Open-path Differential Optical Absorption Spectroscopy (DOAS) is used for measuring concentrations of gaseous criteria pollutants from ppm down to ppt levels. DOAS transmits UV light through the atmosphere; the absorption signatures of various gases in the path of the light beam can then be analyzed from the collected light.

DOAS is a versatile technique. A single instrument can simultaneously measure many pollutants such as O3, NO2, SO2, CH2O, and BTX. Due to its many advantages, open-path DOAS finds widespread use in air quality monitoring.

A prototype DOAS instrument has been constructed to measure ambient NO2, SO2, and O3. The results correlate well with those of a commercial DOAS instrument. A new design is currently being assembled, which will have environmental proofing and added functionality.