The role of size in the multiple scattering correction C for dual-spot aethalometer: a field and laboratory investigation

The dual-spot aethalometer AE33 is a widely used instrument for measuring the aerosol absorption coefficient, but the accuracy of its measurements is heavily dependent on the multiple scattering correction factor (C), which compensates for multiple scattering effects in the filter matrix. Despite its widespread use, the factors influencing variability of C remain poorly understood, particularly in relation to aerosol properties. In this work, we explore the variability of C for the AE33 in a wide range of conditions and aerosol properties by combining chamber experiments with freshly emitted laboratory-generated soot and ambient data from a mountaintop site in Italy (Monte Cimone, CMN). The C factor is derived by comparison with independent filter-based instruments such as the MAAP (Multi-Angle Absorption Photometer) and MWAA (Multi-Wavelength Absorption Analyzer) at CMN and the extinction-minus-scattering (EMS) approach in chamber experiments. The mean C value at a wavelength of 637 nm derived at CMN is 2.35 with a standard deviation of 0.58, while the average values obtained in chamber experiments in different conditions range from 2.89 ± 0.03 to 3.9 ± 0.06. The variability of C at CMN appears to be primarily influenced by the signal-to-noise ratio of the instruments, especially during the colder months when absorption coefficient values fall below 1 Mm⁻¹. In contrast, in the chamber experiments, the variability is mainly driven by particle properties. The C value at 637 nm, derived from measurements at CMN, increases with increasing single scattering albedo (SSA), particularly for SSA values above 0.94, while showing no statistically significant spectral variability. Both ambient and chamber experiments highlight the dependence of the C factor on particle size, with C increasing as particle diameter decreases below 120 nm. This size dependence is relatively small (within 15 %) under ambient conditions dominated by mostly scattering aerosols, but it leads to changes greater than 60 % for highly absorbing soot particles.