---4.3 Optical properties of Arabian dust
In Paper III, we retrieved aerosol particle properties in an understudied dusty region over the United Arab Emirates (UAE) using a ground-based multi-wave-length PollyXT Raman lidar (Engelmann et al., 2016). The night-time lidar observa-tions from the year-long campaign revealed frequent multiple aerosol particle layer structures over the site (Fig. 10). Only in 10% of the cases, a single aerosol layer was present. Two (30%), three (29%) or even more simultaneous layers (31%) were more common. These multiple aerosol particle layers resulted from gravitational waves generated by the sea breeze passing over the mountains and stratifying the atmos-phere over the measurement site. Because of this stratification, isolated dry layers were present in the atmosphere as discussed in Section 4.1.
Figure 10. Night-time geometrical boundaries of the aerosol layers observed between 6th March 2018 and 14th February 2019 at the measurement site in UAE. The color indicates the
number of aerosol layers in the atmosphere. The gaps in the dataset seen from May to Au-gust and between September and November were due to instrumental complications.
Apart from the geometrical aerosol properties, we have also retrieved the AOD and further calculated the contribution of boundary layer (BL) and free-tropospheric (FT) aerosols to the total AOD. The mean AOD amounts to 0.37 ± 0.12 and 0.21 ± 0.11 at 355 and 532 nm, respectively, with higher values occurring during the summer months and lower values during the winter months. The contribution of FT aerosol layers to the total AOD was usually greater than that of the BL. Nevertheless, this behaviour was reversed from November to February. The lower total layer AODs during these months resulted from either the absence of multiple FT layers, or the lower surface wind speeds. We should mention here that, although the AOD values refer to night-time observations, on average the intra-day variation in the region is moderate (Arola et al., 2013; Eck et al., 2008), and therefore the aforementioned val-ues are valid also during daytime.
---The climatological values of the aerosol optical properties and their relevance to the vertical domain are presented in Figure 11. Due to the lower (on average) aerosol load, the β and α-coefficients decreased with increasing altitude. In contrast, almost constant LRs up to 5 km propose rather similar aerosol mixtures. Interestingly, δp at 532 nm wavelength increased or remained constant with altitude excluding aerosol layers above 5 km. This behaviour was also seen at 355 nm wavelength up to 2 km.
The most plausible explanation is that up to 2 km, the night-time residual layers con-tain mixtures of mineral dust and anthropogenic pollution or/and marine aerosols resulting to lower linear particle depolarization values. Hygroscopicity effects were rejected since the mean relative humidity of these aerosol layers was much less than 60 % for 82 % of the cases.
Figure 11. Height-dependent aerosol properties for 0-1, 1-2, 2-3, 3-4 and >5 km altitude. (a) Backscatter coefficient at 355 nm (blue), 532 nm (green) and 1064 nm (red). (b) Extinction coefficient at 355 nm and 532 nm. (c) Lidar ratio at 355 nm and 532 nm. (d) Linear particle
depolarization ratio at 355 nm and 532 nm.
Lastly, we have explored the pure Arabian dust optical properties. This aerosol type exhibits different optical properties than dust originating from Saharan or Asian deserts yet there are currently only four previous lidar studies from the area. The table below summarizes the Arabian dust optical properties as retrieved from this study and all the previous ones (Table 3). Examining the reasons behind the different LR values in Arabian compared to African dust, previous studies linked the optical behaviour to the chemical composition of the dust particles. In fact, Schuster et al.
(2012) showed that the LR behaviour of dust is subject to the percentage of illite in the soil. The content of illite, i.e a K-rich argillaceous component of sedimentary rock, in the dust determines the real refractive index. The real refractive index influences strongly the lidar ratio, an aerosol type parameter. Thus, it is expected that different
---dust types would exhibit different optical characteristics depending on their miner-alogical composition. For example, high content of illite in Saharan soil (up to 35-40 %) results in somewhat higher real refractive index values than that of Arabian dust. To confirm the connection, we have collected dust samples from the area around the measurement site and performed elemental analysis. The fraction of K-rich argillaceous component of sedimentary rocks was well below 5.5 % in the col-lected dust samples, supporting previous theories.
Implications of these findings propose that a universal lidar ratio of 55 sr for dust aerosol particles, as currently used, leads to biased retrievals in ceretain regions. For example, in satellite or ground-based extinction or aerosol typing retrievals as well as separation methods of a lidar signal to its aerosol components. In turn, all the aforementioned products are usually the basic input for advanced methodologies such as the retrieval of CCN/INP concentrations from lidar observations.
Table 3: Aerosol particle properties of the Arabian dust and comparison to previous studies.
Both 355 and 532nm wavelengths are reported in terms of their lidar ratio (LR), linear particle depolarization (δp) and Ångström exponent from the extinction (AE) at 355/532. The mean and standard deviation is shown for each optical property, if available, and the numbers in the brackets show the range of values reported for each parameter.