Michael B. McElroy

Extreme Ultraviolet Observations from Voyager 1 Encounter with Jupiter

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S III, S IV, and O III indicating an electron temperature of 105 K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman α by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (≥ 1000 K) wvith a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.

Three-dimensional climatological distribution of tropospheric OH: Update and evaluation

A global climatological distribution of tropospheric OH is computed using observed distributions of O3, H2O, NOt (NO2 +NO + 2N2O5 + NO3 + HNO2 +HNO4), CO, hydrocarbons, temperature, and cloud optical depth. Global annual mean OH is 1.16×106 molecules cm−3 (integrated with respect to mass of air up to 100 hPa within ±32° latitude and up to 200 hPa outside that region). Mean hemispheric concentrations of OH are nearly equal. While global mean OH increased by 33% compared to that from Spivakovsky et al. [1990], mean loss frequencies of CH3CCl3 and CH4 increased by only 23% because a lower fraction of total OH resides in the lower troposphere in the present distribution. The value for temperature used for determining lifetimes of hydrochlorofluorocarbons (HCFCs) by scaling rate constants [Prather and Spivakovsky, 1990] is revised from 277 K to 272 K. The present distribution of OH is consistent within a few percent with the current budgets of CH3CCl3 and HCFC-22. For CH3CCl3, it results in a lifetime of 4.6 years, including stratospheric and ocean sinks with atmospheric lifetimes of 43 and 80 years, respectively. For HCFC-22, the lifetime is 11.4 years, allowing for the stratospheric sink with an atmospheric lifetime of 229 years. Corrections suggested by observed levels of CH2Cl2 (annual means) depend strongly on the rate of interhemispheric mixing in the model. An increase in OH in the Northern Hemisphere by 20% combined with a decrease in the southern tropics by 25% is suggested if this rate is at its upper limit consistent with observations of CFCs and 85Kr. For the lower limit, observations of CH2Cl2 imply an increase in OH in the Northern Hemisphere by 35% combined with a decrease in OH in the southern tropics by 60%. However, such large corrections are inconsistent with observations for 14CO in the tropics and for the interhemispheric gradient of CH3CCl3. Industrial sources of CH2Cl2 are sufficient for balancing its budget. The available tests do not establish significant errors in OH except for a possible underestimate in winter in the northern and southern tropics by 15–20% and 10–15%, respectively, and an overestimate in southern extratropics by ∼25%. Observations of seasonal variations of CH3CCl3, CH2Cl2, 14CO, and C2H6 offer no evidence for higher levels of OH in the southern than in the northern extratropics. It is expected that in the next few years the latitudinal distribution and annual cycle of CH3CCl3 will be determined primarily by its loss frequency, allowing for additional constraints for OH on scales smaller than global.

Mars: An Evolving Atmosphere

Photochemical reactions in the martian exosphere produce fast atoms of oxygen, carbon, and nitrogen and provide large escape fluxes of these elements. They appear to play a crucial role in the evolution of the martian atmosphere. The relative outgassing rates of H2O and CO2 on Mars are comparable with terrestrial values, although absolute rates for Mars are lower by a factor of 103. Nitrogen is a trace constituent, less than 1 percent, of the present martian atmosphere.

Stability of the Martian Atmosphere

A detailed chemical dynamic model is presented for a moist martian atmosphere. Recombination of carbon dioxide is catalyzed by trace amounts of water. The abundances of carbon monoxide and molecular oxygen should vary in response to changes in atmospheric water and atmospheric mixing.

Atmospheric Chemistry: Response to Human Influence

Present understanding of global atmospheric chemistry is reviewed. Models are presented and compared with a wide range of atmospheric observations, with emphasis on the stratosphere. In general, excellent agreement is found between the calculated and observed distributions of long lived trace gases. The abundances of many shorter lived species are also satisfactorily reproduced, including NO2, HNO3, O, O3, OH and ClO. Discrepancies between theory and observation are examined and their significance assessed. The influence of human perturbations due to combustion, agriculture and chlorocarbon releases is discussed with emphasis on O3. Uncertainties associated with present models are highlighted. Combustion related releases of CO cause a decrease in the abundance of tropospheric OH with consequent increase in the concentrations of CH4, H2, CH3Cl and other halocarbons. CO emissions may become sufficiently large during the next century to induce substantial increases in tropospheric ozone on a global scale. Recombination of nitrogen fixed by agriculture and combustion may lead to an enhanced source of atmospheric N2O with a related impact on stratospheric NOx. Chlorocarbon industry provides an important source of stratospheric chlorine, and enhanced levels of stratospheric Clx and NOx may cause a significant reduction in the abundance of atmospheric O3, by as much as 10% during the next century. Perturbations due to various anthropogenic activities interact in a nonlinear fashion and the influence on atmospheric chemistry is correspondingly complex.

Nitrous Oxide: A Natural Source of Stratospheric NO

Abstract Supersonic transport planes currently under development will cruise in the stratosphere and there is concern about possible environmental effects. In particular, NO emitted by these aircraft may catalytically affect atmospheric ozone. Here we investigate an important natural source of NO, the reaction O(1D) + N2O → 2NO, and compare the natural source with estimates for the source due to a fleet of 500 planes cruising for an average of 7 hr a day. The natural and artificial inputs above 15 km are of comparable magnitude. The natural source corresponds to a net production of NO, averaged over the globe, of about 2 × 107 molecules cm−2 sec−1, and offers a yardstick for judging the possible significance of any artificial input. Additional sources of stratosphere NO, due to downward diffusion from the ionosphere and upward transport from the earths surface, are discussed but have not been quantitatively estimated at this time.

Changing Composition of the Global Stratosphere.

The current understanding of stratospheric chemistry is reviewed with particular attention to the influence of human activity. Models are in good agreement with measurements for a variety of species in the mid-latitude stratosphere, with the possible exception of ozone (O3) at high altitude. Rates calculated for loss of O3 exceed rates for production by about 40 percent at 40 kilometers, indicating a possible but as yet unidentified source of high-altitude O3. The rapid loss of O3 beginning in the mid-1970s at low altitudes over Antarctica in the spring is due primarily to catalytic cycles involving halogen radicals. Reactions on surfaces of polar stratospheric clouds play an important role in regulating the abundance of these radicals. Similar effects could occur in northern polar regions and in cold regions of the tropics. It is argued that the Antarctic phenomenon is likely to persist: prompt drastic reduction in the emission of industrial halocarbons is required if the damage to stratospheric O3 is to be reversed.

The visual dayglow

Abstract New observations of altitude profiles for dayglow emissions at λλ 3914(N 2 +) 5200[N], 5577[O] and 6300[O] have been obtained with rocket borne photometers. These data are of adequate quality to justify an extensive theoretical discussion. The first portion of the paper constitutes a general review of dayglow excitations which may occur through five types of process: dissociative recombination of molecular ions; fluorescence (including resonance scattering) of sunlight; photodissociation of molecules; chemical reactions and collisions with energetic electrons. Only upper limits to yields of λλ 5577 and 6300 from photodissociation of O 2 can be deduced from measurements of total absorption coefficients, since a multiplicity of electronic transitions may contribute to the opacity. Also little detailed information is available on the production of excited states from dissociative recombination. A method for estimating some of the relevant rate coefficients is presented and applied to O( 1 D ) and O( 1 S ). Our analysis of the dayglow observations confirms laboratory measurements of the rate coefficients for N 2 + + O and N 2 + + O 2 reactions. We also obtain estimates of quenching rates for O( 1 D ) and N( 2 D ), in collisions with N 2 and O 2 , respectively. An upper limit of 5R for λ5577 nightglow excited by photodissociation of O 2 by night-sky Lyman-α is obtained from an intercomparison of dayglow and nightglow data. The most important excitation processes are: resonance scattering for λ3914; dissociative recombination and ion-atom interchange for λ5200; three body association and photoelectron collisions for λ5577; and photodissociation, electron collisions and dissociative recombination for ϖ300. A summary of our results is presented in tabular form in Section 5.

Isotopic Composition of Nitrogen: Implications for the Past History of Mars' Atmosphere

Models are presented for the past history of nitrogen on Mars based on Viking measurements showing that the atmosphere is enriched in 15N. The enrichment is attributed to selective escape, with fast atoms formed in the exosphere by electron impact dissociation of N2 and by dissociative recombination of N2+. The initial partial pressure of N2 should have been at least as large as several millibars and could have been as large as 30 millibars if surface processes were to represent an important sink for atmospheric HNO2 and HNO3.

Atmospheric Ozone: Possible Impact of Stratospheric Aviation

Abstract Models for stratosphere temperature and ozone are developed and shown to give good agreement with observational data. The atmosphere is in local radiative equilibrium at heights above about 35 km, and concentrations of ozone above 28 km can be satisfactorily estimated by models assuming photochemical equilibrium. Nitric oxide, formed by photochemical decomposition of nitrous oxide and ammonia, is an important catalyst for recombination of odd oxygen below 50 km, and is responsible for a reduction, by about a factor of 2, in the computed column density of ozone. Possible consequences of nitric oxide and water vapor, exhausted by stratosphere aircraft, are discussed. It is argued that there should be a significant reduction in the concentration of stratospheric ozone, with a related decrease in stratospheric temperature, if the globally averaged aircraft source of nitric oxide exceeds 2 × 107 molecules cm−2 sec−1, approximately half the natural source of stratospheric nitric oxide. An increase in s...