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

Abstract

Dry deposition fluxes ( F L) of NO 3 − and SO 4 2− 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 ( C L) of NO x (NO + NO 2), T-NO 3 (gaseous HNO 3 + particulate NO 3 −) and SO x (gaseous SO 2 + particulate SO 4 2−) around the leaves in a suburban area of Japan, using a modified leaf-washing technique. F L of NO 3 − and SO 4 2− decreased as follows: pine >> cedar > cypress ≥ oak and pine >> cedar > oak ≥ cypress, respectively. F L of NO 3 − for all tree species fluctuated synchronously with C L of T-NO 3. F L of SO 4 2− fluctuated with C L of SO x , but the dominant pollutant deposited (SO 2 or SO 4 2−) appeared to differ for different tree species. Dry deposition conductance ( K L) of T-NO 3 and SO x was derived as an F L/ C L ratio. Seasonal variations of K L likely reflect the gas/particle ratios of T-NO 3 and SO x , which were affected by meteorological conditions such as temperature. Dry deposition velocities ( V d) of T-NO 3 and SO x were obtained as the mathematical product of annual mean K L and the total leaf surface areas in the forests. The comparison of V d among tree species indicated that the loads of acidic air pollutants were higher to coniferous forests than broad-leaved forest because of the higher K L and/or larger leaf surface areas.