Flux-gradient relationships of nitrogen oxides over a ponderosa pine plantation during BEARPEX-2009

During the Biosphere Effects on AeRosols and Photochemistry EXperiment(BEARPEX, June 18 to July 30, 2009), gradients and eddy covariance fluxes of NO2, total peroxy nitrates (∑PNs), total alkyl nitrates (∑ANs), and nitric acid (HNO3) were simultaneously monitored from above a ponderosa pine plantation in the Sierra Nevada Mountains of California using thermal dissociation - laser induced fluorescence (TD-LIF). NO gradient and flux measurements were also made using a home-built O3 chemiluminescence instrument. We report upward fluxes of NO and NO2, downward fluxes of ∑PNs and HNO3, and a bidirectional ∑ANs flux during the daytime. Fluxes of NO2 and HNO3 are qualitatively explained by a simple flux-gradient relationship (the flux and gradient have the same sign); however canopy influenced photochemistry must be considered in a more quantitative description of the fluxes. Moreover, the fluxes and gradients of ∑ANs, ∑PNs, and NO are only consistent when considered in the context of their chemistry.