On the spectral depolarisation and lidar ratio of mineral dust provided in the AERONET version 3 inversion product

Abstract

Abstract. Knowledge of the particle lidar ratio (Sλ) and the particle linear depolarisation ratio (δλ) for different aerosol types allows for aerosol typing and aerosol-type separation in lidar measurements. Reference values generally originate from dedicated lidar observations but might also be obtained from the inversion of AErosol RObotic NETwork (AERONET) sun/sky radiometer measurements. This study investigates the consistency of spectral Sλ and δλ provided in the recently released AERONET version 3 inversion product for observations of undiluted mineral dust in the vicinity of the following major deserts: Gobi, Sahara, Arabian, Great Basin, and Great Victoria. Pure dust conditions are identified by an Ångström exponent <0.4 and a fine-mode fraction <0.1. The values of spectral Sλ are found to vary for the different source regions but generally show an increase with decreasing wavelength. The feature correlates to AERONET, retrieving an increase in the imaginary part of the refractive index with decreasing wavelength. The smallest values of Sλ=35–45 sr are found for mineral dust from the Great Basin desert, while the highest values of 50–70 sr have been inferred from AERONET observations of Saharan dust. Values of Sλ at 675, 870, and 1020 nm seem to be in reasonable agreement with available lidar observations, while those at 440 nm are up to 10 sr higher than the lidar reference. The spectrum of δλ shows a maximum of 0.26–0.31 at 1020 nm and decreasing values as wavelength decreases. AERONET-derived δλ values at 870 and 1020 nm are in line with the lidar reference, while values of 0.19–0.24 at 440 nm are smaller than the independent lidar observations by a difference of 0.03 to 0.08. This general behaviour is consistent with earlier studies based on AERONET version 2 products.

Aerosol-type classification based on AERONET version 3 inversion products

“…Figures 1 and 2, presents the mean spectral SSA and volume particle size distributions for the AERONET sites that are mainly affected by anthropogenic pollution, biomass-burning smoke, and Saharan dust. The spectral 25 behaviour of SSA at these sites strongly resembles the ones presented by Dubovik et al (2002), Eck et al (2009), Giles et al (2012), and Shin et al (2018. Dust particles exhibit strong light absorption at short wavelengths and lower absorption in the visible and near infrared wavelength region (Shin et al, 2018).…”

“…The spectral 25 behaviour of SSA at these sites strongly resembles the ones presented by Dubovik et al (2002), Eck et al (2009), Giles et al (2012), and Shin et al (2018. Dust particles exhibit strong light absorption at short wavelengths and lower absorption in the visible and near infrared wavelength region (Shin et al, 2018). Fine-mode particles and hygroscopic particles such as sulphate show nearly neutral spectral dependence of SSA and overall stronger light-scattering, i.e.…”

“…( 3) used values of δ p nd = 0.02 and δ p d = 0.30 for pure dust for Asian dust (Shin et al, 2018). When PLDR was lower than δ p nd or higher than δ p d , R d was set to 0 or 1, respectively.…”

“…Values of PLDR≥ 0.3 indicate nearly pure dust particles (Freudenthaler et al, 2009;Shin et al, 2018), whereas PLDR< 0.1 is representative for anthropogenic pollution or biomass-burning smoke (Burton et al, 2012). External mixtures of mineral dust and non-spherical particles reveal PLDRs in the range from 0.08 to 0.30.…”

Aerosol measurements with shipborne sun-sky-lunar photometer and collocated multiwavelength Raman polarization lidar over the Atlantic Ocean

“…The extinction coefficient was retrieved by Fernald method with assuming the lidar ratios of 60 sr (355 nm), 45 sr (532 nm) and 54 sr (1064 nm) for the dust layer and 25 sr (355, 532, 1064 nm) for the MBL. The lidar ratios at 355 and 532 nm were selected based on nighttime Raman retrieving results, and the lidar ratio at 1064 nm was from AERONET measurements (Shin et al, 2018). Meanwhile, reference values were tuned to achieve the best agreement of AOD between lidar and shipborne CE318-T.…”