This Small Business Innovative Research Phase I proposal seeks to develop an ultrasensitive, laser-based sulfur dioxide gas sensor system for airborne and ground-based atmospheric monitoring. The proposed instrument will be capable of accurately determining sub-parts-per-billion SO2 concentrations in seconds. This compact, instrument will be capable of long-term autonomous operation and have modest power requirements. The Phase I research will demonstrate the feasibility of the technology by performing measurements on SO2 samples using a bench-scale laboratory instrument that employs a novel, frequency agile UV laser source. The results of these tests will be used to quantify detection limits for a Phase II instrument. Commercial systems based on the Phase II prototype will be developed and marketed during Phase III.