The Model 405 nm provides a direct measurement of NO2 by absorbance of light at 405 nm with a precision of 2 ppb or better. The high precision is achieved by using a White cell with a path length of ~2 m. Other methods such as chemiluminescence require conversion of NO2 to NO using a molybdenum or photolytic converter to measure NOx and then determining NO2 as the difference between NOx and NO. Such methods suffer from inaccuracy due to variations in conversion efficiency and interferences from other compounds that may convert to NO. The Model 405 nm also measures NO by modulating the addition of ozone with ~100% conversion of NO to NO2. NOx is then determined by simply adding NO and NO2.