Atmospheric aerosols affect air quality and, in turn, human and ecosystem well-being (WHO, 2013a), and they also play an important role in the Earth’s climate system (IPCC, 2013). In fact, PM (Particles Mater) pollution is probably the most urgent issue in air quality regulation worldwide, and at the same time, it represents one of the biggest sources of uncertainty in current climate simulations. Therefore, vertically resolved measurements of physical and optical properties of aerosol particles are of great interest, and height-resolved observations of these parameters can only be carried out with lidar techniques. However, for lidars based on elastic scattering, an assumption on aerosol extinction-to-backscatter ratio has to be used to retrieve aerosol optical properties. This makes data so obtained not useful for the classification of aerosols. High Spectral Resolution Lidar (HSRL) is an effective technique to measure aerosol optical properties without any assumptions.
The principle of HSRL technique is to measure the vertical profile of aerosol optical properties by separating the Mie signal backscattered by aerosols from Rayleigh signal backscattered by atmospheric molecules on the basis of Doppler effect. Since the Mie and Rayleigh backscattering spectrum are very close, there are three difficulties in developing HSRL: firstly, laser source should have a very narrow spectral bandwidth; secondly, the receiver needs a filter with less than 1pm spectral resolution to separate Mie and Rayleigh signals; thirdly, the receiver must be locked to laser source with an accuracy of tens of MHz. All these difficulties have been solved and the corresponding subsystems have been developed.
The developed laser used in the HSRL employs a 1kHz pulsed seed laser for amplifying to high power 20Hz pulse laser in order to enhance the stability. The laser system comprises five different stages: a narrow bandwidth oscillator (1 kHz), a CW pre-amplifier (1 kHz), a pulsed amplifier (20 Hz) followed by a booster (20 Hz) and a final second harmonic generation stage. The realized spectral bandwidth of the second harmonic light is less than 50MHz.
The developed spectral analysis system is based on the combination of an interference filter, a planar Fabry-Perot interferometer (PFPI) background filter, and a confocal Fabry-Perot interferometer (CFPI) high spectral resolution filter. The bandwidth of the CFPI has been optimized by a system simulation of HSRL. The realized CFPI has been tested and the results meet the requirements.
A frequency-locking subsystem is used to lock the center transmission wavelength of the spectral analysis system to the wavelength of the laser beam. The developed two-stage frequency-locking technique can be used whether the locking laser is a pulsed laser or a continuous laser. The tests show that it is a robust apparatus, with very good stability.