Nitrogen Oxide Chemistry: Connecting Ambient Concentrations to Mechanisms of Emission, Oxidaton and Aerosol Formation

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

This project supports an integrated interpretation of flux and gradient measurements from the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) during 2007 and 2009. The overall goal is to improve mechanistic understanding of nitrogen exchange between the atmosphere and forests. Nitrogen oxides control the catalytic production of tropospheric O3 directly via NOx (NO + NO2) cycling and indirectly via termination reactions of the HOx (OH + HO2) cycle and associated production of higher oxides of nitrogen. The atmosphere-biosphere exchange of NOy (sum of oxidized N species) also affects NOy budgets on regional and global scales, ecosystem productivity, and carbon cycling. A reliable predictive capability for the physicochemical evolution of the troposphere and related feedbacks involving ecosystems requires that major pathways in the cycling of nitrogen oxides be understood and reliably implemented in models. Building on past results, this project will investigate factors regulating the forests NO2 compensation point, the atmospheric mixing ratio below which NO2 is emitted from the forest and above which NO2 is deposited to the forest. The BEARPEX 2007 and 2009 experiments were designed, in part, to provide quantitative constraints on the nature of these NO2 exchange processes. Vertical gradients in NO and NO2 mixing ratios through the forest canopy were measured in parallel with their corresponding fluxes into and out of the canopy. Fluxes of NO from forest soils were also quantified. These results provide independent evidence for a compensation point but actual values inferred from each vary. In addition, NO and NO2 fluxes measured using similar techniques during a 2004 experiment at the same location yielded a factor of 3 greater compensation point, which suggests that the nature of atmosphere-forest exchange may vary as a function of long-term temporal changes in ambient NOx mixing ratios. In addition to NOx, exchange fluxes of higher oxides of nitrogen during BEARPEX 2009 diverged substantially from those measured in 2004. This project will synthesize these various data and associated process-level constraints. Results will be interpreted based on theoretical approaches including modified Browning analysis and detailed model calculations. The overall goal is to develop a coherent mechanistic understanding of factors regulating nitrogen exchange between the atmosphere and forests based on the available observational constraints. The project will contribute to the training graduate students, including two women, and undergraduate students. It will also foster K-12 science education through collaboration with the Chabot Space and Science Center, a local museum with a strong presence in the Oakland school district. Finally, results will advance understanding of the scientific basis for policy decisions regarding air quality and climate. Findings will be communicated in the scientific literature, to a more general audience through various outreach media, and directly to regulators in California at both the Bay Area Air Quality Management District and the California Air Resources Board.