Abstract
Abstract. Exchange of NOx (NO+NO2) between the atmosphere and biosphere is important for air quality, climate change, and ecosystem nutrient dynamics. There are few direct ecosystem-scale measurements of the direction and rate of atmosphere–biosphere exchange of NOx. As a result, a complete description of the processes affecting NOx following emission from soils and/or plants as they transit from within the plant/forest canopy to the free atmosphere remains poorly constrained and debated. Here, we describe measurements of NO and NO2 fluxes and vertical concentration gradients made during the Biosphere Effects on AeRosols and Photochemistry EXperiment 2009. In general, during daytime we observe upward fluxes of NO and NO2 with counter-gradient fluxes of NO. We find that NOx fluxes from the forest canopy are smaller than calculated using observed flux–gradient relationships for conserved tracers and also smaller than measured soil NO emissions. We interpret these differences as primarily due to chemistry converting NOx to higher nitrogen oxides within the forest canopy, which might be part of a mechanistic explanation for the "canopy reduction factor" applied to soil NOx emissions in large-scale models.
Highlights
The chemistry of nitrogen oxides is a major factor affecting the oxidative capacity of the atmosphere and the global burden of tropospheric ozone (Crutzen, 1973)
Reactive nitrogen oxides are a nutrient (Sparks, 2009; Takahashi et al, 2004, 2005a; Teklemariam and Sparks, 2004; Lockwood et al, 2008) and interactions between available nitrogen in ecosystems and atmospheric nitrogen are many and complex, with exchange processes altering the patterns of nitrogen availability in the biosphere (Townsend et al, 1996; Vitousek and Farrington, 1997; Vitousek et al, 1997; Holland and Lamarque, 1997; Holland et al, 1997; Ollinger et al, 2002a, b; Hietz et al, 2011)
The Biosphere Effects on AeRosols and Photochemistry EXperiment (BEARPEX) included a component designed to provide comprehensive measurements of vertical concentration gradients and fluxes of a wide suite of nitrogen oxides – NO, NO2, total and speciated peroxynitrates, total and speciated alkyl ( RONO2) and multifunctional nitrates, HNO3, and nitrous acid (HONO) – and presented a direct opportunity to test our ideas about canopy-scale NOx exchange
Summary
The chemistry of nitrogen oxides is a major factor affecting the oxidative capacity of the atmosphere and the global burden of tropospheric ozone (Crutzen, 1973). Calculations of ozone by large-scale chemical transport models parameterized with measured NO soil fluxes overpredict O3 concentrations in comparison to aircraft and tower observations (e.g., Lerdau et al, 2000) To match observations, these models invoke a canopy reduction factor of 25–80 % (Jacob and Wofsy, 1990; Yienger and Levy, 1995; Wang and Leuning 1998). The Biosphere Effects on AeRosols and Photochemistry EXperiment (BEARPEX) included a component designed to provide comprehensive measurements of vertical concentration gradients and fluxes of a wide suite of nitrogen oxides – NO, NO2, total and speciated peroxynitrates, total and speciated alkyl ( RONO2) and multifunctional nitrates, HNO3, and nitrous acid (HONO) – and presented a direct opportunity to test our ideas about canopy-scale NOx exchange. We describe relationships between gradients and fluxes, present and interpret evidence for chemical canopy reduction processes, and explore the significance of chemistry within the canopy to the import/export of NOx from the canopy
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