Effects of filter pack sampling conditions on observed ambient concentrations of dry acid deposition species

One technique for determining dry acid deposition fluxes involves measurement of time averaged ambient concentrations of dry acid deposition species using filter packs (FP) coupled with estimates of mean deposition velocities for the exposure period. A critical problem associated with filter pack data comparisons from various field sampling networks is the use of substantially different sampling flow rate and duration protocols. Field experiments were conducted to evaluate the effects of varying sampling flow rates, from 1.5 to 10 standard liters per minute, on ambient concentration measurements of specific dry acid deposition species. Collocated FP samplers were used to determine sampling and analysis data reproducibility and representativeness. Ambient air samples were simultaneously collected using groups of filter packs operated at various flow rates over identical 7 day periods. The species measured were sulfur dioxide, particulate sulfate, nitric acid and particulate nitrate. Statistical data comparisons indicated that variations in flow rates over a five-fold range for the conditions employed did not significantly affect the ambient air concentration estimates of the species of interest. At the same sampling conditions, but different flow rates, collocated samples were highly reproducible and there was no significant sampling bias for the species measured. A comparison of sulfur dioxide (SO2) determinations using two different treated filter configurations did not appear to significantly affect overall ambient air concentration estimates of SO2 or those for the other species measured. It was observed, however, that there was significant retention of ambient SO2 on the Nylon filters used in this field study. Although the fractional retention of SO2 on Nylon filters decreased with increasing sampling flow rate, overall estimates of ambient SO2 concentrations were consistent.