Deposition of H15NO3 vapour to white oak, red maple and loblolly pine foliage: experimental observations and a generalized model

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Abstract
Nitric acid vapour enriched with 15 N (H15 NO3 ) 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 H15 NO3 vapour deposition to foliar surfaces ranged from -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 H15 NO3 vapour deposited to leaves, an average of 39 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 H15 NO3 uptake rates across the leaf cuticle increased with surface nitrate concentrations, but were 1 to 2 orders of magnitude lower (O06 to 0.24 nmol m-2 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.