Design and performance of a field exposure system for evaluation of the ecological effects of SO2 on native grassland

We describe the design and performance of two replicate field exposure systems for evaluating the ecological effects of chronic SO2 exposure on 0.5 ha plots of native Montana grassland. The SO2 was supplied at a constant rate to each plot through a network of 2.5-cm inside diameter aluminum pipes set parallel to and approximately 0.7 m above the ground with 0.8-mm SO2 release holes at 3-m intervals. The resulting SO2 concentrations on the plots varied with micrometeorological conditions. Fumigations were essentially continuous from April through October for 5 yr on one set of four plots and for 4 yr on another set of four plots. Frequency distributions of SO2 concentrations were approximately log-normal and qualitatively similar to those from various area and point sources. The diurnal pattern of SO2 concentrations was strongly correlated with the reciprocal of windspeed. Nighttime concentrations averaged almost double daytime concentrations. The diurnal concentration pattern resembled an area source more closely than a point source, a direct result of using constant SO2 source strength. Plume strikes could be simulated by turning the system on only during pre-specified meteorological conditions. Similarly, seasonal variations in area source concentrations could be simulated by varying source strength on the plots. The data from 5 years use of the system indicate that the fine-scale behavior of SO2 concentrations would be realistic.