What is the purpose of collocated monitoring in ambient air programs?

• A. To compare performance only during startup.
• B. To calibrate the instrument automatically.
• C. To compare performance and quantify drift, and validate calibration.
• D. To replace the main monitors.

Answer: (C)

Collocated monitoring is used to check and improve data quality by running two or more instruments that measure the same pollutant at the same location in parallel. This setup lets you directly compare their readings, so you can see if any instrument is drifting from the others or from a reference standard over time.

The main purpose is to quantify drift and validate calibration. By analyzing how the collocated instruments diverge (or agree) over time, you can determine if there is bias, confirm that calibration procedures are working, and decide whether recalibration or instrument adjustment is needed. This approach also demonstrates the precision and reliability of the measurements being reported.

It’s not intended to replace the main monitors. The collocated setup is a QA/QC tool to verify and maintain data quality, ensuring the data from the primary monitor are accurate and defensible.