Although tropospheric NO, NO , and O rapidly interconvert in a fast photochemical cycle, 23 the processes governing their removal rates, interactions with ecosystems, and human interest in their mitigation are distinctly different. Thus a detailed understanding of their behavior at the atmosphere-biosphere interface is crucial. The reactivity of this family of gases has often confounded measurements in the past, and the question of biospheric emission and uptake of NO remains open(Lerdauetal.,2000).

In order to address these issues, we present concurrent eddy covariance fluxes of NO, NO , 2 and O at the rural, deciduous, mixed hardwood Harvard Forest in central Massachusetts 3 during the summer and fall of 2000. The independent measurements were conducted above the forest canopy, at similar heights, on two towers within several hundred meters of one another. NO was measured using an existing photolysis-chemiluminescence detector in the eddy covariance mode, and NO using a new tunable diode laser absorption spectrometer

(TDLAS). O concentrations, profiles, and fluxes were also measured at the site. At night, 3 NO fluxes were effectively zero while small but persistent downward NO fluxes were 2 observed. Fluxes of NO during the day were generally upward and coupled with downward 2 NO fluxes of similar magnitude. The opposing NO and NO fluxes confirm the predictions 2 of some canopy exchange models that include the effect of the light gradient within the canopy on photo-reactive species (e.g. Gau et al. 1993).