What is the difference between NO2 and NO measurement in air monitoring?

• A. NO is typically measured directly by chemiluminescence; NO2 can be measured directly by certain methods or inferred from NOx after conversion; NOx is often determined after converting NO2 to NO.
• B. NO is never measured.
• C. NO2 is measured only with DOAS, NO is never measured.
• D. NOx is measured only by gas chromatography.

Answer: (A)

Measuring NO and NO2 in air hinges on how their signals are produced and interpreted. NO is typically measured directly by chemiluminescence: it reacts with ozone to form excited NO2 and light, and the light output is proportional to the NO concentration in the sampled air. NO2, on the other hand, can be detected directly with certain methods or, more commonly in routine monitoring, inferred from NOx by converting NO2 to NO and measuring the total NOx as NO. In practice, many instruments determine NOx by passing the sample through a converter that turns NO2 into NO, so the chemiluminescence reading represents NO plus the converted NO2. This approach lets a single chemiluminescence detector quantify total NOx, while other methods (like DOAS) can measure NO2 directly. The other options are not accurate because NO is indeed measured directly in many setups, NO2 has direct measurement methods beyond DOAS, and NOx is not restricted to gas chromatography.