LIDAR INNOVATIONS FOR TECHNOLOGIES AND ENVIRONMENTAL SCIENCES (LITES) – AN REMOTE SENSING INFRASTRUCTURE FACILITY: SETUP AND MEASUREMENTS EXAMPLES

At the University of Hertfordshire, we have been developing a new remote sensing facility (LITES) to explore the feasibility of using Raman and/or fluorescence backscattering for chemical aerosol profiling. This paper provides an overview of the instruments of the facility and measurement examples. LITES includes a ultra-high-energy Nd:YAG/OPO setup, spectroscopic equipment with high spectral resolution, several imaging and single detectors that allow for time-resolved (lidar) signal detection, a Raman/fluorescence microscope, and a suite of gas and aerosol chambers. We present examples of elastic, rotational and vibrational spectroscopic lidar signals, as well as in-situ microscopic spectrums of dust and bio-aerosol compounds.


INTRODUCTION
Lidar is the only remote measurement method that allows for spatially (vertically) and temporally highly-resolved observations of atmospheric state parameters, trace gases and particulate pollution [1,2]. The combination of various lidar techniques (nitrogen Raman and high spectral resolution lidar) with mathematical inversion algorithms nowadays allows for temporally and vertically resolved observations of aerosol optical and microphysical properties [3,4,5]. Differential absorption lidar is another concept that allows for quantitative rangeresolved measurements of the concentration of chemical components in the atmosphere [6]. However, lidar and passive remote sensing instruments currently cannot provide us with vertically-resolved information on the chemical composition of aerosols under ambient atmospheric conditions. We are exploring the potential of a multi-channel high-resolution spectrometric lidar for the vertically-resolved chemical characterization of aerosols and reactive gases by measuring range-resolved Raman and fluorescence/photoluminescence spectra. For that purpose, we have been developing a new remote sensing facility, called Lidar Innovations for Technologies and Environmental Sciences (LITES). The facility combines a unique Lidar Spectroscopy Instrument (LiSsI) with in-situ instruments such as Raman and fluorescence microscope, gas and aerosol chambers.
In this work we present the setup of LITES and show measurement examples. LiSsI allows for the profiling of trace gases, chemical components in particles, and bio-aerosols in atmospheric aerosol pollution in the troposphere through combining different non-linear spectroscopy techniques (photoluminescence, fluorescence, Raman and coherent anti-Stokes Raman spectroscopy) on a single measurement platform.

INSTRUMENTS
The instruments of LITES were designed to operate as a single platform that allows for an easy switch between different experimental setups. Most of the equipment parts can be used for more than one instrument component of LITES. Table 1 summarizes the specifications of the system.

The lidar system
The main components of the lidar subsystems include: -an ultra-high-energy seeded Nd:YAG laser (Continuum Powerlite Furie LD) with SHG and THG. This system emits laser light with energies up to 8 J per pulse (at the fundamental 1064-nm wavelength). This output corresponds to a pulse power of more 500 GW and an average power of more than 50 W; -an Optical Parametric Oscillator (Horizon OPO) that is optically pumped by the main laser. The OPO emits laser light from the deep UV (192 nm) to the near infrared (2750 nm); EPJ Web Conferences 237, 07017 (2020) ILRC 29 https://doi.org/10.1051/epjconf/202023707017 -a customized motorized beam combiner for selecting the wavelengths. The beam combiner allows for switching between seven modes, namely emission of 1) 1064 nm only, 2) 532 nm only, 3) 355nm only, 4) 1064 nm and 532 nm (simultaneously), 5) 1064 nm and 532 nm and 355 nm (simultaneously), as well as 6) optimized OPO pumping, and 7) side-port emission of laser light for non-lidar applications; -a Horiba 1250M imaging spectrometer with four exchangeable gratings, dual entrance ports/slits and two exits ports -one imaging (front) and one high-spectral resolution (side) port/slit; -three imaging detectors that can be used with the spectrometer; -an intensified CCD camera (ICCD, Princeton Instruments PI-MAX4 1024i-HBf); -a 32-channel Licel PMT; -an EMCCD (ANDOR iXon 3); -three kinds of single detectors -Photomultiplier tubes, InGaAs ADP, and silicon diode.
The system allows for using three types of acquisition approaches by which we can detect lidar signal on the basis of a) the use of single detectors, b) simultaneous detection of multichannel spectroscopically-resolved lidar signals, c) determination of Raman/fluorescence spectrums by point-to point swapping of wavelengths, d) swapping of the range of wavelengths for the imaging detectors we use for LITES.
Depending on the setup of the experiment, the lidar instrument can be used as a a) multiwavelength elastic backscatter lidar for measurements of aerosols and temperature from the troposphere to the mesosphere, b) multichannel spectroscopic Raman lidar (using Stokes and anti-Stokes, rotational, and rotationalvibrational Raman scattering), c) multi-channel spectroscopic photoluminescence/fluorescence lidar, d) high spectral resolution lidar (HSRL), e) polarization lidar, or f) infrared absorption and differential absorption lidar (DIAL).

The Raman and fluorescence microscope
This instrument allows for investigations of Raman and fluorescence spectrums of solid-state aerosol samples taken by in-situ particlecollection methods. The instrument is based on an Olympus BX51TRF-6 microscope. The microscope can use the OPO laser emissions or light from a 532-nm CW DPSS laser that is used as pumping source for delivering Raman and fluorescence spectrums. The microscope is equipped with objective lenses with magnification factors x5, x10, x20, x50, and x100. In that way we obtain a spatial resolution better that 1μm. An additional CMOS imaging camera (xiQ, MQ013CG-) is available for acquiring high spatial resolution reflection or transmission images of the samples in the visible range of radiation. The spectrums can be obtained with the main spectrometer HORIBA 1250M and all detection systems available in the LITES facility.

Customized gas and aerosols gas chambers
LITES also includes a suite of gas and aerosol chambers for the investigation of general optical properties of gases and particles, namely ATR (Attenuation, Transmission, Reactance) and Raman and fluorescence spectrums of gases and aerosols particles. The chambers can simulate the conditions of gas mixtures of different types of air-pollutions, reactive trace gases and aerosol particles under real atmospheric conditions up to tropopause heights.

Pure rotational and vibrational-rotational signals
Results of the first pure-rotational Raman scattering measurements that were obtained with the ICCD camera and the Horiba 1250M spectrometer are presented in Figure 2. The emitted laser intensity was 300 mJ at 354.9 nm. The spectrum was averaged over 10 laser shots and the background has been subtracted. The time gate of the camera was set to 0.03-ȝV which corresponds to 4.5-300.0 m distance (black curve) and 2-ȝV which corresponds to 300-600 m distance (red curve). A 355-nm RazorEdge ultrasteep long-pass edge filter (Semrock LP02-355RE-25) was used to suppress elastic scattering. The spectrometer grating was set to 1200 g/mm with a 50 μm entrance slit that allows for a spectral resolution of less than 5 cm -1 . An example of spectra taken with the rotationalvibrational lidar Raman set-up is shown in Fig. 3.  Figure 4 shows one of the first LiSsI Raman measurements performed between 1726 and 1735 UTC on 3 November 2017 using the laser at 354.9 nm with a power of approximately 300 mJ. The signals were taken with a SP32-20 multi-channel PMT in photon counting mode and averaged over 5400 laser shots. The spectrometer grating was set to 1200 g/mm with a 50-μm entrance slit. Aerosols can be seen in the elastic signal (354.9nm) up to 3-km height above ground. Cirrus clouds are clearly visible between 7.0 and 10.5 km height. Profiles from pure rotational Raman lidar signals from nitrogen molecules were detected at two wavelengths up to approximately 9 km height. Raman signals were also observed in the 364.7-nm channel (754 cm -1 Raman shift) up to 1.4 km height. This signal is most probably caused by nitrogen dioxide (NO2) which has a strong Raman peak at 754 cm -1 .

Microscopic Raman spectra of standard samples and mineral dust
Figure.5. Raman spectrums of: a) polystyrene and de-ionized water and b) corundum (Al2O3) Figure 5 presents examples of Raman spectrums obtained by the Raman microscope of calibration standard samples (polystyrene and de-ionized water) and corundum (Al2O3). The spectral resolution is 0.02nm, we used 150mW@532nm laser power and a grating with 2400gr/mm and slit width of 200μm.
3.5 Microscopic fluorescence spectra of bioaerosols. Figure 6 presents examples of measurements of fluorescence spectra of different types of fresh (Cupressus) and aged (Platanus, Cupressus,) pollen taken from different urban locations at the city of Granada (Spain, 680 m a.s.l.). The spectra were obtained using the laser wavelength 532 nm, a grating with 2400 grooves mm -1 DQG ȝP VOLW width. Gold-coated slides (up to 50-nm thick coating) were used to improve signal intensity.