Characterizing the Saharan dust transported to the UK through lidar ratio, particle depolarization, and spectroscopic measurements

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 Ångström 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.