Determination and analysis of in situ spectral aerosol optical properties by a multi-instrumental approach

Abstract. Continuous in situ measurements of aerosol optical properties were conducted from 29 June to 29 July 2012 in Granada (Spain) with a seven-wavelength Aethalometer, a Multi-Angle Absorption Photometer, and a three-wavelength integrating nephelometer. The aim of this work is to describe a methodology to obtain the absorption coefficients (b_abs) for the different Aethalometer wavelengths. In this way, data have been compensated using algorithms which best estimate the compensation factors needed. Two empirical factors are used to infer the absorption coefficients from the Aethalometer measurements: C – the parameter describing the enhancement of absorption by particles in the filter matrix due to multiple scattering of light in the filter matrix – and f, the parameter compensating for non-linear loading effects in the filter matrix. Spectral dependence of f found in this study is not very strong. Values for the campaign lie in the range from 1.15 at 370 nm to 1.11 at 950 nm. Wavelength dependence in C proves to be more important, and also more difficult to calculate. The values obtained span from 3.42 at 370 nm to 4.59 at 950 nm. Furthermore, the temporal evolution of the Ångström exponent of absorption (α_abs) and the single-scattering albedo (ω₀) is presented. On average α_abs is around 1.1 ± 0.3, and ω₀ is 0.78 ± 0.08 and 0.74 ± 0.09 at 370 and 950 nm, respectively. These are typical values for sites with a predominance of absorbing particles, and the urban measurement site in this study is such. The b_abs average values are of 16 ± 10 Mm⁻¹ (at 370 nm) and 5 ± 3 Mm⁻¹ (at 950 nm), respectively. Finally, differences between workdays and Sundays have been further analysed, obtaining higher b_abs and lower ω₀ during the workdays than on Sundays as a consequence of the diesel traffic influence.