A technique is described for low parts-per-trillion (ppt) detection of SO 2 with high temporal resolution compared to current filter sampling methods. A sulfur chemiluminescence detector, equipped with a quartz burner chamber and a sample probe with a critical orifice, was used as a single detector to analyze SO 2 in ambient air by alternately cycling the sample airstream through a SO 2 denuder. The method provides one SO 2 concentration reading every 10 min. The limits of detection and quantitation were found to be 20 and 70 pptv, respectively. Results from field measurements at a remote site on the south rim of the Grand Canyon during November 1991 are presented, and they indicate a highly variable distribution of SO 2 from the limit of detection to near 2 ppbv