Determining the Aethalometer multiple scattering enhancement factor C from the filter loading parameter

Light-absorbing aerosols heat the atmosphere; an accurate quantification of their absorption coefficient is mandatory. However, standard reference instruments (CAPS, MAAP, PAX, PTAAM) are not always available at each measuring site around the world. By integrating all previous published studies concerning the Aethalometers, the AE33 filter loading parameter, provided by the dual-spot algorithm, were used to determine the multiple scattering enhancement factor from the Aethalometer itself (hereinafter C_AE) on an yearly and a monthly basis. The method was developed in Milan, where Aethalometer measurements were compared with MAAP data; the comparison showed a good agreement in terms of equivalent black carbon (R² = 0.93; slope = 1.02 and a negligible intercept = 0.12 μg m^-3) leading to a yearly experimental multiple scattering enhancement factor of 2.51 ± 0.04 (hereinafter C_MAAP). On a yearly time base the C_AE values obtained using the new approach was 2.52 ± 0.01, corresponding to the experimental one (C_MAAP). Considering the seasonal behavior, higher experimental C_MAAP and computed C_AE values were found in summer (2.83 ± 0.12) whereas, the lower ones in winter/early-spring (2.37 ± 0.03), in agreement with the single scattering albedo behavior in the Po Valley. Overall, the agreement between the experimental C_MAAP and C_AE showed a root mean squared error (RMSE) of just 0.038 on the C_MAAP prediction, characterized by a slope close to 1 (1.001 ± 0.178), a negligible intercept (-0.002 ± 0.455) and a high degree of correlation (R² = 0.955). From an environmental point of view, the application of a dynamic (space/time) determination of C_AE increases the accuracy of the aerosol heating rate (compared to applying a fixed C value) up to 16 % solely in Milan, and to 114 % when applied in the Arctic at 80°N.