Experimentally derived estimates of nitric acid dry deposition velocity and viscous sub-layer resistance at a conifer forest

Daytime dry deposition of nitric acid to a conifer forest with a LAI of 5.3 was measured using the relaxed eddy accumulation technique. The observations indicate a mean friction velocity of 0.45 ms(-1) and a mean dry deposition velocity (nu(d)) of 7.5 cm s(-1), with approximately equal aerodynamic (r(a)) and non-aerodynamic resistances (r(b) + r(c)). Mean r(a) is 10 s m(-1), while mean r(b) + r(c) (derived from the difference between the observed nu(d) and a value derived based solely on r(a)) is 13.2 s m(-1). Assuming the surface resistance (r(c)) is zero, the viscous sub-layer resistance (r(b)) from a number of models capture the mean observationally derived value to within +/-40%, but the models underestimate the variability inherent in the measurements. This discrepancy between the modeled and observed sample-to-sample variability of rb does not appear to be accounted for by leaf wetness, stomatal opening or an additional dependence on friction velocity. (C) 2004 Elsevier Ltd. All rights reserved.