NOx = NO + NO2 ½-hour measurements and 24-hour measurements
The results are used for smog warning.
Measurement method (½ hour measurements)
Nitrogen oxides (NO, NOx) are measured by chemiluminescence. The air is sucked together with ozone into a chamber. Here NO is converted to NO2. In the transformation, light of a certain wavelength is emitted which is recorded in a light detector. By alternately sucking the air into the chamber and through a cell that reduces NO2 to NO, both nitrogen monoxide (NO) and nitrogen oxides (NOx = NO + NO2) are determined. NO2 is calculated as the difference between the two measurements.
Technical description regarding ½ hour measurements
Analyzing method: Chemiluminiscence
- Principle: Chemiluminiscent reaction between NO and O3; NO2 is measured after reduction to NO (conversion efficiency better than 96%) and measurement of NO. The NO2 is calculated as the difference between NO with and without conversion. The monitor measures sequentially in one chamber with a swiching time of about 4 seconds, in practise simultaneously.
- Measuring range: On background and roof stations: 0-500 ppb. Otherwise: 0-2000 ppb at 0.6 l/min.
- Lower detection limit: <1 ppb
- Precision ±5% (due to changes of humidity in the air)
- Selectivity: 85% of a PAN and <0.5 % of a NH3 concentration is measured as NOx.
- Routine checks: Automatic zero point check every day around midnight with "zero- air" from a generator. Automatic span check once a week with around 800 ppb NO in N2 from a pressure bottle.
- Calibration at the laboratory: 1) Diluted reference gas (from Messer Griesheim) of 50 ppm of NO in N2. 2) Diluted NO2 gas from a permeation tube. The NO2 to NO converter is checked with gas phase titration.
24 hour nitrogen dioxide (NO2) measurements
Method: KI impregnated filters.
The method is no longer used. It will in some cases be replaced by passive collection on impregnated filters.