How do you calculate PM concentration from a filter weight difference and known sample volume?

• A. Concentration = (flow rate × sampling time) / filter mass gain.
• B. Concentration = (filter mass gain) × (sample volume).
• C. Concentration (µg/m³) = (filter mass gain in µg) / (sample volume in m³).
• D. Concentration = sample volume / filter mass gain.

Answer: (C)

PM concentration is expressed as the mass of particulates per unit volume of air. The filter weight difference gives the mass of particles collected, in micrograms, while the volume of air sampled is the amount that passed through the filter, in cubic meters (often calculated from flow rate times sampling duration). To get concentration, you divide the mass by the volume: PM concentration (µg/m³) = (filter mass gain in µg) / (sample volume in m³).

For example, if the filter gained 40 µg and the sampled air volume is 0.08 m³, the concentration is 40 ÷ 0.08 = 500 µg/m³.

The other approaches mix the quantities incorrectly (multiplying by volume or inverting the ratio), which doesn’t yield a concentration in units of µg/m³.