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Multiwavelength lidar node development and simulation for a regional tropospheric aerosol monitoring network

This work studies multiwavelength lidar node operation requirements to operate in a regional aerosol monitoring network. Some of the parameters taken into account are simplicity and robustness of the system in continuous and remote operation conditions. Sub-system modularity and accessibility is also contemplated. A numerical simulation is performed on a synthetic atmospheric signal to analyze the behaviour of this system in a) the visible (532 nm) and infrared (1064 nm) spectral regions; b) the main atmospheric compound Raman spectral region (nitrogen, oxygen water vapor). Adding depolarization channels in the 532 nm spectral region is also contemplated.


Introduction
The role that aerosols meet in the atmospheric system is very complex and still not well known. A deeper understanding of its temporal and spatial distribution and of the interaction with other geophysical variables, such as water vapor, are necessary to understand better the dynamics of global climate balance [1].
Several remote sensing instruments placed in ground based stations, aircrafts and satellites are being used to collect aerosols information. Among them, the lidar (acronym of LIght Detection and Ranging) technique has the ability to retrieve both spatial and temporal aerosol information by means of their optical properties [2]. This system operates as radar but instead of using radio waves it operates at the infrared to ultraviolet spectral region, in which particle and molecular scattering are more important. These instruments are usually classified by the type of scattered radiation detected by the system. The most common ones are the elastic and inelastic (Raman) backscatter lidars. Elastic lidars detects the same wavelengths as the ones emitted by the laser source [3]. For weak atmospheric aerosol loads, Rayleigh scattering prevails due to the presence of molecules, while in presence of aerosols aerosol (Mie) scattering is predominant. As the elastic lidar equation has two unknown terms (extinction and backscatter), the solution arrives from an a priori assumption that links them [4]. For the Raman case, the atmospheric scattering of the majoritarian atmospheric compounds is proportional to the molecular concentration. Knowing this profile the extinction can be retrieved. For this reason inelastic Raman scattering is measured when possible [5]. The multiwavelegth lidar system (MWLS) operating at the Lidar Division at Villa Martelli [6], is a powerful instrument for aerosol characterization through the use of multiple emission and detection wavelengths as it combines all the above mentioned measuring techniques.
A new system is being developed to perform measurements at remote geographical areas. This lidar has similar capabilities than the MWLS and it also incorporates new features. Two photomultipliers (PMT) at different polarization angles will measure atmospheric-induced laser depolarization to retrieve aerosol geometry parameters for an additional aerosol characterization [7]. This technique contributes to discriminate liquid from solid aerosols, ice crystal presence in clouds, etc. Also a 32 channel spectrometer combined with a multi-cathode photomultiplier module and a photoncounting acquisition system [8] [9] will help to characterize aerosol composition from its range resolved spectra.

Lidar Objectives
The system will be able to retrieve the geophysical variables presented in Table 1. Among the main requirements for the design of the system are:  Mobility: It has to be easily transported to a remote site in which the system must be able to operate for long periods of time.  Continuous and automatic operation: It has to be able to perform long-term measurements with minimal human intervention.  Modularity: It must be scalable design to allow new features to the system.  Node operation features: Operation and data management are oriented to create a base design for an aerosol monitoring network in Argentina.  Scanning Capabilities (Optional): Capable to monitor aerosols profiles at different lines of sights.  Figure 1 shows the lidar main modules as a block diagram.

System emission
The transmitter is a Quantel Brilliant 20, flash pumped Nd:YAG laser. It delivers short pulses (5 ns) of linearly polarized radiation (> 90% ) at a repetition rate of 20 Hz. The energies at the fundamental, second and third harmonic are 350 mJ (1064 nm), 150 mJ (532 nm) and 90 mJ (355 nm) respectively. This laser was chosen because of its relative high energy per pulse at the visible wavelength (used for Raman backscatter detection) and its polarized emission (for aerosols depolarization studies). The laser beam is redirected to the atmosphere by a broadband dielectric mirror having 99% reflection at the 320 to 1100 nm spectral range.

Receiver
The backscattered light is collected using a Celestron C8-A XLT Schmidt-Cassegrain telescope with a primary mirror diameter of 203 mm diameter and a focal length of 2032 mm. In operation conditions a diaphragm at its focal plane acts as a field stop to narrow the field of view (FOV) to 1 mrad. The reflective and antireflective coatings assures high transmission efficiency at visible wavelengths.

Polychromator setup
After reaching the focal plane, a convex lens collimates the incoming radiation. Depending on the type of measurement, the light can be redirected to the range dependent spectrum analyzer or continue to the filter based polychromator. This last discriminates the elastic backscattered wavelengths (Rayleigh and Mie scattering at 355, 532 and 1064 nm) and the Raman backscattered wavelengths created from the interaction of the the 532 nm laser emission and the atmospheric oxygen, nitrogen and water vapor molecules. The polychromator design scheme is presented in figure 2.   The lidar is designed to detect the polarization of the 532 nm elastic backscatter using a polarizing cube beam splitter placed inside the polychromator close to the corresponding photomultipliers.
All the signals are acquired using Licel TR-20-160 AP transient recorder modules operating at 20 MSPS. Raman signals are measured in photon-counting mode (bandwidth of 0 to 300 MHz and a maximum counting rate of 250 MHz) and elastic signals are measured in analog mode (12 bits, 20 MHz sampling frequency A/D converter). The module performs internal summation of multiple profiles up to a maximum of 4096. Final records are sent by an Ethernet connection to the main computer.

Spectrometer description
The Multi Spectral Lidar Detector (MSLD) from Licel Company collects a continuous spectrum divided in 32 equally separated bands (channels). Each one is a photosensitive area of a multianode photomultiplier module connected to a photon-counting unit acquiring 1024 bin profiles (15.36 km length, with a 15 m bin size). The configuration description is presented in Table 5. The polychromator uses a Raman edge filter (filter S, Figure 2) to remove the strong elastic backscatter lidar return at 532 nm, allowing the transmission of the weak inelastic backscatter lidar signals within visible spectrum range (537-750 nm) with an efficiency exceeding 90 %.

System Overlap Function
The overlap function is the range-dependent fraction of the total illuminated air mass that is seen by the detector while the laser beam propagates trough the atmosphere. This function can be simulated by means of optical ray tracing techniques. For this case the free version of OSLO (Optics Software for Layout and Optimization) [10] [11] was used using the simulation parameters from table 6.
This lidar will use a coaxial configuration in which the laser emission axis coincides with the telescope line of sight. In the simulations, the lidar backscattered signals were projected on an 8 mm in diameter circular targe, corresponding to the abovementioned Hamamatsu PMT module up to a distance of 3.75 km distance which can be considered as infinity. Two FOV were used giving an overall transmission of 89.5 % and 82.5 % with a FOV of 0.98 mrad (aperture 1.8 mm) and 0.5 mrad (aperture 1 mm), respectively.

Mobile Laboratory
The mobile laboratory is being built in a 20 feet's shelter and it has two rooms: one for the lidar instrumentation and the other for the acquisition computers, and eventually a lidar operator. This laboratory is shown on Figure 4.    Figure 5 shows the elastic backscatter simulations results, according to the parameters in table 6.   Figure 6 shows the elastic backscatter simulations, according to table 6.

Summary
The description of a new tropospheric aerosol lidar that will be operating as a monitoring network mobile node is presented. Main features include full Raman detection of the atmospheric majoritarian compounds (nitrogen, oxygen and water vapor) in the visible region, depolarization measurements at 532 nm, and the use of a spectrometer for range and wavelength resolved lidar measurements. The overlap function results shows that the system is appropriate for close-to-ground boundary layer studies, reaching overlap maximum at the 450 m with 1 mrad FOV. Simulations show the desired system lidar profile; absolute values are probably overestimates due to the fact that there is still not a measurement of the overall lidar system optical throughput at the detection stage. Finally possible lidar locations are suggested for a lidar network operation, which is the principal goal of this instrument.