Exploiting the features of multi-λ polar photometers to retrieve optical properties of aerosols collected on filters

In the last years, an increasing interest has been paid to the knowledge of aerosol optical properties due to the large uncertainties still affecting the estimates of the role of aerosols in the Earth radiative forcing. In this frame, a number of instruments - both in-situ and filter-based – have been developed to retrieve a robust assessment of the aerosol absorption coefficient, possibly at multi wavelengths. At the state-of-art there is not a gold reference standard and results from intercomparison exercises among the most widely used instruments still underline significant differences and the need of correction algorithms, although many of these commercial instruments are currently used in monitoring networks and research field campaigns. Moreover, in many field campaigns a huge amount of filters is collected for aerosol chemical-physical characterisation but optical absorption properties are usually not included. The research groups of the Universities of Milan and Genoa have developed benchtop multi-λ polar photometers[1,2,3] (called PP_UniMI and MWAA, respectively) which allow the off-line determination of the absorption coefficient (babs) of particles collected, in principle, on any kind of filters; in addition, PP_UniMI has been implemented to characterise optical properties on 1-h resolution samples collected by streaker samplers[2]. It is noteworthy that PP_UniMI and MWAA allow also retrospective analysis on stored filters thus retrieving e.g. data of climatological interest. The measurement methodology relies on the direct measurement of the phase function in the scattering plane and retrieves babs from the application of the adding method and a radiative transfer model to account for multiple scattering, i.e. the same algorithm used by the MAAP (Multi-Angle Absorption Photometer[1,3]). Tests on PP_UniMI and MWAA reliability have been performed vs. filter-based instrumentation (i.e. MAAP) with ambient samples and vs. both in-situ and filter-based measurements in laboratory tests[4]. Multi-l measurements of babs by PP_UniMI have been carried out in a number of field campaigns and results will be shown for selected casestudies including also optical source apportionment. More recently, the polar photometers have been implemented to measure filter spots taken from MAAP and Aethalometer aiming at clarifying the causes underneath the disagreement found between babs obtained by different instrumentation during recent field campaigns[5]. Moreover, the availability of multi-l measurements of filter spots can be used to investigate correction factors used e.g. the enhancement C factor in the Aethalometer AE33, their dependence on the wavelength and on the aerosol chemical composition.