Which monitoring method is commonly used for NOx and O3, and what do they measure?

• A. Beta-attenuation measures NOx; TEOM measures O3.
• B. Chemiluminescence measures NOx; UV photometry measures O3.
• C. Infrared spectroscopy measures NOx; UV photometry measures PM2.5.
• D. UV photometry measures NOx; chemiluminescence measures PM.

Answer: (B)

The question tests how NOx and O3 are typically measured and what each method detects. For NOx, chemiluminescence detectors are used because NO reacts with ozone to form an excited NO2 that emits light as it returns to the ground state. The intensity of that light is proportional to the NO concentration, and with a converter you can measure total NOx (NO plus NO2). For O3, UV photometry is used because ozone strongly absorbs ultraviolet light around 254 nm; the amount of UV light that gets through the sample decreases in proportion to the ozone concentration, giving a direct readout of ozone levels.

That’s why this pairing is standard: chemiluminescence for NOx and UV photometry for O3. The other options mix methods that are typically used for particulates (like PM) or use techniques not standard for NOx or O3 in ambient air monitoring.