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Michael B. McElroy

Abstract

Models for the Venus atmosphere and ionosphere are presented and compared with data obtained in the S-band occultation experiment on Mariner 5. Good agreement is found between model and experiment if the Venus atmosphere is composed of essentially pure CO2. An upper limit of about 10% is derived for the N2 mixing rate. It is argued that atomic oxygen is a minor constituent in the upper atmosphere of Venus.

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Junling Huang and Michael B. McElroy

Abstract

The Hadley system provides an example of a thermally direct circulation; the Ferrel system in contrast provides an example of a thermally indirect circulation. In this study, the authors develop an approach to investigate the key thermodynamic properties of the Hadley and Ferrel systems, quantifying them using assimilated meteorological data covering the period January 1979–December 2010. This analysis offers a fresh perspective on the conversion of energy in the atmosphere from diabatic heating to the production of atmospheric kinetic energy. The results indicate that the thermodynamic efficiency of the Hadley system, considered as a heat engine, has been relatively constant over the 32-yr period covered by the analysis, averaging 2.6%. Over the same interval, the power generated by the Hadley regime has risen at an average rate of about 0.54 TW yr−1; this reflects an increase in energy input to the system consistent with the observed trend in the tropical sea surface temperatures. The Ferrel system acts as a heat pump with a coefficient of performance of 12.1, consuming kinetic energy at an approximate rate of 275 TW and exceeding the power production rate of the Hadley system by 77 TW.

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Michael B. McElroy and John C. McConnell

Abstract

Detailed calculations are presented for the production of O and CO in the Martian atmosphere. The diffusion equations are solved and results compared with Mariner observations of O and CO in the upper Marian atmosphere. An eddy diffusion coefficient of 5 × 108 cm2 sec−1 is required to account for these observations. Transport alone cannot explain the small abundances of CO and O2 in the lower atmosphere of Mars and chemical schemes suggested earlier encounter various difficulties.

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John C. McConnell and Michael B. McElroy

Abstract

Sources and sinks for atmospheric odd nitrogen are discussed and detailed calculations are presented for the altitude range 0–80 km. The mixing ratio of odd nitrogen is approximately 2×10−9 throughout the troposphere and increases in the stratosphere to a value of order 2×10−8 at 40 km. The dominant atmospheric forms of odd nitrogen are HNO3, NO2 and NO. Nitric acid is the major form below 25 km. Nitric oxide is the most abundant constituent during the day and at night is efficiently converted to NO2 below 65 km. Possible modification of stratospheric NOx by supersonic aircraft is discussed and it is concluded that the consequences may be detectable if the globally averaged source of NO from stratospheric aviation should exceed 4×107 molecules cm−2 sec−1.

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Michael B. McElroy and John C. McConnell

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 earth's surface, are discussed but have not been quantitatively estimated at this time.

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Michael B. McElroy, Nien Dak Sze, and Yuk Ling Yung

Abstract

Carbon monoxide, produced in the Venus atmosphere by photolysis of CO2, is removed mainly by reaction with OH. The radical OH is formed in part by photolysis of H2O2, in part by reaction of 0 with H02. Photolysis of HCl provides a major source of H radicals near the visible clouds of Venus and plays a major role in the overall photochemistry. The mixing ratio of 02 is estimated to be approximately 10−7, about a factor of 10 less than a recent observational upper limit reported by Traub and Carleton. A detailed model, which accounts for the photochemical stability of Venus CO2, is presented and discussed.

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Malcolm K. W. Ko, Nien Dak Sze, Mikhail Livshits, Michael B. McElroy, and John A. Pyle

Abstract

A two-dimensional zonal-mean model with parameterized dynamics and an advanced photochemical scheme is used to simulate the stratospheric distributions of atmospheric trace gases including ozone. The model calculates the distributions of 37 species that are photochemically coupled via 140 reactions with rate data from WMO/NASA. A full diurnal treatment is used to calculate the diurnal variations of the short-lived species and the diurnal mean of the production/loss rates for the long-lived species. The calculated concentrations are compared with a wide range of observations with emphasis on the seasonal and latitudinal features. In this work, no post hoc adjustment of the dynamical parameters has been attempted to improve agreement with observations.

In general, the model results are in good agreement with observations, although several discrepancies are noted. Rather than focusing on any individual species, we look for systematic agreements and discrepancies between model and observations for a wide range of species. The model appears to successfully simulate the major features of the mixing ratio surfaces for the long-lived species. However, at the equatorial region, the model tends to underestimate the concentrations of upward diffusing species (e.g., CFCs, CH4, N2O) and overestimate the column abundances of the downward diffusing species (HNO3, HCl, O3). These discrepancies are systematically examined and their implications for transport parameterization assessed.

The model successfully simulates the general latitudinal and seasonal behavior of the local concentration and column abundance of O3. Apart from the overestimation of the column abundances at the equator, the model also underestimates its seasonal contrast at high latitudes. There are difficulties in explaining the observed low concentrations of NO2 in winter at high latitudes. It is shown that errors in the simulation of NO2 concentration in these regions can significantly affect the calculated seasonal and latitudinal behavior of the column abundance of ozone in the middle and high latitudes.

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Michael B. McElroy, Steven C. Wofsy, Joyce E. Penner, and John C. McConnell

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 stratospheric water vapor causes a small increase in the concentration of ozone but cannot compensate for the much larger effects associated with nitric oxide. The more detailed analysis reported here confirms conclusions drawn earlier by Crutzen and Johnston regarding the possible impact of large numbers of supersonic transports. A fleet of 320 Concordes operating for 7 hr a day at 17 km is predicted to lead to a decrease of 1% in the column density of ozone, and similar conclusions presumably apply to the Soviet TU144. Aircraft flying at higher altitudes, with similar exhaust characteristics, would be expected to induce relatively more serious changes in atmospheric ozone.

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Steven C. Wofsy, Gian Paolo Gobbi, Ross J. Salawitch, and Michael B. McElroy

Abstract

Growth of nitric acid trihydrate (NAT) particles on background stratospheric aerosols is examined for an isolated air parcel cooled at a uniform rate. The free energy barrier against nucleation and rates of cooling are varied over a range of probable values. During the process of nucleation, the saturation ratio of HNO3 vapor reaches a maximum value between 2 and 15, corresponding to supercooling by 1–4 K. Significant supersaturation may be maintained after nucleation due to the small surface area of NAT available for vapor deposition.

If cooling rates exceed 0.5–1 K day−1, small (<2 μm radius) particles of NAT are produced. A major fraction of the available condensation nuclei is activated and removal of HNO3 by gravitational settling is slow. If cooling rates are less than 0.5–1 K day−1, the number of aerosols that nucleate is reduced, leading to differential growth of large (>2 μm radius) NAT particles. Gravitational settling of NAT particles could result in removal of HNO3 on time scales close to one week.

Observations of 5 μm radius particles in clouds at temperatures above the water frost point may reflect condensation of NAT on ice particles that fall through a column of air as it is cooled. Rapid condensation of HNO3 on ice particles is promoted by the high supersaturation attained during nucleation and maintained during subsequent cooling. This process provides a mechanism for irreversible removal of HNO3.

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