Implementation of UV rotational Raman channel to improve aerosol retrievals from multiwavelength lidar

Abstract

Vibrational Raman effect is widely used in atmospheric lidar systems, but rotational Raman present several advantages. We have implemented a new setup in the ultraviolet branch of an existing multiwavelength lidar system to collect signal from rotational Raman lines of Oxygen and Nitrogen. We showed that, with an appropriate filter wavelength selection, the systematic error introduced in the particle optical properties due to temperature dependence was less than 4 %. With this new setup, we have been able to retrieve aerosol extinction and backscatter coefficients profiles at 355 nm with 1-h time resolution during daytime and up to 1-min time resolution during nighttime.

A Comparative Analysis of Aerosol Optical Coefficients and Their Associated Errors Retrieved from Pure-Rotational and Vibro-Rotational Raman Lidar Signals

“…As part of the discussion we aim to compare our results with some recent bibliographic endeavors, some of which have already been used for comparison in the temperature analysis Section 2.4. This section is focused on the measurements and the products obtained by [11] at 355 nm, [7] at 532 nm and [21] at 1064 nm.…”

“…They reported a temperatureinduced variation for the backscatter coefficient less than 1% and for the aerosol extinction coefficient error less than 2 Mm −1 . In reference [11] (λ 0 = 354.75 nm), the authors report a temperature variation for the backscatter coefficient was less than 4%. The extinction coefficient error is reported with an always negative variation of up to −1.6 Mm −1 .…”

“…To quantify the temperature-induced variations for the estimation of backscatter and extinction coefficients, we rely on the theory developed in [20] and afterwards employed for pure rotational Raman lidar applications by [7,11]. In these works, expressions for the calculation of backscatter and extinction coefficients are developed considering the temperature dependency:…”

“…To overcome these limitations pure rotational Raman (PRR) scattering can be used instead of vibro-rotational scattering. While PRR scattering has been widely used in laser remote sensing for the measurement of atmospheric temperature [8], its use for aerosol remote sensing is more recent and thus less common [7,[9][10][11] and has been prompted by the availability of compact interference filters able to select a suitable group of N 2 and O 2 PRR spectrum lines, while providing enough rejection at the close excitation wavelength. The small spectral separation between the elastic and PRR lines implies that the spectral dependence of aerosol extinction has virtually no effect on the retrieval of the extinction and backscatter coefficients.…”

“…Also, the total differential backscatter cross-section of both N 2 and O 2 PRR is greater than that of vibro-rotational scattering. N 2 PRR measurements have been used successfully to retrieve extinction coefficients at 532 [7] and 1064 nm [11], and the first tests have been performed at 355 nm [11].…”