Vertical profiles of aerosol absorption coefficient from micro-Aethalometer data and Mie calculation over Milan

Abstract

Vertical profiles of aerosol number–size distribution and black carbon (BC) concentration were measured between ground-level and 500 m AGL over Milan. A tethered balloon was fitted with an instrumentation package consisting of the newly-developed micro-Aethalometer (microAeth® Model AE51, Magee Scientific, USA), an optical particle counter, and a portable meteorological station. At the same time, PM 2.5 samples were collected both at ground-level and at a high altitude sampling site, enabling particle chemical composition to be determined. Vertical profiles and PM 2.5 data were collected both within and above the mixing layer. Absorption coefficient ( b abs) profiles were calculated from the Aethalometer data: in order to do so, an optical enhancement factor ( C), accounting for multiple light-scattering within the filter of the new microAeth® Model AE51, was determined for the first time. The value of this parameter C (2.05 ± 0.03 at λ = 880 nm) was calculated by comparing the Aethalometer attenuation coefficient and aerosol optical properties determined from OPC data along vertical profiles. Mie calculations were applied to the OPC number–size distribution data, and the aerosol refractive index was calculated using the effective medium approximation applied to aerosol chemical composition. The results compare well with AERONET data. The BC and b abs profiles showed a sharp decrease at the mixing height (MH), and fairly constant values of b abs and BC were found above the MH, representing 17 ± 2% of those values measured within the mixing layer. The BC fraction of aerosol volume was found to be lower above the MH: 48 ± 8% of the corresponding ground-level values. A statistical mean profile was calculated, both for BC and b abs, to better describe their behaviour; the model enabled us to compute their average behaviour as a function of height, thus laying the foundations for valid parametrizations of vertical profile data which can be useful in both remote sensing and climatic studies.