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Papers on the photochemistry of the early atmosphere, the troposphere, stratosphere, and upper atmosphere are presented. Consideration is given to the effect of chemical constituents on the climate and atmosphere. Topics discussed include the composition of the atmospheres of Venus, Mars, the outer planets and their satellites, and the chemical changes which occur in comets.
The thermal structure and composition of Uranus, Neptune, Saturn, Jupiter, Titan, and Io are described. Molecular hydrogen is the dominant constituent in the atmosphere of the outer planets. The hydrogen and helium, methane, ammonia and phosphine, and carbon monoxide photochemical reactions of the outer planets are studied. The importance of Jupiter's lightning as a source of organic matter is examined. The aerosol and haze layers of the stratosphere of Jupiter, Saturn, Neptune, and Uranus are observed. The photochemistry of Titan's atmosphere, which includes nitrogen, methane, carbon monoxide, and carbon dioxide reactions, is analyzed. The SO2 atmosphere on Io is discussed.
The oxidation of nonmethane hydrocarbons represents a source of tropospheric ozone that is primarily confined to the boundary layers of several highly industrialized regions. (Each region has an area greater than one million km/sq cm). Using a photochemical model, the global tropospheric ozone budget is reexamined by including the in-situ production from these localized regimes. The results from these calculations suggest that the net source due to this photochemistry, which takes place on the synoptic scale, is approximately as large as the amount calculated for global scale photochemical processes which consider only the oxidation of methane and carbon monoxide. Such a finding may have a considerable impact on our understanding of the tropospheric ozone budget. The model results for ozone show reasonable agreement with the climatological summer distribution of ozone and the oxides of nitrogen at the surface and with the vertical distribution of ozone and nonmethane hydrocarbons obtained during a 1980 field program.
A one-dimensional photochemical model is used to analyze the photochemistries of CH4 and HCN in the primitive terrestrial atmosphere. CH4, N2, and HCN photolysis are examined. The background atmosphere and boundary conditions applied in the analysis are described. The formation of HCN as a by-product of N2 and CH4 photolysis is investigated; the effects of photodissociation and rainfall on HCN is discussed. The low and high CH4 mixing ratios and radical densities are studied.
Photochemistry of ammonia, methane, phosphine, hydrogen sulfide, methylamine, hydrogen cyanide and carbon monoxide in the atmospheres of Jupiter, Saturn and Uranus is discussed. Condensation of ammonia, ammonium hydrosulfide, water, methane, ethane and acetylene below and near the tropopause of these planets is formulated. Whenever necessary, new calculations are included. Candidates for the upper atmospheric hazes, and the reddish-brown chromosphore in the clouds of Jupiter and Saturn are discussed.
An assessment is made of the development status of concepts for cloud and aerosol compositions, vertical and horizontal distributions, and microphysical properties, in the Jovian upper troposphere and stratosphere. Attention is given to several key photochemical species' relationships to aerosol formation as well as their transport process implications, treating photochemistry in the context of comparative planetology and noting differences and similarities among the outer planet atmospheres; since this approach emphasizes observational data, a variegated assortment of ground-based and spacecraft observations is assembled. Current views on the tropospheric distribution of clouds are challenged, and a rationale is presented for alternative accounts.
Models of stratospheric photochemistry are generally tested by comparing their predictions for the composition of the present atmosphere with measurements of species concentrations. These models are then used to make predictions of the atmospheric sensitivity to perturbations. Here the problem of the sensitivity of such a model to chlorine perturbations ranging from the present influx of chlorine-containing compounds to several times that influx is addressed. The effects of uncertainties in input parameters, including reaction rate coefficients, cross sections, solar fluxes, and boundary conditions, are evaluated using a Monte Carlo method in which the values of the input parameters are randomly selected. The results are probability distributions for present atmosheric concentrations and for calculated perturbations due to chlorine from fluorocarbons. For more than 300 Monte Carlo runs the calculated ozone perturbation for continued emission of fluorocarbons at today's rates had a mean value of -6.2 percent, with a 1-sigma width of 5.5 percent. Using the same runs but only allowing the cases in which the calculated present atmosphere values of NO, NO2, and ClO at 25 km altitude fell within the range of measurements yielded a mean ozone depletion of -3 percent, with a 1-sigma deviation of 2.2 percent. The model showed a nonlinear behavior as a function of added fluorocarbons. The mean of the Monte Carlo runs was less nonlinear than the model run using mean value of the input parameters.
LIMS data at vernal equinox conditions are used to study the photochemistry of the upper stratosphere. The results indicate, and it has been recently reported, that with the use of recommended reaction rates, current models underestimate ozone mixing ratio by 20-40 percent. For ozone, good agreement with data is realized with the modification of six key reaction rates within the published limits of uncertainty. These modifications also yield better agreement with data for daytime NO2. Model results for other parameters such as the ratio HNO3/NO2, OH mixing ratio, and the temperature sensitivity of O3 are compared with data.
Laboratory work on the photochemistry of HC3N, C4N2, and CH3CN in relation to their possible role as CN parent molecules in comets is discussed. Photodissociation of HC3N, photolysis of C4N2, and quantum yields of excited CN(B) and CN(A) radicals from the VUV photolysis of CH3CN are considered.
A discharge flow apparatus with a chemiluminescence detector was used to investigate the reaction O + ClO yields Cl + O2(asterisk), where O2(asterisk) = O2(a1Delta g) or O2(b1Sigma + g). It is found that the observed O2(a1Delta g) airglow of Venus cannot be explained in the framework of standard photochemistry using the experimental results obtained here and those reported in the recent literature. The possibility of an alternative source of O atoms derived from SO2 photolysis in the Venus mesosphere is suggested.
The dominant photochemical reactions taking place in the ionospheres of Venus, Saturn, and Comet P/Halley are presented. It is shown that the differences in the ionospheres of these celestial bodies result from the different chemistry, energetics, and dynamics of the respective atmospheres. The role of photochemistry in the formation of the individual ionospheres is discussed.
Experiments described in report revealed potentially useful aspects of photochemistry of 2,5-dibenzoyl-1, 4-dimethylbenzene (DBX) and 2,5-diacetyl-1, 4-dimethylbenzene (DAX). Behavior of these compounds reminiscent of orthoalkylphenyl ketones, studied from similar perspective for more than two decades.
Earth-based observations relevant to the question of photochemistry and vertical mixing are discussed. Phytolysis of methane, the only known photochemically active volatile in the Uranian atmosphere, produces heavier hydrocarbons, the most abundant of which are ethane, acetylene, and the polyacetylenes. Unlike Jupiter and Saturn, these hydrocarbon products condense at the low temperatures prevalent in the middle atmosphere. Contrary to the pre-Voyager notion that the atmosphere of Uranus is remarkable clear, it is found that the aerosols are widely and extensively distributed. Despite its photodestruction, methane remains stable in the Uranian atmosphere. The vertical mixing on Uranus is found to be the least efficient of any of the planetary atmospheres.
The relationship between the biosphere and the atmosphere is examined, emphasizing the composition and photochemistry and chemistry of the troposphere and stratosphere. The reactions of oxygen, ozone, and hydroxyl are reviewed and the fate of the biogenic gases ammonia, methane, reduced sulfur species, reduced halogen species, carbon monoxide, nitric oxide, nitrous oxide, nitrogen, and carbon dioxide are described. A list is given of the concentration and sources of the various gases.
The products of diacetylene's ultraviolet photochemistry over the 245-220 nm region were directly determined in experiments where C4H2 was excited within a small reaction tube attached to a pulsed nozzle. The products formed in the collisions of C4H2* with C4H2 were subsequently ionized by vacuum UV radiation (at 118 nm) in the ion source of a time-of-flight mass spectrometer. It was found that the reaction of C4H2* with C4H2 produces C6H2 (+C2H2), C8H2 (+2H,H2), and C8H3 (+H), confirming the results of Glicker and Okabe (1987). Under certain conditions, secondary products were observed. Mechanisms for the observed reactions are proposed.
Absorption of solar UV irradiance in the spectral interval 120-420 nm is chiefly responsible for radiative heating and photodissociation of important atmospheric constituents (e.g., O2, O3, H2O, NO2, etc.) in the stratosphere, mesosphere, and lower thermosphere. Thus, the absolute value and time perturbations of the UV irradiance could significantly affect the energetics, photochemistry, and subsequent dynamics of these regions. Analysis of preliminary data from the SOLSTICE (UARS) observations for a period of 244 days (3 Oct 1991-2 Jun 1992) is discussed in this paper. The data provide mean daily values of the spectral distribution of the observed irradiances at 1-nm resolution and their solar rotation and semirotation variations. The average amplitudes of the 27-day irradiance oscillations for the 244-day data period were 5.7% at Lyman-alpha (121 nm), 1% at 200 nm, 0.5% at 210 nm, and generally less than 0.2% at wavelengths longer than 280 nm. The average amplitudes of 13.5-day oscillations were, by and large, about half of these values. Solar irradiance variations at 10.7 cm are highly correlated with those at Ly-alpha and other chromospheric emission lines (r = 0.7 to 0.8) and only moderately correlated with irradiances at wavelengths of 180-208 nm (r = 0.5). The correlation decreases as the source region of the irradiance gets closer to the base of the photosphere. At the 2-nm interval 279-281 nm, however, which contains the cores of the Mg II h and k lines, the correlation is again approximately 0.8.
An empirical time-series model for estimating ozone mixing ratios based on Stratospheric Aerosols and Gas Experiment II (SAGE II) monthly mean ozone data for the period October 1984 through June 1991 has been developed. The modeling results for ozone mixing ratios in the 10- to 30- km region in early months of 1993 are presented. In situ ozone profiles obtained by a dual-beam UV-absorption ozone photometer during the Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) campaign, May 1-14, 1993, are compared with the model results. With the exception of two profiles at altitudes below 16 km, ozone mixing ratios derived by the model and measured by the ozone photometer are in relatively good agreement within their individual uncertainties. The identified discrepancies in the two profiles are discussed.
An understanding of the composition, structure, transport and photochemical processes of the present atmosphere of Mars is essential for addressing such fundamental questions as the stability, evolution and the origin of the Martian atmosphere. This paper discusses our current knowledge of the photochemistry and stability of the Martian atmosphere, with special emphasis on new models and work in progress.