SYNERGETIC USE OF ACTIVE AND PASSIVE REMOTE SENSING INSTRUMENTS FOR THE SEASONAL VARIANCE OF AEROSOLS OVER CYPRUS

CEST2013 Athens, Greece Ref no: XXX Proceedings of the 13 th International Conference of Environmental Science and Technology Athens, Greece, 5-7 September 2013 Formatted: English (United States) SYNERGETIC USE OF ACTIVE AND PASSIVE REMOTE SENSING INSTRUMENTS FOR THE SEASONAL VARIANCE OF AEROSOLS OVER CYPRUS Formatted: Font color: Auto NISANTZI A 1., MAMOURI RE 1., AKYLAS E 1., and HADJIMITSIS D.G. 1 1 Cyprus University of Technology, Department of Civil Engineering and Geomatics, 30 Archiepiskopou Kyprianou Str., 3036, Limassol, Cyprus e-mail: argyro.nisantzi@cut.ac.cy Formatted: Font: Not Bold Formatted: Left EXTENDED ABSTRACT Aerosol optical properties consist one of the main uncertainty in climate modeling as aerosol interact with the solar and terrestrial radiation and act as cloud condensation nuclei. Cyprus is a part of the eastern Mediterranean and as a consequence different type of aerosols (desert dust, biomass burning) for different type of sources reach the island. Sun-photometric measurements retrieved from the ground-based AERONET station of Limassol (34 0 67 N, 33 0 04 E) can give the seasonal variance of aerosol optical properties as well the daily lidar measurements depict the vertical distribution of atmosphere in the study area. Thus the combination of two instruments could provide vulnerable knowledge concerning the optical characteristics of the particulate air pollution in the area of interest. Transportation procedures over Limassol affect strongly the seasonal variability of the aerosol loading. For the period April 2010 - November 2012 we used AOD retrievals from the CIMEL sun-photometer and the maximum AOD values are found during the dry season (May-October). The cases where the daily AOD (500nm) exceed the mean value (0.214±0.132) were more than 250, while the maximum value reaches 1.292. For these cases we chose three different aerosol layers reaching Limassol lidar station in order to distinguish different type of particulate pollution by means of aerosol optical properties with respect to Ångström Exponent (α), Aerosol Optical Depth (AOD) and vertical profiles of backscatter coefficient and particle depolarization ratio. It is clearly presented that plumes of biomass-burning and Sahara and Arabia desert dust could be separated by aerosol optical properties. Also the back-trajectories have been calculated at six layers within the atmosphere for the determination of air-masses ending up at Limassol lidar station. Keywords: aerosol optical properties, lidar, sun-photometer Formatted: Indent: Left: 0", First line: 0" Please do not use page numbers

1. INTRODUCTION The Cyprus island is located in the northern corner of the eastern Mediterranean basin and aerosol layers from Sahara and Arabian Desert (Kalivitis et al., 2007) as well European continent reach the island. Dust plume originating in Sahara desert is more common in Mediterranean during the spring period (Gobbi et al., 2000) whereas the meteorological conditions is responsible for the aerosol layers come from different destinations. Passive remote sensing measurements of solar transmission are able to provide some information on the physical and optical properties of aerosols (Dubovik and King, 2000). Unfortunately these are integrated measurements for the whole column of the atmosphere and do not give any information concerning layer and height. This gap filled with lidars which provide the vertical distribution of the free troposphere. Thus the depolarization lidar in conjunction with sun-sky photometer retrievals could provide a robust and practical way to extract information concerning the size and shape of aerosols (Ansmann et al., 2011b). Daily aerosol lidar measurements over Limassol have started on May 2010 and up to now there is not sufficient information on basic optical properties of dust and biomassburning particles in the study area. In this paper three different cases are presented according to the type of aerosols and the source of layer reaching the site of Limassol, Cyprus. These cases are chosen due to the daily AOD (500nm) retrievals from AERONET exceed the mean value (0.214±0.132). Specifically the investigation includes the mineral dust layers from Saharan and Arabia deserts and biomass-burning aerosols from the North East site of Black Sea (Russia). The three aerosol layers are classified by means of their backscatter coefficient at two wavelengths (532 and 1064nm) and by their particle depolarization ratio at 532nm. Additionally AERONET retrievals by means of AOD values and α are used in order to characterize the aerosol optical properties of layers. 2. INSTRUMENTATION Formatted: Tab stops: Not at 0.5" 2.1. Lidar measurements At CUT, Limassol (34.675ºN, 33.043ºE, 10m above sea level) a 2-wavelength Raman lidar is used to perform continuous measurements of atmospheric aerosols in the Planetary Boundary Layer (PBL) and the lower troposphere since 2010. For the period May 2010 to March 2012 the lidar records daily measurements between 08:00 UTC and 09:00 UTC in order to be consistent with Terra MODIS overpass and to perform continuous measurements for the retrieval of the aerosol optical properties over Limassol. From April 2012, when the Raman channel added to the system, lidar records expanded not only for the night measurements but for more than an hour during daytime. The Lidar transmits laser pulses at 532 and 1064 nm simultaneously and collinear with a repetition rate of 20 Hz. This system is based on a small, rugged, flashlamp-pumped Nd-YAG laser with pulse energies around 25 and 56 mj at 1064 and 532 nm, respectively. Elastically backscatter signals at two wavelengths (532nm, 1064nm) are collected with a Newtonean telescope with primary mirror diameter of 200 mm and an overall focal length of 1000 mm. The field of view (FOV) of the telescope is 2 mrad. The lidar covers the whole range starting at the full overlap of the lidar (300 m) up to tropopause level. So far, four channels are detected, one for the wavelength 1064 nm, two for 532 nm and one for 607nm. A special optomechanical designs allows the manual ±45 -rotation of the whole depolarization detector module with respect to the laser polarization for evaluating the depolirization calibration constant of the system. The CUT depolarization lidar operates at 532nm and it is possible to rotate the detection box including the polarization beamsplitter

cube in order to calibrate the instrument (Freudenthaler et al., 2009). Firstly, we record the backscattered lidar signals (P and S) as usual, using the normal orientation of the lidar detection box and for the two other steps we rotate by ±45º the lidar detection box respectively and we record as before the P and S signals. The operation principal of this method is based on the fact that same amount of energy is sent to P and S channels, at opposite directions. The raw signal spatial resolution is down to 7.5 meters. The system follows the quality assurance test according to the EARLINET protocol (Bosenberg et al., 2003). 2.2. AERONET The columnar aerosol optical thickness (AOT) was measured among others aerosol optical properties by a CIMEL sun-sky radiometer, which is part of the AERONET Global Network (http://aeronet.gsfc.nasa.gov). The CIMEL is an automatic Sun-sky scanning filter radiometer allowing the measurements of the direct solar irradiance and sky radiance at wavelengths; 340, 380, 440, 500, 670, 870, 1020 and 1640 nm. Taking into consideration the technical specifications of the instrument, the calibration precision and data accuracy (Holben et al., 1998) the estimated accuracy of the AOT measurements to be presented is about ± 0.02 for the level 2. The CUT_TEPAK AERONET station is located in the old town of Limassol, 500 m from the sea. The sun-photometric station is operated since April 2010 by the Laboratory of Remote Sensing. 2.3. HYSPLIT model/ MODIS Terra and Aqua In order to verify the source of the layers over Limassol site the hybrid single-particle Lagragian integrated trajectory (HYSPLIT) model (Draxler et al., 2003) was used to calculate the backward trajectories (http://ready.arl.noaa.gov/hysplit.php ). Thus for each vertical profile we took 7-days air mass backward trajectories within six levels in the atmosphere. Additionally MODIS Terra and Aqua images were used in order to further show the existence of plume above Cyprus. Formatted: Font color: Auto 3. RESULTS Formatted: Tab stops: Not at 0.5" Since the AERONET first operation was at the 11 th of April 2010, in the Figure 1 are presented the daily values for the AOD (500nm) and α (440-675nm) for the period April 2010 to December 2012. Figure 1.AOD and Angstrom exponent (α) from CUT-TEPAK AERONET sun-photometer (level 1.5 and 2) for the period 2010-2012

According to the scale of α, different colours were used for specific values so as it is clearly illustrated the cases with the same characteristics. The first case of biomassburning plume α has significant high values (1.91) while AOD remain at 0.236. As we expected for the Sahara dust event (Sicard et al., 2011), α retains values ranging between 0 and 0.65, whereas AOD was at 0.32. The mineral dust comes from Arabia has medium α (0.72-0.95) and the daily AOD is around 0.38. Table 1.Layers statistic from lidar measurements Layer (m) δp ΑΟD 532nm AOD 1064nm Biomass-burning 2000-3340 0.14±0.01 0.091 0.014 Sahara dust 1100-4000 0.22±0.02 0.179 0.085 Arabia dust 1800-6200 0.35±0.02 0.245 0.059 3.1. Biomass-burning event: July 2010 Formatted: Tab stops: Not at 0.5" The quasi-permanent flow from the North during summer is a common pattern for the Eastern Mediterranean (Alpert et al., 1990, Kallos et al., 1998). As shown from Figure 2a, the fires above Black Sea in conjunction with the northern origin of air masses transport the smoke plume above the Limassol site (Fig.2b). During the lidar measurement (08:0-09:30 UTC) the AOD value was around 0.22 while the α reached the highest value (Fig.3) indicating the present of small particles. Comparing columnar AOD at 500nm (AERONET) and lidar AOD at 532nm within the layer (Table 1), it is clear that layer s aerosols comprise the 45% of the total AOD. Figure 2. (a) 7-days airmass back-trajectories for Limassol site and (b) Terra MODIS image during the transportation of biomass-burning plume on 19 July 2010 ( Figure 4 shows the profile of backscatter coefficient for two wavelengths (532 and 1064nm). The signals reflect the Planetary Boundary Layer (PBL) up to 900m and biomass-burning layer between 2000 and 3340m. Within the layer the particle depolarization values fluctuate around 14%. It is important to define define the lidar depolarization ratio δ p = // (Freudenthaler et al., 2009) so as to assist the characterization of aerosols as sphericals or non-sphericals. It is well known from Mie theory that spherical particles generate no depolarization so depending on the δ p values we can distinguish aerosols in spherical and non-spherical. Formatted: Indent: Left: 0"

Figure 3.AOD and α from CUT-TEPAK AERONET sun-photometer (level 2) for the 19 th of July 2010 Figure 4.Vertical profiles of backscatter coefficient (532nm, 1064nm) and particle depolarization from Limassol lidar measurements on 19 July 2010. The hatched area depicts the PBL=880m 3.2. Sahara dust event: April 2012 The transportation of air masses from North Africa to the eastern Mediterranean occurs mainly during spring and is directly linked with the eastward passage of frontal low pressure system (Kubilay et al., 2000). As shown in Figure 5a, back-trajectories at all six levels come from North Africa and end up at Limassol area, while Aqua MODIS does not depict the strength of plume (Fig.5b). During the lidar measurement (08:45-11:52 UTC) the AOD value was remaining at 0.32 and the α values were at 0.55 (Fig.6) indicating the present of large particles. Similarly to the previous case the comparison between columnar AOD at 500nm (AERONET) and lidar AOD at 532nm within the layer (Table 1) shows that layer s aerosols comprise the 55% of the total AOD. The profile of backscatter coefficient for two wavelengths (532 and 1064nm) (Fig. 7) depict the Planetary Boundary Layer (PBL) at 950m and the Sahara dust layer between 1100 and 4000m. The particle depolarization values in the layer ranging at 22%. This value is small but it is comparable with those retrieved from the SAMUM campaign (Freudenthaler et al., 2009).

Figure 5.(a) 7-days airmass back-trajectories for Limassol site and (b) Aqua MODIS image during the transportation of Sahara dust on 6 April 2012 Figure 6. AOD and α from AERONET retrievals during the 6 th of April 2012 Figure 7. Vertical profiles of backscatter coefficient (532nm, 1064nm) and particle depolarization from Limassol lidar measurements on 6 April 2012. The hatched area depicts the PBL= 600m. 3.3. Arabia dust event: June 2012 Despite the fact that the dust transportations from sources in the Middle East to the Eastern Mediterranean is more typically in the autumn (Dayan et al., 1986), a summer case has been selected. The back-trajectories at the three levels 2500, 3500 and 5000m coming from Arabia and ends over Limassol lidar site (Fig. 8a). This transportation is obvious in Aqua MODIS image (Fig.8b).

Figure 8.(a) 7-days airmass back-trajectories for Limassol site and (b) Aqua MODIS image during the transportation of Arabia dust on 15 June 2012 During the lidar measurement (08:15-11:50 UTC) the AOD value was fluctuating around 0.39 and the α values reached almost the highest value of the day 0.93 (Fig.9) indicating the present of medium particles. In this case comparing the columnar AOD at 500nm (AERONET) and lidar AOD at 532nm within the layer (Table 1) is shown that aerosols within the layer comprise the 65% of the total aerosols. Figure 9.AOD and α from AERONET during the 15 th of June 2012 Figure 10. Vertical profiles of backscatter coefficient (532nm, 1064nm) and particle depolarization from Limassol lidar measurements on 15 June 2012. The hatched area depicts the PBL=800m. The backscatter profiles of two wavelengths (Fig.10) illustrate the PBL at 960m and the mineral dust layer from Arabia from 1800 to 6200m. The particle depolarization values in

the layer reaching the value of 35% and is comparable with the retrievals of mineral dust from different campaigns (Grob et al., 2011). 4. CONCLUSIONS Formatted: Tab stops: Not at 0.5" The main aim of this work was to present three different cases of layers by means of aerosol properties above Limassol lidar station, where the AOD values exceed the mean AOD value of the site. Aerosol observations of biomass burning plume, which is common during the summer months in Cyprus as the flow from the North penetrate smoke from the fires above Black Sea, showed that the columnar AE reached the value of 2.2 and the particle depolarization ratio within the layer were of the order of 13-15%, indicating the presence of small and non-spherical particles. During the expected spring transportation of Sahara dust the AOD values are higher than 0.3 and is indicative of aerosol dust load, the corresponding α values were at 0.54-0.72 and the particle depolarization ratio was at 22%. This explains clearly the presence of large, non-spherical particles during the dust outbreaks. Finally for the Arabian desert dust case the AOD values present a peak at 0.39 with α values ranged between 0.72 and 0.95 and the particle depolarization ratio in the layer was at 35% indicating the existence of medium non-spherical particles. REFERENCES 1. Kalivitis N., et al. (2007), Dust transport over the eastern Mediterranean derieved from total ozone mapping Spectrometer, Aerosol Robotic Network, and surface measurements, Journal of Geophysical Research, 112, p. D03202. 2. Gobbi G.P., et al., (2000), Altitude-resolved properties of a Saharan dust event over Mediterranean, Atmospheric Environment, 34, 5119-5127. 3. Dubovic O., and King M.D., (2000), A flexible inversion algorithm for retrieval of aerosol optical properties from sun and sky radiance measurements, J. Geophys. Res., 105, 20673-20696. 4. Ansmann A., et al. (2011b), Ash and fine mode particle mass profiles from EARLINET- AERONET observations over central EUROPE after eruptions of Eyjafjallajokull volcano in 2010, J. Geophys. Res., 116, D00U02. 5. Bösenberg J., et al. (2003), EARLINET: a European aerosol research lidar network to establish an aerosol climatology. Max Planck Institute for Meteorology, Hamburg, Report No. 348, 191pp 6. Holben B. N., et al., (1998), AERONET A federated instrument networkand data archive for aerosol characterization, Remote Sens. Environ., 66, 1 16. 7. Draxler R.R. and Rolph G.D., (2003), HYSPLIT (HYdrid Single-Particle Langragian Integrated Trajectory) Model acess via NOAA ARL READY, URL: (http://ready.arl.noaa.gov/hysplit.php), NOAA Air Resources Laboratory, Silver Spring. 8. Sicard M., et al., (2011), Seasonal variability of aerosol optical properties observed by means of a Raman lidar at an EARLINET site over Northeastern Spain, Atmos. Chem. Phys., 11, 175 190. 9. Alpert P., et al., (1998), Quantification of dust-forced heating of the lower troposphere, Nature, 395, 367-370. 10. Kallos G. et al., (1998), On the long-range transport of air pollutants from Europe to Africa, Geophysical Research letters, 25, 619-622 11. Freudenthaler V. et al., (2009), Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006, 61B, 165-179. 12. Kubilay N., et al., (2000) An illustration of the transport and deposition of mineral dust onto the eastern Mediterranean, Atmospheric Environment, 34, 1293-1303. 13. Dayan U. (1986), Climatology of back-trajectories from Israel based on synoptic analysis, Journal of Climate and Applied Meteorology, 25, 591-595. 14. Grob S. et al., (2011), Characterization of Saharan dust, marine aerosols and mixtures of biomass-burning aerosols and dust by means of multi-wavelength depolarization and Raman lidar measurements during SAMUM 2, Tellus B, 63, 706 724. Formatted: Tab stops: Not at 0.25"