Evaluation of black carbon mass concentrations using a miniaturized aethalometer: Intercomparison with a continuous soot monitoring system (COSMOS) and a single-particle soot photometer (SP2)

Ground-based observations of black carbon (BC) aerosols were conducted in spring 2017 in Yokosuka City, south of Tokyo, Japan. Equivalent BC (EBC) mass concentrations were measured using a miniaturized, palm-sized filter-photometer (MicroAeth AE51, EBCAE51) and evaluated against those obtained using a continuous soot monitoring system (COSMOS, EBCCOSMOS) and a single-particle soot photometer (SP2, refractory BC, rBC(SP2)). We propose methods to account for the following key factors affecting AE51 measurements: (1) the non-linearity of light attenuation (ATN) measurements associated with high-particle loading on the filter (i.e., the loading effect); and (2) the positive artifacts in ATN measurements created by non-refractory materials (nonBC) on the filter. To examine these effects, we employed a custom automated system that switches the two sampling lines for the AE51, one of which was heated to 300 degrees C to minimize the sampling of nonBC. Alternate operation of the heated and unheated modes (for 15 min) enabled the separation of the described effects. Consistent with previous studies, in the heated mode, sensitivity decreased linearly with increases in ATN (-0.44% per ATN) in comparison with the other aethalometers. When the loading effect was corrected for, the EBCAE51 in the unheated mode still showed some bias relative to EBCCOSMOS and rBC(SP2). This bias correlated with-and was successfully corrected for using-the mass ratio of nonBC to BC derived from the SP2. This approach can also be applied to fine-mode aerosol (PM2.5) concentrations measured at environmental monitoring networks, thus eliminating the need for additional SP2 data in the data correction.