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) 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 (R = 0.93; slope = 1.02 and a negligible intercept = 0.12 μg m) leading to a yearly experimental multiple scattering enhancement factor of 2.51 ± 0.04 (hereinafter C). On a yearly time base the C values obtained using the new approach was 2.52 ± 0.01, corresponding to the experimental one (C). Considering the seasonal behavior, higher experimental C and computed C 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 and C showed a root mean squared error (RMSE) of just 0.038 on the C 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 (R = 0.955). From an environmental point of view, the application of a dynamic (space/time) determination of C 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.