Biased ozone precursors in the upper troposphere: Evaluation, recommendations, and implications

Ozone is a significant air quality pollutant and the third largest anthropogenic source of climate forcing. Despite its importance, ozone can only be sparsely sampled in the atmosphere. As a result, model simulations are required to fully understand ozone's effects as an air quality pollutant and as a short-lived climate forcer. Simulations predict ozone by modeling the interaction between the environment and chemical precursors. Current simulations have known biases for precursors in the upper troposphere, the altitude at which ozone is most efficient at climate forcing. In present global and regional chemical transport models (CTMs), the limiting ozone precursors -- nitrogen oxides -- are biased low compared to observations in the upper troposphere. Identifying the source(s) of error in a CTM can be difficult if compensating errors in one or several model processes (e.g., emission, transport, deposition, or chemistry) mask symptoms of a model deficiency. Each process, therefore, must be evaluated with minimal influence from other processes. This study develops a framework for isolating the chemistry process in the upper troposphere by combining aircraft observations with statistical physics models. First, this dissertation evaluates the chemistry process in the upper troposphere and quantifies biases that are specific to chemistry. Then, chemical reactions important to tropospheric ozone are evaluated for potential to cause the nitrogen oxides low-bias, and each reaction rate's uncertainty is constrained using Bayesian techniques. This study identifies a revision to a critical reaction that removes nitrogen oxides and radicals that drive ozone production (NO2 + HO· → HNO3). Finally, the downward revision of the reaction rate is implemented and evaluated in a full global CTM. Evaluation in the global CTM improves the partitioning of nitrogen precursors to ozone, while increasing model sensitivity to emissions of nitrogen oxides. The three phases of this dissertation identify chemistry process bias, constrain uncertain reactions, and demonstrate the importance of findings. Improving simulated chemistry in the upper troposphere contributes to the scientific understanding of processes that produce ozone. Improving the simulated processes helps to decrease the uncertainty in simulated future scenarios and the emission reduction tests that form the scientific basis for policy development.