Characterization of a dissociative excited state in the solid state: Photochemistry of poly/methyl methacrylate/ - Photochemical processes in polymeric systems. V
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Gupta, A.↗
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Photochemistry of SO2 in Venus' upper cloud layers
The photochemical and transport processes that occur in the Venusian atmosphere in and above the upper cloud region are characterized with attention to the correlation with the Pioneer Venus spectrometer results for the cloud top SO2 distribution. The model uses upward-flowing SO2 and H2O to replace the sulfur and hydrogen lost through the lower boundary by the settling of sulfuric acid aerosols. Oxygen for the oxidation of SO2 to SO3 prior to hydration to H2SO4 is supplied by the photolysis of CO2. A model with SO2 as the major sulfur-bearing gas is consistent with the Pioneer data.
Winick, J. R.↗
Vertical transport and photochemistry in the terrestrial mesosphere and lower thermosphere /50-120 km/
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.
Allen, M.↗
Chemical kinetics and photochemistry
A compilation of rate constant data for use in stratospheric modeling is presented. The tabulations are divided into two categories: bimolecular and termolecular. Explanations are given for anomalous pressure and temperature dependences.
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Photochemistry of SO2 in the atmosphere of Io and implications on atmospheric escape
Photochemical models of Io's atmosphere are presented with the assumption that SO2 is the major gas and that the SO2 surface pressure is controlled by vapor-pressure equilibrium at the surface. Photolysis of SO2 leads to efficient production of SO, O2, S, and O. Of these products, O and S are likely to be the dominant constituents in the upper atmosphere, and the atmospheric escape is expected to be in atomic form. Nonthermal escape processes are necessary to populate the plasma torus. SO2 surface densities greater than 100-billion per cu cm are needed on the dayside to supply the required amounts of S and O to maintain the torus. Surface chemistry presents an interesting possibility, namely, the conversion of O to O2, in which case O2 will be the dominant constituent throughout the nightside and over the polar caps with a surface O2 density of 10-billion per cu cm.
Kumar, S.↗
The effects of isotropic multiple scattering and surface albedo on the photochemistry of the troposphere
The present study is believed to be the first investigation which combines a detailed tropospheric photochemical model with a radiative code that realistically considers the important physical processes in the cloudless troposphere which control the transfer of incoming solar radiation. It is found in most cases that the inclusion of the multiple scattering code significantly alters the photolytic reaction frequencies. This in turn alters the vertical distribution of some key tropospheric species. In the case of ozone a decrease was found, while O(1D) and OH showed increased concentrations. The increases are especially evident for the species which are highly dependent on photolysis processes that occur shortward of about 320 nm. The photolysis frequencies for ozone photolysis calculated with the Anderson-Meier model are in general in excellent agreement with recent measurements
Augustsson, T. R.↗
Ozone photochemistry and OH - A two-dimensional model study
Recent measurements of key HO(x) reaction rates are presented which imply that OH concentrations at altitudes below 30 km are lower than those predicted by current photochemical models. The consequences of the lower OH abundances are examined with respect to calculated perturbations of ozone by added nitrogen oxides, chlorine, and water vapor. Consideration is also given to the perturbation of stratospheric ozone by continuing release of chlorofluoromethanes in light of the new HO(x) reaction rates. The ozone loss is found to be reduced by about 25% compared with earlier estimates, mainly because the reduced OH abundance below 30 km slows the reaction between OH and HCl.
Whitten, R. C.↗
Observations and photochemistry of O/++/ in the daytime thermosphere
An analysis is conducted for the Atmosphere Explorer C observations of O(++) in the daytime thermosphere which supercedes that of Breig et al (1977) and restricts attention to five orbits of data from the high-grain mode of the magnetic ion-mass spectrometer. The present investigation adopts a model for O(++) ionospheric chemistry which has been revised in light of recent laboratory measurements of a fast loss rate for O(++) through reaction with N2. An improved procedure is also introduced which analyzes the basic ion currents recorded by the magnetic ion-mass spectrometer and has special application for the very low concentrations near and below 200 km. It is found that large scale features in the low altitude data are best represented with an additive term which is proportional to the N2 or O2 density, with the preferred interpretation as a contaminant signal induced by neutral particles impinging on instrument or spacecraft surfaces.
Breig, E. L.↗
The photochemistry and dynamics of a dusty cometary atmosphere
The solving of a simultaneous set of differential equations representing conservation of number density, momentum, and energy together with solar radiation transfer in the streams which result in photolytic processes and the heating of the nucleus yields a self-consistent solution of the dynamical and thermal structure of an H2O-dominated two-phase dusty gas cometary atmosphere. Two models are considered for the transfer of solar radiation through the circumnuclear dust halo. The first considers only the direct extinction by the dust, and in the second the diffuse radiation field due to multiple scattering by the dust halo, which compensates for radiation removed by direct absorption when the optical depth is near unity, is approximated by neglecting the attenuation of radiation given off by the dust. It is shown that while dust attenuation has a strong effect on the H2O production rate, it also increases the electron density in the inner coma over the unattenuated case.
Marconi, M. L.↗
Photochemistry of NH3, CH4 and PH3 - Possible applications to the Jovian planets
It is found that the photolysis of NH4 at 185 nm in the presence of a two-fold excess of CH4 results in the loss of about 0.25 mole of CH4 per mole of NH3 decomposed. The loss is shown to arise from the abstraction of hydrogen atoms from CH4 by photolytically generated hot hydrogen atoms. It is concluded that NH3 photolysis in the H2-abundant atmosphere of Jupiter is not responsible for the presence of the carbon compounds observed there, such as ethane, acetylene, and hydrogen cyanide, but may have had a role in the early atmosphere of Titan. Also, it is found that the photolysis of PH3 with a 206 nm light source gives P2H4, which in turn is converted to a red-brown solid. The course of the photolysis is not changed appreciably when the temperature is lowered to 157 K except that the concentration of P2H4 increases, while the presence of H2 has no effect on the P2H4 yield. Photolysis of 9:1 NH3:PH3 is found to give a rate of decomposition of PH3 that is comparable with that observed by the direct photolysis of PH3 and comparable amounts of the red-brown solid and P2H4 are observed. In addition, the implications of these results for the structures of the compounds responsible for the wide array of colors observed in the atmosphere of Jupiter are examined.
Ferris, J. P.↗
HCN formation on Jupiter - The coupled photochemistry of ammonia and Acetylene
HCN formation in the upper troposphere and lower stratosphere of Jupiter is presently modeled in terms of UV pyrolysis of the C2H5N isomer aziridine, which is a product of the NH2 and C2H3 radicals that originate from ammonia photolysis and the addition of H atoms to acetylene, respectively. The sensitivity of the HCN column density to the individual rate constants and the eddy diffusion coefficient profile is considered, along with the possibility that additional HCN-yielding pathways may exist. Both ammonia and phosphine are strongly depleted by photolysis.
Kaye, J. A.↗
Chlorine nitrate photochemistry - Photolysis products and kinetics of the reaction Cl + ClONO2 yields Cl2 + NO3
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Margltan, J. J.↗
The photochemistry of anthropogenic nonmethane hydrocarbons in the troposphere
A lumped, nonmethane hydrocarbon (NMHC) chemical mechanism is presently applied to a one-dimensional photochemical model of the troposphere. The profiles of OH, HO2, NO(x), and HNO3, showed only slight changes when NMHC chemistry was added. The integrated column of peroxyacetylnitrate (PAN), when NMHC chemistry was included, comprised 17 percent of the odd nitrogen budget. Advection is noted as an important possible mechanism for the removal of PAN at midlatitudes. The inclusion of such intermediate lifetime species as aldehydes and olefins has both provided additional sources of short-lived NMHC radicals, such as the peroxyacetyl radical that is the radical precursor of PAN, and offered a more detailed description of the concentrations of short-lived species and the overall NMHC chemistry.
Brewer, D. A.↗
Formation and photochemistry of methylamine in Jupiter's atmosphere
In the upper troposphere and lower stratosphere of Jupiter, translationally hot H atoms are produced in the photolysis of ammonia, phospine, and acetylene which react with methane to form methyl radicals. The latter combine with NH2 to form methylamine. It is presently shown that the combined production of methylamine and subsequent photolysis to HCN is unlikely to account for the HCN observed near Jupiter's tropopause. The recommendation of NH2 and C2H3 radicals to yield C2H5N, followed by photolysis to HCN, is the preferred path. An upper limit column density on CH3PH2 is estimated to be about 10 to the 13th/sq cm, as compared to 10 to the 15th/sq cm for CH3NH2.
Kaye, J. A.↗
The reaction NH2 + PH3 yields NH3 + PH2 - Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter
The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be indepenent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then contrasted with those for the corresponding reactions of H and OH with PH3.
Bosco, S. R.↗
The reaction NH2 + PH3 yields NH3 + PH2: Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter
The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be independent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then constrasted with those for the corresponding reactions of H and OH with PH3.
Bosco, S. R.↗
Photochemistry of methane in the earth's early atmosphere
The photochemical behavior of methane in the early terrestrial atmosphere is investigated with a detailed model in order to determine how much CH4 might have been present and what types of higher hydroocarbons could have been formed. It is found that any primordial methane accumulated during the course of earth accretion would have been dissipated by photochemical reactions in the atmosphere in a geologically short period of time after the segregation of the core. Abiotic sources of methane are not likely to have been large enough to sustain CH4 mixing ratios as high as 10 to the -6th, the threshold for a possible methane greenhouse, with a CO-rich atmosphere being a possible exception. After the origin of life an increasing biogenic source of methane may have driven CH4 mixing ratios well above 10 to the 6th. The rise of atmospheric oxygen in the early Proterozoic may have led to a more rapid photochemical destruction of methane, lowering the mixing ratio to its present value.
Kasting, J. F.↗
Laser diagnostics of photochemistry and reaction dynamics
Experiments will be described which illustrate how tunable dye lasers are used to determine the quantum state distributions of the free radicals that are produced in photochemical systems. Other experiments will be described to show how lasers can be used to accurately measure the effects of internal energy upon the rate of chemical reactions.
Jackson, W. M.↗