The continuous sulfur dioxide detector (CSD) is a sensitive instrument for reliable measurements at high time resolution in the atmosphere. This new instrument is based on a SO2 measurement technique utilizing the sulfur chemiluminescence detector, previously validated in a rigorously blind experiment sponsored by the National Science Foundation. Simplified sample handling, denuder separation technology, and the intrinsic sensitivity and fast response of the detector permit measurement at levels below 100 parts per trillion by volume in tens of seconds with the CSD. The CSD provides a differential measurement where response from ambient SO2 is determined by the difference between air containing SO2 and air scrubbed of SO2, where both air samples contain other detectable sulfur species. Digital signal post processing with phase-locked amplification of the detector signal enhances the precision and temporal resolution of the CSD. Oversampling of the detector signal at 10 Hz permits subsequent data retrieval to be adapted to changing ambient levels by either enhancing signal to noise when sulfur dioxide levels are low or by maximizing temporal resolution of derived data when levels are high. The instrument has advantages over existing instruments based on chromatographic separation in that the CSD provides accurate and reliable measurements at low parts per trillion by volume sulfur dioxide with high time resolution. The CSD is compact and automated and does not require cryogenic materials, making this instrument suitable for remote field locations. The high temporal resolution, specificity for SO2, and sensitivity of the CSD make it a good candidate for installation on an aircraft. Airborne studies of SO2 with a sensitive, fast time response instrument may offer new insight into the understanding of phenomena such as gas-to-particle conversion, long-range transport of pollutants, and the oxidation of biogenically produced sulfur gases. ¿ 2001 American Geophysical Union