This Small Business Innovation Research Phase I project will develop a method for detecting nitrogen dioxide (NO2), one of the most important components of nitrogen oxide species (NOy) in the troposphere. Direct measurements of NO2 are crucial for understanding the chemical processes that determine natural levels of tropospheric ozone and for assessing potential effects of human-influenced emissions. The difficulties associated with direct measurements of NO2 in the unpolluted troposphere arise from low ((100 pptv) concentrations and the potential for interferences from other more abundant nitrogen compounds. This proposal would develop a direct spectroscopic method using infrared absorption with a tunable diode laser light source which would directly detect absolute levels of NO2 in the clean troposphere. A key component of the proposed research plan is the development of a long path length, low volume absorption cell which would have sufficient length ( 600 M) to provide adequate sensitivity and yet have a minimal volume ((10 liters) to enable rapid sampling and a minimal instrument response time. Modulation of the trace species at the cell inlet would allow real-time background subtraction to further increase the sensitivity. Automated signal processing using curve fitting to known spectral line parameters would provide absolute concentration levels without using calibration gasses. The resulting instrument would have an NO2 detection limit of 4 ppt with a signal averaging time of 100 s.
