In two previous University of Washington field programs, airborne measurements of SO2 using carbonate-impregnated filters and a Teco pulsed-fluorescence analyzer showed excellent agreement over a range of ambient concentrations from 2 to 127 ppbv. As part of the Gas-Phase Sulfur Intercomparison Experiment (GASIE), ambient air, diluted fivefold to tenfold with zero air, was sampled in the concentration range of 0.02 to 4 ppbv. With the bulk of the measurements in the range of 40 to 230 parts per trillion by volume (pptv), agreement between the two techniques was again very good Teco=1.07(filter)+4.5 pptv, r=0.93>. Using careful precleaning, impregnation, storage, and handling techniques for the filter substrates, at sub-100 pptv concentrations, the filter method is capable of an accuracy of better than ¿10% with a ¿7 pptv uncertainty (due to blank variability) for 6 m3 samples. In addition, the Teco model 43S is capable of rather precise measurements of sub-100 pptv concentrations (approximately ¿16 pptv) provided a suitable averaging time is employed (at least 10 min).¿ 1997 American Geophysical Union