The Chemical Kinetics of CO2Atmospheres

Abstract

This review examines laboratory observations of the ultraviolet absorption and dissociation processes of CO₂ and the reactions of the dissociation products. The following conclusions about photolytic formation of CO and O atoms are reached. In the pressure and temperature conditions found in planetary atmospheres, CO₂ is dissociated at the rate implied by the absorption measurements made at higher pressure in the laboratory. Although most of the O atoms produced in the λ < 1650 Å dissociation process are initially formed in the O(¹D) state, these atoms are quenched to ground state O atoms after a few collisions with CO₂. There is no persuasive laboratory evidence to support the hypothesis that a metastable CO₃ species is formed in the O(¹D)-CO₂ quenching process. The possible fates of the CO and O atoms after they are formed in the photolysis process are examined to see if known reactions can account for the observations of low O atom concentrations in the upper atmosphere and low overall mixing ratios of O₂ and CO on Mars and Venus. It is concluded that there is no known two-body process that can remove O atoms fast enough to provide, through chemistry, a concentration as low as that observed in the outer atmospheres of Mars and Venus. It is therefore probable that the O atoms are removed from the upper atmospheres by downward transport. However, a comparison of known three-body loss rates shows that at lower altitudes the O atoms will be recombined to form O₂ rather than CO₂ which raises the question of how CO₂ is regenerated from CO and O₂. Comparison of measured rate constants suggests that regeneration of CO₂ on Venus can probably take place via direct reaction between O₂ and CO if the atmosphere is efficiently mixed. There does not appear to be any known gas phase process capable of explaining the low mixing ratios of CO and O₂ in the Martian atmosphere. The possibility of heterogeneous recombination mechanisms is briefly discussed.