Aerosol-type classification based on AERONET version 3 inversion products

© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License.This study proposes an aerosol-type classification based on the particle linear depolarization ratio (PLDR) and single-scattering albedo (SSA) provided in the AErosol RObotic NETwork (AERONET) version 3 level 2.0 inversion product. We compare our aerosol-type classification with an earlier method that uses fine-mode fraction (FMF) and SSA. Our new method allows for a refined classification of mineral dust that occurs as a mixture with other absorbing aerosols: pure dust (PD), dust-dominated mixed plume (DDM), and pollutant-dominated mixed plume (PDM). We test the aerosol classification at AERONET sites in East Asia that are frequently affected by mixtures of Asian dust and biomass-burning smoke or anthropogenic pollution. We find that East Asia is strongly affected by pollution particles with high occurrence frequencies of 50 % to 67 %. The distribution and types of pollution particles vary with location and season. The frequency of PD and dusty aerosol mixture (DDM+PDM) is slightly lower (34 % to 49 %) than pollution-dominated mixtures. Pure dust particles have been detected in only 1 % of observations. This suggests that East Asian dust plumes generally exist in a mixture with pollution aerosols rather than in pure form. In this study, we have also considered data from selected AERONET sites that are representative of anthropogenic pollution, biomass-burning smoke, and mineral dust. We find that average aerosol properties obtained for aerosol types in our PLDR–SSA-based classification agree reasonably well with those obtained at AERONET sites representative for different aerosol types.Peer reviewe

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

Knowledge of the particle lidar ratio (Sγ) and the particle linear depolarisation ratio (δγ) 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 Sγ and δγ 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 Angström exponent < 0:4 and a fine-mode fraction < 0:1. The values of spectral Sγ 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 Sγ D 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 Sγ 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 δγ shows a maximum of 0.26-0.31 at 1020 nm and decreasing values as wavelength decreases. AERONET-derived δγ 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.Peer reviewe

3+2 + X: what is the most useful depolarization input for retrieving microphysical properties of non-spherical particles from lidar measurements using the spheroid model of Dubovik et al. (2006)?

The typical multiwavelength aerosol lidar data set for inversion of optical to microphysical parameters is composed of three backscatter coefficients (β) at 355, 532, and 1064 nm and two extinction coefficients (α) at 355 and 532 nm. This data combination is referred to as a 3β+2α or 3+2 data set. This set of data is sufficient for retrieving some important microphysical particle parameters if the particles have spherical shape. Here, we investigate the effect of including the particle linear depolarization ratio (δ) as a third input parameter for the inversion of lidar data. The inversion algorithm is generally not used if measurements show values of δ that exceed 0.10 at 532 nm, i.e. in the presence of non-spherical particles such as desert dust, volcanic ash, and, under special circumstances, biomass-burning smoke. We use experimental data collected with instruments that are capable of measuring δ at all three lidar wavelengths with an inversion routine that applies the spheroidal light-scattering model of Dubovik et al. (2006) with a fixed axis-ratio distribution to replicate scattering properties of non-spherical particles. The inversion gives the fraction of spheroids required to replicate the optical data as an additional output parameter. This is the first systematic test of the effect of using all theoretically possible combinations of δ taken at 355, 532, and 1064 nm as input in the lidar data inversion. We find that depolarization information of at least one wavelength already provides useful information for the inversion of optical data that have been collected in the presence of non-spherical mineral dust particles. However, any choice of δλ will give lower values of the single-scattering albedo than the traditional 3+2 data set. We find that input data sets that include δ355 give a spheroid fraction that closely resembles the dust ratio we obtain from using β532 and δ532 in a methodology applied in aerosol-type separation. The use of δ355 in data sets of two or three δλ reduces the spheroid fraction that is retrieved when using δ532 and δ1064. Use of the latter two parameters without accounting for δ355 generally leads to high spheroid fractions that we consider not trustworthy. The use of three δλ instead of two δλ, including the constraint that one of these is measured at 355 nm does not provide any advantage over using 3+2+δ355 for the observations with varying contributions of mineral dust considered here. However, additional measurements at wavelengths different from 355 nm would be desirable for application to a wider range of aerosol scenarios that may include non-spherical smoke particles, which can have values of δ355 that are indistinguishable from those found for mineral dust. We therefore conclude that – depending on measurement capability – the future standard input for inversion of lidar data taken in the presence of mineral dust particles and using the spheroid model of Dubovik et al. (2006) might be 3+2+δ355 or 3+2+δ355+δ532.Peer reviewe

Utilization of the depolarization ratio derived by AERONET Sun/sky radiometer data for type confirmation of a mixed aerosol plume over East Asia

This is an accepted manuscript of an article published by Taylor & Francis Group in International Journal of Remote Sensing on April 24, 2016. Subject to 12 months' embargo period, embargo end date: April 24, 2017 Young Min Noh, Sung-Kyun Shin, Kwonho Lee, Detlef Müller & Kwanchul Kim (2016) Utilization of the depolarization ratio derived by AERONET Sun/sky radiometer data for type confirmation of a mixed aerosol plume over East Asia, International Journal of Remote Sensing, 37(10): 2008-2025, DOI: 10.1080/01431161.2016.1176274This article confirms the utilization of depolarization ratio derived by ground-based Aerosol Robotic Network (AERONET) Sun/sky radiometer data obtained during a high-PM10 episode at Gwangju, Korea (35.10° N, 126.53° E) in April 2009, in order to determine the nature and source of the atmospheric aerosol associated with this event. Integrated monitoring using satellite and depolarization light detection and ranging (lidar) data, together with model analysis, was also completed for the period of the high-PM10 event. The Sun/sky radiometer-derived particle depolarization ratio values are similar to the lidar-derived values, and these values highlight the effect of dust particles on aerosol observation. High particle depolarization ratios (12.5– 14.2%) were observed when the aerosol plume transported from the west between 5 and 7 April. In contrast, lower particle depolarization ratios (5.8–9.8%) were detected when the aerosol plume was transported from the north on other observation days. Different optical properties are also shown according to the variation of depolarization ratio. High values in the real part of the refractive index (1.47–1.49 at 440 nm), lower values in the imaginary part of the refractive index (0.007–0.009 at 440 nm), and a high proportion of coarser particles were observed during the high depolarization ratio period. In contrast, the atmospheric aerosol transported from the north showed characteristics more commonly associated with smoke, with lower values in the real part of the refractive index (1.41–1.48 at 440 nm), higher values in the imaginary part of the refractive index (0.008–0.011), and a high proportion of fine particles. This indicates that the Sun/sky radiometer-derived depolarization ratio is a useful parameter when estimating the effect of dust particles during high-PM10 events.Peer reviewedFinal Accepted Versio