EARLINET observations of Saharan dust intrusions over the northern Mediterranean region (2014–2017): properties and impact on radiative forcing

Abstract

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.

Radiative Effect and Mixing Processes of a Long-Lasting Dust Event over Athens, Greece, during the COVID-19 Period

“…Such local scale processes inside the PBL cannot be easily reproduced at the mesoscale resolution of the model (0.3° × 0.3°), especially for regions of complex topography [87], thus explaining the discrepancy between modeled and measured values, especially at ground level. Figure 6 presents the MODIS "True Image" alongside the Combined Value-Added Aerosol Optical Depth normalized at 550 nm (AODMODIS) product from both Terra and Aqua satellites per day of the long-lasting dust event (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). The cloud formation over Southern Europe is in accordance with wind vectors at 2 km, as estimated by the WRF model (see Figure 3), especially for the period 14-20 May.…”

“…Figure 9. LR355 values reported by various campaigns that correspond to pure (either directly from the source or long-range transportation) and mixed/polluted dust, along with the values reported in this study (red circles inside a grey frame)] [20,38,90,91,93,95,97,98] The aforementioned observation regarding the two distinct transportation phases of the dust plume is further supported by the comparison of the LR355 values, obtained during the two phases of the event and reported here, with the corresponding literature values regarding pure and polluted dust (Figure 9). During the first time period (contains 11 and 14 May), the mean LR355 obtained by our ground-based measurements is very close to pure dust observations reported by dedicated campaigns in the African region (SAMUM and SHADOW), but also long-range transported pure dust over Barbados (SALTRACE).…”

“…This extra circulation also affected the dust load (dry deposition of the coarser particles), as depicted by the mean values of αaer and βaer and AOD, which reduced to around half (α355 = 45 ± 18 Mm −1 , α532 = 40 ± 18 Mm −1 , β355 = 1.4 ± 0.4 Mm −1 sr −1 , β532 = 1.5 ± 0.5 Mm −1 sr −1 , β1064 = 0.7 ± 0.3 Mm −1 sr −1 ) the corresponding values observed during the 1st phase (α355 = 122 ± 60 Mm −1 , α532 = 122 ± 62 Mm −1 , β355 = 2.1 ± 1.0 Mm −1 sr −1 , β532 = 2.5 ± 1.4 Mm −1 sr −1 , β1064 = 1.3 ± 0.8 Mm −1 sr −1 ). The mean values of δp355 and LR also decreased, depicting that the air mass circulation played a crucial role in the aging of the dust particles from almost pure [36,91,[93][94][95][96] to long-range transported [20,33,37,39,90,97,98]. Finer particles reached over Athens after 15 May, as shown by the increase in the mean AEα355/532 and AEβ532/1064 values.…”

“…Furthermore, a recent study shows a link between air pollution and an increased risk of death from COVID-19 [17]. Therefore, it is of particular importance to study the spatio-temporal variability of particles' shape, concentration, size distribution, and refractive index, and other geometrical and physico-chemical properties [18][19][20], especially under specific conditions where local air pollution sources were minimized, as we will discuss in this section.…”

“…In Southeastern Europe, Saharan dust intrusions frequently occur in the free troposphere from spring to autumn due to the high cyclonic activity over Northern Africa [23][24][25][26][27][28][29]. The majority of these events over the Mediterranean usually last from a few days up to one week during the autumn and summer periods [27,[30][31][32][33][34][35][36] In the past, several studies of the vertical variability of dust aerosol geometrical and physico-chemical properties have been performed in the European continent by the synergy of lidars, in situ instruments, sun photometers, satellite observations and model simulations, mainly in the frame of the European Aerosol Research Lidar Network (EARLINET) [20,32,[37][38][39][40][41][42][43][44]. However, no such studies have been performed under specific conditions with very low local air pollution levels, such as those which prevailed during the COVID-19 confined period in Europe [45][46][47][48].…”