Determination of backscatter ratio and depolarization ratio by mobile lidar measurements in support of EARTHCARE and AEOLUS missions

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1 ÓPTICA PURA Y APLICADA. Sección Especial / Special Section: V Workshop on Lidar Measurements in Latin America Determination of backscatter ratio and depolarization ratio by mobile lidar measurements in support of EARTHCARE and AEOLUS missions Dimitrios T. Kokkinos (1,*), Georgios Tzeremes (2), Errico Armandillo (2) 1. Laboratory of Laser Remote Sensing, NTUA Athens, Greece. 2. Opto Electronics Section, ESA ESTEC, Keplerlaan 1, 2200AG, Noordwijk, The Netherlands. (*) Dimitrios.Kokkinos@esa.int; ag_kokkinou@yahoo.com S: miembro de SEDOPTICA / SEDOPTICA member Recibido / Received: 16/11/2010. Versión revisada / revised versión: 02/02/2011. Aceptado / Accepted: 02/03/2011 ABSTRACT: In preparation of the forthcoming ESA spaceborne lidar missions, ADM and EarthCARE, ESA support mobile lidar (ESMOL) has conducted a preliminary campaign to assess the impact of multi channel as well as multi instrument observation on the retrieval of key atmospheric parameters. This paper reports on a test campaign which was performed with ESMOL, supplemented with a Sunphotometer, for the measurement of the backscattering ratio and the depolarization ratio of low mid altitude aerosol clouds. A post processing code was developed in order to localise and integrate these parameters along cloud patterns, capable of generating an average overview of the aerosol characteristics. This campaign took place during a period of 8 months in Noordwijk, the Netherlands. ESMOL uses a Raman LIDAR (Raymetrics LR 211 UV D40) system transmitting at 355 nm and 532 nm, and simultaneously monitoring four channels, 355 nm polarized, 355 nm cross polarized, 387 nm and 532 nm. Its measurements were also post calibrated by a sun photometer with five filters at 340 nm, 380 nm, 500 nm, 936 and 1020 nm as well as data of daily weather conditions from meteorological databases. Key words: ESA, Lidar, Aerosol Cloud, Depolarization Ratio, Angstrom. REFERENCES AND LINKS [1] V. Freudenthaler, M. Esselborn, M. Wiegner, B. Heese, M. Tesche, A. Ansmann, D. Müller, D. Althausen, M. Wirth, A. Fix, G. Ehret, P. Knippertz, C. Toledano, J. Gasteiger, M. Garhammer, M. Seefeldner, Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006", Tellus B 61, (2008). [2] S. K Solanki, Y. C Unruh, A model of the wavelength dependence of solar irradiance variations, Astron. Astrophys. 329, (1997). [3] R. J. Hogan, Fast approximate calculation of multiply scattered lidar returns, Appl. Opt. 45, (2006). [4] X. Wang, A. Boselli, L. D'Avino, R. Velotta, N. Spinelli, P. Bruscaglioni, A. Ismaelli, G. Zaccanti, An algorithm to determine cirrus properties from analysis of multiple scattering influence on lidar signals, Appl. Phys. B 80, (2005). [5] L. R. Bissonnette,G. Roy, F. Fabry Range height scans of lidar depolarization for characterizing properties and phase of clouds and precipitation, J. Atmos. Ocean. Tech. 18, (2001). [6] M. Hess, P. Koepke, I. Schult, Optical properties of aerosols and clouds: The software package OPAC, B. Am. Meteorol. Soc. 79, (1998). [7] D. G. Kaskaoutis, H. D. Kambezidis, A. D. Adamopoulos, P. A. Kassomenos, On the characterization of aerosols using the Angstrom exponent in the Athens area, J. Atmos. Sol. Terr. Phys. 68, (2006). [8] J. Ackermann, The extinction to backscatter ratio of tropospheric aerosol: A numerical study, J. Atmos. Ocean. Tech. 15, (1998). [9] E. W. Eloranta, A practical model for the calculation of multiply scattered lidar returns, Appl. Opt. 37, (1998). Opt. Pura Apl. 44 (1) (2011) 93 Sociedad Española de Óptica

2 ÓPTICA PURA Y APLICADA Introduction In order to generate data sets for the depolarization ratio that shall be used from future ESA space lidar missions, a novel algorithm was developed. This algorithm uses as input the raw data for the backscatter coefficient of the polarized 355 nm and the 355 nm cross polarized channels. The first step of this algorithm is the detection of the aerosol cloud pattern. This is achieved by detecting large boundary variations of the cross polarized backscatter coefficient at 355 nm with the algorithm is primarily isolating the cloud pattern. This is typically occuring in a smaller segment of the whole data set, both in interval of data collection as well as the height of detection. The second function of this algorithm is the integrationaveraging over time of all the data in the specific segment. As a result the the depolarization ratio data do not correspond to a single instance within an aerosol layer but constitute the average depolarization ratio of the aerosol cloud. The developed algorithm integrates three additional functions. The first function is elimination of channel contamination caused by the beam splitter cube. The beam splitter cube is separating the two orthogonally polarized 355 nm channels [1]. The issue appears due to fact that 10% of the 355 nm polarized signal is permitted to the cross polarized detector (less than 1% of the cross polarized signal is directed to the polarized detector so it is ignored). This signal cannot be eliminated by hardware filters, since it is in the same wavelength, as the collected one and an additional polarizer doesnt fit in the detecting scheme. The algorithm calculates the signal of the polarized channel before the absorption of the PMT tube filters and subsequently subtracts portion of it (approximately 8% of the polarized) from the cross polarized channel measurement to correct the beam splitter cube passthrough.this issue has been resolved by changing the polarisation of the transmitter. The second function of this algorithm is a noise reduction function. It was observed that during solar zenith measurement, the input at the 532 nm channel (while the transmitter at 532 nm was blocked) contained significant signal (approximately 3%above electronics noise). The algorithm treats the received signal from the 532 nm modified accordingly (by extrapolating from the lidar input parameters and the sun emission spectra curve [2]) as sun emission noise for the 355 nm channel. The last function of the developed algorithm is the removal of multipath scattering effects according to the applications of Hogan and Wang depending on the aerosol cloud pattern [3,4]. The final goal of the measurement campaign is to measure the parameters of the atmospheric aerosol inhabitants using mainly the 355nm wavelength in conjunction with a sun photometer. The sun photometer provides information about the aerosol optical depths and the Ångström coefficients for different spectral regions. Moreover the optical depths measured by the sun photometer are used to calculate the lidar ratio from the integration of the total backscatter coefficient. A calculation of depolarisation ratio of the 355 nm channel provides information regarding the size and shape of the aerosols. The higher values of depolarization ratio are linked with nonspherical shapes when single scattering from aerosols is assumed whereas also suggest strong multiscattering from spherical shapes such as water droplets. [5] The measurement material is expected to be a mixture of water ice clouds with aerosol layers on the upper or on the lower parts. The uncertainty of depolarisation ratio is kept at 10%. 2. The raymetrics LR 211 UV D40 intrument description The Raymetrics LR 211 UV D40 is a four channel Raman lidar that operates at 355 nm polarized, 355 crosspolarized, 387 nm Raman shifted, and 532 nm channels. It is an integration of the laser head and a receiving/ signal acquisition unit. Fitted into two cabinets, one contains the telescope and the mini optical table, while the second contains the electronics and the cooling of the system. This configuration permits easy mounting in a van or an airplane, since the two cabinets can have a separation distance up to two meters. Opt. Pura Apl. 44 (1) (2011) 94 Sociedad Española de Óptica

3 ÓPTICA PURA Y APLICADA The emitter The emitter consists of three subunits, the laser source the reflective mirrors and the beam expander unit. The laser source is a pulsed Nd:YAG laser operating at 1064n nm and its two harmonics.table I shows the wavelengths operated as well as the pulse duration, pulse energy and divergence for each wavelength separately. The pulse repetition rate is 10 Hz. The beam divergence at 86.5% is 0.8 mrad for both the second and third harmonics before the beam expanders. The laser source is divided into two parts. The laser head incorporated in the optical box and the power supply cooling system integrated in the electronics cabinet. TABLE I Wavelength parameters Wavelength Pulse duration Energy Divergence 355 nm 8.3 ns 72 mj 0.25 mrad 532 nm 9.8 ns 47 mj 0.25 mrad 1064 nm 11.9 ns 95 mj 0.25 mrad The reflective mirrors provide high reflection for the required wavelengths and allow easy microalignment during system operation. The beam expander unit contains two beam expanders with beam expansion factor equal to 4, for the two wavelengths of interest. This reduces the beam divergence of the two beams down to 0.2 mrad at the exit windows of the bistatic system (separate windows for each wavelength, symmetrically positioned on one side of the telescope). The expanders are replaceable and can be removed or substituted if required. The primary harmonic is not currently used and it is dumped in a tilted black dumper, minimizing the reflections The receiving / signal acquisition system The receiving system consists of the telescope and a wavelength separation unit. The telescope is of Casegrainian type. Its primary mirror has a diameter of 400 mm, coated with HR coatings and with very small thermal expansion coefficient. The secondary mirror has a diameter of 9 mm with similar optical and thermal properties. The field of view of the telescope is adjustable and is currently fixed at 1 mrad. The wavelength separation unit (WSU) separates the three wavelengths (532nm, 387 nm, 355 nm) as well as the polarized and cross polarized channels at 355 nm. Mirror 445DCSP reflects only the 532 nm channel. Mirror 370DRSP reflects only the 387 nm channel, while the polarization tube is separating the perpendicularly polarized 355 nm channels, schematics available in Fig. 1. As the input signal collected by the telescope saturates the PMTs, filters are installed in front of each channel. For the 532 nm channel, two BG 7 filters reduce the original signal down to 36% its initial power. For the 355 nm polarized channel, two different filters, a BG 28 and a UG 1 reduce the original signal to 42% its initial power. For the Raman shifted and the cross polarized channels no filters where used. Fig. 1. Schematics of the optics at the reception site. 3. Results The Ångström parameters are calculated in the UV region from the sunphotometer measurements.low values of Ångström exponent (accompanied with high AOD value) are related to coarse particles suspended in the atmosphere. Higher Ångström values (accompanied with lower AOD values) are related to fine particles, mainly of anthropogenic origin. Although cloud parameters should not show wavelength dependence concerning the optical depths, the results have large values of Ångström that imply the existence of quantities Opt. Pura Apl. 44 (1) (2011) 95 Sociedad Española de Óptica

4 ÓPTICA PURA Y APLICADA. Fig. 2. The 3D image of the backscatter coefficient of 355 nm cross polarized channel in arbitrary units during a typical day with maritime clouds. of aerosol particles specially in the lower atmosphere (less than 5 km). The lidar measurements of backscatter coefficient point out towards the same conclusion. Strong scattering layers are shown that cause high depolarisation when they are combined with water clouds. Occasionally industrial aerosols were on the atmosphere during measurements that came from industrial sites. The optical depths for those occasions could be as high as 1.0. There are two predominant cloud patterns in the Netherlands during spring time. The North winds push dense maritime clouds from the north sea that hinder laser penetration. The calculated backscatter coefficient is high for these mixed clouds with aerosol populated layers and the depolarization ratio reaches values as high as 50%. In the UV region the Ångström parameter is about 1.5, affected by the high humidity in lower troposphere. Figures 2 5 show the backscatter coefficient of cross polarized 355 nm channel, the mean backscatter coefficient profile and the depolarization ratio respectively, for a typical day as described earlier. The depolarization ratio and the backscatter coefficient are averaged from 14:30 till 16:00 [6]. Fig. 3. The mean backscatter coefficient plot in m 1 sr 1 within the cloud from 2 km up to 3 km averaged from 14:30 till 16:00. Fig. 4. The mean backscatter coefficient plot in m 1 sr 1 within the cloud from 6 km up to 10 km averaged from 14:30 till 16:00. Opt. Pura Apl. 44 (1) (2011) 96 Sociedad Española de Óptica

5 ÓPTICA PURA Y APLICADA. ratio and the backscatter coefficient are averaged from 11:40 till 12:00. To cross check the analysis results from the lidar software the optical depths were also calculated from the backscatter coefficients using the Riemann integral. The results are very well in agreement as shown in Fig. 9. Fig. 5. The mean depolarization ratio from 1 Km up to 10 km averaged from 14:30 till 16:00. The West, SW and SE winds on the other hand carry thin clouds with strong reflectivity at low altitudes (typically less than 5 km). These clouds generated in urban and industrial areas demonstrate an average depolarization ration of 0.3 and UV region Ångström parameter of about 1.2. These clouds are considered to be either mixture of ice particles and pollutants during the coldest days, or mostly aerosol pollutant particles of semi oriented crystal shape [7]. Figures 6 8 show the backscatter coefficient of cross polarized 355 nm channel, the mean backscatter coefficient profile and the depolarization ratio respectively, for a typical day as described earlier. The depolarization 4. Conclusions The key achievement presented in this paper is the development of an algorithm that automatically detects an aerosol cloud and averages the depolarization ratio within the specific cloud pattern. Close to that a campaign was performed and data were collected for application in the upcoming ESA missions, Aeolus and EarthCARE. The campaign shall resume in coming Spring season, incorporating night measurements as well as the 387 nm Raman channel. Summarizing the results,winter spring measurements are affected by the local relative humidity which is typically more than 80% [8]. According to the results of Ångström parameters measured by the sun photometer as well as depolarization ratio, the aerosol clouds are predominantly composed of water or ice particles with a mixture of salt or urban aerosols depending on the wind direction. Concerning the multiscattering effect the developed algorithm simulates the effect of reducing the field of view Fig. 6. The 3D image of the backscatter coefficient of 355 nm cross polarized channel in arbitrary units during a typical day with pollution clouds. Opt. Pura Apl. 44 (1) (2011) 97 Sociedad Española de Óptica

6 ÓPTICA PURA Y APLICADA. by extracting a proportion of signal which corresponds to the multiscattered photons. This is based on the fact that there is a statistical distribution on the grade and magnitude of the multiscattering effect along the surface of field of view which depends on the penetration depth of the medium that causes the effect [9]. Fig. 7. The mean backscatter coefficient plot in m 1 sr 1 within the cloud from 1 km up to 3 km averaged from 11:40 till 12:00. Fig. 8. The mean depolarization ratio from 1 km up to 3 km averaged from 11:40 till 12:00. The results were evaluated according to bibliography regarding the values of depolarization ratio, backscatter coefficient, aerosol optical depth and Ångström exponent. Table II displays mean values of aerosol optical depths for different measurements of aerosol cloud patterns. Data from profiles with clean troposphere were applied to calibrate the depolarization ratio calculation procedure assuming as in literature that 0.01% depolarization corresponds to pure molecular troposphere. TABLE II Aerosol optical depth Wavelength Oct. Nov. Dec. 340 nm nm Min nm Max nm nm Min nm Max Wavelength Oct. Nov. Dec. 340 nm nm Min nm Max nm nm Min nm Max Fig. 9. Aerosol optical depth ratio measured from lidar data to photometer data in respect to the total backscatter coefficient. Opt. Pura Apl. 44 (1) (2011) 98 Sociedad Española de Óptica