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.

Project Report

This project aimed at three interrelated questions: 1) How do organic molecules emitted from forests (for example the molecules that give forests their characteristic smells) interact with nitrogen oxides? Do those interactions affect the flow of nitrogen oxides into and out of the forest canopy? 2) Do chemical reactions of forest organics and nitrogen oxides produce aerosol? How much? Under which circumstances? 3) Do these reactions affect ozone formation locally at the forest or regionally when the molecules are transported on the winds. We learned a variety of new things about the chemistry of the atmosphere while pursuing answers to these questions. Notable among them. First, we find that chemical reactions within the forest are extremely important to understanding the flow of molecules from the forest floor to the free atmosphere above the forest. The role of chemical reactions within the forest has been ignored or at least the scope of the influence of chemistry was not fully appreciated. Second, we made new observations both in a cold, non-forest environment (as a control experiment) and in a hot forested environment. The new observations lay a foundation for challenging our understanding of the chemistry. Third, we presented a comprehensive review of the atmospheric observations of organic nitrate molecules, molecules whose chemistry we have studied with NSF support for over a decade and which are now recognized to be much more important than when we began. Fourth, we have used several different approaches to study the organic nitrates produced during the oxidation of isoprene. Isoprene is one of the most important natural emissions to the atmosphere and resolving confusion about the products of isoprene chemistry remains one of the outstanding challenges in our field. Finally, we studied aerosol formation and growth in our laboratory learning about the basic chemical mechanisms that contribute to shrinking of organic aerosol and to growth of water droplets. Broader impacts of the research effort include: a) the education of graduate students, teaching them both what it means to do cutting edge research and also the practice of state-of-the-art experimental methods, b) interactions with K-12 schools in our community. The research director for this project and his graduate students have visited many local classrooms to talk about the research and high school students have come to visit our laboratories, and c) a deeper understanding of this chemistry improves our ability to make predictions about how the atmosphere will respond to a changing climate and/or changing emissions of the precursors to ozone and aerosol. These improved predictions are useful to formulating policy to eliminate human exposure to unhealthy air and to reducing economic damage to agriculture, cattle raising and other industries from exposure to ozone and aerosol.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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Sylvia A. Edgerton
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University of California Berkeley
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