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Aerosol Optical Properties: Models, Methods & Measurements

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A special issue of Atmosphere (ISSN 2073-4433). This special issue belongs to the section "Aerosols".

Deadline for manuscript submissions: closed (28 February 2018) | Viewed by 35259

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Special Issue Editor

Dr. Jonathan E. Thompson

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Guest Editor

Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, USA
Interests: aerosols and particulate matter; human health effects; atmospheric chemistry
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Atmospheric particulate matter scatters and absorbs significant quantities of sunlight in the Earth’s atmosphere, thus leading to alterations of the Earth’s radiative balance. For climate scientists to understand the net effect of aerosol, key optical variables of aerosol must be known. Researchers employ many methods to collect such information, such as direct optical measurements on atmospheric aerosol, well-constrained laboratory studies, or building complex models of aerosol particle geometry and mixing state and employing computational methods, such as the discrete dipole approximation to estimate optical properties.

For this Special Issue, we aim to provide the community a valuable resource by organizing the most recent contributions to the study of aerosol optics. Such creative works may take the form of exceptional literature review articles that outline recent developments in the field. Alternatively, authors may describe the development and application of novel measurement methods for study of aerosol optics. Additional contributions might include manuscripts that focus on summary ambient measurements and/or the transformation of particle properties during atmospheric processing. Lastly, laboratory and modeling studies are welcome contributions to this Special Issue. In short, all contributions that improve our understanding of aerosol optics are welcome.

Prof. Dr. Jonathan E. Thompson
Guest Editor

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Keywords

light scattering
light absorption
aerosol
climate forcing
T-matrix
discrete dipole approximation
nephelometry
photoacoustic spectroscopy
cavity ring-down spectroscopy

Published Papers (7 papers)

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Research

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20 pages, 7607 KiB

Open AccessEditor’s ChoiceArticle

Lidar Measurements of Dust Aerosols during Three Field Campaigns in 2010, 2011 and 2012 over Northwestern China

by Tian Zhou, Hailing Xie, Jianrong Bi, Zhongwei Huang, Jianping Huang, Jinsen Shi, Beidou Zhang and Wu Zhang

Atmosphere 2018, 9(5), 173; https://doi.org/10.3390/atmos9050173 - 05 May 2018

Cited by 29 | Viewed by 5246

Abstract

Ground-based measurements were carried out during field campaigns in April–June of 2010, 2011 and 2012 over northwestern China at Minqin, the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) and Dunhuang. In this study, three dust cases were examined, and the statistical results of dust occurrence, along with physical and optical properties, were analyzed. The results show that both lofted dust layers and near-surface dust layers were characterized by extinction coefficients of 0.25–1.05 km⁻¹ and high particle depolarization ratios (PDRs) of 0.25–0.40 at 527 nm wavelength. During the three campaigns, the frequencies of dust occurrence retrieved from the lidar observations were all higher than 88%, and the highest frequency was in April. The vertical distributions revealed that the maximum height of dust layers typically reached 7.8–9 km or higher. The high intensity of dust layers mostly occurred within the planetary boundary layer (PBL). The monthly averaged PDRs decreased from April to June, which implies a dust load reduction. A comparison of the relationship between the aerosol optical depth at 500 nm (AOD₅₀₀) and the Angstrom exponent at 440–870 nm (AE_440–870) confirms that there is a more complex mixture of dust aerosols with other types of aerosols when the effects of human activities become significant. Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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14 pages, 2418 KiB

Open AccessArticle

Spatial Factor Analysis for Aerosol Optical Depth in Metropolises in China with Regard to Spatial Heterogeneity

by Hui Shi, Qingqing He and Wenting Zhang

Atmosphere 2018, 9(4), 156; https://doi.org/10.3390/atmos9040156 - 20 Apr 2018

Cited by 7 | Viewed by 4031

Abstract

A substantial number of studies have analyzed how driving factors impact aerosols, but they have been little concerned with the spatial heterogeneity of aerosols and the factors that impact aerosols. The spatial distributions of the aerosol optical depth (AOD) retrieved by Moderate Resolution Imaging Spectrometer (MODIS) data at 550-nm and 3-km resolution for three highly developed metropolises, the Beijing-Tianjin-Hebei (BTH) region, the Yangtze River Delta (YRD), and the Pearl River Delta (PRD), in China during 2015 were analyzed. Different degrees of spatial heterogeneity of the AOD were found, which were indexed by Moran’s I index giving values of 0.940, 0.715, and 0.680 in BTH, YRD, and PRD, respectively. For the spatial heterogeneity, geographically weighted regression (GWR) was employed to carry out a spatial factor analysis, where terrain, climate condition, urban development, and vegetation coverage were taken as the potential driving factors. The results of the GWR imply varying relationships between the AOD and the factors. The results were generally consistent with existing studies, but the results suggest the following: (1) Elevation increase would more likely lead to a strong negative impact on aerosols (with most of the coefficients ranging from −1.5~0 in the BTH, −1.5~0 in the YRD, and −1~0 in the PRD) in the places with greater elevations where the R-squared values are always larger than 0.5. However, the variation of elevations cannot explain the variation of aerosols in the places with relatively low elevations (with R-squared values approximately 0.1, ranging from 0 to 0.3, and approximately 0.1 in the BTH, YRD, and PRD), such as urban areas in the BTH and YRD. (2) The density of the built-up areas made a strong and positive impact on aerosols in the urban areas of the BTH (R-squared larger than 0.5), while the R-squared dropped to 0.1 in the places far away from the urban areas. (3) The vegetation coverage led to a stronger relief on the AOD in parts of the YRD and PRD (with coefficients less than −0.6 and ranging from −0.4~−0.6, respectively) where there is greater vegetation coverage, and led to a weaker relief on the AOD in the urban area of the PRD with a coefficient of approximately −0.2~−0.4. Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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19 pages, 7354 KiB

Open AccessArticle

Long-Term Analysis of Aerosol Optical Depth over the Huaihai Economic Region (HER): Possible Causes and Implications

by Yang Shen, Lianpeng Zhang, Xing Fang, Zhuowen Zhao, Xing Li, Jiahui Wang and Qi Chai

Atmosphere 2018, 9(3), 93; https://doi.org/10.3390/atmos9030093 - 04 Mar 2018

Cited by 7 | Viewed by 4470

Abstract

The quality of air is increasingly affecting regional climate and human activity. Collection 6 aerosol products retrieved from the Moderate-Resolution Imaging Spectroradiometer (MODIS) onboard the Terra satellite were validated based on CE-318 sun photometric data to analyze their applicability in the Huaihai economic region (HER) at the Xuzhou and Shouxian sites. The spatio-temporal variations of aerosol optical depth (AOD) were also analyzed over HER from 2000 to 2016, with analyses of the correlation with potential driving factors, including meteorology, vegetation and human factors. HER is an economic cooperation organization with multiple industrial structures, containing coal resource-based cities, a national transportation hub and agricultural and high-altitude areas, which shows regional differences in AOD. The results suggest that MODIS Terra AOD products show good agreement with ground observations, with correlation coefficients of above 0.84 in HER, and the main pollutants for high AOD values are fine particles (the mean Ångström exponent was 1.16). The average annual change in AOD varied with a weak growth trend over the past 17 years, while a transition in 2012 made the tendency change from upward to downward due to the extensive cooperation of cities in the joint prevention and control of the deterioration of the ecological environment. The largest monthly mean AOD value appeared in June, which resulted from significant agricultural residue burning. The spatial distribution of multi-year average AOD occurred with a banded high-value center, extending from the north-west to the south-east. The high aerosol loadings were located in resource-based cities, and industrially developed and south-eastern coastal areas, whereas the regions with relatively low AOD in HER were distributed in the southern agricultural and northern high-altitude areas. The AOD value in the western, northern, and eastern coastal areas of HER showed a significant increasing trend, while no area exhibited a decreasing trend. The average wind speed has the largest negative correlation with the AOD value in terms of the natural driving factors, and GDP (gross domestic product) was more positively correlated with AOD with respect to the human factors, in comparison with population density. Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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3055 KiB

Open AccessArticle

Optical Properties of Biomass Burning Aerosols: Comparison of Experimental Measurements and T-Matrix Calculations

by Samin Poudel, Marc N. Fiddler, Damon Smith, K. M. Flurchick and Solomon Bililign

Atmosphere 2017, 8(11), 228; https://doi.org/10.3390/atmos8110228 - 21 Nov 2017

Cited by 13 | Viewed by 6179

Abstract

The refractive index (RI) is an important parameter in describing the radiative impacts of aerosols. It is important to constrain the RI of aerosol components, since there is still significant uncertainty regarding the RI of biomass burning aerosols. Experimentally measured extinction cross-sections, scattering cross-sections, and single scattering albedos for white pine biomass burning (BB) aerosols under two different burning and sampling conditions were modeled using T-matrix theory. The refractive indices were extracted from these calculations. Experimental measurements were conducted using a cavity ring-down spectrometer to measure the extinction, and a nephelometer to measure the scattering of size-selected aerosols. BB aerosols were obtained by burning white pine using (1) an open fire in a burn drum, where the aerosols were collected in distilled water using an impinger, and then re-aerosolized after several days, and (2) a tube furnace to directly introduce the BB aerosols into an indoor smog chamber, where BB aerosols were then sampled directly. In both cases, filter samples were also collected, and electron microscopy images were used to obtain the morphology and size information used in the T-matrix calculations. The effective radius of the particles collected on filter media from the open fire was approximately 245 nm, whereas it was approximately 76 nm for particles from the tube furnace burns. For samples collected in distilled water, the real part of the RI increased with increasing particle size, and the imaginary part decreased. The imaginary part of the RI was also significantly larger than the reported values for fresh BB aerosol samples. For the particles generated in the tube furnace, the real part of the RI decreased with particle size, and the imaginary part was much smaller and nearly constant. The RI is sensitive to particle size and sampling method, but there was no wavelength dependence over the range considered (500–680 nm). Our values for the RI of fresh (white pine) biomass burning aerosols ranged from 1.33 + i0.008 to 1.74 + i0.008 for 200-nm, 300-nm, and 400-nm diameter particles. These are within the range of RI values in the most recent study conducted during the Fire Laboratory at Missoula Experiments (FLAME I and II), which were 1.55 to 1.80 for the real part, and 0.01–0.50 for the imaginary part, for fresh BB aerosols with diameters of 200–570 nm. There is no clear trend on the dependence of the RI values on particle size. The RI values derived from measurements of aerosols produced from the combustion of hydrocarbons and diesel cannot be used for BB aerosols. Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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13961 KiB

Open AccessArticle

Aerosol Optical Properties over China from RAMS-CMAQ Model Compared with CALIOP Observations

by Tong Wu, Meng Fan, Jinhua Tao, Lin Su, Ping Wang, Dong Liu, Mingyang Li, Xiao Han and Liangfu Chen

Atmosphere 2017, 8(10), 201; https://doi.org/10.3390/atmos8100201 - 17 Oct 2017

Cited by 8 | Viewed by 4490

Abstract

The horizontal and vertical distributions of aerosol optical properties over China in 2013–2015 were investigated using RAMS (Regional Atmospheric Modeling System)-CMAQ (Models-3 Community Multiscale Air Quality) simulations and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) observations. To better understand the performance of the RAMS-CMAQ model over China, comparisons with the ground-based Sun photometers AERONET (Aerosol Robotic Network), MODIS (Moderate Resolution Imaging Spectroradiometers) data and the on-board Lidar CALIOP were used for comprehensive evaluations, which could characterize the abilities of the model to simulate the spatial and vertical distributions of the AOD (Aerosol Optical Depth) as well as the optical properties for four seasons. Several high value areas (e.g., the Sichuan Basin, Taklamakan Desert, North China Plain, and Yangtze River Delta) were found over China during the study period, with the maximum mean AOD (CALIOP: ~0.7; RAMS-CMAQ: >1) in the Sichuan district. Compared with AODs of AERONET, both the CALIOP and RAMS-CMAQ AODs were underestimated, but the RAMS-CMAQ data show a better correlation with AERONET (AERONET vs. RAMS-CMAQ R: 0.69, AERONET vs. CALIOP R: 0.5). The correlation coefficients between RAMS-CMAQ and CALIOP are approximately 0.6 for all four seasons. The AEC (Aerosol Extinction Coefficient) vertical profiles over major cities and their cross sections exhibit two typical features: (1) most of the AEC peaks occurred in the lowest ~0.5 km, decreasing with increasing altitude; and (2) the RAMS-CMAQ AEC underestimated the region with high AODs in the northwest of China and overestimated the region with high AODs in the east–central plain and the central basin regions. The major difference in the AEC values of RAMS-CMAQ and CALIOP is mainly caused by the level of relative humidity and the hygroscopic growth effects of water-soluble aerosols, especially, in the Sichuan district. In general, both the column and vertical RAMS-CMAQ aerosol optical properties could be supplemented efficiently when satellite observations are not available or invalid over China in the applications of climate change and air pollution. Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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5824 KiB

Open AccessArticle

Validation of MODIS-Aqua Aerosol Products C051 and C006 over the Beijing-Tianjin-Hebei Region

by Ke Wang, Xuejin Sun, Yongbo Zhou and Chuanliang Zhang

Atmosphere 2017, 8(9), 172; https://doi.org/10.3390/atmos8090172 - 14 Sep 2017

Cited by 10 | Viewed by 3429

Abstract

The recently released MODerate resolution Imaging Spectroradiometers (MODIS) Collection 6(C006) includes several significant improvements, which are expected to do well in analyzing aerosols and using the observations for air pollution application. The C006 Aerosol Optical Depth (AOD) retrievals should be validated completely before they will be applied to specific research. However, the validation of C006 AOD retrievals at a regional scale is limited. Therefore, this study evaluated the performance of the MODIS-Aqua Collection 51 (C051) and C006 AOD retrievals over the Beijing-Tianjin-Hebei region in China from 2006 to 2015 using ground-based Sun photometers. The algorithms of the AOD product include Dark Target (DT) and Deep Blue (DB). The results indicated that the improvements in DT C006 were slight, as the expected error (EE) increased by almost 9% over the two sites, and the DT C051 and DT C006 AOD were overestimated for both sites. DB C006 presented an improvement over DB C051, and a better correlation was observed between the collocated DB C006 retrievals and Sun photometer data (R ranged from 0.9343–0.9383). There was an increase in the frequency from DT C051 to DT C006, in the range 0.6–1.5, over the two sites; moreover, the AOD from the DB retrievals had a very narrow range (0.1–0.3). The spatial distribution of the AOD values was high (AOD > 0.7) over the southeastern region and low (AOD < 0.3) over the northwestern region. Changes in the DT C006 algorithm resulted in an increased AOD (0.085) for the region. The AOD values in spring and summer were higher than those in fall and winter. By subtracting the C051 AOD from the corresponding C006 values, greater positive changes (~0.2) were found in the southeastern areas during summer, presumably as the updated cloud-masking allowed heavy smoke retrievals. The accuracy of the AOD retrievals depended on the assumptions of surface reflectance and the selection of the aerosol model. The use of the DB C006 algorithm is recommended for the Beijing and Xianghe sites. Because of the limitations of the DT algorithm over sparsely vegetated surfaces, the DT C006 product is recommended for Xianghe. Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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Review

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1430 KiB

Open AccessReview

Parameterization of the Aerosol Upscatter Fraction as Function of the Backscatter Fraction and Their Relationships to the Asymmetry Parameter for Radiative Transfer Calculations

by Hans Moosmüller and John A. Ogren

Atmosphere 2017, 8(8), 133; https://doi.org/10.3390/atmos8080133 - 25 Jul 2017

Cited by 27 | Viewed by 6452

Abstract

Simple analytical approximations for aerosol radiative forcing generally contain the aerosol upscatter fraction (the fraction of scattered light that is scattered into the upper hemisphere), while ambient measurements generally yield the backscatter fraction, and theoretical calculations of scattering phase functions often yield the asymmetry parameter. Therefore, simple analytical relationships and parameterizations relating these three parameters are very valuable for radiative transfer calculations. Here, we review published parameterizations, mostly based on the Henyey-Greenstein phase function, and evaluate their goodness and range of validity. In addition, we give new parameterizations that are valid over the full range of backscatter fractions that are possibly encountered in the ambient atmosphere (i.e., 0 to 0.5). Full article

(This article belongs to the Special Issue Aerosol Optical Properties: Models, Methods & Measurements)

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