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 ana lysed to estimate nitric oxide emission rates and median non-stomatal controlled canopy resistance R(fs) = R(c)(night). Temperature coeffici ents for NO emission rates were estimated from literature values and a ssumed to be 0.30 (June 1992), 0.25 (August/September 1992), 0.00 (Jan uary/February 1993) and 0.20 ng N m(-2) s(-1) K-1 (May 1993) for surfa ce temperatures above 5 degrees C. Median R(fs) was found to be in the order of 700 s m(-1) during the vegetation period (the range of media n values at different surface temperatures was 500-950 s m(-1) with no visible trend in temperature). R(fs) is supposed to describe uptake o f NO2 by soil and foliage (except stomata). During daytime, stomatal r esistance R(st) controls uptake of NO2. Two models were fitted to the measurements of R(st):R(st)(H2O) = 100 + 30,000/K-in resulted for the Turner and Begg model used by Baldocchi et al. (1987, Atmospheric Envi ronment 21, 91-101) and R(st)(H2O) = 100 x [1 + (200/K-in)(2)] resulte d for the Wesely (1989, Atmospheric Environment 23, 1293-1304) model. K-in is the short-wave incoming radiation (W m(-2)), and R(st)(H2O) = R(st)(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 N O2 fluxes occurred during the first morning hours, clearly before noon . Median minimum stomatal resistances R(st)(min) of 120s m(-1) (for NO 2) were found for relatively moist conditions and up to 256 s m(-1) fo r relatively dry soil. The stomatal uptake of NO2 was found to be at m aximum 6.8 times more efficient than uptake by soil and foliage when s oil water availability was good. With relatively dry soil, this ratio dropped to not more than 3.6.