Engineering topics
Publications and source records attributed to Allen, M..
Simultaneous in situ measurements of stratospheric NO(sub 2), HNO(sub 3), HCI, and CH(sub 4) from 34 to 24 km were made in August 1992 from Palestine, Texas, using the Balloon-borne Laser In-Situ Sensor (BLISS)tunable diode laser spectrometer.
Carbon dioxide comprises over 95 percent of the Mars atmosphere, despite continuous photolysis of CO2 by solar ultraviolet (UV) radiation. Since the direct recombination of CO and O is spinforbidden, the chemical stability of CO2 in the Martian atmosphere is thought to be the result of a HO(x)-catalyzed recombination scheme. Thus the rate of CO oxidation is sensitive to the abundance and altitude distribution of OH, H, and HO2. Most Martian atmospheric models assume that HO(x) abundances are governed purely by gas phase chemistry. However, it is well established that reactive HO(x) radical are adsorbed by a wide variety of surfaces. The authors have combined laboratory studies of H, OH, and HO2 adsorption on inorganic surfaces, observational data of aerosol distributions, and an updated photochemical model to demonstrate that adsorption on either dust or ice aerosols is capable of reducing HO(x) abundances significantly, thereby retarding the rate of CO oxidation.
Some of the most interesting chemistry in the Solar System involves changes in the oxidation state of the simple carbon species. The chemical pathways for the conversion of CH4 to CO and CO2 are for the most part known. The reverse process, the reduction of CO to CH4, is, however, poorly understood. This is surprising in view of the importance of the reduction process in the chemistry of the Solar System. Recently we investigated the chemical kinetics of a hitherto unsuspected reaction. It is argued that the formation of the methoxy radical (CH3O) from H+H2CO may play an essential role in the reduction of CO to CH4. The rate coefficient for this reaction has been estimated using the approximate theory of J. Troe and transition state theory. We will discuss the implications of this reaction for the chemistry of CO on Jupiter, in the solar nebula, for interpreting the laboratory experiments of A. Bar-Nun and A. Shaviv and A. Bar-Nun and S. Chang, and for organic synthesis in the prebiotic terrestrial atmosphere. The possible relation of CO reduction in the solar nebula and polyoxymethylene observed in comet Halley will be discussed.
Observed formaldehyde column densities of 1 x 10 to the 12th - 3 x 10 to the 13th/sq cm in cloud envelopes along lines of sight with A(V) = 1-4 mag can not be explained with the current understanding of interstellar gas phase chemistry. However, these column densities can be reproduced by a simple time-dependent model in which H2CO is supplied to the gas phase by the erosion of icy grain mantles. The release of H2CO from the grain mantles must occur on time scales comparable to the time scales for mixing from the cloud interior to the cloud envelope. Thus, in low-density regions of clouds, it appears that formaldehyde is the second molecule whose gas phase source is primarily ejection from grains. This simple model suggests understanding gas phase steady state in clouds on macroscopic, rather than microscopic, spatial scales.
An investigation is conducted in order to determine the relative importance of several modeled processes in controlling the magnitude and phase of the mesospheric ozone response. A detailed one-dimensional modeling study of the mesospheric ozone response to solar UV flux variations is conducted to remove some of the deficiencies in previous studies. This study is also used to examine specifically the importance of solar zenith angle, self-consistent calculation of water vapor abundance, and temperature feedback with a nonlocal thermodynamic equilibrium radiation model. The photochemical model is described, and the assumptions made for the purpose of comparing model results with the observed ozone response obtained from a statistical analysis of Solar Mesosphere Explorer data (Keating et al., 1987) are discussed. The numerical results for the theoretical ozone response are presented. The results of selected time-dependent calculations are considered to illustrate the degree to which a relatively simple model of the mesosphere is able to capture the major characteristics of the observed response.
A high-resolution (0.059/cm) M-band spectrum has been obtained of the embedded young stellar object GL490. The spectrum shows interstellar absorption in the fundamental vibrational band, v = 1-0, of (C-12)O. Two strong and narrow (10 km/s) velocity components are present. One, at the velocity of GL490 (vLSR = -16 km/s), is likely gas in the molecular cloud within which GL490 is embedded. The other component is blueshifted by 13 km/s relative to GL490. An observation of emission from the J = 3-2 transition of HCO(+) using a 20-arcsec beam supports the view that the blueshifted gas is near the central object. The -29-km/s feature is interpreted as a recently ejected shell. It is conjectured that the extended outflows of cold molecular gas seen by millimeter CO emission observations are driven by sporadic outbursts rather than by continuous flows from the central object.
The bulk of O sub 3 destruction in the Antarctic stratosphere takes place in the lower stratosphere between 15 and 25 km. Both O sub 3 and the halogen reservoir species have their origins in the higher altitude region (20 to 30 km) in the equatorial and mid-latitude stratosphere. Using the Caltech-JPL two-dimensional residual circulation model, researchers investigate the growth of stratospheric halogen due to the increase of CFCl sub 3 and CF sub 2 Cl sub 2.
Methane and ammonia abundances in the coma of Halley are derived from Giotto ion mass spectrometer data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto high-intensity spectrometer ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01 = 0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(Q2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.
Morning and afternoon mesospheric ozone profiles (50-90 km) measured by the Solar Mesosphere Explorer (SME) satellite are analyzed with one-dimensional photochemical models. The observed ozone abundances are 40 percent and 100 percent greater than the model ozone abundances at 50 and 80 km, respectively. Diurnal model calculations are compared with SME observations of ozone profiles at about 0400 and 1400 LT for high northern summer latitudes. Analysis of the ratios of these early morning and midafternoon ozone profiles provides the additional constraint that larger odd-oxygen production rates are required if lower odd-hydrogen activity is invoked to increase model O3 abundances. The increase in odd-oxygen production must be solar zenith angle independent in the mesosphere, ruling out significant changes in the Schumann-Runge band O2 opacities from Allen and Frederrick (1982).
A photochemical model for the atmosphere of Jupiter, including 1-D vertical eddy diffusive transport, was developed. It extends from the upper troposphere through the homopause. The hydrocarbon chemistry involves species containing up to four carbon atoms (and polyynes through C8H2). The calculations show that a large fraction of photochemical carbon may be contained in molecules with more than two carbon atoms. At the tropopause, C2H6 is the major photochemical species and C2H2, C3H8, and C4H10 are of comparable abundance and down from C2H6 by a factor of ten. These species may be detectable with the mass spectrometer of the Galileo Probe. The vertical distributions of the photochemical species are sensitive to the magnitude of eddy diffusive mixing in the troposphere and stratosphere and the details of the interface region.
The simultaneous measurements of atomic O, O2, O3, and N2 between 90 and 110 km obtained by a rocket-borne mass spectrometer flown during the Aladdin 74 program provide an opportunity to test directly the simple atmosphperic chemical model first proposed more than 50 years ago by S. Chapman, which is today the core of the complex chemistry controlling stratospheric O3. The Aladdin 74 O3 results are found to be significantly in excess of model abundances computed using the most current values of the key chemical parameters. The calculations may possibly be in error due to uncertainties in these key parameters and/or in the measurement of the related species. However, the extra O3 may be a consequence of reactions between ground state O2 and electronically excited O2 resulting from the recombination of atomic O.
The Caltech one-dimensional photochemical model is used to analyze important questions concerning the upper stratospheric O3 and the key HO(x), NO(x), and ClO(x) free radicals. The model is described and first order effects of the inclusion of diffuse radiation in a spherical rather than plane parallel atmosphere at solar zenith angles close to 90 deg are assessed. A comparison is made between photochemical theory and observations for upper stratospheric O3, where local photochemistry rather than dynamics should control the O3 abundance. The photochemical equilibrium relation for odd oxygen is studied, including the important radicals involves. It is concluded that a significant model ozone deficit exists in the upper stratosphere. Possible causes for such a discrepancy are investigated in light of current model and experimental uncertainties. In particular, the observational ratio of atomic oxygen to zone appears to disagree with the model results.
Measurements were made of the J = 1 - 2 rotational transition of terrestrial mesospheric CO both in emission and in absorption against the moon on January 25-26, 1982. A CO mixing profile was obtained from the high signal-to-noise ratio emission spectrum. With the inclusion of these most recent spectra and spectra measured by Kunzi and Carlson (1982), further evidence is found suggesting seasonal variation of mesospheric CO as originally reported by Clancy et al. (1982). This seasonal variation may be the consequence of hemispheric circulation in the upper atmosphere.
Updated chemical schemes and estimates of key rate coefficients are used in the present investigation of the photochemistry of Titan atmosphere C- H- and O-atom containing simple molecules, according to a model incorporating exospheric boundary conditions, vertical transport, and condensation processes at the tropopause. It is suggested that the composition, climatology, and evolution of the Titan atmosphere are controlled by five major processes: CH4 photolysis and photosensitized dissociation, H-to-H2 conversion and hydrogen escape, higher hydrocarbon synthesis, nitrogen and hydrocarbon coupling, and oxygen and hydrocarbon coupling. The model accounts for the minor species concentrations observed by Voyager instruments. Implications of abiotic organic synthesis on Titan for the origin of life on earth are briefly discussed.
An assessment is made of the ability of current theory to explain the phenomenology of upper atmospheric ozone as revealed by the sizeable body of measurements presently available. The chemical processes affecting the vertical distribution of ozone are reviewed, and simple analytical expressions for the ozone concentrations at different altitudes are derived which approximate the key elements of the ozone chemistry. These equations provide simple explanations of the sensitivity of model computations to the choice of rate constants and climatological patterns. Model calculations are compared with a detailed measurement of an ozone profile, and the model is modified to assess the variation in ozone expected to result from perturbations in key climatological processes. These predictions are then compared with the variability observed in midlatitude measurements in order to verify the model description of ozone processes in the upper atmosphere.
Accurate calculations of the atmospheric opacity and the photodissociation rate of molecular oxygen in the Schumann-Runge bands (175-205) are necessary for modeling chemistry in the terrestrial upper atmosphere. The present investigation is concerned with a single simple parameterization of effective cross sections which can be used to calculate both O2 opacity and dissociation rates. Use is made of a zenith angle dependent factor which accounts for variations shown in detailed calculations. The conducted analysis is based on the results of Frederick and Hudson (1980). Attention is given to molecular oxygen effective cross sections and nitric oxide effective cross sections. It is found that the depth of the atmosphere to which solar radiation in the 175-200 nm spectral region penetrates is a sensitive function of the rotational line widths in the Schumann-Runge bands. The oscillator strength for each band measures the cross section integrated over the band while the line width determine how the absorption is distributed in wavenumber.
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A study is conducted of the coupled effects of kinetics, solar cycle flux variations, and vertical transport on the distribution of long-lived hydrogen-carbon-oxygen compounds in the terrestrial mesosphere and lower thermosphere, using a one-dimensional aeronomy model. The calculations account for the important chemical reactions and use rocket measurements of the solar flux at solar minimum and maximum. Photodissociation rates appropriate for the mesosphere are determined with a spherical shell atmosphere formalism. Detailed corrections for the O2 Schumann-Runge bands and the temperature dependence of the CO2 cross sections are used. An eddy diffusion profile is derived which is in agreement with the Aladdin 74 mass spectral measurements of atomic O, O2, CO2, and Ar in the lower thermosphere and observations of the O3 minimum at about 80 km.