SEVENTEEN-YEAR SYSTEMATIC MEASUREMENTS OF DUST AEROSOL OPTICAL PROPERTIES USING THE EOLE NTUA LIDAR SYSTEM (2000-2016)

A comprehensive analysis of the seasonal variability of the optical properties of Saharan dust aerosols over Athens, Greece, is presented for a 17-year time period (2000-2016), as derived from multi-wavelength Raman lidar measurements (57 dust events with more than 80 hours of measurements). The profiles of the derived aerosol optical properties (aerosol backscatter and extinction coefficients, lidar ratio and aerosol Ångström exponent) at 355 nm are presented. For these dust events we found a mean value of the lidar ratio of ~52±13 sr at 355 nm and of ~58±8 sr (not shown) at 532 nm (2-4 km a.s.l. height). For our statistical analysis, presented here, we used monthly-mean values and time periods under cloud-free conditions. The number of dust events was greatest in late spring, summer, and early autumn periods. In this paper we also present a selected case study (04 April 2016) of desert dust long-range transport from the Saharan desert.


INTRODUCTION
Dust events occur in the Eastern Mediterranean region mostly from early spring to early autumn, while during winter months dust transport over the area remains quite low as previously shown [1][2][3][4][5]. Aerosol lidar measurements are carried out in Athens since the year 2000. Each year Saharan dust events occurs that heavily influence the atmospheric conditions over the city of Athens. The optical properties of the resulting dust aerosol for the time-period 2000-2016, are presented in this work.

INSTRUMENTATION
The Raman lidar system (EOLE) of the National Technical University of Athens (NTUA), in the frame of the European Aerosol Research Lidar network (EARLINET) was used to derive the vertical profiles of aerosols over Athens, Greece. The lidar system is located at the Laser Remote Sensing Unit (LRSU) of NTUA (37.9 o N, 23.6 o E, 200 m a.s.l.). LRSU is equipped with an advanced 6-wavelength Raman lidar system, with a very recently added (2016), depolarization channel at 355 nm ( Fig. 1), able to perform independent and simultaneous measurements of the vertical profiles of the aerosol backscatter (at 355, 532 and 1064 nm) and extinction coefficient (at 355 and 532 nm). The data processing was based on the Single Calculus Chain (SCC) developed within EARLINET [6] and on manual retrievals. Air-mass back-trajectory analysis from the HYSPLIT and the BSC DREAM models was used to verify the lidar measurements of dust events.

METHODOLOGΥ
The NTUA aerosol lidar measurements during Saharan dust outbreaks [1,4] are based on early warnings, using forecast model data, provided by the DREAM model [7]. However, the dust lidar measurements are performed only under cloud-free weather conditions. Also, due to the weak Raman signals, the complete set of aerosol products (3 backscatter and 2 extinction coefficient profiles) can be derived for tropospheric layers only during nighttime. Thus, the number of the observed dust days is always fewer than the forecasted ones. In Fig. 2 we present the monthly and seasonal distribution of the dust cases percentage of occurrences observed by EOLE, for the period 2000-2016.
In this study the main emphasis has been given on the mean aerosol properties at 355 nm, as the 1 st Stokes of the laser radiation at 355 nm shifted by N 2 (at 387 nm) was the first Raman channel to be implemented by EOLE in 2003, some years before the implementation of the 2 nd Raman channel (at 607 nm). Thus, the longest database on the aerosol properties over Athens refers to the 355 nm channel. We note here, that according to Groß et al. [3], during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment (SALTRACE) in Barbados in June and July 2013, the lidar ratio (LR) within the pure dust layer was found to be wavelength independent, with mean values of 53±5 sr at 355 nm. According to Groß et al., 2015 [3], during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment in Barbados in June and July 2013, the lidar ratio (LR) within the pure dust layer was found to be wavelength independent with mean values of 53±5 sr at 355 nm.

STATISTICAL ANALYSIS
In Fig. 3 (upper and middle graphs) we present the seasonal evolution (from spring to winter) of the vertical profiles of the aerosol optical properties (backscatter-β aer and extinction-α aer coefficients) at 355 nm, as well as their mean values (red lines) and the corresponding statistical error (red error bars). The seasonal variation of the lidar ratios and their mean values (red lines) are presented in Fig. 3 (lower graph) for the height range between 2 and 4 km, where usually most of the Saharan dust layers are observed [4].

CASE STUDY OF 04 APRIL 2016
The dust event of 04 April 2016 observed over Athens was forecasted by the BSC-DREAM model for 12:00 and 18:00 UTC (Fig. 5a). The dusty air masses are distinguished by their green-yellow color, spreading from N-NW-W Saharan towards Italy and Greece. The corresponding dust load over Athens was of the order of 0.2 g/m 2 . According to the BSC-DREAM forecast, the vertical profile of the dust concentration [shown in shows the presence of dust particles from near ground up to 4-4.5 km height (also between 4.5-6 km at 18:00 UTC), with maximum mass concentration of ~25 µg/m 3 , around 1.5 km height, indicating an episode of rather low intensity. Indeed, EOLE detected dust layers in the range between 1 and 3 km and 5.5-7 km height.   In Fig. 6 we present the spatio-temporal evolution of the range-corrected lidar signal at 1064 nm obtained over Athens on 04 April 2016 (11:45-19:21 UTC). The aerosol layers are clearly visible, mainly from 1-3 km and around 5.5-6.5 km height.
Indeed, SeaWiFS detected the arrival of Saharan dust over Greece on 04 April 2016 (Fig. 9).
Moreover, the AERONET sun photometer of the National Observatory of Athens detected the arrival of   Indeed, the true color image provided by the Seaviewing Wide Field-of-view Sensor-SeaWiFS (Fig. 10) on 04 April 2016, clearly shows the desert dust sweeping Greece from SW to the NE (Athens is marked by asterisk).

CONCLUSIONS
Multi-wavelength aerosol Raman lidar data (57 dust events) have been analyzed for the period 2000-2016, regarding the main aerosol optical properties (β aer , α aer , LR, AE a and AE b ) of Saharan dust particles over the city of Athens. Mean values of 52±13 sr and 58±8 sr, were found for LR at 355 and 532 nm, respectively.
The height range, where usually most of the layers related to Saharan dust intrusions were observed, was found between 2 and 4 km, while the occurrence of the dust events was greater during spring and summer months.

ACKNOWLEDGMENTS
The LRSU-NTUA multi-wavelength Raman lidar is founding member of the EARLINET network (www.earlinet.org). The research leading to these results has received funding from the European Union