A LABORATORY INVESTIGATION OF THE ROUTES OF HNO3 DRY DEPOSITION TO CONIFEROUS SEEDLINGS

White pine, Norway spruce and red spruce seedlings were exposed to nitric acid vapor concentrations of 10 to 120 ppb in constant stirred tank reactors. Nitric acid dry deposition rates were determined from both the change in nitric acid concentration in the reactor flow stream and from the amount of nitrogen recovered from the seedlings. Nitric acid labeled with N-15 was used to distinguish dry-deposited nitrogen in the plant from the nitrogen that was already present. It was found that dry deposition occurs via three routes: surface deposition, trans-cuticular deposition, and stomatal deposition. Resistance to surface deposition is very low (< 4.8 m2-s mol-1) for a freshly washed surface, but increases as the surface adsorption sites are occupied. Resistance to trans-cuticular uptake averaged 206 m2-s mol-1. Stomatal resistance can be calculated from the rate of water diffusion out of the plant. Eighty per cent of the nitric acid deposited via the transcuticular and stomatal routes was assimilated by the plant. However, none of the nitric acid deposited on the surface was assimilated. In rural areas with coniferous forests, the combination of low ambient nitric acid concentrations and low initial surface resistance means that most nitric acid will be dry deposited on the tree surface, and thus will not be directly assimilated.