Biosphere-atmosphere exchange of reactive nitrogen oxides between the atmosphere and a ponderosa pine forest

Anthropogenic activities including production of nitrogen fertilizers, planting of nitrogen fixing organisms and combustion processes affect the global biogeochemical cycle of nitrogen (N), and have caused increased emission of NO and N2O to the atmosphere and increased deposition of biologically available N to terrestrial ecosystems. As a result, there are numerous changes in ecosystem function, water quality, air pollution and global climate change. However, the mechanisms that control the exchange of reactive nitrogen oxides between the biosphere and atmosphere remain poorly understood due to the lack of adequate measurement techniques. In order to improve our understanding of biosphere-atmosphere exchange of nitrogen oxides, I describe the development and application of thermal dissociation - laser induced fluorescence (TD-LIF) coupled with the eddy covariance technique to measure fluxes of NO2, total peroxy and peroxy acyl nitrates (SigmaPNs), total alkyl and multifunctional alkyl nitrates (SigmaANs), and total gaseous and semi-volatile HNO 3. A thorough evaluation of the sensitivity and accuracy of the technique is presented.; Measurements of fluxes and mixing ratios of these species were made over a ponderosa pine plantation in the mid-elevation Sierra Nevada at the University of California-Blodgett Forest Research Station through a full annual cycle. Winter observations exhibited the expected patterns of downward (deposition) fluxes of SigmaANs, SigmaPNs and HNO3, and upward (emission) fluxes of NO2. These results are the first direct evidence that organic nitrates are a significant (∼60%) component of total NOy deposition. In the summer, HNO3, SigmaPNs and NO 2 fluxes were upward, and only SigmaANs fluxes were downward, contrary to expectations. I present a detailed analysis of the hypothesis that chemical reactions between ozone (O3) and unsaturated volatile organic compounds (VOC) result in unusually high OH within the forest canopy that interacts with the nitrogen oxides and causes the observed fluxes. Both the measured HNO3 and SigmaPN fluxes are consistent with an in-canopy OH of 3 x 107 molecules cm-3. The OH acts to oxidize NO2 and aldehydes, resulting in the upward fluxes of these two species. The downward SigmaANs flux is explained by a combination of deposition, chemical production and chemical loss to OH. Ecosystem emissions of NO x are due only in part to soil emissions, and imply significant HONO or NOx production in the forest canopy, both of which are poorly understood.; The mechanisms identified in this research show that the forest canopy atmosphere is a highly reactive, oxidizing environment in which HOx and NOx cycles are closely coupled. As a result of the larger than expected oxidative rates, secondary organic aerosol formation is likely more rapid within the forest canopy than current models predict, regional tropospheric chemistry is faster than current expectations and reactive nitrogen is likely retained by the forest in greater amounts than models suggest. This research has provided significant new insights into the processes at work in the forest; however, a more complete understanding of this forest canopy, including the chemical and ecological processes at work and their consequences, will require new field and laboratory experiments aimed at more thoroughly investigating the mechanisms proposed in this thesis.