Measurements of nitrogen oxides and ozone fluxes by eddy covariance at a meadow: evidence for an internal leaf resistance to NO₂

Abstract. Nitrogen dioxide (NO₂) plays an important role in atmospheric pollution, in particular for tropospheric ozone production. However, the removal processes involved in NO₂ deposition to terrestrial ecosystems are still the subject of ongoing discussion. This study reports NO₂ flux measurements made over a meadow using the eddy covariance method. The measured NO₂ deposition fluxes during daytime were about a factor of two lower than a priori calculated fluxes using the Surfatm model without taking into account an internal (also called mesophyllic or sub-stomatal) resistance. Neither an underestimation of the measured NO₂ deposition flux due to chemical divergence or an in-canopy NO₂ source nor an underestimation of the resistances used to model the NO₂ deposition explained the large difference between measured and modelled NO₂ fluxes. Thus, only the existence of the internal resistance could account for this large discrepancy between model and measurements. The median internal resistance was estimated to be 300 s m⁻¹ during daytime, but exhibited a large variability (100–800 s m⁻¹). In comparison, the stomatal resistance was only around 100 s m⁻¹ during daytime. Hence, the internal resistance accounted for 50–90% of the total leaf resistance to NO₂. This study presents the first clear evidence and quantification of the internal resistance using the eddy covariance method; i.e. plant functioning was not affected by changes of microclimatological (turbulent) conditions that typically occur when using enclosure methods.