Eddy covariance fluxes and vertical concentration gradient measurements of NO and NO 2 over a ponderosa pine ecosystem: observational evidence for within-canopy chemical removal of NO x
Exchange of NO x (NO+NO 2) 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 NO x. As a result, a complete description of the processes affecting NO x 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 NO 2 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 NO 2 with counter-gradient fluxes of NO. We find that NO x 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 NO x to higher nitrogen oxides within the forest canopy, which might be part of a mechanistic explanation for the "canopy reduction factor" applied to soil NO x emissions in large-scale models.