Comparison of surrogate surface techniques for estimation of sulfate dry deposition

The dry deposition rates of sulfate particles to artificial surfaces within and above a mature hardwood forest were measured over an annual range of synoptic weather conditions. Artificial, or ‘surrogate’, surfaces representing both rough and smooth textural types included deposition buckets, petri dishes, filter paper, Teflon configurations and polycarbonate membranes. Ambient concentrations of sulfate and sulfur dioxide were also monitored.The artificial surfaces were evaluated on the basis of the magnitude of the sulfate dry deposition rates and measurement precision. Correlations between techniques and the magnitude of the deposition velocities identified technique similarities. Ambient concentrations of the sulfur oxides and the deposition rates were not well correlated. For diverse reasons, many of the techniques were found to have limited reliability. The petri dish, bucket inside and filter plate surfaces were found to represent the most precise devices for the estimation of dry deposition to smooth, complex and rough artificial surfaces, respectively. Seasonal averages for samplers exposed at all heights were 11.2, 27.7 and 71.2μg SO₄²⁻m⁻²h⁻¹, yielding mean deposition velocities to surfaces exposed within the forest canopy of 0.03,0.11 and 0.14 cm s⁻¹ and an annual estimate of the potential dry deposition to a foliated hardwood forest of 4.0, 11.5 and 21.0 kg SO₄²⁻ha⁻¹ for the petri dish, bucket inside and filter plate surfaces, respectively. The indirect ratio between deposition rates and velocities results from varying concentrations of ambient sulfate between sampling periods. The accuracy of the filter plate data is suspect due to a significant correlation with sulfur dioxide concentrations. Sulfur concentration and deposition rate gradients indicate the forest is providing a net sink for sulfur pollutants during periods with foliage.The wide range of dry deposition rates estimated from the variety of deposition surfaces emphasizes the uncertainty of the artificial surface measurement techniques. In spite of these limitations, surrogate surfaces provide an estimate of sulfate flux rates not currently obtainable from natural surfaces.