Take the example of a complex mixture of particles collecting on our filter media. When the particulates collect on the filter, their mass may be influenced by interaction with airborne gases (acids, water vapor) or other particles in the sample air stream, or possibly by the filter media itself. The thermodynamic conditions of the sample air stream and the area around the sample filter influence the degree to which these ongoing reactions may occur. All these processes collectively comprise what we define as filter dynamics, and they may result in a positive or negative sampling artifact component of the PM mass concentration. The higher the time resolution of the PM measurement system, the more easily the PM mass concentration change resulting from filter dynamics can be observed.
Regulatory methods employed for the determination of PM generate data that are defined to be an “indicator” of airborne PM. Regulatory standards are then based upon this indicator measurement. Note that the PM indicator measurement is to some degree determined by the definition, and it is not based purely upon scientific considerations. Almost all regulatory methods are “filter based” methods, centered around the measurement of PM collected on a sample filter.
The dynamics explained above describe how airborne particles can change because of their interaction with immediate surroundings, including the filter. Over a long time period, the net result of collecting these particles and measuring them is in a mass gain. This is true even though there may have been shorter periods when particles were losing mass due to negative artifacts. Only the faster-responding continuous methods of PM measurements are capable of seeing the short-term positive and/or negative mass changes that combine to make up the longer-term 24-hour measurement. A couple of examples help to highlight these points.
Example 1
A plume from a source such as a burn barrel, a wood-burning appliance or even a diesel vehicle engulfs a monitoring site. Should this happen, the short-term mass concentration will rise dramatically, followed by a sharp decrease after the plume ceases. The sharp decrease may be associated with a period (1 to 2 hours or more) of continuing net loss on the sample filter while the collected particulate matter continues to equilibrate to the thermodynamic conditions at the sample filter.
Example 2
Small water droplets from fog or mist are collected on the sample filter but evaporate over time. If the evaporative mass is greater than the collected mass, then net negative mass concentration is observed.