Distinguish between a gravimetric PM sample and a real-time PM monitor.

• A. A gravimetric PM sample uses the same approach as a real-time monitor by providing continuous estimates in the field.
• B. A gravimetric PM sample uses a filter collected over a defined period and weighed in a lab to determine mass concentration, while a real-time monitor provides continuous, near-instantaneous estimates of PM using optical or other sensing methods (and may require field calibration).
• C. A gravimetric PM sample measures mass by beta attenuation.
• D. A real-time monitor measures mass by filtering and weighing in a lab.

Answer: (B)

The key idea is that gravimetric sampling and real-time monitoring measure PM in two fundamentally different ways: one is an integrated, post-collection mass measurement, the other is a continuous, in-the-field estimate.

With gravimetric sampling, PM is drawn onto a pre-weighed filter at a known flow rate for a defined period. After the sampling period, the filter is brought back to the laboratory and weighed with a precise balance. The difference in weight, divided by the volume of air that passed through the filter, gives the mass concentration for that sampling interval. This method provides an integrated value over the entire collection period and relies on careful handling and conditioning of the filter and accurate metrological procedures. It does not deliver real-time data.

Real-time PM monitors, on the other hand, continuously sample air and estimate PM concentration in near real time using sensing principles such as light scattering or other optical or electrical methods. These readings are displayed as ongoing concentration values and can be influenced by particle properties and environmental conditions, often requiring field calibration to relate the instrument signal to true mass concentration. They do not depend on weighing a collected sample in a lab.

So, the best description is that a gravimetric sample uses a filter collected over a defined period and weighed in a lab to determine mass concentration, while a real-time monitor provides continuous, near-instantaneous estimates of PM using optical or other sensing methods (and may require field calibration).