Model 405 nm NO2/NO/NOx Monitor™
Direct Measurement of NO2 by Absorbance
FEM-Approved for NO2 Compliance Monitoring
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). SD card data logging is now a standard feature.
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). 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. The paper may be downloaded here, and the patent may be downloaded here.
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. The paper may be downloaded here, and the patent may be downloaded here.
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.
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Specifications
| Certifications | US EPA Federal Equivalent Method (FEM) for NO2 (0-500 ppb, 20-30°C) CE |
| Principle of Measurement | Direct absorbance of NO2 at 405 nm |
| Measurement Modes | NO2 only; NO only; NO2, NO and NOx |
| Linear Dynamic Range | 0-10,000 ppb (0-10 ppm) for NO2; 0-2,000 ppb (0-2 ppm) for NO (FEM approved for 0-500 ppb NO2 for 20-30°C) |
| Resolution | 0.1 ppb |
| Precision (1σ rms noise) | < 0.5 ppb or 0.5% of reading (with adaptive filter1) |
| Accuracy | Greater of 2 ppb or 2% of reading |
| Limit of Detection (2σ) | < 1 ppb (with adaptive filter1) |
| Flow Rate (nominal) | 1.5 Liter/min |
| Flow Rate Requirement | Minimum: 1.4 Liter/min; Maximum: 1.6 Liter/min |
| Response Time, 100% of Step Change | 10 s for 5-s averaging 20 s with adaptive filter1 |
| Measurement Frequency | 0.2 Hz (once every 5 s) |
| Averaging Times | 5 s, 1 min, 5 min, 1 hr |
| SD Card Logger Capacity | Minimum 2 GB (> 2-year capacity for 5-s measurement mode) |
| Internal Data Logger Capacity | 8,192 data lines (5-s avg = 0.47 days; 1-min avg = 5.7 days; 5-min avg = 1.0 mo; 1-hr avg = 0.94 yr) |
| Concentration Units | ppb, ppm, pphm |
| Pressure Units | mbar |
| Temperature Units | °C |
| T and P Corrected | Yes |
| Operating Temperature Range | 10 to 50°C (FEM approved for 20-30°C for NO2) |
| Power Requirement; Supplied by 5-amp 110/220 VAC Power Pack (provided) or Battery |
11-14 V dc or 120/240 V ac, 1.4 A at 12 V, 17 watt Max: 2.9 A at 12 V, 35 watt (warmup) |
| Size | Rackmount: 17" w x 14.5" d x 5.5" h (43 x 37 x 14 cm) |
| Weight | 18.6 lb (8.4 kg) |
| Data Outputs | LCD; RS232; SD card; 0-2.5 V Analog Outputs for NO and for NO2 |
| Data Transfer Baud Rates | 2400, 4800, 19200 |
| Output Ranges | User-Defined Scaling Factor in Menu |
| DewLine™ | Yes |
| Long-Life Pump | Yes: 15,000 hr, 1.7 yr of continuous operation |
| Flow Meter | Yes |
| Options | Bluetooth for wireless data transmission; USB output (in place of RS232); touchscreen |
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.
Comparison: See a comparison of the specifications of 2B Tech's Model 405 nm with other manufacturers' nitrogen dioxide monitors.