Part I:
This thesis consists of two chapters concerning photochemical processes in the atmospheres of earth and Mars. The first chapter is a comprehensive study of the photochemistry of the martian atmosphere. Classical models of the Mars atmosphere have neglected an important property of carbon dioxide, namely that the photoabsorption cross section decreases with lower temperature. Accounting for this effect yields a smaller photolysis rate for CO_2 and more importantly, an enhanced photolysis rate for water vapor. Both effects combine to yield carbon monoxide mixing ratios smaller by a factor of four than observations indicate. We propose modifications in the rate coefficients for two key reactions, CO + OH and OH + HO_2, in order to resolve this discrepancy. We note that similar revisions have been proposed to reconcile models and observations of ozone in the terrestrial mesosphere. Other investigators have proposed a heterogeneous sink of odd hydrogen radicals on aerosols; we find that such a sink is unnecessary. Finally, we have performed the first time dependent calculation to examine the mechanism by which the escape of atomic oxygen controls the escape flux of hydrogen from the atmosphere. We show that this coupling operates over a time scale of 10^5 years.
In chapter two we investigate the formation and evolution of low ozone anomalies in the northern winter stratosphere using a Lagrangian photochemical model. The UARS spacecraft has observed pockets of low ozone in the 6 to 10 millibar altitude range, where the effects of dynamics and chemistry on the ozone budget are comparable. We employ the Lagrangian model to compute the ozone loss rate within an isolated parcel of air as it travels along a specified trajectory. Since we have decoupled the dynamics and chemistry, disagreement between the model and observations should reflect deficiencies in the chemistry. We find that the model consistently overestimates the ozone loss rate above about 7 millibars altitude, which is a common feature of most current photochemical models. Below 10 millibars altitude, the model is in good agreement with the observations, indicating that the description of chemistry is valid in the low to mid stratosphere.
Part II:
Photochemical models have historically overestimated ozone loss rates in the upper stratosphere and lower mesosphere, where ozone is photochemically controlled. Thus it is evident that there is some missing chemistry in current models. Current understanding of the factors controlling ozone in the low to mid stratosphere is that dynamical influences play a large role in determining the ozone abundance. It is difficult to test the chemistry in the models at these altitudes since the contributions from transport and chemistry must be separated.
The Microwave Limb Sounder (MLS) aboard the Upper Atmosphere Research Satellite (UARS) has observed pockets of low ozone in the winter polar stratosphere outside the polar vortex. These pockets occur in the 6 to 10 millibar altitude range, where the effects of dynamics and chemistry on the ozone budget are comparable. The formation and evolution of these anomalies are investigated using a Lagrangian photochemical model, where the chemistry within an isolated parcel of air is simulated as it travels along a specified trajectory. Since we have decoupled the dynamics and chemistry, disagreement between the model and observations should reflect deficiencies in the chemistry.
We find that the model consistently overestimates the ozone loss rate above about 7 millibars, where the ozone deficit tends to manifest itself. Below 10 millibars altitude, the model is in good agreement with the observations, indicating that the description of chemistry is valid in the low to mid stratosphere. Individual trajectories have many uncertainties associated with them; in order to present more quantitative conclusions the results computed along many trajectories should be taken together to minimize errors.
