Abstract. Measurements performed in western Africa (Senegal) during the SHADOW 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 the 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 in the
imaginary part of the CRI at 440 nm from approximately 0.0045 to 0.0025. This
decrease is in line with a change in the relationship between the lidar
ratios (the extinction-to-backscattering ratio) at 355 and 532 nm
(S355 and S532). For instance in the first half of April,
S355∕S532 is as high as 1.5 and the backscatter Ångström 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 of 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. The second phase of the SHADOW campaign was
focused on evaluation of the lidar ratio of smoke and estimates of its
dependence on relative humidity (RH). For five studied smoke episodes the
lidar ratio increases from 44±5 to 66±7 sr at 532 nm and
from 62±6 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 an
increase in the imaginary part of the smoke particles in the ultraviolet
(UV) range.
Variability in lidar-derived particle properties over West Africa due to changes in absorption: towards an understanding
