DEPOSITION OF H15NO3 VAPOR TO WHITE OAK, RED MAPLE AND LOBLOLLY-PINE FOLIAGE - EXPERIMENTAL-OBSERVATIONS AND A GENERALIZED-MODEL

Nitric acid vapour enriched with N-15 ((HNO3))-N-15 was volatilized into the cuvette of an open-flow gas exchange system containing red maple (Acer rubrum L.), white oak (Quercus alba L.), or loblolly pine (Pinus taeda L.) seedling shoots to facilitate direct measurements of total foliar deposition, and subsequent assessments of the rate of HNO3, movement across the cuticle (transcuticular uptake). Total (HNO3)-N-15 vapour deposition to foliar surfaces ranged from < 5 to 27 nmol m-2 s-1, the variability being largely accounted for by differences in HNO3 concentrations and leaf conductance. Mean whole-leaf conductance to HNO3 ranged between 0.9 and 3.4 mm s-1 for hardwoods and between 6 and 34 mm s-1 for loblolly pine. Of the total (HNO3)-N-15 vapour deposited to leaves, an average of 3 9 to 48% was immediately 'bound' into hardwood foliage whereas only 3% was bound to loblolly pine needles. This implies that rain events might extract greater amounts of HNO3-derived nitrate in throughfall from conifer canopies as compared to hardwood canopies. Post-exposure (HNO3)-N-15 uptake rates across the leaf cuticle increased with surface nitrate concentrations, but were 1 to 2 orders of magnitude lower (0.06 to 0.24 nmol m-1 s-1) than total HNO3 deposition during exposures. A generalized leaf-level model of HNO3 deposition to foliage capable of simulating deposition pathways to sorption sites on the leaf surface, and to the metabolically active leaf interior via transcuticular or stomatal pathways is formulated and suggested for use in planning future work on HNO3 deposition.