Variability of Lidar-Derived Particle Properties Over West Africa Due to Changes in Absorption: Towards an Understanding

Abstract. Measurements performed in Western Africa (Senegal) during the SHADOW-2 field campaign are analyzed to show that spectral dependence of the imaginary part of the complex refractive index (CRI) of dust can be revealed by lidar-measured particle parameters. Observations in April 2015 provide good opportunity for such study, because, due to high optical depth of the dust, exceeding 0.5, the extinction coefficient could be derived from lidar measurements with high accuracy and contribution of other aerosol types, such as biomass burning, was negligible. For instance, in the second half of April 2015, AERONET observations demonstrated a temporal decrease of the imaginary part of CRI at 440 nm from approximately 0.0045 to 0.0025. This decrease is in line with a change in relationship between lidar ratios (the extinction-to-backscattering ratio) at 355 nm and 532 nm (S355 and S532). In the first half of April, S355 / S532 is as high as 1.5 and the backscatter Angstrom exponent Aβ, is as low as −0.75, while after 15 April S355 / S532 = 1.0 and Aβ is close to zero. The aerosol depolarization ratio δ532 for the whole April exceeded 30 % in the height range considered, implying that no other aerosol, except dust, occurred. The performed modeling confirmed that the observed S355 / S532 and Aβ values match the spectrally dependent imaginary part of the refractive index as can be expected for mineral dust containing iron oxides. West Africa is also known for significant biomass burning aerosol emissions during the dry season in the Sahel region. The second phase of the SHADOW-2 campaign was focused on evaluation of lidar ratio of smoke and estimates of its dependence on relative humidity (RH). For considered five smoke episodes the lidar ratio increases from 44 ± 5 sr to 66 ± 7 sr at 532 nm and from 62 ± 6 sr to 80 ± 8 sr at 355 nm, when RH varied from 25 % to 85 %. Performed numerical simulations demonstrate, that observed ratio S355 / S532, exceeding 1.0 in the smoke plumes, can indicate to increase of the imaginary part of the smoke particles in UV.