Rotational vibrational-rotational Raman differential absorption lidar for atmospheric ozone measurements: methodology and experiment

Depth profiling for the identification of unknown substances and concealed content at remote distances using time-resolved stand-off Raman spectroscopy

Time-resolved stand-off Raman spectroscopy was used to determine both the position and identity of substances relative to each other at remote distances (up to tens of meters). Spectral information of three xylene isomers, toluene, and sodium chlorate was obtained at a distance of 12 m from the setup. Pairs and triplets of these samples were placed at varying distances (10-60 cm) relative to each other. Via the photon time of flight the distance between the individual samples was determined to an accuracy of 7% (corresponding to a few cm) of the physically measured distance. Furthermore, at a distance of 40 m, time-resolved Raman depth profiling was used to detect sodium chlorate in a white plastic container that was non-transparent to the human eye. The combination of the ranging capabilities of Raman LIDAR (sample location usually determined using prior knowledge of the analyte of interest) with stand-off Raman spectroscopy (analyte detection at remote distances) provides the capability for depth profile identification of unknown substances and analysis of concealed content in distant objects. To achieve these results, a 532 nm laser with a pulse length of 4.4 ns was synchronized to an intensified charge-coupled device camera with a minimum gate width of 500 ps. For automated data analysis a multivariate curve resolution algorithm was employed.

Improvements of the aerosol algorithm in ozone lidar data processing by use of evolutionary strategies

The differential absorption lidar (DIAL) at the Institut für Meteorologie und Klimaforschung has been upgraded for precise ozone and aerosol studies in the entire troposphere and the lower stratosphere. Its excellent technical performance offers the opportunity to apply improved data processing. The existing inversion algorithm is extended to derive the optical coefficients from the backscatter profiles for three wavelengths. Correlating the correction terms of the DIAL equation and the ozone concentration yields the wavelength dependence of the backscatter and extinction coefficients of the aerosol. Under some conditions, in particular if homogeneous layers are present, the backscatter-to-extinction ratio and the reference value can also be retrieved. We find the solutions by applying evolutionary strategies. From the optical coefficients obtained in this way the ozone concentration can be calculated with substantially reduced error.

Combined Raman lidar for the measurement of atmospheric temperature, water vapor, particle extinction coefficient, and particle backscatter coefficient

The lidar of the Radio Science Center for Space and Atmosphere (RASC; Kyoto, Japan) make use of two pure rotational Raman (MR) signals for both the measurement of the atmospheric temperature profile and the derivation of a temperature-independent Raman reference signal. The latter technique is new and leads to significant smaller measurement uncertainties compared with the commonly used vibrational Raman lidar technique. For the measurement of temperature, particle extinction coefficient, particle backscatter coefficient, and humidity simultaneously, only four lidar signal are needed the elastic Cabannes backscatter signal, two RR signals, and the vibrational Raman water vapor signal. The RASC lidar provides RR signals of unprecedented intensity. Although only 25% of the RR signal intensities can be used with the present data-acquisition electronics, the 1-s -statistical uncertainty of nighttime temperature measurements is lower than for previous systems and is < 1K up to 11-km height for, e.g., a resolution of 500 m and 9 min. In addition, RR measurements in daytime also have become feasible.

Error analysis of Raman differential absorption lidar ozone measurements in ice clouds

A formalism for the error treatment of lidar ozone measurements with the Raman differential absorption lidar technique is presented. In the presence of clouds wavelength-dependent multiple scattering and cloud-particle extinction are the main sources of systematic errors in ozone measurements and necessitate a correction of the measured ozone profiles. Model calculations are performed to describe the influence of cirrus and polar stratospheric clouds on the ozone. It is found that it is sufficient to account for cloud-particle scattering and Rayleigh scattering in and above the cloud; boundary-layer aerosols and the atmospheric column below the cloud can be neglected for the ozone correction. Furthermore, if the extinction coefficient of the cloud is ?0.1 km(-1), the effect in the cloud is proportional to the effective particle extinction and to a particle correction function determined in the limit of negligible molecular scattering. The particle correction function depends on the scattering behavior of the cloud particles, the cloud geometric structure, and the lidar system parameters. Because of the differential extinction of light that has undergone one or more small-angle scattering processes within the cloud, the cloud effect on ozone extends to altitudes above the cloud. The various influencing parameters imply that the particle-related ozone correction has to be calculated for each individual measurement. Examples of ozone measurements in cirrus clouds are discussed.