On the concentration differences between PM_2.5 FEM monitors and FRM samplers

BAM-1020 and TEOM-FDMS have undergone rigorous testing and analysis protocols to become the Federal Equivalent Method (FEM) monitors and serve as reliable near real-time monitors for compliance with the National Ambient Air Quality Standards and references for low-cost PM_2.5 sensor calibration. However, differences between the FEM and FRM (Federal Reference Method) data still exist, which cause inconsistency in PM_2.5 measurements. This study carried out the field tests across five geographically diverse stations in different seasons in Taiwan with 265 daily samples collected by the collocated BAM-1020 and TEOM-FDMS and the FRM sampler and found that the biases between the FEM and FRM values increased with the decreasing PM_2.5 concentrations and varied with ambient conditions. The measurement uncertainties exist in the BAM-1020 were mainly due to the aerosol water content, while the TEOM-FDMS always over-measured PM_2.5 compared to the FRM sampler since it corrects for the evaporation loss of semi-volatile particle materials. To reduce the biases between the FEM monitors and FRM samplers, empirical equations based on PM_2.5 concentrations (μg m⁻³), temperature (^oC), and relative humidity (%) were derived to convert the FEM data to the FRM data. After correction, the mean normalized biases were decreased from +1.67 ± 12.43% to +0.63 ± 8.75% for the BAM-1020 and from +13.86 ± 14.50% to −0.85 ± 9.0% for the TEOM-FDMS. Also, the same empirical equation was used to convert the FRM PM_2.5 values to the “true” or “actual” PM_2.5 values represented by the TEOM-FDMS with the bias reduced from −10.76 ± 11.42% to +1.33 ± 8.44% after conversion.