Over the past decade, the importance of understanding the sources and sinks of carbon dioxide and other greenhouse gases has been recognized. In particular, airborne measurements of CO2 profiles throughout the troposphere and lower stratosphere have provided a great deal of useful information, but the instrumentation used has been restricted to airplane or large stratospheric-type balloon gondola platforms due to the size, weight and power requirements of these instruments. While a more widespread measurement campaign using smaller, less expensive balloon sondes could provide very important data, such an approach has been limited by the lack of suitable instrumentation. In this SBIR program, Southwest Sciences proposes to developed a lightweight, inexpensive greenhouse gas sensor suitable for balloon sonde measurements, yet exhibiting specifications that approach those of the much larger and expensive research instruments used on current airborne platforms. Using a novel ratiometric measurement technique, this sensor will provide dry air mixing ratios of CO2 without the need for concurrent measurements of temperature, pressure or moisture.