Transfer resistances of NO2 determined from eddy correlation flux measurements over a litter meadow at a rural site on the swiss plateau

Nocturnal transfer resistances of nitrogen dioxide were separately analysed to estimate nitric oxide emission rates and median non-stomatal controlled canopy resistance Rfs = Rc(night). Temperature coefficients for NO emission rates were estimated from literature values and assumed to be 0.30 (June 1992), 0.25 (August/September 1992), 0.00 (January/February 1993) and 0.20 ng N m−2 s−1 K−1 (May 1993) for surface temperatures above 5°C. Median Rfs was found to be in the order of 700 s m−1 during the vegetation period (the range of median values at different surface temperatures was 500–950 s m−1 with no visible trend in temperature). Rst is supposed to describe uptake of NO2 by soil and foliage (except stomata). During daytime, stomatal resistance Rfs controls uptake of NO2 Two models were fitted to the measurements of Rst: Rst (H2O = 100 + 30,000/Kin resulted for the Turner and Begg model used by Baldocchi et al. (1987, Atmospheric Environment, 21, 91–101) and Rst (H20) = 100 x [1 + (200/Kin2] resulted for the Wesely (1989, Atmospheric Environment, 23, 1293–1304) model. Kin is the short-wave incoming radiation (W m−2), and Rst(H2O) = Rst (NO2)/1.6 (scaled with the ratio of the diffusion coefficients). The hyperbolic fit of Baldocchi et al. fitted the data slightly better at low incoming radiation and is therefore preferred, since maximum NO2 fluxes occurred during the first morning hours, clearly before noon. Median minimum stomatal resistances Rst(min) of 120 s m−1 (for NO2 were found for relatively moist conditions and up to 256 s m−1 for relatively dry soil. The stomatal uptake of NO2 was found to be at maximum 6.8 times more efficient than uptake by soil and foliage when soil water availability was good. With relatively dry soil, this ratio dropped to not more than 3.6.