Which method is often used to measure NOx in ambient air quality standards?

The method commonly used to measure nitrogen oxides (NOx) in ambient air is chemiluminescence. This technique is particularly effective because it takes advantage of the specific reaction between nitrogen dioxide (NO2) and ozone (O3), which produces light as a byproduct. The intensity of this emitted light is directly correlated to the concentration of NOx in the air being sampled.

Chemiluminescence measurement offers high sensitivity and specificity for detecting NOx, making it ideal for monitoring compliance with ambient air quality standards. It is widely used in regulatory compliance monitoring due to its ability to provide real-time data and its effective response to low concentrations of NOx.

Other methods, such as chemical absorption, infrared spectroscopy, and gas chromatography, are less favored for measuring NOx in ambient air. Chemical absorption can be complex and less direct for measuring gaseous pollutants like NOx. Infrared spectroscopy is better suited for gases that strongly absorb infrared light, but NOx primarily does not have strong absorption in the infrared spectrum. Gas chromatography typically requires a sample to be collected and processed, which could delay results and is not as efficient for continuous monitoring of ambient air quality compared to chemiluminescence.