Evaluation of Bias in the Measurement of Condensable Particulate Matter with Method 202

ABSTRACT

In the present study, positive and negative biases when measuring condensable particulate matter (CPM) was measured according to US EPA Method 202. Four factors were considered to measure and improve CPM overestimation: SO₂ absorption, the volume of condensate, oxygen content, and residence time. Negative biases were evaluated using weight loss caused by static electricity, vapor pressure of the particles, rinsing volume, beaker size, and filter paper holder. Finally, a forced ventilation chamber was designed to reduce the drying time of the organic sample. The results revealed that artifacts produced by SO₂ could not be eliminated. The homemade condensate-divert impinger did not reduce the amount of artifacts because the oxidation of SO₂ already occurred when it came into contact with water while passing through the Graham condenser. The residence time, volume of condensate, and oxygen content can all increase CPM overestimation. Therefore, field sampling should be performed as quickly as possible. Furthermore, electrostatic neutralization of the beaker should be performed before weighing it. Because of the high volatility of the material with high vapor pressure, nitrogen purging may reduce the particle mass with high vapor pressure. Furthermore, when the CPM sample was transferred from the beaker to the tin, a smaller beaker size led to lower residual mass. A 4% particle loss was measured according to the leakage of particles between the filter paper and the filter paper holder provided by the original manufacturer. A gasket is therefore recommended to minimize leakage. Finally, it took only 1.5–2.5 h for the organic sample to dry when it was placed in the forced ventilation chamber, and a sample recovery rate of >98.5% was achieved.  Although the artifact produced by SO₂ is unavoidable, estimation of PM_2.5 can be more precise by considering our proposed improvements.