Engineering topics
Publications and source records attributed to Mcelroy, M. B..
Ionization of thermal and nonthermal oxygen atoms above the plasmapause on Venus supplies an escape flux for O averaging 6 x 10 to the 6th atoms/sq cm-sec. Hydrogen and oxygen atoms escape with stoichiometry characteristic of water. It is argued that escape of H is controlled by the oxidation state of the atmosphere, regulated by escape of O.
The composition of the atmosphere is influenced both directly and indirectly by biological activity. Evidence is presented here to suggest that nitrification in soil is a potentially significant source of both NO and N2O. Between 0.3 and 10% of the ammonium oxidized by cultures of the soil bacterium Nitrosomonas europaea is converted to these gases. The global source for NO associated with nitrification could be as large as 15,000,000 tonnes N/yr, with a source for N2O of 5,000,000-10,000,000 tonnes N/yr. Nitric oxide has a key role in tropospheric chemistry, participating in a complex set of reactions regulating OH and O3. Nitrous oxide is a dominant source of stratospheric NO and has a significant influence on climate.
Processes that affect the concentration of tropospheric OH are considered, taking into account linkages between the chemistry of OH and the chemistry of H, HO2, and H2O2. A review is presented of observational data of special relevance to OH, notably the distributions of CO, CH4, O3, H2O, HNO3, NO, and NO2. Most of the results presented in connection with the description of the model were obtained by solving time-dependent continuity equations. Rates for photolytic processes were allowed to vary diurnally with insolation. It is found that a well-calibrated and reliable model for OH places important constraints on global budgets for a variety of gases including CH4, H2, and CH3Cl in addition to CO. Accurate measurements of species such as CH3CCl3 can provide valuable checks on global models that must integrate over a variety of atmospheric conditions. However, emission rates for the relevant gases must be adequately quantified.
Parallel investigations were conducted relating to: the sensitivity of 1-D photochemical model simulated column ozone perturbations due to a projected fleet of 1000 aircraft cruising 7 hours per day at altitudes of 15-16 and 18-19 km to uncertainties in kinetic rate constant data determining modeled OH concentrations and eddy diffusivity profile parameterization and a comparison of the inherent strengths and weaknesses of Eulerian and Langrangian averaging processes in the development of multidimensional models and investigation of approaches to applying the Generalized Lagrangian Mean (GLM) formalism to zonal-mean models. The role of multiple scattering and Earth curvature in the evaluation of diurnally dependent photodissociation rates and trace species variations was examined.
The influence of oxygen concentration on the production of NO2(-) and N2O by nitrifying marine bacteria of the genus Nitrosomonas is investigated. Pure cultures of the ammonium-oxiding bacteria isolated from the Western Tropical Atlantic Ocean were grown at oxygen partial pressures from 0.005 to 0.2 atm, and concentrations of N2O in the air above the growth medium and dissolved NO2(-) were determined. Decreasing oxygen concentrations are observed to induce a marked decrease in NO2(-) production rates and increase in N2O evolution, leading to an increase of the relative yield of N2O with respect to NO2(-) from 0.3% to nearly 10%. Similar yields of N2O at atmospheric oxygen levels are found for nitrifying bacteria of the genera Nitrosomonas, Nitrosolobus, Nitrosospira and Nitrosococcus, while nitrite-oxydizing bacteria and a dinoflagellate did not produce detectable quantities of N2O. Results support the view that nitrification is a major source of N2O in the environment.
The oxidation of COS and CS2 by reaction with hydroxyl radicals is investigated as a possible source of atmospheric SO2 in remote marine regions. Calculations of the vertical profiles of SO2 were performed based on a one-dimensional photochemical model of the formation and destruction of SO2 by various processes for observed O3, CO, CH4 and H2O profiles at 15 deg S. Variations in the rate of SO2 destruction, the chosen deposition velocity and the loss due to aerosols are shown to lead to similar SO2 profiles, which indicate higher mixing ratios at high altitude, while the oxidation of dimethyl sulfide or hydrogen sulfide can not account for the profiles observed. Possible diffuse sources of CS2 and COS are indicated, and it is concluded that the oxidation of COS and possibly CS2 may provide an explanation for the existence of a uniform background level of SO2.
The composition of the Venusian atmosphere was studied using a mass spectrometer on the Pioneer Venus sounder probe. The single-focusing magnetic-sector spectrometer scanned the mass range from hydrogen through mercury with a dynamic range of six decades. Data taken by the mass spectrometer were compared with those of a gas chromatograph, resulting in slight discrepancies due to the use of a sputter ion pump acting as a sink for entry of rare gases through the inlet leak. A surprisingly large concentration of primordial Ar-36 and Ar-38 was discovered in a ratio of 5 to 1. It was concluded that the large excess of primordial argon was a valid result and that the mixing of HCl in the lower Venusian atmosphere was less than a few parts per million. Arguments against the sun as the source for excess primordial gases on Venus were presented. Concentrations of other elements such as Ne, Kr, He, S, and O were discussed. Although the mass peaks in the spectrum were real, it was not clear whether all of the chemical reactions (i.e., COS production) actually occurred in the atmosphere. Until further analysis can be made, it will be uncertain how the inlet system, which is at atmospheric temperature, affected the results.
Reactions between nitrogen and water in the air surrounding lightning discharges can provide an important source of nitric oxide even under conditions where oxygen is a minor atmospheric constituent. Estimates are given for the associated source of soluble nitrite and nitrate. It is shown that lightning and subsequent atmospheric chemistry can provide a source of nitrate for the primitive ocean as large as one million tons of nitrogen per year, sufficient to fill the ocean to its present level of nitrate in less than one million years.
The Pioneer Venus orbiter electron temperature probe was used to obtain altitude profiles of electron temperature and density in the ionosphere of Venus. Elevated temperatures at times of low solar wind flux might indicate support for a certain model. According to this model, less than 5% of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 km where electron cooling to the neutral atmosphere proceeds rapidly. The patterns of electron temperatures and densities at higher solar wind fluxes are considered, the variability of the ionopause height in the late afternoon is noted, and the role of an induced magnetic barrier in the neighborhood of the ionopause is discussed.
Initial examination of data from the neutral mass spectrometer on the Pioneer Venus sounder probe indicates that the abundances of argon-36, argon-38, and neon-20 in the Venus atmosphere are much higher than those of the corresponding gases in Earth's atmosphere, although the abundance of radiogenic argon-40 is apparently similar for both planets. The lower atmosphere of Venus includes significant concentrations of various gaseous sulfur compounds. The inlet leak to the mass spectrometer was temporarily blocked by an apparently liquid component of the Venus clouds during passage through the dense cloud layer. Analysis of gases released during the evaporation of the droplets shows the presence of water vapor to some compound or compounds of sulfur.
Data are presented which suggest the complexity of the aquatic nitrogen cycle as it affects N2O. The data are from studies made in the central and south-east tropical regions of the Pacific Ocean and in Chesapeake Bay. The data indicate that oxidation of ammonium and amino nitrogen and nitrification form the principle source for marine N2O. It is estimated that the yearly global yield for oceanic N2O is less than about 10 to the 7th power tons. The consumption of atmospheric N2O by the open ocean has not been evidenced, although data from the south-east tropical Pacific and Chesapeake Bay show the consumption of dissolved N2O in low-oxygen conditions. Preliminary observations have also indicated the consumption of atmospheric N2O by aquatic systems such as freshwater pond and a tidal saltmarsh.
Global atmospheric chemistry is surveyed, and the agreement of models with observed distribution of gases is considered. The influence of human perturbations due to combustion, agriculture, and chloro-carbon releases is examined with emphasis on ozone. Effects of combustion-related releases of CO on the abundances of other gases as well as possible effects of CO on tropospheric ozone are discussed. Other topics include the contribution of the chlorocarbon industry to stratospheric chloride and the recombination of nitrogen fixed by agriculture and combustion.
A technique for solving the problem of photoelectron spatial transport and spectral degradation is presented. The developed model, which allows for a relatively complete description of the relevant transport and collisional physics, can be applied to study processes at either low or high altitude, and may be used to investigate interhemispheric transport. The technique involves dividing the spatial dimension into a set of discrete ordinates and calculating the photoelectron intensity for a specific energy at each point in space and at a set of angle points. The selected range of the energy spectrum was covered completely by a set of energy bins, and the photoelectron intensity in the highest energy bin was calculated first by assuming that the only source term was derived from photoionization. The method is similar to a method used in radiative transfer theory. Spatial and angular redistribution of electrons play a dominant role at heights above 400 km.
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Radiative recombination of N and O provides a significant source for auroral emission in the gamma and delta bands of NO with selective population of vibrational levels in the A 2 Sigma(+) and C 2 Pi states. This mechanism may account for emissions detected near 2150 A. Models are derived for the auroral ionosphere and include estimates for the concentrations of N and NO. The concentration of NO is estimated to have a value of about 100 million per cu cm near 140 km in an IBC III aurora. The corresponding density for N is about 50 million per cu cm, and the concentration ratio NO(+)/O2(+) has a value of about 5.5.
A critical analysis is carried out for models which may be fine tuned to give agreement with observational constraints for O2, CO, and O3 in the Martian lower atmosphere and which are in accord also with upper atmospheric data for O and CO. The models have a number of features in common: dynamic mixing must be exceedingly rapid at heights above 90 km; if heterogeneous processes are not assumed to play a major role in the removal of atmospheric CO and O2, mixing must be rapid at heights between 30 and 40 km. The value implied for the diffusion coefficient in this region depends on the assumptions made regarding the rates for reaction of OH with HO2 to form H2O and the rate of reaction of HO2 with itself to form H2O2. If these reaction rates are taken to have values similar to those used in current models of the earth's stratosphere, the eddy diffusion coefficient at 40 km above the Martian surface should be about 5 by 10 to the seventh power sq cm/sec.
The upper atmospheric mass spectrometers flown on Viking 1 and 2 are described, and results obtained for the composition and structure of Mars' upper atmosphere are summarized. Carbon dioxide is the major constituent of the atmosphere at all heights below 180 km. The thermal structure of the upper atmosphere is complex and variable with average temperatures below 200 K for both Viking 1 and 2. The atmosphere is mixed to heights in excess of 120 km. The isotopic composition of carbon and oxygen in the Martian atmosphere is similar to that in the terrestrial atmosphere: N-15 is enriched in Mars' atmosphere by a factor of 1.62 + or - 0.16.
Viking measurements of the upper atmosphere of Mars have indicated thermospheric temperatures below 200 K, colder than those originally calculated by remote sensing experiments. It is suggested that dynamical coupling of the upper and lower regions of the Martian atmosphere may account for the variability in thermospheric temperature. The absorption of extreme ultraviolet solar radiation may account for observed ionospheric features, and may provide a source of fast N and O atoms escaping the planet's gravitational field. Measurements of the isotopic composition of O and N may be used to place constraints on models of the evolution of the Martian atmosphere. It is suggested that previous abundance of N2 may have exceeded that of CO2 in the present atmosphere, and that large sources of H2O may exist on the planet. The present degassing rate for nitrogen is felt to be less than the time-averaged degassing rate by at least a factor of 20.