Kvælstofoxider (NO,NO_x) måles med kemiluminescens. Luften suges sammen med ozon ind i et kammer. Her omdannes NO til NO₂. Ved omdannelsen udsendes lys med en bestemt bølgelængde, som registreres i en lysdetektor. Ved skiftevis at suge luften direkte ind i kammeret og gennem en celle, der reducerer NO₂ til NO, bestemmes både kvælstofmonoxid (NO) og kvælstofoxider (NO_x=NO+NO₂). NO₂ beregnes som differensen mellem de to målinger. 

• 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. 

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