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

Conspectus The atmosphere-biosphere exchange ofnitrogen oxides playsa key role in determining the composition of reactive nitrogen interrestrial vegetated environments. The emission of nitric oxide (NO)from soils is an important atmospheric source of reactive nitrogen.NO is rapidly interconverted with NO2, making up the chemicalfamily NO x (NO x & EQUIV; NO2 + NO). NO x furtherreacts with the oxidation products of volatile organic compounds (VOCs)to form the functionalized nitrogen oxide groups acyl peroxynitrates(APNs = R(O)O2NO2) and alkyl nitrates (ANs =RONO2). Both canopy-level field measurements and laboratorystudies suggest that the absorption of nitrogen dioxide NO2 and APNs by vegetation is a significant sink of atmospheric NO x , removing a large fraction of global soil-emittedNO( x ) and providing key control on theamounts and lifetimes of NO x and reactivenitrogen in the atmosphere. Nitrogen oxides influence the productionof surface O-3 and secondary aerosols. The balance of theemission and uptake of nitrogen oxides thus provides a mechanism forthe regulation of regional air quality. The biosphere, via this biogeochemicalcycling of nitrogen oxides, is becoming an increasingly importantdetermining factor for airborne pollutants as much of the world continuesto reduce the amount of combustion-related nitrogen oxide emissions.Understanding the function of the biosphere as a source and sink ofreactive nitrogen is therefore ever more critical in evaluating theeffects of future and current emissions of nitrogen oxides on humanand ecosystem health. Laboratory measurements of the foliardeposition of NO2 and other reactive nitrogen species suggestthat there is a substantialdiversity of uptake rates under varying environmental conditions andfor different species of vegetation that is not currently reflectedin the widely utilized chemical transport models. Our branch chambermeasurements on a wide variety of North American tree species highlightthe variability in the rates of both photosynthesis and nitrogen oxidedeposition among several different nitrogen oxide compounds. Box-modelingand satellite measurement approaches demonstrate how disparities betweenour understanding of nitrogen oxide foliar exchange in the laboratoryand what is represented in models can lead to misrepresentations ofthe net ecosystem exchange of nitrogen. This has important implicationsfor assumptions of in-canopy chemistry, soil emissions of NO, canopyreductions of NO x , lifetimes of tracegases, and the impact of the biosphere on air quality.