Eddy covariance fluxes and vertical concentration gradient measurements of NO and NOsub2/sub over a ponderosa pine ecosystem: observational evidence for within-canopy chemical removal of NOsubx/sub

摘要

Exchange of NO_x (NO+NO₂) between the atmosphere andbiosphere is important for air quality, climate change, and ecosystemnutrient dynamics. There are few direct ecosystem-scale measurements of thedirection and rate of atmosphere–biosphere exchange of NO_x. As aresult, a complete description of the processes affecting NO_xfollowing emission from soils and/or plants as they transit from within theplant/forest canopy to the free atmosphere remains poorly constrained anddebated. Here, we describe measurements of NO and NO₂ fluxes andvertical concentration gradients made during the Biosphere Effects onAeRosols and Photochemistry EXperiment 2009. In general, during daytime weobserve upward fluxes of NO and NO₂ with counter-gradient fluxes of NO.We find that NO_x fluxes from the forest canopy are smaller thancalculated using observed flux–gradient relationships for conserved tracersand also smaller than measured soil NO emissions. We interpret thesedifferences as primarily due to chemistry converting NO_x tohigher nitrogen oxides within the forest canopy, which might be part of amechanistic explanation for the "canopy reduction factor" applied to soilNO_x emissions in large-scale models.