We propose to use a Fabry-Perot interferometer (FPI) in a pure rotational Raman lidar to isolate return signals that are due to pure rotational Raman scattering from atmospheric nitrogen against the sky background. The main idea of this instrumental approach is that a FPI is applied as a frequency comb filter with the transmission peaks accurately matched to a comb of practically equidistant lines of a pure rotational Raman spectrum (PRRS) of nitrogen molecules. Thus a matched FPI transmission comb cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines of nitrogen molecules while it is transparent to light within narrow spectral intervals about these lines. As the width of the spectral gaps between the lines of the PRRS of nitrogen molecules is -114 times the width of an individual spectral line, cutting out of the sky background from these gaps drastically improves the signal-to-background ratio of the pure rotational Raman lidar returns. This application of the FPI enables one to achieve daytime temperature profiling in the atmosphere with a pure rotational Raman lidar in the visible and near-UV spectral regions. We present an analysis of application of the FPI to filtering out the pure rotational Raman lidar returns against the solar background. To demonstrate the feasibility of the approach proposed, we present temperature profiles acquired during a whole-day measurement session in which a Raman lidar equipped with a FPI was used. For comparison, temperature profiles acquired with Vaisala radiosondes launched from the measurement site are also presented.

Implementation of the pure-rotational Raman (PRR) lidar method for simultaneous measurement of atmospheric temperature, humidity, and aerosol extinction and backscatter coefficients is reported. The isolation of two wavelength domains of the PRR spectrum and the suppression of the elastically scattered light is carried out by a double-grating polychromator. Experiments involving elastic backscatter from dense clouds and a solid target confirm the high level of suppression of the elastic light in the corresponding acquisition channels of the two selected PRR domains. Calibration of the temperature channel was done both by comparison with an experimentally verified atmospheric temperature model profile and by inter-comparison with radiosondes. Night-time temperature profiles with high vertical resolution were obtained up to the lower stratosphere. The PRR temperature profile combined with the water vapor mixing ratio obtained from the ro-vibrational Raman channel is used to estimate the relative humidity.PACS 42.68.Wt; 42.68.Mj; 33.20.Fb IntroductionSimultaneous measurements of vertical profiles of atmospheric temperature, water vapor, and aerosol optical properties (i.e. backscatter and extinction coefficients) are required for the retrieval and interpretation of the relative humidity (RH) of the atmosphere, as well as the height and the dynamics of the planetary boundary layer (PBL). Vertical temperature and humidity profiles are usually obtained worldwide by systematic radiosonde measurements. The temporal resolution of these observations is quite low, typically two radiosonde launches a day, with a single data readout per height bin. As a result, the measured profiles are often not representative of some important weather phenomena, such as the development of a convective boundary layer or the passage of a cold front, which cannot be resolved due to the lack of time resolution. Furthermore, the standard radiosondes are not equipped with instruments for aerosol measurements.Alternative remote-sensing techniques like lidar can be very useful for supplying temperature, humidity, and aerosol u Fax: +41-21-693-5145, E-mail: valentin.simeonov@epfl.ch data with high temporal and spatial resolution. The two lidar techniques used for temperature profiling are the Rayleigh and the pure-rotational Raman methods. The Rayleigh approach [1] exploits the proportionality of the molecular lidar signal to the atmospheric density. It requires density and pressure data at a relatively high altitude (30-40 km) as a starting point for the retrieval and assumes the existence of hydrostatic equilibrium throughout the entire atmospheric column below this point. In addition, the method is not applicable for atmospheric layers with a significant aerosol load [2] and can be mainly used in stratospheric regions which are essentially free of aerosols [3]. The Rayleigh method can also be used for lower altitudes if a vibrational Raman signal from atmospheric nitrogen is employed to compensate for the aerosol influence [4].Cooney [5...

We propose to use a Fabry-Perot interferometer (FPI) as a comb frequency filter to isolate pure rotational Raman spectra (PRRS) of nitrogen molecules. In making the FPI's free spectral range equal to the spectral spacing between the lines of nitrogen PRRS, which are practically equidistant, one obtains a device with a comb transmission function with the same period. However, to match the FPI transmission comb completely with the comb of nitrogen PRRS lines one should tune the wavelength of the radiation used to excite the PRRS of nitrogen exactly to the position of any minimum in the FPI transmission comb. Thus to achieve this task for the case of nitrogen PRRS one must take the FPI's free spectral range Dnu(f)= 4B(N(2)) and the wavelength of the exciting radiation such that (1/lambda(exc)) = 4B(N(2))(k + 1/2), where B(N(2)) is the rotational constant of the nitrogen molecule and k is an arbitrary integer number. In this case all (odd and even) pure rotational Raman lines of nitrogen will pass through the FPI while the line of exciting radiation is being suppressed. Additionally, a FPI cuts out the spectrally continuous sky background light from the spectral gaps between the PRRS lines.

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