The Changing Role of Reactive Nitrogen in the Troposphere

Abstract

Nitrogen is the most abundant molecule in the Earth’s atmosphere and is one of the essential ingredients for life as we know it. Human activities, especially over the last century, have radically perturbed the natural nitrogen cycle, primarily via emissions of reduced nitrogen from the production and use of fertilizer for agriculture and of oxidized nitrogen from the combustion of fossil fuels. Nitrogen oxides play a central role in driving tropospheric chemistry and are a key ingredient of fine particulate matter, acid rain, and ozone. However, despite this long-understood importance of reactive oxidized nitrogen (NOy) species, even modern chemical transport models have struggled to accurately represent their chemistry.

This thesis spans three projects that seek to characterize and explain possible sources of this uncertainty. The first project presents a comprehensive budget of reactive oxidized nitrogen in the troposphere using a state-of-the-science chemical transport model, and observational constraints for this budget from remote troposphere flight campaign data. We also provide modeled estimates for the chemical fluxes between key NOy species, finding that species beyond those that have been the foci of previous work play a crucial role in driving overall chemical cycling. In the second project we explore the sensitivity of this NOy budget to uncertain multiphase chemistry, including the photolysis of nitrate aerosol, the reactive uptake of nitrogen dioxide on aerosol surfaces, and the uptake of nitric acid on dust. We find that these processes may have substantial regional or temporal importance, but they have limited effects on the global NOy budget and are insufficient to explain inter-model discrepancies. Finally, we investigate the utility of long-term wet and dry deposition measurements made in the continental United States as a constraint on regional anthropogenic emissions trends of acid rain precursors (nitrogen and sulfur oxides). We find that dry deposition fluxes follow anthropogenic emissions trends, and wet deposition fluxes are likely more representative of total regional emissions (natural and anthropogenic). Taken together, these studies provide novel, holistic constraints on reactive oxidized nitrogen and identify key chemical processes that govern the fate of NOy in the troposphere. As anthropogenic emissions continue to decline and the effects of climate change intensify, these insights and such a framework will be useful in accurately predicting future atmospheric chemistry and composition.