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 NO2 Converter, 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 by the U.S. Environmental Protection Agency as a Federal Equivalent Method (FEM) for NO2 compliance monitoring (EQNA-0217-243) and is QAL1 certified by TÜV Rheinland as conforming to European Standards EN-15267 and -14211 (certificate 0000062070). SD card data logging is now a standard feature.

Theory of Operation

The following figure is a simplified schematic diagram of the Model 405 nm NOx Monitor. Sample air is continuously drawn through the instrument by an air pump at a flow rate of ~1.5 L/min. The Scrubber Valve alternately bypasses and sends the sample air through a NO2 scrubber to remove all NO2 in the sample, thus allowing the measurement of a light intensity in the absence (Io) of NO2 and presence (I) of NO2. The Beer-Lambert Law is then used to calculate the concentration of NO2 from I and Io.

Nitric oxide is measured by bypassing the NO2 Scrubber and measuring the light intensity while adding (I) or not adding (Io) ozone to convert NO to NO2 according to the reaction:.

NO + O3 → NO2 + O2

As in our ozone monitors, a Nafion tube is used to equilibrate humidity during I and Io measurements, so that any water vapor interference due to refractive effects is eliminated. NOx is obtained by adding the measurements of NO2 and NO. The instrument may be operated in continuous NO2, NO or NOx mode, or in a mode where NO2 and NO are alternately measured, once every 20 seconds.

We recently published a paper in Atmospheric Measurement Techniques describing our patented folded tubular photometer and its application in the Model 405 nm NO2/NO/NOx Monitor.

Applications

The Model 405 nm NO2/NO/NOx Monitor is ideal for applications that require precise and accurate measurement of NO and NO2, such as air quality monitoring both outdoors and indoors. Its portability and relatively low power consumption make it particularly well suited for field deployments. An example application, in which the Model 405 nm was deployed on a tall tower, is described below.

Download Application Study: Investigating Wintertime Haze in Beijing

Specifications:

 1An adaptive filter may be selected from the menu, making signal averaging similar to competing NOx monitors. Specifications above are for default parameters: Change Difference = 40 ppb, Change Percent = 10%, Short Filter = 4 pts (20 s), Long Filter = 36 pts (3 min). Adaptive filter parameters may be adjusted by the user.

Industrial Technical Services from Mississauga, Ontario is a single source Instrumentation and Automation company. We provide certified calibrations, PLC programming, custom controls and Panels.