B Amp;r Automation South Africa

[Faustina Bartsch]

Twoversions 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 3 + 2 Raman lidar PollyXT (3 + 2: three backscatter and two extinction coefficients), the second generation of Polly, is described. The extended capabilities of PollyXT 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 PollyXT.

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 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.

According to the plan for implementation of a Global Atmospheric Watch (GAW) Aerosol Lidar Observation Network (Bsenberg 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 ngstrm 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 (PollyXT) 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 Mller 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 (Mller 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; Mller et al. 2005) and on the numerous field campaigns we conducted during the past 10 yr in Europe (e.g., Ansmann et al. 2002; Mller et al. 2002; Wandinger et al. 2002), Asia (Franke et al. 2003; Mller et al. 2003; Ansmann et al. 2005; Tesche et al. 2007), and Africa (Tesche et al. 2009).

The prototype Polly was developed between April 2002 and September 2003. This lidar is set up on an optical table of 700 mm 1000 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.

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 90. 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 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 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 45 amounts to 96% at 532 nm and the reflectivity amounts to 88% at 607.3 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 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 532.1 and 607.3 nm, respectively, and the suppression of the light at other wavelengths is at least 105 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 108. The transmission windows of both filters have a full width at half maximum (FWHM) of 0.5 nm.

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