Calibration and intercomparison of filter-based measurements of visible light absorption by aerosols

Data on light absorption by atmospheric particles are scarce relative to the need for global characterization. Most of the existing data come from methods that measure the change in light transmission through a filter on which particles are collected. We present a calibration of a recently developed filter-based instrument for continuous measurement of light absorption (model PSAP, Radiance Research, Seattle, WA) that has been incorporated in several measurement programs. This calibration uses a reference absorption determined as the difference between light extinction and light scattering by unaltered (suspended) particles. In addition, we perform the same calibration for two other common filter-based methods: an Integrating Plate and the Hybrid Integrating Plate System, For each method, we assess the responses to both particulate light scattering and particulate light absorption. We find that each of the instruments exhibits a significant response to nonabsorbing aerosols and overestimates absorption at 550 nm by suspended particles by about 20-30 %. We also present correction factors for the use of the PSAP.