Eddy covariance fluxes and vertical concentration gradient measurements of NO and NO2 over a ponderosa pine ecosystem: observational evidence for within-canopy chemical removal of NOx

摘要

Exchange of NO (NO+NO) 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. As aresult, a complete description of the processes affecting NOfollowing 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 NO fluxes andvertical concentration gradients made during the Biosphere Effects onAeRosols and Photochemistry EXperiment 2009. In general, during daytime weobserve upward fluxes of NO and NO with counter-gradient fluxes of NO.We find that NO 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 tohigher nitrogen oxides within the forest canopy, which might be part of amechanistic explanation for the "canopy reduction factor" applied to soilNO emissions in large-scale models.