•Optical properties are computed for fractal-like aggregate black carbon (BC) aerosols coated with different substances. Two models are used for these aerosols: (i) the coated aggregate model (model ...I), where coating is added to the voids and surroundings of BC fractal-like aggregates, and (ii) the closed-cell aggregate model (model II), where coating is added concentrically to each monomer of BC fractal-like aggregates. Our results favor choosing the coated aggregate model (model I) to simulate scattering and absorption by coated BC aerosols because this model is morphologically more realistic, and because this model yields mass absorption cross section (MAC) and backscattering linear depolarization ratio (LDR) values that are consistent with field measurements. Moreover the corresponding computed degree of linear polarization (DoLP) and LDR values are very sensitive to changes in the coating volume fraction (fvol) and the coating refractive index (m). With the same absorbing BC core, the MAC value increases steadily with increasing fvol and m values. For example, using a density of 1.8 g/cm3 and BC refractive index 1.95 + i0.79, the calculated MAC values for uncoated BC aggregates range from 6.2 to 6.8 m2/g at 0.55 µm. When coating material is applied to the BC aggregates, the calculated MAC values for model i particles increase to between 9.8 and 13.2 m2/g (depending on m) when fvol = 87.5%. The backscattering LDR values also tend to increase with the increasing m values for the shapes and sizes considered in our study. For model i particles, the backscattering LDR values span a wide range of 4.2% to 27.8% at a wavelength of 0.35 µm at fvol = 87.5% when m increases from 1.33 to 1.55. Our results are relevant to analyses of polarimetric and lidar observations of smoke particles, especially when these particles undergo hygroscopic growth.
Optical properties are computed for fractal-like aggregate black carbon (BC) aerosols coated with different substances. Two models are used for these aerosols: (i) the coated aggregate model (model I), where coating is added to the voids and surroundings of BC fractal-like aggregates; and (ii) the closed-cell aggregate model (model II), where coating is added concentrically to each monomer of BC fractal-like aggregates. Our results favor choosing the coated aggregate model (model I) to simulate scattering and absorption by coated BC aerosols because this model is morphologically more realistic, and because this model yields mass absorption cross section (MAC) and backscattering linear depolarization ratio (LDR) values that are consistent with field measurements. Moreover the corresponding computed degree of linear polarization (DoLP) and LDR values are very sensitive to changes in the coating volume fraction (fvol) and the coating refractive index (m). With the same absorbing BC core, the MAC value increases steadily with increasing fvol and m values. For example, using a density of 1.8 g/cm3 and BC refractive index 1.95 + i0.79, the calculated MAC values for uncoated BC aggregates range from 6.2 to 6.8 m2/g at 0.55 µm. When coating material is applied to the BC aggregates, the calculated MAC values for model I particles increase to between 9.8 and 13.2 m2/g (depending on m) when fvol = 87.5%. The backscattering LDR values also tend to increase with the increasing m values for the shapes and sizes considered in our study. For model I particles, the backscattering LDR values span a wide range of 4.2–27.8% at a wavelength of 0.35 µm at fvol = 87.5% when m increases from 1.33 to 1.55. Our results are relevant to analyses of polarimetric and lidar observations of smoke particles, especially when these particles undergo hygroscopic growth.
•On-line coupling of HF5 to depolarized MALS is demonstrated.•HF5/D-MALS applied to study of a Teflon latex.•Depolarization ratio of latex measured continuously across fractogram.
Introduced here is ...the on-line coupling of hollow-fiber flow field-flow fractionation (HF5) to depolarized multi-angle static light scattering (D-MALS). HF5 is a size-based separation alternative to size-exclusion and hydrodynamic chromatography and asymmetric flow field-flow fractionation. HF5 can separate larger sizes than its chromatographic counterparts and provides several advantages over its fractionation counterpart, including reduced sample consumption and greater ease of operation. D-MALS is a variant of MALS in which the depolarized scattering from the analyte solution is measured at a variety of angles simultaneously. Measurements of depolarized scattering have previously been employed in studying the optical properties of solutions or suspensions, to determine the length of rod-like analytes, and to gain increased accuracy in the determination of analyte molar mass. The coupling HF5/D-MALS allows for the depolarization ratio of a solution or suspension to be measured continuously across the fractogram. This is demonstrated here for a Teflon latex the size range of which extends beyond that accessible to commercial size-exclusion columns. The results presented provide the first reported on-line HF5/D-MALS coupling, showing the feasibility of the technique as well as its realized potential for providing continuous depolarization measurements, inter alia.
Active lidar remote sensing has been used to obtain detailed and quantitative information about the properties of aerosols. We have analyzed the spatio-temporal classification of aerosols using the ...parameters of particle linear depolarization ratio and single scattering albedo from Aerosol Robotic Network (AERONET) over seven megacities of Asia namely; Lahore, Karachi, Kanpur, Pune, Beijing, Osaka, and Bandung. We find that pollution aerosols dominate during the winter season in all the megacities. The concentrations, however, vary concerning the locations, i.e., 70–80% pollution aerosols are present over Lahore, 40–50% over Karachi, 90–95% over Kanpur and Pune, 60–70% and over Beijing and Osaka. Pure Dust (PD), Pollution Dominated Mixture (PDM), and Dust Dominated Mixture (DDM) are found to be dominant during spring and summer seasons.This proposes that dust over Asia normally exists as a mixture with pollution aerosols instead of pure form. We also find that black carbon (BC) dominated pollution aerosols.
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•Aerosol classification has been performed over seven megacities of Asia.•Pollution aerosols dominate during the winter season.•Dust over Asia was observed to exist in the form of a mixture.
A severe prolonged regional haze episode occurred over the Yangtze River Delta (YRD) from 2 to 14 December 2013. In this paper, we have discussed the probable cause of haze episode and aerosol ...characteristics. Analysis of visibility and meteorological parameters suggest that the meteorological conditions play a very important role in the accumulation of aerosol particles that cause haze. The height of the planetary boundary layer (PBL) is considered as one of the key factors together with other factors in the haze formation.
The Principal Component Analysis (PCA) clearly shows accumulation process of air pollutants from vehicular, industrial and anthropogenic sources are major contributors for the haze formation. The aerosol optical properties (aerosol optical depth – AOD, Ångström exponent – α and Ultra Violet Aerosol Index – UVAI) are analyzed to study the temporal and spatial variations of aerosol loadings. The results show concentrations of fine mode particles during the haze episode, the dust could have been also entered in the region from the northwestern China. Analysis of vertical aerosol profiles indicate that dust aerosols concentrated in the lower part of the haze layers.
•A severe prolonged regional haze episode occurred in YRD region, China.•Principal influential factors of visibility were different in haze episode.•Accumulation process of air pollutants was discussed based on PCA method.•Characteristics of aerosol properties and loading were analyzed.
The lack of aerosol shape data is one of the causes for the uncertainty of aerosol radiative forcing. In this paper, the parameterization of aerosol particle shape is successfully implemented based ...on the written computer program using satellite and ground observation data. The method can use aerosol optical (microphysical) properties and linear depolarization ratio from multiple data sources for obtaining aspect ratio based on the ellipsoidal model, or aspect ratio and roundness based on the super-ellipsoidal model. Subsequently, the distribution characteristics of aerosol depolarization ratio have been analyzed, the aerosol nonsphericity in global regions have been inverted based on the method, and the solar radiance of different study regions has been calculated according to the aerosol nonsphericity and verified by the sun photometer data. The results show that the aerosol depolarization ratio data is mainly distributed below 6 km. In urban regions, the depolarization ratio is generally below 20%, while in desert regions, it is mostly distributed between 20% and 40%. The aerosol aspect ratio based on the ellipsoidal model obtained by inversion ranges from 1.00 to 1.55 and vary considerably with region, generally less than 1.20 in urban regions and generally greater than 1.20 in desert regions. Meanwhile, the aerosol aspect ratio and roundness based on the super-ellipsoidal model obtained by inversion range from 1.00 to 1.40 and 1.00 to 1.30, respectively. The aspect ratio and roundness in urban regions are distributed between 1.00 and 1.15 (1.10), while in desert regions are generally greater than 1.10. Besides, the calculation results of solar radiance show that the parameterized data of aerosol nonsphericity can provide an effective shape input for aerosol optical modeling.
•An inversion code for the nonsphericity of aerosol particle system has been written.•The spatial distribution of the aerosol depolarization ratio have been analyzed.•The aerosol nonsphericity in different study regions has been parameterized.
•A semianalytic Monte Carlo (MC) polarization radiative transfer model is developed to study the multiple scattering effect in polarized oceanic lidar.•A mutual validity of the MC model and ...experimental results was established.•The multiple scattering effect enhances the depolarization of seawater.•The depolarization ratio increases with increasing penetration depth, receiver's field of view, single scattering albedo and beam attenuation coefficient of seawater.
The polarization characteristic of oceanic lidar returns includes a great deal of seawater information, which plays an important role in subsurface layer detection and seawater optical property retrieval. Along with the microphysical properties of particles, the multiple scattering effect between laser light and particles in seawater is also a source of polarization, which obscures the original depolarization properties of the seawater. However, modeling multiple scattering in polarized oceanic lidar with analytical methods is a complex problem. In this paper, a semianalytic Monte Carlo (MC) polarization radiative transfer model is established to assess the impact of multiple scattering on the backscattering depolarization measurement of oceanic lidar. The MC-simulated lidar returns of both the parallel and perpendicular channels and the depolarization ratios coincide well with the experimental results from three stations in the Yellow Sea of China by shipborne polarized lidar. Further analyses show that the multiple scattering effect enhances the depolarization of seawater; specifically, the depolarization ratio of the lidar return increases with increasing penetration depth, receiver's field of view (FOV), single scattering albedo and beam attenuation coefficient of seawater. This semianalytic polarized MC model is useful for understanding the mechanism of polarized multiple scattering in seawater and for further improving the precision of optical property retrieval using oceanic polarized lidar.
Desert dust and sand storms from the Sahara can sometimes be transported thousands of kilometers, causing social and economic damages throughout their path, deteriorating the air quality, and ...affecting human health. The seasonal climatology and transport pathways of dust aerosols are, therefore, essential to a deeper understanding of the impact of dust on the affected region. This study investigated the three-dimensional climatology of dust aerosol distribution and its transportation characteristics over the Sahara, Mediterranean, and Europe using multiple satellites and reanalysis data. The results showed a strong spatial variation of dust aerosols, with the highest loading over the Sahara Desert and a gradual latitudinal decrease toward the north with the lowest dust load over Europe. Temporally, higher dust activities were observed during summer and spring in terms of dust occurrence frequency and contribution to the total aerosol loading. Dust aerosols significantly increased over Central Sahara and decreased over Central Europe. Saharan dust was efficiently transported toward Europe during summer and spring, mainly at higher altitudes of 2–6 km. It could reach as north as the latitude of ∼42° N in the transect of 5° – 15° E in the layer 2–4 km during summer, while in winter, the occurrence of dust was at lower altitudes (< 2 km) in some areas in Europe, north of 45° N. The dust transport pathway toward Europe showed a long-standing dust layer mainly appearing over the Mediterranean from the near-surface up to 3 to 4 km during summer and spring, contributing to the meridional transport of dust. The dust layer characteristics and the planetary boundary layer (PBL) height above Europe showed the likelihood of dust intrusion into Europe's PBL during summer. This work also highlighted the main elements affecting dust distribution and transport toward Europe: wind, topography, and the PBL height.
•Climatology of dust profile and optical depth from Sahara to Europe were presented.•Characteristics of the Saharan dust seasonal transport toward Europe were revealed.•A long-standing dust layer and its variations were shown over the study region.•Changes in dust physical features from Sahara to its surroundings were exhibited.
The types of aerosols are essential for estimating radiative forcing effects, improving the lidar retrieval algorithms, and identifying aerosol sources. However, it is difficult to categorize the ...aerosols with similar optical properties based on lidar techniques. In this study, an approach was proposed to distinguish desert dust (DD), mineral dust (MD), air pollution aerosols (APA), and their mixtures (DD&MD, DD&APA, MD&APA, DD&MD&APA). We established the fluorescence–Mie polarization lidar (FMPL) system to categorize three types of main aerosols using the customized fluorescence-to-Mie ratio (FMR) and volume depolarization ratio (VDR) in the experiments from September 2017 to December 2018. Then, the mixture of the aerosols was classified by combining backward trajectory analysis. The feasibility of this method was verified through five typical cases in the paper, and it is proved that the method could be employed to study air pollution issues based on FMPL, which could provide references for the meteorological investigation.
•We established a fluorescence–Mie polarization lidar system.•We defined Fluorescence-to-Mie ratio (FMR).•Aerosols were divided into seven types according to their FMR and VDR.
The contribution of black carbon in absorption aerosol optical depth (BC AAOD) can be retrieved using depolarization ratio (DPR, δp), single scattering albedo (SSA, ω), and Ångstrom exponent (AE, å). ...AErosol RObotic NETwork (AERONET) inversion products provide ground-based long-term observations of aerosol optical properties based on the sun photometer's observations. In this study, the AERONET version 3 level 2.0 products are utilized to approximate BC AAOD on a global scale during the recent two decades (2000–2018). We compared our AERONET-retrieved BC AAODs with the ones provided by the Modern-Era Retrospective analysis for Research and Applications, version 2.0 (MERRA-2) data, at the selected AERONET site's location and observation time. We found the highest MERRA-2 and AERONET-derived BC AAOD over densely populated countries such as China and India. Further, Nigeria, one of the largest sources of biomass burning and a leading gas-flaring country, showed increased values of BC AAOD. Both datasets confirmed a drop in BC AAOD over Sao-Paulo, Beijing, and Seoul from 2000 to 2018. AERONET and MERRA-2 data indicated higher BC variations during cold months in Beijing, Kanpur, Seoul, Ilorin, and Sao-Paulo. We concluded that the high BC values over the populous countries mainly owe to the considerable BC emission from transportation, industrial, and residential sectors in response to the high population. A comparison of MERRA-2 and our AERONET-derived retrieval methodology suggested lower values of MERRA-2 BC AAOD compared to AERONET-derived variations in Sao-Paulo.
•Depolarization ratio single scattering albedo, and Ångstrom exponent are used to calculate black carbon light absorption.•The simulated MERRA-2 BC AAOD variations are illustrated against AERONET-retrieved BC AAODs.•Higher BC AAOD was found in China and India, and Nigeria.•MERRA-2 BC AAOD is lower than AERONET-derived BC AAOD in Sao-Paulo.•MERRA-2 and AERONET-derived BC AAOD indicated a drop in BC AAOD over Sao-Paulo, Beijing, and Seoul.
Asian mineral dust is one of the main aerosol sources in the Earth-atmosphere system, which generates significant effect on air quality, human health, and climate change. Meanwhile, knowledge of ...vertical optical properties of dust aerosol is crucially needed for identification of the dust source and improved understanding of radiative effect in climate model. In the study, triple-wavelength polarization Raman lidar observation combined with photometer, radiosonde and simultaneous model data was performed at Kashi in the northwestern of Tarim Basin. Taklimakan desert, located in the center of Tarim Basin, is the largest desert of Asia. Base on the measurement and model, two typical dust sources in different altitude were identified in the study, namely Taklimakan desert (East Road), Central Asia desert and Middle East desert (West Road). Particle size distribution of photometer shows that these cases were all coarse-mode-dominated with effective radius larger than 1.7 μm. The lidar observations revealed particle linear depolarization ratios (PLDR) of the Taklimakan dust ranged from 0.28 to 0.34 at 355 nm, 0.33 to 0.35 at 532 nm and 0.29-0.35 at 1064 nm, while lidar ratios (LR) ranged from 47 to 54 sr at 355 nm and from 42 to 51 sr at 532 nm wavelength. Spectral variation of LR and PLDR for Asian dust and Saharan dust was analyzed. All observed Asian dust present the consistent spectral variation that lidar ratio at 355 nm is higher than that of 532 nm, which however is not the case for Saharan dust. Both Saharan and Asian dust measurement PLDR at 532 nm is larger than that of 355 nm and 1064 nm. The measured dust properties provide particularly valuable information for dust simulation and dust climate model for different dust source.
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