The Model 405 nm 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. NO is measured by selective conversion with ~100% efficiency using the highly selective reaction of NO with ozone (O3). Total NOx is obtained by addition of NO and NO2.

Unlike chemiluminescence instruments where NO2 must be converted to NO using either a molybdenum or photolytic converter with highly variable efficiency, in the Model 405 NOx Monitor nitrogen dioxide is measured directly by absorbance, analogous to an ozone monitor. Because NO2 has a much lower absorption cross section than ozone, a folded cell with corner mirrors is used to produce a long absorbance path of ~2 m to achieve approximately the same sensitivity for NO2 as we do for ozone in our UV-based-absorbance monitors. The wavelength of 405 nm was chosen because no other species found in ambient air has significant absorbance at that wavelength, making the Model 405 nm extremely selective for NO2.

Although our Model 410 Nitric Oxide Monitor can measure NO and NOx when used in combination with our Model 401 NO2Converter, the Model 405 nm is the preferred method for NO2 because it is a direct method and requires less power, while providing an accurate measurement of NO as well.

The Model 405 nm NO2/NO/NOx Monitor is designated as a Federal Equivalent Method (FEM) for NO2 compliance monitoring (EQNA-0217-243).  SD card data logging is now a standard feature.