Compact dual-wavelength depolarization lidar for aerosol characterization over the subtropical North Atlantic

Abstract

We present a comprehensive characterization of the optical properties of various aerosol types based on data collected using a compact dual-wavelength depolarization elastic lidar (532 and 808 nm, CIMEL CE376). This study evaluates the vertical distribution and temporal evolution of aerosols observed in the subtropical North Atlantic region, covering Saharan dust, volcanic aerosols, and fresh and aged wildfire plumes. Measurements were conducted between August 2021 and August 2023, using a modified two-wavelength Klett inversion method to derive the aerosol backscatter and extinction coefficients from CE376 lidar measurements. To assess the performance of the CE376 system, an intercomparison campaign with an MPL-4B lidar (MPLNET) was conducted, with both systems were collocated at the Izaña Atmospheric Research Centre (Canary Islands, Spain). Both instruments depicted the vertical aerosol structure similarly. Discrepancies were primarily attributed to errors in determining the overlap function and depolarization calibration in each instrument, as well as the greater influence of solar background radiation on the CE376 system during daylight. The absolute difference in the volume depolarization ratio (δv) was 0.003, which decreased to 0.002 when only nighttime data were analyzed. The combination of the two channels of the CE376 provided valuable insights into particle linear depolarization (δP), extinction Ångström exponent (EAE (532/808)) and attenuated color ratio (ACR (808/532)). Fresh Saharan dust particles, characterized by large, non-spherical morphology and a well-mixed vertical layer, exhibited the lowest EAE (532/808), the highest ACR (808/532) and δP (532) >0.15. In contrast, smaller particles with quasi-homogeneous morphology were attributed to sulfate aerosols from the early stages of the Cumbre Vieja volcano eruption and aged Canadian wildfire plumes traveling across the Atlantic. These aerosols showed the lowest δP (0.03 for volcanic sulfate and 0.08 for aged wildfire aerosols) and the highest EAE (532/808) (1.5 and 1.2, respectively). Intermediate values of these retrieved parameters were associated with a heterogenous mixture of ash, soot and charred vegetation from fresh local forest wildfires. The retrieved properties underscore the suitability of the CE376 micro-lidar for continuous monitoring and characterization of the temporal and vertical distribution of atmospheric aerosols.