The evolution and variability of atmospheric ozone over geological time

The rise of atmospheric O₃ as a function of the evolution of O₂ has been investigated using a one-dimensional steady-state photochemical model based on the chemistry and photochemistry of O_x(O₃, O, O(¹D)), N₂O, NO_x(NO, NO₂, HNO₃), H₂O, and HO_x(H, OH, HO₂, H₂O₂) including the effect of vertical eddy transport on the species distribution. The total O₃ column density was found to maximize for an O₂ level of 10^?1 present atmospheric level (PAL) and exceeded the present total O₃ column by about 40%. For that level of O₂, surface and tropospheric O₃ densities exceeded those of the present atmosphere by about an order of magnitude. Surface and tropospheric OH densities of the paleoatmosphere exceeded those of the present atmosphere by orders of magnitude. We also found that in the O₂-deficient paleoatmosphere, N₂O (even at present atmospheric levels) produces much less NO_x than it does in the present atmosphere.     

Ozone and photochemistry of the Martian lower atmosphere

The calculation of number densities of CO₂, H₂O and N₂ photolysis products was carried out for the Martian atmosphere at heights up to 60 km. The ozone distributed in the atmosphere as a layer of 10 km width with [O₃] max = 2.5 × 10⁹ cm³ at height of 35 km which agree well with the results of u.v. observations on the evening terminator from the Mars-5 satellite. The calculated densities of O₂, CO and H₂O are also in good agreement with the measured data. The eddy diffusion coefficient is equal to 3 × 10⁶ in the troposphere (h ? 30 km) and 10⁸ cm² s^?1 above 40 km. The dependence of the total ozone content on water vapour amount in the atmosphere is considered; the hypothesis about the influence of water ice aerosol on the ozone formation is proposed to explain the high concentrations of ozone in the morning.     

Photochemistry of the Martian atmosphere

A variety of models are explored to study the photochemistry of CO₂ in the Martian atmosphere with emphasis on reactions involving compounds of carbon, hydrogen, and oxygen. Acceptable models are constrained to account for measured concentrations of CO and O above 90 km, with an additional requirement that they should be in accord with observations of CO, O₂, and O₃ in the lower atmosphere. Dynamical mixing must be exceedingly rapid at altitudes above 90 km, with effective eddy diffusion coefficients in excess of 10⁷ cm² sec^?1. If recombination of CO₂ is to occur mainly by gas phase chemistry, catalyzed by trace quantities of H, OH, and HO₂, mixing must be rapid over the altitude interval 30 to 40 km. The value implied for the diffusion coefficient in this region is a function of assumptions made regarding the rates for reaction of OH with HO₂ to form H₂O and of the rate for reaction of HO₂ with itself to form H₂O₂. If rates for these reactions are taken to have values similar to rates used in current models for the Earth's stratosphere, the eddy diffusion coefficient at 40 km on Mars should be about 5 × 10⁷ cm² sec^?1, consistent with Zurek's (1976) estimate for this parameter inferred from tidal theory. Surface chemistry could have an influence on the abundances of atmospheric CO and O₂, but a major effect would imply sluggish mixing at all altitudes below 50 km and in addition would carry implications for the magnitude of the rates for reaction of OH with HO₂ and HO₂ with itself.     

The global distribution of O3 on mars

Models are developed to describe the photochemistry of ozone on Mars. Catalytic reactions involving H, OH and HO₂ play a major role at low latitudes where they ensure a vertical column density for O₃ of less than 2 × 10^?4 cm atm. The source for odd hydrogen (H + OH + HO₂) is relatively smaller at high latitudes in winter due to the small concentrations of H₂O present there at that time. Odd hydrogen is also efficiently removed from the high-latitude winter atmosphere by condensation of H₂O₂. The role of catalytic chemistry is reduced accordingly and the vertical column density of O₃ may be as large as 5.7 × 10^?3 cm atm in accord with earlier observations carried out by Barth and co-workers with instruments on Mariner 9.     

Photochemistry of nitrogen in the Martian atmosphere

Models are developed for the photochemistry of a CO₂H₂ON₂ atmosphere on Mars and estimates are given for the concentrations of N, NO, NO₂, NO₃, N₂O₅, HNO₂, HNO₃, and N₂O as a function of altitude. Nitric oxide is the most abundant form of odd nitrogen, present with a mixing ratio relative to CO₂ of order 10^?8. Deposition rates for nitrite and nitrate minerals could be as large as 3× 10⁵ N equivalent atoms cm^?2 sec^?1 under present conditions and may have been higher in the past.     

Molecular emission in dense massive clumps from the star-forming regions S231–S235

The paper is concerned with the study of the star-forming regions S231–S235 in radio lines of molecules of the interstellar medium—carbon monoxide (CO), ammonia (NH₃), cyanoacetylene (HC₃N), in maser lines—methanol (CH₃OH) and water vapor (H₂O). The regions S231–S235 belong to the giant molecular cloudG174+2.5. The goal of this paper is to search for new sources of emission toward molecular clumps and to estimate their physical parameters from CO and NH₃ molecular lines. We obtained new detections ofNH₃ andHC₃Nlines in the sources WB89673 and WB89 668 which indicates the presence of high-density gas. From the CO line, we derived sizes, column densities, and masses of molecular clumps. From the NH₃ line, we derived gas kinetic temperatures and number densities in molecular clumps. We determined that kinetic temperatures and number densities of molecular gas are within the limits 16–30 K and 2.8–7.2 × 10³ cm^?3 respectively. The shock-tracing line of CH₃OH molecule at a frequency of 36.2 GHz was detected in WB89 673 for the first time.     

Representative abundances of a few positive ions in the innermost coma of comet Halley

The production rate of H₂O molecules at a heliocentric distance of 1 AU for comet Halley and the abundance ratio with respect to water (H₂O) of parent molecules at the cometary nucleus from the paper of Yamamoto (1987) have been used to compute the number densities of positive ions viz. H₃O⁺, H₃S⁺, H₂CN⁺, H₃CO⁺, CH₃OH ₂ ⁺ and NH ₄ ⁺ at various cometocentric distances within 600 kms from the nucleus.The role of proton transfer reactions in producing major ionic species is discussed. A major finding of the present investigation is that NH ₄ ⁺ ion which may be produced through proton transfer reactions is the most abundant ion near the nucleus of a comet unless the abundance of NH₃ as a parent is abnormally low. Using the quoted value of Q(NH₃)/Q(H₂O) for comet Halley and the life times of NH₃ and H₂O molecules, the abundance ratio N(NH₃)/N(H₂O) is found to be one-third of that used in the present paper. The consequent proportionate decrease in the NH ₄ ⁺ ions does not, however, affect its superiority in number density over other ions near the nucleus.The number density of the next most abundant ion viz. H₃O⁺ is found to be 4 × 10⁴ cm^-3 at the nucleus of comet Halley and decreases by a factor of two only upto a distance of 600 K ms from the nucleus. The ionic mass peak recorded by VEGA and GIOTTO spacecrafts atm/q = 18 is most probably composite of the minor ionic species H₂O⁺, as its number density = 10² cm^-3 remains virtually constant in the inner coma and of NH ₄ ⁺ , the number density of which at large cometocentric distances may add to the recorded peak atmlq = 18. The number densities of other major ions produced through proton transfer from H₃O⁺ are also discussed in the region within 600 K ms from the nucleus of comet Halley.     

Martian isotopic ratios and upper limits for possible minor constituents as derived from Mariner 9 infrared spectrometer data

The Mariner 9 infrared spectrometer obtained data over a large part of Mars for almost a year beginning late in 1971. Mars' infrared emission spectrum was measured from 200 to 2000 cm^?1 with an apodized resolution of 2.4 cm^?1. No significant deviation from terrestrial ratios of carbon (¹²C/¹³C) or oxygen (¹⁶O/¹⁸O; ¹⁶O/¹⁷O) isotopes was observed on Mars. The ¹²C/¹³C isotopic ratio was found to be terrestrial with an uncertainty of 15%. Upper limits have been calculated for several minor constituents. With an effective noise equivalent radiance of 1.2 × 10^?9 W cm^?2 sr^?1/cm^?1, new upper limits in centimeter-atmospheres of 2 × 10^?5 for C₂H₂, 4 × 10^?3 for C₂H₄, 3 × 10^?3 for C₂H₆, 2 × 10^?4 for CH₄, 1 × 10^?3 for N₂O, 1 × 10^?4 for NO₂, 4 × 10^?5 for NH₃, 1 × 10^?3 for PH₃, 7 × 10^?4 for SO₂, and 1 × 10^?4 for OCS have been derived.     

Phosphine photochemistry in the atmosphere of Saturn

A model is presented for the photochemistry of PH₃ in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH₃ and hydrocarbon photochemistry, specifically the C₂H₂ catalyzed photodissociation of CH₄. PH₃ is rapidly depleted with altitude (scale height ～35 km) in the upper troposphere when K～10⁴cm²sec^?1; an upper limit for K at the tropopause is estimated at ～10⁵cm²sec^?1. If there is no gas phase P₂H₄ because of sublimation, P₂ and P₄ formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H₂ + M → PH₃ + M is exceedingly slow. An upper limit P₄ column density of ～2×10¹⁵cm^?2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P₂H₄, P₂ and P₄ may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T?100°K. Potentially observable amounts of the organophosphorus compounds CH₃P₂H₂ and HCP are predicted, with column densities of >10¹⁷ cm^?2 and production rates of ～2×10⁸cm^?2sec^?1. The possible importance of electronically excited states of PH_x and additional PH₃/hydrocarbon photochemical coupling paths are also considered.     

Some problems in Venus' aeronomy

Models are presented for the height distribution of various photochemically active gases in Venus' upper atmosphere. Attention is directed to the chemistry and vertical transport of odd hydrogen (H, OH, HO₂, H₂O₂), odd oxygen (O, O₃), free chlorine (Cl, ClO, ClOO, Cl₂), CO, O₂, H₂ and H₂O. Supply of O₂ may play a limiting role in the formation of a possible H₂SO₄ cloud on Venus. The supply rate is influenced by both chemical and dynamical processes in the stratosphere, and an analysis of recent spectroscopic data for O₂ implies a lower limit to the appropriate eddy coefficient of about 3 × 10⁵ cm²/sec. The abundances of thermospheric O and CO are determined largely by vertical mixing, and an analysis of Mariner 10 measurements of Venus' Lyman α airglow suggests that the eddy coefficient in the lower thermosphere may be as large as 5 × 10⁷cm²sec. The corresponding values for the mixing ratios of O and CO at the ionospheric peak are approximately 1 per cent. The Lyman α data could be reconciled with larger values for thermospheric O, and smaller values for the vertical eddy coefficient, if non-thermal loss processes were to play a dominant role in hydrogen escape, and if the corresponding flux were to exceed 10⁷ atoms/cm²/sec. A sink of this magnitude would imply major depletion of Venus' atmospheric water over geologic time, and would appear to require mixing ratios of H₂O in the lower atmosphere in excess of 4 × 10^?4. The extensive component to the Lyman α emission measured by Mariner 5 may be due to resonance scattering of sunlight by hot atoms formed by charge transfer with O⁺. The H scale height, therefore, may reflect the temperature of positive ions in Venus' topside ionosphere.     

Chemical budgets of the stratosphere

A review is given of the stratospheric budgets of odd oxygen, odd nitrogen, nitrous oxide, methane and carbonyl sulfide. The stratospheric column production rate of NO by the reaction N₂O + O(¹D) → 2 NO is 1.1–1.9 × 10⁸ molecules cm^?2 s^?1. The stratospheric loss rates for N₂O, CH₄ and COS are equal to 0.9–1.4 × 10⁹, 1 × 10¹⁰ and 0.5 × 10⁷ molecules cm^?2 s^?1, respectively. From currently available information on the global distributions of N₂O and CH₄ there are some indications of about two times smaller OH concentrations below 35 km than those which are calculated based on the latest compilation of kinetic data.Most significantly, however, it is shown that photochemical models and available ozone observations cannot be reconciled and that there may be particularly severe problems in the 25–35 km region. This issue is thoroughly discussed.Volcanic emissions of SO₂ to the stratosphere may locally lead to much enhanced ozone concentrations and heating rates. These may influence the dynamic behaviour of volcanic plumes before their dispersion over large volumes of the stratosphere.     

Ion chemistry in the cometary atmosphere

Dirty ice of a second kind (major components, H₂O, CO, and N₂; minor components less than several percents, NH₃, CH₄, and other organic substances such as HCN, CH₃CN etc.) is assumed for the composition of volatiles in the cometary nucleus. The consistency with the observations of molecular ions and daughter molecules in the cometary atmosphere is argued by taking into account various ion-molecular reactions and dissociative recombinations. There is a satisfactory agreement for the second kind of dirty-ice model, but the presence of large amounts of CH₄ and NH₃ is found to be rather in contradiction with observational evidence. A velocity of 8 km s^?1 for the hydrogen atoms, derived from analysis of the hydrogen Lyman-alpha corona around comets, is found from the dissociative recombination of H₃O⁺, the dominant constituent of cometary ionosphere, in accordance with H₃O⁺+e ^?→OH+H+H.     

The role of ion-molecule reactions in the conversion of N2O5 to HNO3 in the stratosphere

We have investigated the role of several ion-molecule reactions in the conversion of N₂O₅ to HNO₃. In the proposed conversion, an N₂O₅ molecule would react with an H₂O molecule clustered to an inert ion to produce two HNO₃ molecules. Subsequent clustering of an H₂O molecule to the inert ion would make the reaction catalytic. If such an ion-catalysed conversion of N₂O₅ to HNO₃ occurs, it would probably play a role in the stratospheric chemistry at high latitudes in winter. In this paper we present reaction rate constant measurements made in a flowing afterglow apparatus for hydrated H₃O⁺, H⁺(CH₃CN)_m (m = 1, 2, 3), and several negative ions reacting with N₂O₅. Slow rate constants were found for these ions for hydration levels that are predominant in the stratosphere. With the known stratospheric ion density, these slow rate constants preclude significant N₂O₅ conversion by ion-molecule reactions.     

La lune mange-t-elle les couleurs?

The hydrogen radicals play an important role in the photochemistry of the troposphere of the Earth. The chemistry of OH, HO₂ and H₂O₂ is linked directly to the photodissociation of O₃ through the production of O(¹D). Gaseous H₂O₂ (hydrogen peroxide may be removed by heterogeneous reactions involving aerosols and liquid water. During the day and the night the solubility of ambient H₂O₂ in water is estimated and the oxidant capacity of H₂O₂ may explain the bleaching properties of the dew used in the past. This phenomenon may also explain some old maxims concerning the properties of the Moon's light to corrupt colors.     

Photochemistry of the stratosphere of Venus: Implications for atmospheric evolution

The photochemistry of the stratosphere of Venus was modeled using an updated and expanded chemical scheme, combined with the results of recent observations and laboratory studies. We examined three models, with H₂ mixing ratio equal to 2 × 10^?5, 5 × 10^?7, and 1 × 10^?13, respectively. All models satisfactorily account for the observations of CO, O₂, O₂(¹Δ), and SO₂ in the stratosphere, but only the last one may be able to account for the diurnal behavior of mesospheric CO and the uv albedo. Oxygen, derived from CO₂ photolysis, is primarily consumed by CO₂ recombination and oxidation of SO₂ to H₂SO₄. Photolysis of HCl in the upper stratosphere provides a major source of odd hydrogen and free chlorine radicals, essential for the catalytic oxidation of CO. Oxidation of SO₂ by O occurs in the lower stratosphere. In the high-H₂ model (model A) the OO bond is broken mainly by S + O₂ and SO + HO₂. In the low-H₂ models additional reactions for breaking the OO bond must be invoked: NO + HO₂ in model B and ClCO + O₂ in model C. It is shown that lightning in the lower atmosphere could provide as much as 30 ppb of NO_x in the stratosphere. Our modeling reveals a number of intriguing similarities, previously unsuspected, between the chemistry of the stratosphere of Venus and that of the Earth. Photochemistry may have played a major role in the evolution of the atmosphere. The current atmosphere, as described by our preferred model, is characterized by an extreme deficiency of hydrogen species, having probably lost the equivalent of 10²–10³ times the present hydrogen content.     

Cosmic ray synthesis of organic molecules in Titan's atmosphere

We have studied the possible synthesis of organic molecules by the absorption of galactic cosmic rays in an N₂CH₄H₂ Titan model atmosphere. The cosmic-ray-induced ionization results in peak electron densities of 2 × 10³ cm^?3, with NH₄⁺, C₃H₉⁺, and C₄H₉⁺ being among the important positive ions. Details of the ion and neutral chemistry relevant to the production of organic molecules are discussed. The potential importance of $\text{N}\text{(}{}^2\text{D)}$ reactions with CH₄ and H₂ is also demonstrated. Although the integrated production rate of organic matter due to the absorption of the cosmic ray cascade is much less than that by solar ultraviolet radiation, the production of nitrogen-bearing organic molecules by cosmic rays may be greater.     

Ion-ion mutual neutralization and ion-neutral switching reactions of some stratospheric ions

Following the recent mass spectrometric observations of the ambient stratospheric positive and negative ions we have carried out co-ordinated laboratory experiments using a selected ion flow tube apparatus and a flowing afterglow apparatus for the following purposes: (i) to consider whether CH₃CN is a viable candidate molecule for the species X in the observed stratospheric ion series H⁺ (H₂O_n (X)_m and (ii) to determine the binary mutual neutralization rate coefficients α_i for the reactions ofH⁺ (H₂O₄ and H⁺(H₂O)(CH₃CN)₃ with several of the negative ion species observed in the stratosphere. We conclude from (i) that CH₃CN is indeed a viable candidate for X and from (ii) that the α_i for stratospheric ions are within the limited range (5–6) × 10^?8 cm³ s^?1.     

The upper ionosphere of Titan

Photoionization of the upper atmosphere of Titan by sunlight is expected to produce a substantial ionospheric layer. We have solved one-dimensional forms of the mass, momentum, and energy conservation equations for ions and electrons and have obtained electron number densities of about 10³ cm^?3, using various model atmospheres. The significant ions in a CH₄H₂ atmosphere are H⁺, H₃⁺, CH₅⁺, CH₅⁺, CH₃⁺, and C₂H₅⁺. Electron temperatures may be as high as 1000°K, depending on the abundance of hydrogen in the high atmosphere. Interaction of the solar wind with the ionosphere is also discussed.     

Carbon dioxide on planetary bodies: Theoretical and experimental studies of molecular complexes

An absorption band at ∼4.26 μm wavelength attributed to the asymmetric stretching mode of CO in CO₂ has been found on two satellites of Jupiter and several satellites of Saturn. The wavelength of pure CO₂ ice determined in the laboratory is 4.2675 μm, indicating that the CO₂ on the satellites occurs either trapped in a host material, or in a chemical or physical complex with other materials, resulting in a blue shift of the wavelength of the band. In frequency units, the shifts in the satellite spectra range from 3.7 to 11.3 cm⁻¹. We have performed ab initio quantum chemical calculations of CO₂ molecules chemically complexed with one, two, and more H₂O molecules and molecules of CH₃OH to explore the possibility that the blue shift of the band is caused by chemical complexing of CO₂ with other volatile materials. Our computations of the harmonic and anharmonic vibrational frequencies using high levels of theory show a frequency shift to the blue by 5 cm⁻¹ from pure CO₂ to CO-H₂O, and an additional 5 cm⁻¹ from CO₂-H₂O to CO₂-2H₂O. Complexing with more than two H₂O molecules does not increase the blue shift. Complexes of CO₂ with one molecule of CH₃OH and with one CH₃OH plus one H₂O molecule produce smaller shifts than the CO₂-2H₂O complex. Laboratory studies of CO₂:H₂O in a solid N₂ matrix also show a blue shift of the asymmetric stretching mode.     

Models of the cometary coma in which abundances are calculated for various heliocentric distances

One-dimensional radial models of the chemistry in cometary comae have been constructed for heliocentric distances ranging from 2 to 0.125 AU. The coma's opacity to solar radiation is included and photolytic reaction rates are calculated. A parent volatile mixture similar to that found in interstellar molecular clouds is assumed. Profiles through the coma of number density and column density are presented for H₂O, OH, O, CN, C₂, C₃, CH, and NH₂. Whole-coma abundances are presented for NH₂, CH, C₂, C₃, CN, OH, CO⁺, H₂O⁺, CH⁺, N₂⁺, and CO₂⁺.