The Model 405nm NOx Monitor (“nm” for “nanometer” and for “NOx Monitor”) is designed for the direct measurement of atmospheric nitrogen dioxide (NO2), nitric oxide (NO) and total reactive oxides of nitrogen (NOx = NO + NO2) in the concentration range 0-10,000 ppb (0-10 ppm) for NO2 and 0-2,000 ppb (0-2 ppm) for NO with high precision and accuracy. In this instrument NO2 is measured directly by absorbance at 405 nm.