Laboratory-based calibration procedure for filter-based absorption photometers using photothermal interferometry and the extinction-minus-scattering method

We compared aerosol light absorption measurements by a photothermal interferometer (PTI) and by extinction minus scattering (EMS) in a laboratory setting using test aerosols with single scattering albedo (SSA) in the range from about 0.1 to 1. The EMS method reported higher light absorption coefficients, babs, by 25% up to a factor of 2 compared to PTI, with the deviation increasing when coating fresh soot with secondary organic matter. In a second step, the attenuation measured by numerous filter-based absorption photometers and in-situ-measuring instruments was calibrated against PTI at the wavelengths of 450 and 808 nm to determine reliable calibration factors for each instrument. The attenuation coefficient measured by the filter-based instruments, batn, was larger by a factor of 4 − 9 than the reference babs measurements of PTI depending on the instrument model and filter tape. We determined similar multiple-scattering correction factors C for the aethalometer AE33 to those reported by recent studies using PTI as a reference method, but almost two times higher than those reported by older studies referenced to the Multi-Angle Absorption Photometer (MAAP). In-situ-measuring instruments showed a reasonable agreement with PTI despite the low aerosol absorption coefficients which were close to the limit of detection for some of the photoacoustic instruments. In view of the revised Air Quality Directive of the European Union, our study highlights the need for traceable aerosol light absorption measurements to harmonize calibration procedures across Europe.