Physical Sciences Inc. proposes to develop a compact, rugged, rapid-response, autonomous sensor for in-situ monitoring of ambient O3 from UAVs. Our innovation is to combine newly available UV light emitting diodes (LEDs) with miniaturized, low power, high sensitivity signal detection electronics to create a next generation, UAV-class, photometer for O3. The advent of UV LEDs enables the development of a very compact and highly sensitive monitor for ambient O3. An LED-based sensor has many advantages over currently available technologies and is highly suitable for deployment in UAVs. The Phase I program will demonstrate the feasibility of a breadboard sensor and create a detailed conceptual plan for a fieldable prototype. The TRL at the end of Phase I will be level 4. The Phase II program will fabricate a prototype that can be field demonstrated on an aircraft. The TRL at the end of Phase II will be level 6. Successful completion of Phases I and II will result in a rigorously validated prototype sensor that can monitor ambient O3 with high speed and precision. The sensor architecture can be easily modified to measure other species. Using new mid-IR LEDs, the photometer can monitor trace gases such as CO2 and CO.