Measurements of Atmosphere-Biosphere Exchange of Oxidized Nitrogen and Implications for the Chemistry of Atmospheric NO x .

ConspectusThe atmosphere-biosphere exchange of nitrogen oxides plays a key role in determining the composition of reactive nitrogen in terrestrial vegetated environments. The emission of nitric oxide (NO) from soils is an important atmospheric source of reactive nitrogen. NO is rapidly interconverted with NO 2 , making up the chemical family NO x (NO x ≡ NO 2 + NO). NO x further reacts with the oxidation products of volatile organic compounds (VOCs) to form the functionalized nitrogen oxide groups acyl peroxynitrates (APNs = R(O)O 2 NO 2 ) and alkyl nitrates (ANs = RONO 2 ). Both canopy-level field measurements and laboratory studies suggest that the absorption of nitrogen dioxide NO 2 and APNs by vegetation is a significant sink of atmospheric NO x , removing a large fraction of global soil-emitted NO x and providing key control on the amounts and lifetimes of NO x and reactive nitrogen in the atmosphere. Nitrogen oxides influence the production of surface O 3 and secondary aerosols. The balance of the emission and uptake of nitrogen oxides thus provides a mechanism for the regulation of regional air quality. The biosphere, via this biogeochemical cycling of nitrogen oxides, is becoming an increasingly important determining factor for airborne pollutants as much of the world continues to reduce the amount of combustion-related nitrogen oxide emissions. Understanding the function of the biosphere as a source and sink of reactive nitrogen is therefore ever more critical in evaluating the effects of future and current emissions of nitrogen oxides on human and ecosystem health.Laboratory measurements of the foliar deposition of NO 2 and other reactive nitrogen species suggest that there is a substantial diversity of uptake rates under varying environmental conditions and for different species of vegetation that is not currently reflected in the widely utilized chemical transport models. Our branch chamber measurements on a wide variety of North American tree species highlight the variability in the rates of both photosynthesis and nitrogen oxide deposition among several different nitrogen oxide compounds. Box-modeling and satellite measurement approaches demonstrate how disparities between our understanding of nitrogen oxide foliar exchange in the laboratory and what is represented in models can lead to misrepresentations of the net ecosystem exchange of nitrogen. This has important implications for assumptions of in-canopy chemistry, soil emissions of NO, canopy reductions of NO x , lifetimes of trace gases, and the impact of the biosphere on air quality.