Why is a collocation period used to estimate bias in PM measurements?

• A. It measures only the highest recorded concentration.
• B. It compares measurements against a reference to quantify systematic differences, enabling bias correction.
• C. It is used to calibrate the weather station.
• D. It determines the color of the sampling enclosure.

Answer: (B)

The main idea is to quantify systematic differences by comparing two instruments side by side during a collocation period. When two PM monitors sample the same air at the same time, any consistent discrepancy between them reveals bias—the predictable amount by which one instrument tends to read higher or lower than the reference. By averaging the differences (or fitting an appropriate relationship) across the collocation period, you obtain a bias estimate that can be used to correct past measurements and improve future accuracy. This is not about capturing the single highest concentration, nor about calibrating a weather station, nor about choosing the color of a sampling enclosure.