Rui Song

Abstract:

On 1 May 2024, a Saharan dust plume was observed over Hatfield, UK, and analyzed using a novel spectroscopic lidar system (LITES) developed at the University of Hertfordshire. To the best of our knowledge, this represents the first coordinated study over the UK combining lidar optical properties at 532 nm with a spectrally resolved analysis of a long-range transported Saharan dust event. A distinct dust layer was observed between 2–5 km, with lidar-derived optical thickness values ranging from 0.19 to 0.26 at 532 nm, indicating a low-to-moderate Saharan dust intrusion. Within this layer, the lidar ratio was 53 ± 14 sr and the particle linear depolarization ratio was 0.27 ± 0.02, confirming the presence of non-spherical dust particles. After 4 h, despite the reduction in dust loading, the lidar ratios (mean: 51 ± 11 sr) and particle linear depolarization ratios (0.28 ± 0.01) remained characteristic of Saharan dust. Complementary AERONET observations, collected approximately 10 km from the lidar site, recorded low Å ngström exponent values (0.05–0.2 at 440–870 nm) and a high total AOD of 0.5, with coarse-mode AOD contributing 0.37 (74%) at 500 nm, further confirming the dominance of coarse-mode Saharan dust. Spectrally resolved lidar signals revealed distinct Raman-shifted features near 571.0 nm ± 1.3 nm (1284 ± 40 cm−1) and higher backscatter signals in the 542.9–554.4 nm (377-759 cm−1) range, using a primary emission wavelength of 532 nm. These spectral features persisted throughout the dust layer but were absent in the Raman spectrum of a low-level cloud that was embedded in the dust layer during the lidar observation time. These dust spectral features furthermore disappeared above the dust layer, confirming their attribution to Saharan dust. The spectrally resolved observations are consistent with the Raman spectra of dust samples, which were measured in the LITES laboratory using Raman spectroscopy for direct comparison.

Abstract:

Atmospheric aerosols have pronounced effects on climate at both regional and global scales, but the magnitude of these effects is subject to considerable uncertainties. A major contributor to these uncertainties is an incomplete understanding of the vertical structure of aerosol, largely due to observational limitations. Spaceborne lidars can directly observe the vertical distribution of aerosols globally and are increasingly used in atmospheric aerosol remote sensing. As the first spaceborne high-spectral-resolution lidar (HSRL), the Atmospheric LAser Doppler INstrument (ALADIN) on board the Aeolus satellite was operational from 2018 to 2023. ALADIN data can be used to estimate aerosol extinction and co-polar backscatter coefficients separately without an assumption of the lidar ratio. This study assesses the performance of ALADIN's aerosol retrieval capabilities by comparing them with Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) measurements. A statistical analysis of retrievals from both instruments during the June 2020 Saharan dust event indicates consistency between the observed backscatter and extinction coefficients. During this extreme dust event, CALIOP-derived aerosol optical depth (AOD) exhibited large discrepancies with Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua measurements. Using collocated ALADIN observations to revise the dust lidar ratio to 63.5 sr, AODs retrieved from CALIOP are increased by 46 %, improving the comparison with MODIS data. The combination of measurements from ALADIN and CALIOP can enhance the tracking of aerosols' vertical transport. This study demonstrates the potential for spaceborne HSRL to retrieve aerosol optical properties. It highlights the benefits of spaceborne HSRL in directly obtaining the lidar ratio, significantly reducing uncertainties in extinction retrievals.