Dry deposition of acidic air pollutants to tree leaves, determined by a modified leaf-washing technique

Dry deposition fluxes (FL) of NO3− and SO42− to leaf surfaces were measured for Japanese red pine (Pinus densiflora), Japanese cedar (Cryptomeria japonica), Japanese cypress (Chamaecyparis obtusa), and Japanese white oak (Quercus myrsinaefolia), together with atmospheric concentrations (CL) of NOx (NO + NO2), T-NO3 (gaseous HNO3 + particulate NO3−) and SOx (gaseous SO2 + particulate SO42−) around the leaves in a suburban area of Japan, using a modified leaf-washing technique. FL of NO3− and SO42− decreased as follows: pine >> cedar > cypress ≥ oak and pine >> cedar > oak ≥ cypress, respectively. FL of NO3− for all tree species fluctuated synchronously with CL of T-NO3. FL of SO42− fluctuated with CL of SOx, but the dominant pollutant deposited (SO2 or SO42−) appeared to differ for different tree species. Dry deposition conductance (KL) of T-NO3 and SOx was derived as an FL/CL ratio. Seasonal variations of KL likely reflect the gas/particle ratios of T-NO3 and SOx, which were affected by meteorological conditions such as temperature. Dry deposition velocities (Vd) of T-NO3 and SOx were obtained as the mathematical product of annual mean KL and the total leaf surface areas in the forests. The comparison of Vd among tree species indicated that the loads of acidic air pollutants were higher to coniferous forests than broad-leaved forest because of the higher KL and/or larger leaf surface areas.