 in the upper stratosphere and mesosphere is necessary for this layer to form. Recent laboratory measurements of H2SO4 and SO3 photolysis rates are consistent with such conversion, though an additional source of SO2 may be required. Nucleation throughout the polar winter extends the top of the aerosol layer to higher altitudes, despite strong downward transport of ambient air. This finding may be important to heterogeneous chemistry at the top of the aerosol layer in polar winter and spring.

Mills, Michael J.↗

NASA NTRS1997

Carl Sagan and the Exploration of Mars and Venus

Inspired by childhood readings of books by Edgar Rice Burroughs, Carl Sagan's first interest in planetary science focused on Mars and Venus. Typical of much of his career he was skeptical of early views about these planets. Early in this century it was thought that the Martian wave of darkening, a seasonal albedo change on the planet, was biological in origin. He suggested instead that it was due to massive dust storms, as was later shown to be the case. He was the first to recognize that Mars has huge topography gradients across its surface. During the spacecraft era, as ancient river valleys were found on the planet, he directed studies of Mars' ancient climate. He suggested that changes in the planets orbit were involved in climate shifts on Mars, just as they are on Earth. Carl had an early interest in Venus. Contradictory observations led to a controversy about the surface temperature, and Carl was one of the first to recognize that Venus has a massive greenhouse effect at work warming its surface. His work on radiative transfer led to an algorithm that was extensively used by modelers of the Earth's climate and whose derivatives still dominate the calculation of radiative transfer in planetary atmospheres today. Carl inspired a vast number of young scientists through his enthusiasm for new ideas and discoveries, his skeptical approach, and his boundless energy. I had the privilege to work in Carl's laboratory during the peak of the era of Mars' initial exploration. It was an exciting time, and place. Carl made it a wonderful experience.

Toon, Owen B.↗

NASA NTRS1996

Dehydration of the Upper Troposphere and Lower Stratosphere by Subvisible Cirrus Clouds Near the Tropical Tropopause

The extreme dryness of the lower stratosphere is believed to be caused by freeze-drying of air as it enters the stratosphere through the cold tropical tropopause. Previous investigations have been focused on dehydration occurring at the tops of deep convective cloud systems, However, recent observations of a ubiquitous stratiform cirrus cloud layer near the tropical tropopause suggest the possibility of dehydration as air is slowly lifted by large-scale motions, In this study, we have evaluated this possibility using a detailed ice cloud model. Simulations of ice cloud formation in the temperature minima of gravity waves (wave periods of 1 - 2 hours) indicate that large numbers of ice crystals will likely form due to the low temperatures and rapid cooling. As a result, the crystals do not grow larger than about 10 microns, fallspeeds are no greater than a few cm/s, and little or no precipitation or dehydration occurs. However, ice cloud's formed by large-scale vertical motions (with lifetimes of a day or more) should have,fever crystals and more time for crystal sedimentation to occur, resulting in water vapor depletions as large as 1 ppmv near the tropopause. We suggest that gradual lifting near the tropical tropopause, accompanied by formation of thin cirrus, may account for the dehydration.

Jensen, Eric J.↗

NASA NTRS1996

The 1996 Subsonic Aircraft: Contrail and Cloud Effects Special Study

During April 1996 NASA, in conjunction with the DOE, will sponsor a multi-aircraft field campaign to better understand the microphysical and radiative properties of cirrus clouds, the origins of ice nuclei and cloud condensation nuclei in the upper troposphere, and the possible role that the commercial subsonic aircraft fleet might play in altering cloud or aerosol properties. The NASA ER-2 aircraft will be used as a remote sensing platform, while the NASA DC-8 aircraft will be used as an in situ measurement platform. In situ observations will include a full set of size distribution measurements from nano-meter to millimeter sizes, ice water content measurements, gas phase and condensed phase chemical measurements, ice crystal optical phase function measurements, lidar observations of cloud top and cloud base, and atmospheric state measurement. The ER-2 will have lidar, microwave ice water path measurements, as well as visible and infrared spectral measurement. In this presentation the highlights of the mission will be presented. The goal will be to address fundamental questions such as the mode of nucleation of cirrus clouds, the composition of the nuclei on which cirrus form, the degree to which aircraft impact cirrus cloud properties.

Toon, Owen B.↗

NASA NTRS1996

Comparisons Between the Formation of Polar Stratospheric Clouds and Cirrus Clouds

Analyses of lidar data taken during the 1989 AASE I program show that polar stratospheric clouds most often contain frozen particles, but sometimes contain spherical, presumably liquid, particles. In this paper the mechanisms that lead to the freezing of particles, and to the formation of liquid particles will be discussed based largely upon analyses of existing data. For example, trajectory studies help shed light on the conditions that lead to particle freezing. During April of 1996 an experiment using the NASA DC-8 and ER-2 aircraft will be conducted over the central U.S. Some of the goals of the project are to better understand the mechanisms of cirrus cloud formation, and to investigate the properties of freezing nuclei in the upper troposphere. Results from this field program will be presented as they apply to the topic of ice crystal nucleation. Comparisons will be drawn between the mechanisms that lead to new particle formation in the polar stratosphere, and the mechanisms that lead to new particles formation in the upper troposphere.

Toon, Owen B.↗

NASA NTRS1996

SUCCESS Studies of the Impact of Aircraft on Cirrus Clouds

During April of 1996 NASA will sponsor the SUCCESS project to better understand the impact of subsonic aircraft on the Earth's radiation budget. We plan to better determine the radiative properties of cirrus clouds and of contrails so that satellite observations can better determine their impact on Earth's radiation budget. We hope to determine how cirrus clouds form, whether the exhaust from subsonic aircraft presently affects the formation of cirrus clouds, and if the exhaust does affect the clouds whether the changes induced are of climatological significance. We seek to pave the way for future studies by developing and testing several new instruments. We also plan to better determine the characteristics of gaseous and particulate exhaust products from subsonic aircraft and their evolution in the region near the aircraft. In order to achieve our experimental objectives we plan to use the DC-8 aircraft as an in situ sampling platform. It will carry a wide variety of gaseous, particulate, radiative, and meteorological instruments. We will also use a T-39 aircraft primarily to sample the exhaust from other aircraft. It will carry a suite of instruments to measure particles and gases. We will employ an ER-2 aircraft as a remote sensing platform. The ER-2 will act as a surrogate satellite so that remote sensing observations can be related to the in situ parameters measured by the DC-8 and T-39. The mission strategy calls for a 5 week deployment beginning on April 8, 1996, and ending on May 10, 1996. During this time all three aircraft will be based in Salina, Kansas. A series of flights, averaging one every other day during this period, will be made mainly near the Department of Energy's Climate and Radiation Testbed site (CART) located in Northern Oklahoma, and Southern Kansas. During this same time period an extensive set of ground based measurements will be made by the DOE, which will also be operating several aircraft in the area to better understand the radiative properties of the atmosphere. Additional flights will be made over the Rocky Mountains, to investigate wave clouds. Flights will also be made over the Gulf of Mexico to utilize an oceanic background for remote sensing measurements. The results of this mission will be presented in this talk.

Toon, Owen B.↗

NASA NTRS1995

Spectroscopic Evidence Against Nitric Acid Trihydrate in Polar Stratospheric Clouds

Heterogeneous reactions on polar stratospheric clouds (PSC's) play a key role in the photochemical mechanism thought to be responsible for ozone depletion in the Antarctic and Arctic. Reactions of PSC particles activate chlorine to forms that are capable of photochemical ozone destruction, and sequester nitrogen oxides (NOx) that would otherwise deactivate the chlorine. Although the heterogeneous chemistry is now well established, the composition of the clouds themselves is uncertain. It is commonly thought that they are composed of nitric acid trihydrate, although observations have left this question unresolved. Here we reanalyse infrared spectra of type 1 PSCs obtained in Antarctica in September 1987, using recently measured optical constants of the various compounds that might be present in PSCs. We find these PSCs were not composed of nitric acid trihydrate but instead had a more complex compositon, perhaps that of a ternary solution. Because cloud formation is sensitive to their composition, this finding will alter our understanding of the locations and conditions in which PSCs form. In addition, the extent of ozone loss depends on the ability of the PSCs to remove NOx permanently through sedimentation, The sedimentation rates depend on PSC particle size which in turn is controlled by the composition and formation mechanism.

Toon, Owen B.↗

NASA NTRS1995

A Model Simulation of Pinatubo Volcanic Aerosols in the Stratosphere

A one-dimensional, time-dependent model is used to study the chemical, microphysical, and radiative properties of volcanic aerosols produced by the Mount Pinatubo eruption on June 15, 1991. Our model treats gas-phase sulfur photochemistry, gas-to-particle conversion of sulfur, and the microphysics of sulfate aerosols and ash particles under stratospheric conditions. The dilution and diffusion of the volcanic eruption clouds are also accounted for in these conditions. Heteromolecular homogeneous and heterogeneous binary H2SO4/H2O nucleation, acid and water condensational growth, coagulation, and gravitational sedimentation are treated in detail in the model. Simulations suggested that after several weeks, the volcanic cloud was composed mainly of sulfuric acid/water droplets produced in situ from the SO2 emissions. The large amounts of SO2 (around 20 Mt) injected into the stratosphere by the Pinatubo eruption initiated homogeneous nucleation which generated a high concentration of small H2SO4/H2O droplets. These newly formed particles grew rapidly by condensation and coagulation in the first few months and then reach their stabilized sizes with effective radii in a range between 0.3 and 0.5 micron approximately one-half year after the eruption. The predicted volcanic cloud parameters reasonably agree with measurements in term of the vertical distribution and lifetime of the volcanic aerosols, their basic microphysical structures (e.g., size distribution, concentration, mass ratio, and surface area) and radiative properties. The persistent volcanic aerosols can produce significant anomalies in the radiation field, which have important climatic consequences. The large enhancement in aerosol surface area can result in measurable global stratospheric ozone depletion.

Zhao , Jing-xia↗

NASA NTRS1995

A Model for Particle Microphysics, Turbulent Mixing, and Radiative Transfer in the Stratocumulus-Topped Marine Boundary Layer and Comparisons with Measurements

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics module that resolves the size distributions of aerosols and cloud droplets, a turbulence module that treats vertical mixing between layers, and a multiple wavelength radiative transfer module that calculates radiative heating rates and cloud optical properties. The results of a 12-h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an approx. 500-m thick, summertime marine stratocumulus cloud layer by Nicholls. However, in this case, the model predictions of turbulent fluxes between the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine stratus layer made under gale conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties are generally reproduced by the model, although it underpredicts the entrainment of overlying air at cloud top under gale conditions. Sensitivities of the model results are explored. The vertical profile of cloud droplet concentration is sensitive to the lower size cutoff of the droplet size distribution due to the presence of unactivated haze particles in the lower region of the modeled cloud. Increases in total droplet concentrations do not always produce less drizzle and more cloud water in the model. The radius of the mean droplet volume does not correlate consistently with drizzle, but the effective droplet radius does. The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input condensation nuclei and by omitting the effect of cloud-top radiative cooling on the condensational growth of cloud droplets. The omission of infrared scattering produces noticeable changes in cloud properties. The collection efficiencies for droplets less than 30-micrometers radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizontal wind also has a significant effect on a 12-h model simulation of cloud structure. Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplification is that the model assumes horizontal homogeneity, and, therefore, does not resolve updrafts and downdrafts. Likely consequences of this simplification include overprediction of the growth of droplets by condensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and underprediction of peak supersaturations.

Ackerman, Andrew S.↗

NASA NTRS1995

A Model for Particle Microphysics,Turbulent Mixing, and Radiative Transfer in the Stratocumulus-Topped Marine Boundary Layer and Comparisons with Measurements

A detailed 1D model of the stratocumulus-topped marine boundary layer is described. The model has three coupled components: a microphysics module that resolves the size distributions of aerosols and cloud droplets, a turbulence module that treats vertical mixing between layers, and a multiple wavelength radiative transfer module that calculates radiative heating rates and cloud optical properties. The results of a 12-h model simulation reproduce reasonably well the bulk thermodynamics, microphysical properties, and radiative fluxes measured in an approx. 500-m thick, summertime marine stratocumulus cloud layer by Nicholls. However, in this case, the model predictions of turbulent fluxes between the cloud and subcloud layers exceed the measurements. Results of model simulations are also compared to measurements of a marine stratus layer made under gate conditions and with measurements of a high, thin marine stratocumulus layer. The variations in cloud properties are generally reproduced by the model, although it underpredicts the entrainment of overlying air at cloud top under gale conditions. Sensitivities of the model results are explored. The vertical profile of cloud droplet concentration is sensitive to the lower size cutoff of the droplet size distribution due to the presence of unactivated haze particles in the lower region of the modeled cloud. Increases in total droplet concentrations do not always produce less drizzle and more cloud water in the model. The radius of the mean droplet volume does not correlate consistently with drizzle, but the effective droplet radius does. The greatest impacts on cloud properties predicted by the model are produced by halving the width of the size distribution of input condensation nuclei and by omitting the effect of cloud-top radiative cooling on the condensational growth of cloud droplets. The omission of infrared scattering produces noticeable changes in cloud properties. The collection efficiencies for droplets less than 30-micron radius, and the value of the accommodation coefficient for condensational droplet growth, have noticeable effects on cloud properties. The divergence of the horizontal wind also has a significant effect on a 12-h model simulation of cloud structure. Conclusions drawn from the model are tentative because of the limitations of the 1D model framework. A principal simplification is that the model assumes horizontal homogeneity, and, therefore, does not resolve updrafts and downdrafts. Likely consequences of this simplification include overprediction of the growth of droplets by condensation in the upper region of the cloud, underprediction of droplet condensational growth in the lower region of the cloud, and under-prediction of peak supersaturations.

Ackerman, Andrew S.↗

NASA NTRS1995

Origin of condensation nuclei in the springtime polar stratosphere

An enhanced sulfate aerosol layer has been observed near 25 km accompanying springtime ozone depletion in the Antarctic stratosphere. We use a one-dimensional aerosol model that includes photochemistry, particle nucleation, condensational growth, coagulation, and sedimentation to study the origin of the layer. Annual cycles of sunlight, temperature, and ozone are incorporated into the model. Our results indicate that binary homogeneous nucleation leads to the formation of very small droplets of sulfuric acid and water under conditions of low temperature and production of H2SO4 following polar sunrise. Photodissociation of carbonyl sulfide (OCS) alone, however, cannot provide sufficient SO2 to create the observed condensation nuclei (CN) layer. When subsidence of SO2 from very high altitudes in the polar night vortex is incorporated into the model, the CN layer is reasonably reproduced. The model predictions, based on the subsidence in polar vortex, agree with in situ measurements of particle concentration, vertical distribution, and persistence during polar spring.

Zhao, Jingxia↗

NASA NTRS1995

Origin of Condensation Nuclei in the Springtime Polar Stratosphere

Zhao, Jingxia↗

NASA NTRS1995

Spectroscopic Evidence Against Nitric Acid Trihydrate in Polar Stratospheric Clouds

Heterogeneous reactions on polar stratospheric clouds (PSC's) play a key role in the photochemical mechanisms thought to be responsible for ozone depletion in the Antarctic and the Arctic. Reactions on PSC particles activate chlorine to forms that are capable of photochemical ozone destruction, and sequester nitrogen oxides (NOx) that would otherwise deactivate the chlorine. Although the heterogeneous chemistry is now well established, the composition of the clouds themselves is uncertain. It is commonly thought that they are composed of nitric acid trihydrate, although observations have left this question unresolved. Here we reanalyse infrared spectra of type I PCS's obtained in Antarctica in September 1987, using recently measured optical constraints of the various compounds that might be present in PSC's. We find that these PSC's were not composed of nitric acid trihydrate but instead had a more complex composition perhaps that of a ternary solution. Because cloud formation is sensitive to their composition, this finding will alter our understanding of the locations and conditions in which PSCs form. In addition, the extent of ozone loss depends on the ability of the PSC's to remove NOx permanently through sedimentation. The sedimentation rates depend on PSC particle size which in turn is controlled by the composition and formation mechanism.

Toon, Owen B.↗

NASA NTRS1995

Direct Radiative Forcing by Anthropogenic Airborne Mineral Aerosols

Although airborne mineral aerosols are traditionally considered a natural component of the earth-atmosphere climate system, an examination of dust production mechanisms and dust source regions reveals that a significant fraction of the dust loading could be due to human activities. We find that the global mean direct solar radiative forcing by anthropogenically generated mineral aerosols may be comparable to the direct solar radiative forcing by other anthropogenic aerosols. The net regional forcing by mineral aerosols can greatly exceed the regional forcing by sulfate aerosol and can even be comparable to the net forcing by clouds. An improved understanding of the radiative forcing due to mineral aerosols requires better knowledge of the regional patterns of dust production rates and dust optical depths, more comprehensive measurements of dust microphysical and optical characteristics, as well as a comprehensive assessment of the role of dust aerosols on infrared radiative forcing.

Sokolik, Irina N.↗

NASA NTRS1994

Infrared optical constants of H2O ice, amorphous nitric acid solutions, and nitric acid hydrates

We determined the infrared optical constants of nitric acid trihydrate, nitric acid dihydrate, nitric acid monohydrate, and solid amorphous nitric acid solutions which crystallize to form these hydrates. We have also found the infrared optical constants of H2O ice. We measured the transmission of infrared light throught thin films of varying thickness over the frequency range from about 7000 to 500/cm at temperatures below 200 K. We developed a theory for the transmission of light through a substrate that has thin films on both sides. We used an iterative Kramers-Kronig technique to determine the optical constants which gave the best match between measured transmission spectra and those calculated for a variety of films of different thickness. These optical constants should be useful for calculations of the infrared spectrum of polar stratospheric clouds.

Toon, Owen B.↗

NASA NTRS1994

Real refractive indices of infrared-characterized nitric-acid/ice films: Implications for optical measurements of polar stratospheric clouds

The infrared spectra of nitric-acid/ice films representative of polar stratospheric clouds (PSCs) were collected with simultaneous optical interference measurements to determine the real refractive indices at lambda = 632 nm. Ice and amphorous nitric-acid/ice films were prepared by condensation of water and nitric acid vapors onto a wedged Al2O3 substrate. The real refractive indices of these films were determined from the optical interference of a reflected helium-neon laser during film growth. The indices of the amphorous films varied smoothly from n = 1.30 for ice to n = 1.49 for nitric acid, similar to observations in previous work. We were unable to obtain the refractive index of crystlline films during adsorption because of optical scattering caused by surface roughness. Therefore crystlline nitric acid hydrate films were prepared by annealing amphorous nitric-acid/ice films. Further heating caused desorption of the crystalline hydrate films. During desorption, the refractive indices for ice, NAM (nitric acid monohydrate), alpha- and beta-NAT (nitric acid trihydrate) films were measured using the optical interference technique. In agreement with earlier data, the real refractive indices for ice and NAM determined in desorption were n = 1.30 +/- 0.01 and n = 1.53 +/- 0.03, respectively. The real refractive indices for alpha- and beta-NAT were found to be n = 1.51 +/- 0.01 and n greater than or equal to 1.46, respectively. Our measurements also suggest that the shape of crystalline nitric acid particles may depend on whether they nucleate from the liquid or by vapor deposition. If confirmed by future studies, this observation may provide a means of distinguishing the nucleation mechanism of crystalline PSCs.

Middlebrook, Ann M.↗