What is the purpose of collocated sampling in ambient air QA/QC?

• A. To calibrate a single instrument against a standard.
• B. To evaluate precision and identify biases by running two instruments at the same site and comparing results.
• C. To monitor weather conditions at the site.
• D. To assess the overall air quality index.

Answer: (B)

Collocated sampling uses two monitors placed at the same site and operated at the same time to compare their readings. This directly shows how consistent instruments are with each other (precision) and reveals any systematic differences between them (bias). By analyzing the paired data over a period, QA/QC personnel can detect drift, calibration issues, or instrument-specific biases and ensure data from different monitors are reliable and comparable.

Calibrating a single instrument against a standard is a separate process focused on accuracy of that instrument alone. Monitoring weather conditions or calculating an air quality index doesn’t address how two instruments agree with each other, which is the essence of collocation.