Sulfur dioxide (SO2) is a major air pollutant that can contribute to the production of particulate sulfate and increase the acidity in the environment. SO2 is detected by cavity ring-down spectroscopy (CRDS) utilizing the SO2 absorption in the 308 nm region. A ferrous sulfate scrubber and a sodium carbonate annular denuder are used to reduce background interferences and to obtain quantitative values of SO2. The method is characterized using SO2 standards in the laboratory and compared to a commercial pulsed fluorescence analyzer (PFA). A limit of detection of 3.5 ppb/10 s (S/N = 2) is demonstrated. Ambient measurements are attempted to demonstrate this technique.