![]() Post-exposure H15NO3 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-2 s-1) than total HNO3 deposition during exposures. This implies that rain events might extract greater amounts of HNO3-derived nitrate in throughfall from conifer canopies as compared to hardwood canopies. Of the total H15NO3 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. 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. Total H15NO3 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. Nitric acid vapour enriched with 15N (H15NO3) 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).
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