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An acidic precipitation delivery system is described that was designed and constructed for use in a field investigation of the response of red spruce saplings (Picea rubens Sarg.) to the interactive stresses of ozone and acid rain. The system utilizes hydraulic, solid-cone spray nozzles to produce simulated rainfall with droplet size distributions approximating natural rain events, which are of low intensity, i.e., about 1-1.5 cm hr(-1), and are relatively uniform in distribution of volume over a 2.4 m diameter plot. Three different pH treatments (3.1, 4.1, 5.1) were dispensed randomly to each of three treatment subplots located in twelve open-top field chambers and three ambient control chambers. Storage capacity of the system permitted a 2.3 hr rain event. Construction materials used were chosen for resistance to the corrosive nature of the rain simulant, stability to ambient UV radiation, and resistance to penetration by sunlight. Simulated events were not synchronized to ambient events, but were scheduled to prevent moisture deficits.

SUMMARYA method is presented for the rapid and quantitative assessment of the transfer of calcium (Ca^+) and hydrogen (H*) ions between the surface of a leaf and simulated acidic rain. A small volume (144 /il) of pH 3-5 simulated ram solution was confined within a rigid. Teflon capillary on the foliar surface. A liquid membrane-type, neutral carrier-based, ion-selective microelectrode either for Ca^^ or for H^ was placed withm the capillary with the tip of the microelectrode at least 100//m from the foliar surface. This allowed spatial and temporal aspects of ion gradients generated by ion diffusion from the foliar surface to be studied. Results indicate that, throughout a 60-minute period after contact of simulated rain with the adaxial foliar surface of spinach (Spinacea oleracea L. cv. Marathon) or Japanese pachysandra {Pachysandra terminalis Siebold & Zucc)., negligible amounts of foliarderived calcium diffused into the simulated rain solution, and there was virtually no neutralization of the pH 3-5 rain simulant present in the capillary. A discussion of these results with respect to previous methods of investigation and theoretical possibilities of ion diffusion and cuticular permeability is presented.

Quantitative estimates of the variation in response to air pollutants in natural populations of trees must be considered in predicting the magnitude of an effect or the risk of an effect's occurrence. This subject was investigated in 2-year-old black spruce (Piceamariana (Mill.) B.S.P.) exposed for 78 continuous hours to gaseous hydrogen fluoride at concentrations of 2.3, 4.2, or 8.1 μg% F•m−3 and with 3-year-old white spruce (Piceaglauca (Moench) Voss) exposed for 50 continuous hours to hydrogen fluoride at 2.6, 5.2, or 11.1 μg F•m−3 in open-top, field exposure chambers. Tolerances to hydrogen fluoride induced foliar injury followed a lognormal distribution for black spruce and white spruce with, respectively, estimated median tolerances of 3.45 and 13.24 μg F•m−3 and standard deviations corresponding to 1.70- and 2.14-fold changes in concentration of hydrogen fluoride. The curves for the occurrence of moderate and severe degrees of injury in black spruce paralleled that for the occurrence of any hydrogen fluoride induced injury but were displaced towards greater exposures (1.29-fold for moderate and 1.77-fold for severe injury). The distribution of foliar injury on shoots indicated that a portion of the variation in tolerance might be attributed to phenologic differences among the trees. A knowledge of the sources of variation and their magnitudes is necessary to recognize both incidence and severity of pollutant-induced injury in dose-response relationships.