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 m s−1and a mean dry deposition velocity (vd) of 7.5 cm s−1, with approximately equal aerodynamic (ra) and non-aerodynamic resistances (rb+rc). Mean ra is 10 s m−1, while mean rb+rc (derived from the difference between the observed vd and a value derived based solely on ra) is 13.2 s m−1. Assuming the surface resistance (rc) is zero, the viscous sub-layer resistance (rb) 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.