What is the primary purpose of a collocated sampler in ambient particulate matter monitoring?

• A. To assess precision and bias by comparing concurrent measurements at the same site and support data quality objectives.
• B. To calibrate the meteorological sensor at the site.
• C. To replace the reference monitor with the collocated unit.
• D. To measure only chemical composition of PM.

Answer: (A)

Collocated sampling is a QA/QC practice in ambient PM monitoring. Placing two samplers at the same site during the same time period allows you to see how closely they agree with each other. This comparison gives you two key performance measures: precision, meaning how repeatable the measurements are from one instrument to another, and bias, meaning any systematic difference where one unit tends to read higher or lower than the other or than the reference standard. By evaluating these aspects, you can determine whether the measurement system meets your data quality objectives and decide if calibration, maintenance, or method adjustments are needed. It’s not about calibrating meteorological sensors, it doesn’t replace the reference monitor, and while collocated data can inform about mass concentration and composition, the primary purpose is to verify reliability and accuracy of the PM measurements through concurrent, side-by-side comparisons.