Differences in 24-h average PM_2.5 concentrations between the beta attenuation monitor (BAM) and the dichotomous sampler (Dichot)

In this study, PM_2.5 concentrations were measured simultaneously by a Dichotomous sampler (Dichot, Model SA241, Andersen Inc., Georgia, USA) (PM_2.5,D) and a beta attenuation monitor (BAM) (PM_2.5,B) at each of three air monitoring stations in Taiwan. Results show that PM_2.5,B concentrations measured by the VEREWA-F701 BAM (without federal equivalent method (FEM) designation) are consistently higher than PM_2.5,D concentrations by 58.4±37.4% at Jhongshan station, while less overestimation exists at Sinjhuang and Judong stations, which is 29.8±20.2 and 28.4±19.0%, respectively, where the earlier version of the Met-One BAM-1020s (without FEM designation) are used. Different factors influencing the overestimation, which include aerosol water content, volatilization of inorganic species and positive artifacts due to acid gas adsorption by the glass fiber filter tapes were studied for the BAM-1020. Results show that the overestimation is mainly caused by the positive artifacts due to acid gas adsorption by the glass fiber filter tapes used in the BAM-1020. Aerosol water content and volatilization loss of inorganic semi-volatile species are found to be less important to the overestimation.