EARLINET observations of Saharan dust intrusions over the northern Mediterranean region (2014–2017): properties and impact on radiative forcing
2020
Abstract. Remote sensing measurements of aerosols using depolarization Raman lidar
systems from four EARLINET (European Aerosol Research Lidar Network) stations
are used for a comprehensive analysis of Saharan dust events over the
Mediterranean basin in the period 2014–2017. In this period, 51 dust
events regarding the geometrical, optical and microphysical properties of
dust were selected, classified and assessed according to their radiative forcing
effect on the atmosphere. From west to east, the stations of Granada,
Potenza, Athens and Limassol were selected as representative Mediterranean
cities regularly affected by Saharan dust intrusions. Emphasis was given on
lidar measurements in the visible (532 nm) and specifically on the
consistency of the particle linear depolarization ratio ( δp532 ), the extinction-to-backscatter lidar ratio
( LR532 ) and the aerosol optical thickness
( AOT532 ) within the observed dust layers. We found
mean δp532 values of 0.24±0.05 , 0.26±0.06 ,
0.28±0.05 and 0.28±0.04 , mean LR532 values of
52±8 , 51±9 , 52±9 and 49±6 sr and mean
AOT532 values of 0.40±0.31 , 0.11±0.07 , 0.12±0.10 and 0.32±0.17 , for Granada, Potenza, Athens
and Limassol, respectively. The mean layer thickness values were found to
range from ∼ 1700 to ∼ 3400 m a.s.l.
Additionally, based also on a previous aerosol type classification scheme
provided by airborne High Spectral Resolution Lidar (HSRL) observations and
on air mass backward trajectory analysis, a clustering analysis was
performed in order to identify the mixing state of the dusty layers over the studied area. Furthermore, a synergy of lidar measurements and modeling was
used to analyze the solar and thermal radiative forcing of airborne
dust in detail. In total, a cooling behavior in the solar range and a significantly
lower heating behavior in the thermal range was estimated. Depending on the
dust optical and geometrical properties, the load intensity and the solar
zenith angle (SZA), the estimated solar radiative forcing values range from
−59 to −22 W m −2 at the surface and from −24 to −1 W m −2 at
the top of the atmosphere (TOA). Similarly, in the thermal spectral range
these values range from + 2 to + 4 W m −2 for the surface and from
+ 1 to + 3 W m −2 for the TOA. Finally, the radiative forcing seems
to be inversely proportional to the dust mixing ratio, since higher absolute
values are estimated for less mixed dust layers.
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