Impact of aerosol microphysical properties on mass scattering cross sections

We assessed the sensitivity of simulated mass scattering cross sections (αλsca) of three aerosol species to perturbed particle microphysical properties and derived constraints on these microphysical properties, suitable for the north-western Mediterranean basin, from a comparison between code calculations and observations. In detail, we calculated αλsca of mineral dust, organic carbon and sulfate at three wavelengths in the visible range (λ1=0,450μm; λ2=0,525μm; λ3=0,635μm) with a T-matrix optical code, considering ±20% perturbations on size distribution, refractive index and mass density (respect to reference values mainly taken from the OPAC database), and spheroids with two different axial ratios as shape perturbations (reference shape: sphere). Then, we compared the simulation results with a set of observed αλsca of mineral dust, aged organics and ammonium sulfate sources, available at the same three wavelengths of the code calculations. These observations, provided by the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), have been derived through Multilinear Regression (MLR) analysis from measurements of aerosol mass concentrations and optical properties, collected during a 4-year campaign at the Montseny regional background station (Spain) and representative of the north-western Mediterranean basin. We observed quite different impacts of the microphysical perturbations on αλsca values and spectral dependence for different aerosol species, due mainly to the different size of the particles respect to the visible wavelengths. Moreover, by means of a compatibility test on best fit parameters, we constrained the mineral dust log-normal size distribution to a geometric radius and a standard deviation of rg=3,583·10−1μm and σg=1,600, respectively (effective radius: reff=6,221·10−1μm), the organic carbon log-normal size distribution parameters to rg=2,544·10−2μm and σg=1,760 (reff=5,656·10−2μm), the organic carbon real refractive index to nR=(1,576;1,576;1,576) at (λ1;λ2;λ3), the sulfate log-normal size distribution parameters to rg=8,340·10−2μm and σg=1,624 (reff=1,501·10−1μm) and the sulfate real refractive index to nR=(1,547;1,545;1,543) at (λ1;λ2;λ3), in the north-western Mediterranean basin. Regarding the other perturbed microphysical properties, we found the reference prescriptions to be suitable for this geographical region, according to the same analysis procedure.