Abstract
This paper aims to quantify the improvement obtained with a purely rotational Raman (PRR) channel over a vibro-rotational Raman (VRR) channel, used in an aerosol lidar with elastic and Raman channels, in terms of signal-to-noise ratio (SNR), effective vertical resolution, and absolute and relative uncertainties associated to the retrieved aerosol optical (extinction and backscatter) coefficients. Measurements were made with the European Aerosol Research Lidar Network/Universitat Politècnica de Catalunya (EARLINET/UPC) multi-wavelength lidar system enabling a PRR channel at 353.9 nm, together with an already existing VRR (386.7 nm) and an elastic (354.7 nm) channels. Inversions were performed with the EARLINET Single Calculus Chain (SCC). When using PRR instead of VRR, the measurements show a gain in SNR of a factor 2.8 and about 7.6 for 3-h nighttime and daytime measurements, respectively. For 3-h nighttime (daytime) measurements the effective vertical resolution is reduced by 17% (20%), the absolute uncertainty (associated to the extinction) is divided by 2 (10) and the relative uncertainty is divided by 3 (7). During daytime, VRR extinction coefficient is retrieved in a limited height range (<2.2 km) preventing the SCC from finding a suitable calibration range in the search height range. So the advantage of using PRR instead of VRR is particularly evidenced in daytime conditions. For nighttime measurements, decreasing the time resolution from 3 to 1 h has nearly no effect on the relative performances of PRR vs. VRR.
Highlights
Aerosol remote sensing with elastic-only lidar instruments has the drawback that the effects of the aerosol backscatter coefficient and the aerosol extinction coefficient appear in an indistinguishable way—without more or less plausible further assumptions—in the received signal [1]
In most cases this technique is limited to nighttime measurements because the noise induced by the daytime background solar radiation passing through the Raman-channel interference filter swamps the Raman signal provided by the low differential backscatter cross-section of the vibro-rotational Raman (VRR) spectrum [7]
In N1 and N2 aerosols were of local origin and accumulated mostly in the planetary boundary layer (PBL): AOD440 is equal to 0.15 and AE440–675 to 1.27
Summary
Aerosol remote sensing with elastic-only lidar instruments has the drawback that the effects of the aerosol backscatter coefficient and the aerosol extinction coefficient appear in an indistinguishable way—without more or less plausible further assumptions—in the received signal [1]. The principle lies in that, for a purely molecular atmosphere, the law followed by the molecule-specific Ramanshifted radiation collected by the lidar receiver is known, as it only depends (assuming that it does not fall in the absorbing spectrum of an atmospheric gas) on the species number concentration and the molecular scattering; departures from this known law can be related to the extinction introduced by the aerosols. This technique has been used for nearly three decades [4,5,6]. The significant wavelength shift of the VRR spectrum with respect to the excitation wavelength introduces an additional source of uncertainty, as an assumption about the spectral dependence of aerosol extinction is needed for the retrieval of both the extinction and backscatter coefficients [3]
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