Recent improvements to the carbonate-impregnated filter technique for measuring low-level SO2 concentrations have resulted in dramatically improved performance. The improvements are (1) a better cleaning procedure for the paper filter substrates, resulting in approximately 60% reduction of their sulfate blank, (2) the use of an ion-exchange resin to remove the carbonate matrix from the sample extract, resulting in a 100% increase in the signal-to-noise ratio, (3) the use of high-purity glycerol in the filter impregnate, resulting in approximately 10% further reduction of blanks, and (4) improved chromatographic and standardization procedures for more accurate quantification of sample peaks. Combined, these improvements allow measurements to be made of SO2 concentrations in marine background air with a 2&sgr; uncertainity of ¿6 parts per trillion by volume (pptv) and, based on this a 3&sgr; detection limit of 9 pptv for air volumes of 4 m3 (which can be collected in 15 min aboard our research aircraft). Measurements in polluted air show better than 95% collection efficiency, even at concentrations as high as 100 ppbv. Vertical profiles of SO2 measured during three research flights off the Washington coast (one in clean marine air) showed concentrations ranging from 15 to 86 pptv in the mixed layer and from 40 to 93 pptv in the free troposphere. ¿ American Geophysical Union 1991