Portable Raman Lidar Polly XT for Automated Profiling of Aerosol Backscatter, Extinction, and Depolarization

Transcription

1 2366 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 26 Portable Raman Lidar Polly XT for Automated Profiling of Aerosol Backscatter, Extinction, and Depolarization DIETRICH ALTHAUSEN, RONNY ENGELMANN, HOLGER BAARS, BIRGIT HEESE, ALBERT ANSMANN, AND DETLEF MÜLLER Leibniz Institute for Tropospheric Research, Leipzig, Germany MIKA KOMPPULA Finnish Meteorological Institute, Kuopio, Finland (Manuscript received 5 March 2009, in final form 27 April 2009) ABSTRACT Two versions of the portable aerosol Raman lidar system (Polly) are presented. First, the two-channel prototype is depicted. It has been developed for the independent and simultaneous determination of particle backscatter and extinction coefficient profiles at 532 nm. Second, the Raman lidar Polly XT (3 1 2: three backscatter and two extinction coefficients), the second generation of Polly, is described. The extended capabilities of Polly XT are due to the simultaneous emission of light with three wavelengths, more laser power, a larger main receiver mirror, and seven receiver channels. These systems are completely remotely controlled and all measurements are performed automatically. The collected data are transferred to a home server via the Internet and are displayed on a Web page. This paper describes the details of the optical setup, the housekeeping of the systems, and the used data retrieval routines. A measurement example taken close to Manaus, Brazil, on 15 August 2008 shows the capabilities of Polly XT. 1. Introduction The complexity of atmospheric aerosols expressed by their highly variable particle number concentrations, multimodal size distributions, variable shape characteristics, complex chemical composition and mixing behavior, and the correspondingly large temporal and spatial (horizontal and vertical) variability in the aerosol characteristics are the main reasons for the high uncertainties in our quantitative understanding of the role of atmospheric aerosol in environmental, weather, and climate-related processes. The International Panel on Climate Change Fourth Assessment Report (Forster et al. 2007) has identified aerosol radiative forcing and the impact of aerosols on cloud and precipitation processes as one of the major unknowns in our understanding of climate change. Practically all long-range transport of aerosols occurs at elevated height levels Corresponding author address: Dietrich Althausen, Leibniz Institute for Tropospheric Research, Permoserstr. 15, D Leipzig, Germany. dietrich@tropos.de decoupled from the ground. A global climatology of the mesoscale and large-scale aerosol transport based on long-term datasets of vertically resolved aerosol distributions does, however, not exist. A combination of surface-based (in situ and remote sensing) and satellite observations is needed to satisfy our current observational need. Vertical profiling of aerosols with lidar is a natural complement to total column aerosol observations made by surface sun photometers (Welton et al. 2005) as well as from satellites (Kaufman et al. 2003). Advanced lidar systems, which determine the aerosol optical properties in a quantitative way and permit the estimation of main microphysical properties, are well suited for providing ground truth for the retrieval of aerosol products from passive and active sensors in space. For continuous long-term observations, autonomous lidar operation is required. Several approaches have been undertaken in this direction. The Raman lidar at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) site in Oklahoma is an operational instrument that profiles tropospheric water vapor and aerosol optical properties throughout the DOI: /2009JTECHA Ó 2009 American Meteorological Society

2 NOVEMBER 2009 A L T H A U S E N E T A L diurnal cycle autonomously (Turner et al. 2001). The National Aeronautics and Space Administration (NASA) Micro-Pulse Lidar Network (MPLNET) presently consists of globally distributed, compact, autonomous, and eye-safe micropulse lidars (Welton et al. 2001). The aim of MPLNET is to acquire long-term observations of aerosol and cloud vertical profiles at unique geographic sites collocated with sun photometers in the NASA Aerosol Robotic Network (AERONET). Razenkov et al. (2002) built an autonomous high-spectral-resolution lidar for long-term unattended observations of arctic clouds and haze. The lidar is equipped with polarization channels so that the depolarization ratio is determined, which permits us to discriminate ice crystals from liquid drops or to identify nonspherical dust particles. Sugimoto et al. (2008) set up the Japanese National Institute for Environmental Studies (NIES) lidar network in east and southeast Asia. The network consists of about 20 automatically operating lidars and also measure the height profile of the depolarization ratio. According to the plan for implementation of a Global Atmospheric Watch (GAW) Aerosol Lidar Observation Network (Bösenberg and Hoff 2008), lidar measurements should include the identification of aerosol layers in the troposphere and stratosphere, vertical profiles of optical properties with known and specific precision (backscatter and extinction coefficients at selected wavelengths, lidar ratio, and Ångström exponents), aerosol type (e.g., dust, maritime, fire smoke, urban haze), and microphysical properties (e.g., volume and surface concentrations, size distribution parameters, refractive index). This was the main motivation for the development of the comparably small, thus compact, automated two-channel portable aerosol Raman lidar system (Polly; Althausen et al. 2004). Since 2004, this system has been deployed in field campaigns in southern (Ansmann et al. 2005) and northern China (Tesche et al. 2007, 2008; Wendisch et al. 2008) for some months; since June 2005 it is running unattended at the Leibniz Institute for Tropospheric Research (IfT), Leipzig, Germany. The diurnal cycle of the boundary layer evaluation was studied based on a 1-yr dataset (Baars 2007; Baars et al. 2008). From the experience with Polly, the extended version (Polly XT ) has been developed together with the Finnish Meteorological Institute (FMI). The lidar has seven channels (Althausen et al. 2008) and allows the determination of the particle backscatter coefficient at three wavelengths, the particle extinction coefficient at two wavelengths, and the depolarization at one wavelength. As demonstrated by Müller et al. (2001), a dataset consisting of backscatter coefficients at 355, 532, and 1064 nm and extinction coefficients at 355 and 532 nm allows the estimation of microphysical properties from the measured spectrally resolved optical properties with an inversion algorithm (Müller et al. 1999a,b). The development of the compact, automated Raman lidars are based on our long-term experience in aerosol Raman lidar observations of clouds and aerosols (Ansmann et al. 1990, 1992; Althausen et al. 2000; Mattis et al. 2004, 2008; Müller et al. 2005) and on the numerous field campaigns we conducted during the past 10 yr in Europe (e.g., Ansmann et al. 2002; Müller et al. 2002; Wandinger et al. 2002), Asia (Franke et al. 2003; Müller et al. 2003; Ansmann et al. 2005; Tesche et al. 2007), and Africa (Tesche et al. 2009). The aim of this paper is to describe our latest developments of small, compact, and automated Raman lidars at the IfT in Leipzig, Germany. In section 2 the low-cost Raman lidar system Polly is described. Section 3 presents our results of setting up the improved, autonomous measuring, and remotely controllable Raman lidar Polly XT. In this section, the cabinet and all devices are described and the optics and electronics inside are explained and some features belonging to the automatization of the lidars are discussed. The obtained aerosol products, the basics of the procedural methods, and concepts used for the automatic retrieval algorithm techniques are discussed in section 4. The Polly XT systems took already long-term observations in Brazil (IfT Polly XT system, January November 2008) and in India (FMI Polly XT system, March 2008 March 2009). A measurement example from the observations in Brazil is presented in section The prototype Polly The prototype Polly was developed between April 2002 and September This lidar is set up on an optical table of 700 mm mm and housed in a weatherproof cabinet. It can easily be transported and installed in the field. The optical setup is depicted in Fig. 1. The flashlamp-pumped frequency-doubled Nd:YAG laser of type CFR200-GRM (Quantel) generates light pulses at the wavelength of 532 nm. The laser pulses have an energy of 120 mj at 532 nm and a repetition rate of 15 Hz. The divergence of the laser beam is #1.5 mrad. An 8-times beam expander reduces the divergence of the emitted radiation. For the beam expander, optical elements from stock were chosen to ensure low costs. A plano-concave lens with a focal length of 240 mm diverges the beam. Behind this lens, a two-lens laser achromat (Linos Photonics) with a focal length of 310 mm collimates the beam. All theses lenses were antireflection coated ensuring low power in any back reflections. A biaxial setup was chosen because of the relatively small diameter of the receiver telescope. Two flat mirrors (M1 and M2) are used behind the beam expander to direct the

3 2368 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 26 FIG. 1. Optical setup of the Raman lidar system Polly. expanded beam with the 55-mm diameter into the atmosphere on an axis that is as close as mechanically possible to the receiver axis. These 100-mm mirrors (Comar Instruments) are coated with enhanced aluminum that has a reflectivity.94% at 550 nm and at an entrance angle of 458. The maximal peak valley distance over free aperture with a 90-mm diameter is less than l/10 at nm. Finally, because of the stable mountings of the mirrors, the axis of the emitted beam is at a distance of 200 mm to the receiver axis. Two quartz plates in the roof of the cabinet are utilized for the protection of the optics from the environment. A small one with the 100-mm diameter is used in the transmitted laser path and another with the 250-mm-diameter quartz plate covers the receiver telescope. Thicknesses of 10 and 20 mm, respectively, were chosen to avoid distortions resulting from bending. For less transmission losses, the receiver quartz plate is antireflection coated. The receiver telescope is of Newtonian type (cf. Fig. 1). The primary mirror (PM; Astrooptik Philipp Keller) has as a 200-mm diameter. The focal length of 800 mm results in an f-number of 4 and hence the telescope represents a compact and fast telescope. The elliptical secondary mirror (SM; Linos Photonics) deflects the light by 908. The 2.5-mm lateral offset of the secondary mirror results because of the focal length of the telescope, the diameter of the primary mirror, and the distance between the telescope axis and the pinhole of 160 mm (Engelmann 2003). An iris diaphragm (PH) is used as a field stop to realize a variable field of view (FOV) between 1.25 and 3.75 mrad. Behind the iris the light is collimated to a 19-mm diameter by a plano-convex lens (L0; Edmund Industrie Optik) with a focal length of 75 mm. This lens has a MgF 2 antireflection coating. The full-angle divergence of the received light behind the telescope increases to mrad because of the beam diameter downsize to 19 mm. This large full-angle divergence results in a maximum possible light path within the receiver of about 600 mm using optics with a 50-mm diameter. Behind the collimator the light is separated according to its wavelength by a beam splitter (BSP; L.O.T.-Oriel). The elastically backscattered light at the wavelength of 532 nm is transmitted, whereas the inelastically backscattered light at the wavelength of 607 nm is reflected. The distance between the lens and the beam splitter is 150 mm. The size of the beamsplitter is 50 mm 3 50 mm, which is sufficiently large for the 25-mm maximum beam diameter at the position of the beamsplitter. The (nonpolarized) transmission of the beam splitter at 458 amounts to 96% at 532 nm and the reflectivity amounts to 88% at nm. Within the elastic channel a neutral density filter (NF) with the optical thickness of 2 is used to deflect 99% of the channel light toward a lens (L3) and finally onto a camera chip (CAM). The remaining 1% light intensity is transmitted toward a variable assembly of neutral density filters in this channel. Another mirror (M6) is used to direct the inelastic backscattered light onto its detector. The photomultiplier tubes (PMTs) are used in the photon-counting regime. To protect the PMTs from overloading, neutral density filters are placed in front of the photomultipliers. Measurements with the final layout of the system yielded that in the inelastic channel a neutral filter with an optical thickness of 0.5 is appropriate which results in an attenuation of the inelastically backscattered light by a factor of 3.2. The elastically

4 NOVEMBER 2009 A L T H A U S E N E T A L backscattered light intensity may vary much more in intensity due to different particle load in the atmosphere. Hence, a filter cascade was built with neutral filters of optical thickness of 4, 2, 1, and 0.5. In total, seven different settings between optical thicknesses of 0 up to 7.5 can be set remotely in steps of 0.5. Behind the neutral density filters in each channel, the light is transmitted through the interference filters IF1 and IF2, respectively. These filters (Barr) suppress the sky background and any possible cross talk between the channels. The maximum transmission of the bandpass filters IF1 and IF2 is at the central wavelength of and nm, respectively, and the suppression of the light at other wavelengths is at least 10 5 for both filters. For Raman lidar detection an additional light suppression is required at the emitted wavelengths (Clauder 1996). The light suppression of IF2 at the wavelength of 532 nm is The transmission windows of both filters have a full width at half maximum (FWHM) of 0.5 nm. Finally, the backscattered light is imaged by the lenses L1 and L2 (Edmund Industrie Optik) onto the PMTs. These lenses are plano-convex lenses with a focal length of 75 mm and a MgF 2 antireflection coating. The photosensitive areas of the PMTs are 8 mm in diameter. ZEMAX (available online at ZEMAX is a registered trademark of ZEMAX Development Corporation) calculations yielded that the image plane is 65 mm behind the surface of the last lens. The PMT is installed at a 55-mm distance from the last lens. This yields to a 5-mm light spot on the detector when imaging a point source at infinity. Hamamatsu PMT models R5600P-03 and R5600P are used for 532 and 607 nm, respectively. The photomultipliers have a quantum efficiency of 7.5% at 532 nm and of 1.2% at 607 nm. The backscattered photons are transferred to pulses by the PMTs. These pulses are amplified, discriminated, and counted. The preamplified pulses have a FWHM of ns. The bin width of the data acquisition card (Fast ComTec) is 250 ns, which represents a spatial resolution of 37.5 m. The photons are received statistically (Poisson) and cause pulses with no constant time delay. The measured count rate N m differs from the real count rate N r. These quantities are interdependent via the deadtime t. Having count rates of less than 20 megacounts per second (Mcps), the equations for the paralyzable and nonparalyzable counters for the relation between the real count rate and the measured count rate lead to the same result (Whiteman 2002). To ensure a relative error#5%, the maximum measured count rate has to be set below 12.5 Mcps. The overlap of the system is adjusted by aligning the mirrors on the emitter side. The alignment is rechecked from time to time with the help of a charge-coupled device (CCD) camera depicted as CAM in Fig. 1. The system software of Polly ensures autonomous operation and automatic measurements, including system housekeeping, data acquisition, and product delivery routines. Housekeeping comprises, for instance, the housing temperature recording by two negative temperature coefficient (NTC) sensors, precipitation detection, data transfer, overlap monitoring by the camera, and data logging from a meteorological station attached to the cabinet. Finally, the operator supplies the measurement start and stop times, and the operator starts the software. The entire measuring procedure from switching on the laser up, taking the measurements, retrieval and transferring of data, and quicklooks to the Web page runs automatically. 3. The system Polly XT Meanwhile, Raman lidar systems (3 1 2: three backscatter coefficients at 355, 532, and 1064 nm and two extinction coefficients at 355 and 532 nm) with depolarization measurement capabilities are state of the art. Their data allow the retrieval of microphysical particle properties by an inversion. Hence, after the prototype Polly, a new generation of portable lidar systems was developed. This new lidar is called Polly XT because of its extended performance and capabilities and will be explained in this section. a. Cabinet, air conditioning, and external devices Figure 2 shows the opened Polly XT instrument. The cabinet (Knuerr) originates from telecommunication equipment housing. This weatherproof cabinet is thermally insulated by double walls. For transport, six wheels were installed to ensure that two persons can easily move the system. On top of the cabinet a precipitation sensor (Thiess Clima), a temperature sensor (Lufft), and a roof cover (Eigenbrodt) are mounted. The precipitation sensor assures a proper shutdown of the system during a rain event. The roof cover protects the quartz plates on the emitter output and on the receiver input. These quartz plates avoid air exchange between the cabinet inside and the environment. In this way the system is kept clean and an efficient air conditioning is realized. The air conditioning system (LG) is an inverter system with regulated cooling power between 1300 and 4000 W and heating power between 1300 and 5000 W. Thus, the system can be used in different climatic environments. Like the prototype, the system is connected by only two cables: one for electrical power and one for Internet

5 2370 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 26 FIG. 2. Cabinet with open doors showing the major parts of the Raman lidar system Polly XT : (1) laser head, (2) laser power supply, (3) beam expander, (4) receiver telescope, (5) receiver with seven channels, (6) power supply for PMT cooling unit for the 1064-nm channel, (7) computer with data acquisition and interface cards, (8) UPS, (9) air conditioner, (10) sensors for outdoor temperature and rain, and (11) roof cover. connection to control the system remotely and to transfer the data automatically. Inside the cabinet, an optical table supports the laser head, the emitter optics, and the receiver optics with the telescope and the seven channels. The optical table is mounted 58 off the vertical in the rear of the cabinet. The optical setup is shown in Fig. 3 and is explained below. Beneath the optics, the computer, the power supply for the laser, and an uninterruptible power supply (UPS) are mounted. On the left door, the inner part of the air conditioning system is fixed. It is connected to the external heat exchanger via flexible pipes. The computer itself houses peripheral component interconnect (PCI) cards for the data acquisition and also all interfaces to control the entire system. Additional to the lidar inside the cabinet, a pressure sensor and the datalogger for pressure and the outdoor temperature measurements are installed. If no vertical profiles of temperature and pressure are available from radiosonde data, these measurements are used to calculate air density profiles from a standard atmosphere to take into account the Rayleigh scattering from the air molecules. b. Optics The layout on the optical table is depicted in Fig. 3. The emitter parts are indicated with E, whereas R is used for the receiver optics parts. The laser is a Nd:YAG type Inlite III (Continuum). The repetition rate of the laser pulses is 20 Hz. The energy per laser pulse is 450 mj at 1064 nm. The divergence of the emitted laser beam is less than 1.5 mrad. Because the laser has a compact design and is ruggedly built up, it is well suited for field deployment. Also, the possibility of controlling the laser via a serial port is essential for the choice of this laser. This feature contributes to the automation of the complete system. Together with the laser the second harmonic generation (SHG) and third harmonic generation (THG) crystals are assembled on a metal plate (E0) to ensure proper and stable alignment. In this way the laser pulses are emitted at 355, 532, and 1064 nm simultaneously. The transmitted energy is approximately 180 mj at 1064 nm, 110 mj at 532 nm, and 60 mj at 355 nm. The emitted radiation is linearly polarized at 355 nm.

6 NOVEMBER 2009 A L T H A U S E N E T A L FIG. 3. Optical setup of Polly XT. Details are explained in the text. Two quartz prisms (E1 and E2) turn the beam into the upward direction. The front and back surfaces of the prisms are antireflection coated for the respective laser wavelengths. An achromatic beam expander (E3) enlarges the beam diameter from about 6 to about 45 mm before the beam is directed into the atmosphere, resulting in a beam divergence of,0.2 mrad. The backscattered light is collected with a Newtonian telescope (R1 and R2). Its primary mirror (Astrooptik Philipp Keller) has a 300-mm diameter and a 900-mm focal length. The coating is silver with a protecting layer. The secondary mirror is a flat elliptical mirror (Edmund Optics). Its minor and its major axes have lengths of 76.2 and mm, respectively. The coating is enhanced aluminum. The pinhole (R3) defines the receiver FOV of 1 mrad. Behind the pinhole, an achromatic lens (R4; Melles Griot) collimates and transmits the light to seven channels. The focal length of the achromatic lens is 60 mm. Dichroitic beamsplitters (DBS) R5, R6, R8, and R9 separate the light according to its wavelengths. The DBS were selected to reflect light with shorter wavelengths and transmit light with longer wavelengths because it is easier to produce DBS with higher transmission values at the longer wavelengths than at the shorter wavelengths. R5 works as a high reflector for the ultraviolet (UV), whereas R8 acts as a high reflector for light with the wavelength of 532 nm. The transmissions of R5, R6, and R8 were simulated before purchasing and remeasured afterward. For R9, only the calculated transmission is available. These measured transmission values of R5, R6, and R8 and the calculated transmission values of R9 are listed in Table 1. The beamsplitter R7 transmits 70% of the intensity to the channel that is used for detecting the depolarized backscattered light and 30% of the intensity to the channel that is used for detecting the total backscattered intensity at 355 nm. The beamsplitters R11 and R12 have 50% transmission and 50% reflection properties at 532 nm. Because R8 and R9 reflect light at wavelengths of 532 and 607 nm, respectively, these optical elements are also used for R13 and R10, respectively. R14 depicts a polarizer that is placed in front of the 355 nm-channel to ensure detection of cross-polarized light at 355 nm only. Regarding the optics usage, either the transmission or the reflectivity values of R1 R14 are summarized in Table 1. The optical specifications of the interference filters (Barr) are listed in Table 2. The filters for the elastic channels at , , and nm have a bandwidth (FWHM) of 1 nm, whereas the bandwidth is 0.3 nm for the inelastic channel filters at and nm. For lidar application, another essential filter parameter is the light suppression at wavelengths which

7 2372 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 26 TABLE 1. Reflectivity (r) and transmission (t) of the optical elements at , , , 607.4, and nm. Either transmission or reflectivity values are given as the elements are used in the respective channel. The values of R1, R2, R7, R11, and R12 are from the manufacturer s catalog, whereas the data for R4 were calculated using the material data. The incidence angle of the unpolarized light is 458. The error of the measurement is #0.5%. The transmission of R9 was calculated for unpolarized light and an incidence angle of 458. The calculated values are read with 1% precision from the offered curves. Here, p 5 parallel and s 5 perpendicular. Optical element R1 r r r r r R2 r 0.88 r r 0.98 r 0.97 r 0.95 R4 t 0.26 t 0.92 t t t R5 r p 1; s 1 r p 1; s 1 t p 0.96; s 0.88 t p 0.96; s 0.85 t p 0.98; s 0.83 R6 r 1 t 0.92 R7 r 0.3; t 0.7 R8 r 1 t 0.89 t R9 r $ 0.99 t 0.95 R10 r $ 0.99 R11 r 0.5, t 0.5 R12 r 0.5, t 0.5 R13 r 1 R14 t 0.22 must not be detected. The demand is a suppression of relative to the transmission maximum in the wavelength range between 200 and 1200 nm. Also needed for Raman signal detection is a suppression of relative to the transmission maximum at the wavelength of the emitted laser light. In that way, we avoid crosstalk of elastically backscattered light into the Raman signals. Transmission measurements were performed to check the properties of the filters we use. These measurements showed that all the filters have transmission values below the detection limit of Behind each interference filter a plano-convex lens with a 60-mm focal length is mounted in front of each PMT, except in the 1064-nm channel where a lens with a 100-mm focal length is placed in front. The lenses for the visible channels and the 1064-nm channel are antireflection coated with a typical average residual reflectance of #0.3%. The photon-counting PMTs are Hamamatsu H5783P for the UV channels (355, 355s, and 387). The cathode sensitivity of these PMTs is at least 50 ma lm 21, which is a radiant sensitivity of U44.3 ma W 21 at 420-nm wavelength. That radiant sensitivity results in a quantum efficiency of 13%. The dark counts of these PMTs are #27 s 21, which cause low noise and hence a signal-to-noise ratio that is sufficiently high for the measurements. The Hamamatsu H7422P-40 PMTs are used for the visible channels (532 and 607 nm). These PMTs are also used in the photon-counting mode. The cathode radiant sensitivities are U172 ma W 21 at 550 nm, which is equal to quantum efficiencies of U39%. The dark counts of these PMTs are #40 s 21, which is an acceptable signalto-noise ratio for the measurements. The PMTs H7422P-40 are cooled by thermoelectric coolers (Hamamatsu). The detection of the backscattered light at 1064-nm wavelength is more challenging. A PMT is used because of the need of having a detector with a large photosensitive area. This PMT is Hamamatsu R3236; it has a cathode radiant sensitivity U0.5 ma W 21 at 1064 nm, which is equal to a quantum efficiency of U0.06%. The dark count rate is #1000 s 21 at 2308C. The PMT R3236 is cooled by a thermoelectric cooler with a heat-to-air exchanger (Products for Research). The analog channel (denoted as 532a in Fig. 3) is used to visualize the layered structure of the atmosphere with higher temporal resolution. Here, the PMT module Hamamatsu H is used. Its multialkali cathode has a maximum anode current of 100 ma and a typically dark current of 0.4 na. Thus, this PMT offers a higher dynamic range than the 14-bit data acquisition. The pulses of the PMTs, except the PMT in the 1064-nm channel, have #1.5 ns duration. Taking this into account, measurements with less than 10 Mcps have a relative error of less than 1.5%. The PMT in the 1064-nm channel has pulses with 4 ns duration, which yields a relative error of 4.2%. TABLE 2. Measured central wavelength, transmission at peak wavelength, and bandwidth of interference filters. The variabilities are due to two measurements at two pieces of the same coating lot. Central wavelength (nm) Transmission at peak (%) Bandwidth (FWHM; nm)

8 NOVEMBER 2009 A L T H A U S E N E T A L FIG. 4. The electronic layout of Polly XT. Details are explained in the text. Absorptive neutral density filters (Edmund Optics) are installed in front of each detection channel to attenuate the received light to the respective count rates. While running the system, it is necessary to install neutral density filters in each channel to reduce the maximum count rate to 10 Mcps. Therefore, there is still a buffer to adjust for optimum count rates if the system performance degrades in time by means of laser power, optical transmission, or detector quantum efficiencies. Then, compensation can be achieved by removing some of the filters. The installed camera (CAM) is used for monitoring and adjusting the overlap of the emitted beam to the receiver FOV. The lens (Linos) in front of the camera has a focal length of 60 mm. c. Electronics and remote control The signals of the photon-counting PMTs are adapted to the data acquisition (DAQ) cards by preamplifiers (Phillips Scientific). These preamplifiers have a voltage gain of 10 and a 3-dB bandwidth between 100 khz and 1.8 GHz. The current amplifier (Femto-Messtechnik) that is used for the analog channel 532a has a transimpedance of 100 kv A 21 and a 3-dB bandwidth of 10 MHz. A sketch is shown in Fig. 4 that illustrates the essential electronic parts together with the system parts and the sensors. The entire lidar is controlled by a single computer, which is equipped with several interfaces. The laser is controlled with the system computer via an RS232 interface. Remote power management of the laser is realized by a power distribution stripe with LAN capabilities. The data are acquired with three 2-channel photoncounting cards (FAST ComTec). These cards have a 400-MHz count rate capability and 200-ns bin width. We also use a 2-channel, 14-bit transient recorder (Spectrum). This latter card has a maximum digitizing rate of 20 MSamples per second. The DAQ is triggered by a fast photodiode (Thorlabs) that responds to the laser pulse. A USB multifunction DAQ (NI-6009) is utilized to control the roof cover, to monitor the data of the rain sensor, to acquire the data of the temperature sensors inside the cabinet, and to record the data from an additional external laser power meter. The resistances of two platinum 100 (Pt 100) (resistance thermometer of platinum having 100-ohm resistance at 08C) sensors that are used to monitor the temperature inside the cabinet are converted to voltages by tranducers (LKM electronic) before they are passed to the analog-to-digital converters (ADCs) of the USB-multifunction DAQ. During the lidar measurements, the air temperature and the air pressure are measured by a datalogger system (Lufft) and transferred to the system computer with an RS232 interface. Inside the receiver a circuitry with a microprocessor has been installed for controlling the high voltage at each PMT, the status of the H PMTs, and the Peltier coolers of those PMTs. Thus, it is possible to switch on and off the complete lidar from the system computer and start the measuring program. The computer and the UPS are 19-in. rack system units. The UPS (Newave USV Systems) has a power of 3 kva and consists of a controller and a battery rack. Finally, according to the measuring schedule, the DAQ software opens the roof, starts the laser, monitors the system status, starts data acquisition, and records the data. All data are stored in Network Common Data Form (NetCDF). Having all this hardware, it is straightforward to extend such a system to a completely remote-controlled unit. Collected data, major system settings, and status information can be accessed via an Internet connection. The Internet connection covers, for instance, the following features: d computer remote access, directly and via a keyboard, video, and mouse (KVM) switch; d laser control; d high-voltage control for the PMTs; d housekeeping (e.g., rain detection, temperature control); d UPS monitoring; and d connection to a meteorological station. The monitoring of the temperatures inside the cabinet helps especially during the system setup and installation at field sites. 4. Data analysis In this section, the determination methods for particle parameters are summarized starting from the measured quantities of the Polly XT instrument and ending with the

9 2374 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 26 publication on the institute s homepage. First, the preconditioning of the signals and the derivation of the particle optical properties are described. Second, the determination of the microphysical particle parameters by a separate inversion algorithm is explained. At the end, a brief description of an automatic algorithm for the determination of the planetary boundary layer (PBL) top height is given, as well as some remarks to the Web publishing. a. Particle backscatter, extinction coefficient, lidar ratio, and error determination For further retrieval, the raw signals are preconditioned by temporal averaging and background correction. After this, an overlap correction is applied (Wandinger and Ansmann 2002). The particle extinction a P l and the particle backscatter b P l 0 coefficient profiles are determined from 0 the elastic backscattered lidar signal P l0 (z) and the inelastic backscattered lidar signal P lr (z) by using the Raman lidar method (RLM; Ansmann et al. 1990). Here, either the U.S. Standard Atmosphere, 1976 or radiosonde data are used to account for the molecular backscattering (Elterman 1968). During daytime, only measurements up to 2-km height are possible because of daylight background. Then, only the elastically backscattered signal can be used and the Fernald Klett method (FKM; Fernald 1984; Klett 1981) is employed for the determination of particle backscatter b P l coefficient profiles. The intensive particle properties are determined, too: namely, the lidar ratio, 0 S P l 5 a P 0 l /b P 0 l ;theångström exponents of the extinction 0 coefficient, Å P a (355, 532) 5 ln(ap l 355 /a P l 532 )/ln(532/355); and the backscatter coefficient, ÅP b (355, 532) 5 ln(bp l 355 / b P l 532 )/ln(532/355) and ÅP b (532, 1064) 5 ln(bp l 532 /b P l 1064 )/ ln(1064/ 532). The total (superscript T) and the particle (superscript P) depolarization D are computed with the polarization lidar method (PLM; Murayama et al. 1999; Freudenthaler et al. 2009) by using the signal P? 355 (z)ofthe 355s channel, the signal P 355 (z) of the 355 channel, and b P l 355. During all procedures, the statistical nature of the backscattered light is taken into account for the error analysis. Table 3 summarizes the retrieved quantities, the used input signals, and the applied methods. b. Concepts for the automatic retrieval algorithm To automatically get the reference value for the particle backscatter coefficient b P l 0 (z), first, measurements periods that can be analyzed must be identified and discriminated from measurements periods that do not allow a further analysis. This is done similarly to Turner et al. (2002) by using the aerosol scattering ratio ASR(z) 5 [b M l (z) 1 bp l (z)]/bm l (z) P l 0 (z)/p lr (z), where b l M (z) is the molecular backscatter coefficient. Because P lr (z) is only available from Raman measurements, for TABLE 3. Basic optical particle properties and required inputs (explained in the text). The retrieval methods are RLM, FKM, and PLM. Parameter Input Method b P 355 (z) P 355(z), P 387 (z) RLM, FKM b P 532 (z) P 532(z), P 607 (z) RLM, FKM b P 1064 (z) P 1064(z), P 607 (z) RLM, FKM a P 355 (z) P 387(z) RLM a P 532 (z) P 607(z) RLM S P 355 a P 355, bp 355 S P 532 a P 532, bp 532 Å P b (355, 532) b P 355, bp 532 Å P b (532, 1064) b P 532, bp 1064 Å P a (355, 532) a P 355, ap 532 D T 355 (z) P? 355 (z), P 355 (z) PLM D P 355 (z) P? 355 (z), P 355 (z), bp l PLM 355 daytime data retrieval P lr (z) is estimated by using a synthetic molecular signal and a constant factor. This factor is retrieved from comparisons of daytime with nighttime measurements. Within the reference height interval (RHI), the slope and the standard deviation of ASR must be below the determined thresholds. Both requirements ensure measurements without optically thick aerosol layers and clouds below and in the RHI. The algorithm for the determination of z 0 and b P l (z 0 0 ) starts in setting the RHI between 7000 and 9000 m. If this height range can be supposed to be relatively clean and cloud free, then it will be used for the determination of the reference value. If not, then the RHI is set to lower values until a minimum top height of 4000 m. Having the reference height z 0 fixed, the iteration of Fernald Klett and Raman algorithm starts with the start value of b P l 0,input (z 0 ) km 1 sr 1. From the derived backscatter profile, also the mean value within the RHI b P l 0,calc (z 0 ) is computed. For each iteration step, the input value of b P l 0,input (z 0 ) is increased by 10% until the relative difference between b P l 0,input (z 0 ) and b P l 0,calc (z 0 ) is less than 5%. c. Microphysical particle properties by inversion from the optical properties A data inversion algorithm is applied to derive particle microphysical properties from particle backscatter coefficients measured at three wavelengths (355, 532, and 1064 nm) and particle extinction coefficients measured at two wavelengths (355 and 532 nm; Müller et al. 1999b, 2001; Veselovskii et al. 2002). Ansmann and Müller (2005) present a complete summary of the inversion algorithm, including a detailed, updated description of all inversion steps. A list of further references is given there as well. The inversion code provides approximations of volume size distributions. These size distributions are subsequently used to calculate particle effective radius,

10 NOVEMBER 2009 A L T H A U S E N E T A L volume and surface-area concentration, and the complex refractive index. Information on volume size distribution and complex refractive index is used to calculate the particle single-scattering albedo (ssa) at 532-nm wavelength with a Mie scattering code (Bohren and Huffman 1983). d. Determination of planetary boundary layer height Additional to the lidar products concerning the optical particle parameters, an automated algorithm for the height determination of the PBL top (based on Brooks 2003) was implemented successfully. This algorithm uses the wavelet covariance transform technique. The wavelet is the Haar function, a step function defined by the translation b and the dilation a. By performing the covariance transform between the Haar function and the range-corrected lidar signals, the algorithm seeks significant steps in the lidar signal. The first significant step is defined as the PBL top height. This method depends on the parameters a and b. Further details and results from a 1-yr analysis of PBL top height determined with Polly can be found in Baars et al. (2008). e. Web publishing Near-real-time data analysis is guaranteed by automatic transfer of the measurement data via the Internet to a server in Leipzig. There, the automatic algorithm processes the data and presents a quicklook of the temporal development of the range-corrected signal and the PBL height at the Web server (available online at 5. Measurement example After completion of construction and first test measurements in fall 2007, the Polly XT s were installed in FIG. 5. Field site near Manaus, Brazil. (left) The satellite dish for Internet connection and (right) the Polly XT system with a tent for the service personnel. Brazil and India in the frame of the European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI; Kulmala et al. 2009). The measurements in Brazil were taken from January to November The observations in India started in March 2008 and ended in March To demonstrate the operational capability, an example from an evening measurement at the Brazilian field site near Manaus (28499S, 60829W, height about 80 m) on 15 August 2008 is presented and discussed in the following. The Brazilian field site is shown in Fig. 5. The temporal development of the range-corrected lidar signal at 1064 nm between 2235 and 2335 UTC 15 August 2008 is shown in Fig. 6. In the upper tropical troposphere between 8- and 16-km height, cirrus clouds were observed. In the lower troposphere, a lofted aerosol FIG. 6. Temporal development of the range-corrected signal at 1064-nm wavelength between 2235 and 2335 UTC 15 Aug 2008 (LT 5 UTC 2 4 h).

11 2376 J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y VOLUME 26 FIG. 7. Measurement between 2235 and 2335 UTC 15 Aug The data smoothing of the particle (a) backscatter, (b) extinction, and (d) Ångström coefficient is 750 m below 2.8 km and 1510 m above. (c) The particle lidar ratio is smoothed with 1510 m. (e) The vertical bars at the values of ssa and effective radius r eff indicate the data height range that was used for inversion. layer at about 2.5 km and the 1.5-km-deep residual layer can be seen. In Figs. 7a,b, the corresponding profiles of the particle backscatter and extinction coefficient at 355-, 532-, and 1064-nm wavelengths of the aerosol layers for the 1-h time period are plotted. For the retrieval of these data, the radiosonde data from the Manaus airport (3.158S, W, height about 84 m) at 0000 UTC 16 August 2008 are used. Reference values for the particle backscatter coefficient of 0 km 21 sr 21 at 355 and 532 nm and km 21 sr 21 at 1064 nm are used for the reference height interval between 7000 and 7500 m. Very low depolarization (not shown) was detected up to a height of 7 km during this evening. Clearly, aerosol layer structures can be identified up to the height of about 4600 m in the backscatter profile. This parameter has been retrieved by the Raman method, which is insensitive to the influence of the overlap correction. This allows a data analysis down to 400 m above ground. In contrast, the determination of the particle extinction coefficient is affected by the overlap function of the system; hence, the raw data were corrected before calculating the extinction coefficient. Nevertheless, the values of the extinction coefficient below 1000 m should be used carefully. Having these particle parameters determined independently, the lidar ratio profile and the Ångström exponent of the extinction are calculated (Figs. 7c,d). The lidar ratio shows values from 40 to 60 sr 21 for both wavelengths. The extinction-related Ångström exponent Å a P (355, 532) and the backscatter-related Ångström exponent Å b P (355, 532) are rather high (close to 2) and thus indicate small particles. According to backward trajectories, the lofted aerosol layer (2 5-km height) was advected from biomass burning areas south of the lidar site. The inversion of the microphysical data from the optical data yields to an effective radius of the particle size distribution of mm and a single-scattering albedo of (cf. Fig. 7e). The small particle size and the corresponding values for the single-scattering albedo also lead to the conclusion that the observed aerosol load is dominated by biomass burning aerosol advected from regional fire spots near the field site (local/ regional smoke). In contrast to flaming fires, smoldering fires cause aerosols that show a high single-scattering albedo (Müller et al. 2005). This is also supported by backtrajectory analysis. Acknowledgments. Parts of this work were supported by the European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) with Contract REFERENCES Althausen, D., D. Müller, A. Ansmann, U. Wandinger, H. Hube, E. Clauder, and S. Zörner, 2000: Scanning 6-wavelength 11-channel aerosol lidar. J. Atmos. Oceanic Technol., 17, , R. Engelmann, R. Foster, P. Rhone, and H. Baars, 2004: Portable Raman lidar for determination of particle backscatter and extinction coefficients. Proc. 22nd Int. Laser Radar Conf., Basilicata, Italy, European Space Agency, SP-561, 8386.,, H. Baars, B. Heese, and M. Komppula, 2008: Portable Raman lidar Polly XT for automatic profile measurements of aerosol backscatter and extinction coefficient. Proc. 24th Int. Laser Radar Conf., Boulder, CO, NOAA NASA, Ansmann, A., and D. Müller, 2005: Lidar and atmospheric aerosol particles. Lidar: Range-Resolved Optical Remote Sensing of the Atmosphere, C. Weitkamp, Ed., Springer, , M. Riebesell, and C. Weitkamp, 1990: Measurements of atmospheric aerosol extinction profiles with a Raman lidar. Opt. Lett., 15,

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