scholarly journals Determination of the refractive index of insoluble organic extracts from atmospheric aerosol over the visible wavelength range using optical tweezers

2017 ◽  
Author(s):  
Rosalie H. Shepherd ◽  
Martin D. King ◽  
Amelia Marks ◽  
Neil Brough ◽  
Andrew D. Ward
Keyword(s):  
Refractive Index ◽  
Wavelength Range ◽  
Atmospheric Aerosol ◽  
Organic Aerosol ◽  
Wood Smoke ◽  
Angstrom Exponent ◽  
Organic Extracts ◽  
Ångström Exponent ◽  
Aqueous Core ◽  
Real Refractive Index

Abstract. Optical trapping combined with Mie spectroscopy is a new technique used to record the refractive index of insoluble organic material extracted from atmospheric aerosol samples over a wide wavelength range with sub-nanometer resolution. The refractive index of the insoluble organic extracts was shown to follow a Cauchy equation between 460 to 700 nm for organic aerosol extracts collected from urban (London) and remote (Antarctica) locations. Cauchy coefficients for the remote sample were for the Austral summer and gave the Cauchy coefficients to be A = 1.467 and B = 1000 nm2 with a real refractive index of 1.489 at a wavelength of 589 nm. Cauchy coefficients for the urban samples varied with season, with extracts collected during summer having Cauchy coefficients of A = 1.465±0.005 and B = 4625±1200 nm2 with a representative real refractive index of 1.478 at a wavelength of 589 nm, whilst samples extracted during autumn had larger Cauchy coefficients of A=1.505 and B = 600 nm2 with a representative real refractive index of 1.522 at a wavelength of 589 nm. The refractive index of absorbing aerosol was also recorded. The technique applied in the presented study allowed the absorption Ångstrom exponent to be determined for wood smoke and humic acid aerosol extract. Typical values of the Cauchy coefficient for the wood smoke aerosol extract were A = 1.541±0.03 and B = 14800±2900 nm2 resulting in a real refractive index of 1.584±0.007 at a wavelength of 589 nm and an absorption Ångstrom exponent of 7.0. The measured values of refractive index compare well with previous monochromatic or very small wavelength range measurements of refractive index. A one-dimensional radiative-transfer calculation of the top of the atmosphere albedo was applied to model an atmosphere containing a 3 km thick layer of aerosol comprising of pure water, insoluble organic aerosol or an aerosol consisting of an aqueous core-with an insoluble organic shell. The calculation demonstrated that the top of the atmosphere albedo increases by 0.01 to 0.04 for pure organic particles relative to water particles of the same size and the top of the atmosphere albedo increases by 0.03 for aqueous core-shell particles as volume fraction of the shell material increases by 25 percent.

scholarly journals Determination of the refractive index of insoluble organic extracts from atmospheric aerosol over the visible wavelength range using optical tweezers

2018 ◽  
Vol 18 (8) ◽  
pp. 5235-5252 ◽  
Author(s):  
Rosalie H. Shepherd ◽  
Martin D. King ◽  
Amelia A. Marks ◽  
Neil Brough ◽  
Andrew D. Ward
Keyword(s):  
Refractive Index ◽  
Wavelength Range ◽  
Atmospheric Aerosol ◽  
Organic Aerosol ◽  
Angstrom Exponent ◽  
Organic Extracts ◽  
Ångström Exponent ◽  
Aqueous Core ◽  
Ångstrom Exponent ◽  
Real Refractive Index

Abstract. Optical trapping combined with Mie spectroscopy is a new technique used to record the refractive index of insoluble organic material extracted from atmospheric aerosol samples over a wide wavelength range. The refractive index of the insoluble organic extracts was shown to follow a Cauchy equation between 460 and 700 nm for organic aerosol extracts collected from urban (London) and remote (Antarctica) locations. Cauchy coefficients for the remote sample were for the Austral summer and gave the Cauchy coefficients of A = 1.467 and B = 1000 nm2 with a real refractive index of 1.489 at a wavelength of 589 nm. Cauchy coefficients for the urban samples varied with season, with extracts collected during summer having Cauchy coefficients of A=1.465±0.005 and B=4625±1200 nm2 with a representative real refractive index of 1.478 at a wavelength of 589 nm, whilst samples extracted during autumn had larger Cauchy coefficients of A = 1.505 and B = 600 nm2 with a representative real refractive index of 1.522 at a wavelength of 589 nm. The refractive index of absorbing aerosol was also recorded. The absorption Ångström exponent was determined for woodsmoke and humic acid aerosol extract. Typical values of the Cauchy coefficient for the woodsmoke aerosol extract were A=1.541±0.03 and B=14800±2900 nm2, resulting in a real refractive index of 1.584 ± 0.007 at a wavelength of 589 nm and an absorption Ångström exponent of 8.0. The measured values of refractive index compare well with previous monochromatic or very small wavelength range measurements of refractive index. In general, the real component of the refractive index increases from remote to urban to woodsmoke. A one-dimensional radiative-transfer calculation of the top-of-the-atmosphere albedo was applied to model an atmosphere containing a 3 km thick layer of aerosol comprising pure water, pure insoluble organic aerosol, or an aerosol consisting of an aqueous core with an insoluble organic shell. The calculation demonstrated that the top-of-the-atmosphere albedo increases by 0.01 to 0.04 for pure organic particles relative to water particles of the same size and that the top-of-the-atmosphere albedo increases by 0.03 for aqueous core-shell particles as volume fraction of the shell material increases to 25 %.

scholarly journals Extinction-related Angström exponent characterization of submicrometric volume fraction in atmospheric aerosol particles

2019 ◽  
Vol 228 ◽  
pp. 270-280 ◽  
Author(s):  
A. Quirantes ◽  
J.L. Guerrero-Rascado ◽  
D. Pérez-Ramírez ◽  
I. Foyo-Moreno ◽  
P. Ortiz-Amezcua ◽  
...  
Keyword(s):  
Atmospheric Aerosol ◽  
Volume Fraction ◽  
Aerosol Particles ◽  
Angstrom Exponent ◽  
Atmospheric Aerosol Particles ◽  
Ångström Exponent ◽  
Ångstrom Exponent

scholarly journals Detection of Saharan dust and biomass burning events using near real-time intensive aerosol optical properties in the northwestern Mediterranean

2016 ◽  
Author(s):  
M. Ealo ◽  
A. Alastuey ◽  
A. Ripoll ◽  
N. Pérez ◽  
M. C. Minguillón ◽  
...  
Keyword(s):  
Optical Properties ◽  
Biomass Burning ◽  
Mineral Dust ◽  
Organic Aerosol ◽  
Saharan Dust ◽  
Aerosol Optical Properties ◽  
Angstrom Exponent ◽  
Ångström Exponent ◽  
Dust Events ◽  
Ångstrom Exponent

Abstract. The study of Saharan dust events (SDE) and biomass burning (BB) emissions are both topic of great scientific interest since they are frequent and important polluting scenarios affecting air quality and climate. The main aim of this work is evaluating the feasibility of using near real-time in situ aerosol optical measurements for the detection of these atmospheric events in the Western Mediterranean Basin (WMB). With this aim, intensive aerosol optical properties (SAE: scattering Ångström exponent, AAE: absorption Ångström exponent, SSAAE: single scattering albedo Ångström exponent, and g: asymmetry parameter) were derived from multi-wavelength aerosol light scattering, hemispheric backscattering and absorption measurements performed at regional (Montseny; MSY, 720 m a.s.l.) and continental (Montsec; MSA, 1570 m a.s.l.) background sites in the WMB. A sensitivity study aiming at calibrating the measured intensive optical properties for SDE and BB detection is presented and discussed. The detection of Saharan dust events (SDE) by means of the SSAAE parameter and Ångström matrix depended on the altitude of the measurement station, and on SDE intensity. At MSA (mountain-top site) SSAAE detected around 85% of SDE compared with 50% at MSY station, where pollution episodes dominated by fine anthropogenic particles frequently masked the effect of mineral dust on optical properties during less intense SDE. Furthermore, an interesting feature of SSAAE was its capability to detect the presence of mineral dust after the end of SDE. Thus, resuspension processes driven by summer regional atmospheric circulations and dry conditions after SDE favored the accumulation of mineral dust at regional level having important consequences for air quality. On average, SAE, AAE and g ranged between -0.7 and 1, 1.3 and 2.5, and 0.5 and 0.75, respectively, during SDE. Based on the Aethalometer model, biomass burning (BB) contribution to equivalent black carbon (BC) accounted for 36% and 40% at MSY and MSA respectively. Linear relationships were found between AAE and %BCbb, with AAE values reaching around 1.5 when %BCbb was higher than 50%. BB contribution to organic matter (OM) at MSY was around 30%. Thus FF combustion sources showed important contributions to both BC and OM in the region under study. Results for OM source apportionment showed good agreement with simultaneous biomass burning organic aerosol (BBOA) and hydrocarbon-like organic aerosol (HOA) calculated from Positive Matrix Factorization (PMF) applied to simultaneous Aerosol Mass Spectrometer (ACSM) measurements. A wildfire episode was identified at MSY, showing AAE values up to 2 when daily BB contributions to BC and OM were 73% and 78% respectively.

scholarly journals The absorption Ångström exponent of black carbon: from numerical aspects

2018 ◽  
Vol 18 (9) ◽  
pp. 6259-6273 ◽  
Author(s):  
Chao Liu ◽  
Chul Eddy Chung ◽  
Yan Yin ◽  
Martin Schnaiter
Keyword(s):  
Particle Size ◽  
Refractive Index ◽  
Black Carbon ◽  
Numerical Study ◽  
Geometric Mean ◽  
Optical Parameter ◽  
Angstrom Exponent ◽  
Wide Range ◽  
Ångström Exponent ◽  
Ångstrom Exponent

Abstract. The absorption Ångström exponent (AAE) is an important aerosol optical parameter used for aerosol characterization and apportionment studies. The AAE of black carbon (BC) particles is widely accepted to be 1.0, although observational estimates give quite a wide range of 0.6–1.3. With considerable uncertainties related to observations, a numerical study is a powerful method, if not the only one, to provide a better and more accurate understanding on BC AAE. This study calculates BC AAE using realistic particle geometries based on fractal aggregate and an accurate numerical optical model (namely the multiple-sphere T-matrix method), and considers bulk properties of an ensemble of BC particles following lognormal size distributions. At odds with the expectations, BC AAE is not 1.0, even when BC is assumed to have small sizes and a wavelength-independent refractive index. With a wavelength-independent refractive index, the AAE of fresh BC is approximately 1.05 and relatively insensitive to particle size. For BC with geometric mean diameters larger than 0.12 µm, BC AAE becomes smaller when BC particles are aged (compact structures or coated by other non-absorptive materials). For coated BC, we prescribe the coating fraction variation based on a laboratory study, where smaller BC cores are shown to develop larger coating fractions than those of bigger BC cores. For both compact and coated BC, the AAE is highly sensitive to particle size distribution, ranging from approximately 0.8 to even over 1.4 with wavelength-independent refractive index. When the refractive index is allowed to vary with wavelength, a feature with observational backing, the BC AAE may show an even wider range. For different BC morphologies, we derive simple empirical equations on BC AAE based on our numerical results, which can serve as a guide for the response of BC AAE to BC size and refractive index. Due to its complex influences, the effects of BC geometry is better to be discussed at certain BC properties, i.e., known size and refractive index.

scholarly journals Seasonal cycle, size dependencies, and source analyses of aerosol optical properties at the SMEAR II measurement station in Hyytiälä, Finland

2011 ◽  
Vol 11 (9) ◽  
pp. 4445-4468 ◽  
Author(s):  
A. Virkkula ◽  
J. Backman ◽  
P. P. Aalto ◽  
M. Hulkkonen ◽  
L. Riuttanen ◽  
...  
Keyword(s):  
Refractive Index ◽  
Seasonal Cycle ◽  
Complex Refractive Index ◽  
Size Distributions ◽  
Grid Cells ◽  
Weighted Mean ◽  
Angstrom Exponent ◽  
Mean Diameter ◽  
Ångström Exponent ◽  
Ångstrom Exponent

Abstract. Scattering and absorption were measured at the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II) station in Hyytiälä, Finland, from October 2006 to May 2009. The average scattering coefficient σSP (λ = 550 nm) 18 Mm−1 was about twice as much as at the Pallas Global Atmosphere Watch (GAW) station in Finnish Lapland. The average absorption coefficient σAP (λ = 550 nm) was 2.1 Mm−1. The seasonal cycles were analyzed from hourly-averaged data classified according to the measurement month. The ratio of the highest to the lowest average σSP and σAP was ~1.8 and ~2.8, respectively. The average single-scattering albedo (ω0) was 0.86 in winter and 0.91 in summer. σSP was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003–10 μm. Assuming that the particle density was 1.5 g cm−3, the PM10 mass scattering efficiency was 3.1 ± 0.9 g m−2 at λ = 550 nm. Scattering coefficients were also calculated from the number size distributions by using a Mie code and the refractive index of ammonium sulfate. The linear regression yielded σSP(modelled) = 1.046 × σSP(measured) for the data with the low nephelometer sample volume relative humidity (RHNEPH = 30 ± 9 %) and σSP(modelled) = 0.985 × σSP(measured) when RHNEPH = 55 ± 4 %. The effective complex refractive index was obtained by an iterative approach, by matching the measured and the modelled σSPand σAP. The average effective complex refractive index was (1.517 ± 0.057) + (0.019 ± 0.015)i at λ = 550 nm. The iterated imaginary part had a strong seasonal cycle, with smallest values in summer and highest in winter. The contribution of submicron particles to scattering was ~90 %. The Ångström exponent of scattering, σSP, was compared with the following weighted mean diameters: count mean diameter (CMD), surface mean diameter (SMD), scattering mean diameter (ScMD), condensation sink mean diameter (CsMD), and volume mean diameter (VMD). If αSP is to be used for estimating some measure of the size of particles, the best choice would be ScMD, then SMD, and then VMD. In all of these the qualitative relationship is similar: the larger the Ångström exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing αSP and CsMD did not have any clear relationship with αSP. Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean σSP and σAP associated with the grid cells in Eastern Europe were in the range 20–40 Mm−1 and 4–6 Mm−1, respectively. The respective geometric means of σSP and σAP in the grid cells over Norwegian Sea were in the range 5–10 Mm−1 and <1 Mm−1. The source areas associated with high αSP values were norther than those for σSP and σAP. The trajectory statistical approach and a simple wind sector classification agreed well.

scholarly journals STUDY OF AFRICAN DUST WITH MULTI-WAVELENGTH RAMAN LIDAR DURING THE “SHADOW” CAMPAIGN IN SENEGAL

2016 ◽  
Author(s):  
I. Veselovskii ◽  
P. Goloub ◽  
T. Podvin ◽  
V. Bovchaliuk ◽  
Y. Derimian ◽  
...  
Keyword(s):  
Refractive Index ◽  
West Africa ◽  
Imaginary Part ◽  
Depolarization Ratio ◽  
Raman Lidar ◽  
Angstrom Exponent ◽  
Ångström Exponent ◽  
The Mean ◽  
532 Nm ◽  
Ångstrom Exponent

Abstract. West Africa and the adjacent oceanic regions are very important locations for studying dust properties and their influence on weather and climate. The SHADOW (Study of SaHAran Dust Over West Africa) campaign is performing a multi-scale and multi-laboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instruments at an observation site located at IRD (Institute for Research and Development) Center, Mbour, Senegal (14° N, 17° W). In this paper, we present the results of lidar measurements performed during the first phase of SHADOW which occurred in March-April, 2015. The multiwavelength Mie-Raman lidar acquired 3β + 2α + 1δ measurements during this period. This set of measurements has permitted particle intensive properties such as extinction and backscattering Ångström exponents (BAE) for 355/532 nm wavelengths corresponding lidar ratios and depolarization ratio at 532 nm to be determined. The mean values of dust lidar ratios during the observation period were about 53 sr at both 532 nm and 355 nm, which agrees with the values observed during the SAMUM 1 and SAMUM 2 campaigns held in Morocco and Cape Verde in 2006, 2008. The mean value of particle depolarization ratio at 532 nm was 30 ± 4.5 %, however during strong dust episodes this ratio increased to 35 ± 5 %, which is also in agreement with the results of the SAMUM campaigns. The backscattering Ångström exponent during the dust episodes decreased to ~ −0.7, while the extinction Ångström exponent though being negative, was greater than −0.2. Low values of BAE can likely be explained by an increase in the imaginary part of the dust refractive index at 355 nm compared to 532 nm. The dust extinction and backscattering coefficients at multiple wavelengths were inverted to the particle microphysics using the regularization algorithm and the model of randomly oriented spheroids. The analysis performed has demonstrated that the spectral dependence of the imaginary part of the dust refractive index may significantly influence the inversion results and should be taken into account.

scholarly journals Remote sensing of soot carbon – Part 2: Understanding the absorption Angstrom exponent

2015 ◽  
Vol 15 (15) ◽  
pp. 20911-20956 ◽  
Author(s):  
G. L. Schuster ◽  
O. Dubovik ◽  
A. Arola ◽  
T. F. Eck ◽  
B. N. Holben
Keyword(s):  
Refractive Index ◽  
Near Infrared ◽  
Carbonaceous Aerosols ◽  
Angstrom Exponent ◽  
Aerosol Absorption ◽  
Coarse Mode ◽  
Ångström Exponent ◽  
Soot Carbon ◽  
Definition Of ◽  
Ångstrom Exponent

Abstract. Recently, some authors have suggested that the absorption Angstrom exponent (AAE) can be used to deduce the component aerosol absorption optical depths (AAOD) of carbonaceous aerosols in the AERONET database. This "AAE approach" presumes that AAE &amp;ll; 1 for soot carbon, which contrasts the traditional small particle limit of AAE = 1 for soot carbon. Thus, we provide an overview of the AERONET retrieval, and investigate how the microphysics of carbonaceous aerosols can be interpreted in the AERONET AAE product. We find that AAE &amp;ll; 1 in the AERONET database requires large coarse mode fractions and/or imaginary refractive indices that increase with wavelength. Neither of these characteristics are consistent with the current definition of soot carbon, so we explore other possibilities for the cause of AAE &amp;ll; 1. We note that AAE is related to particle size, and that coarse mode particles have a smaller AAE than fine mode particles for a given aerosol mixture of species. We also note that the mineral goethite has an imaginary refractive index that increases with wavelength, is very common in dust regions, and can easily contribute to AAE &amp;ll; 1. We find that AAE &amp;ll; 1 can not be caused by soot carbon, unless soot carbon has an imaginary refractive index that increases with wavelength throughout the visible and near infrared spectrums. Finally, AAE is not a robust parameter for separating carbonaceous absorption from dust aerosol absorption in the AERONET database.

scholarly journals Detection of Saharan dust and biomass burning events using near-real-time intensive aerosol optical properties in the north-western Mediterranean

2016 ◽  
Vol 16 (19) ◽  
pp. 12567-12586 ◽  
Author(s):  
Marina Ealo ◽  
Andrés Alastuey ◽  
Anna Ripoll ◽  
Noemí Pérez ◽  
María Cruz Minguillón ◽  
...  
Keyword(s):  
Optical Properties ◽  
Biomass Burning ◽  
Mineral Dust ◽  
Organic Aerosol ◽  
Western Mediterranean ◽  
Saharan Dust ◽  
Aerosol Optical Properties ◽  
Angstrom Exponent ◽  
Ångström Exponent ◽  
Ångstrom Exponent

Abstract. The study of Saharan dust events (SDEs) and biomass burning (BB) emissions are both topics of great scientific interest since they are frequent and important polluting scenarios affecting air quality and climate. The main aim of this work is evaluating the feasibility of using near-real-time in situ aerosol optical measurements for the detection of these atmospheric events in the western Mediterranean Basin (WMB). With this aim, intensive aerosol optical properties (SAE: scattering Ångström exponent, AAE: absorption Ångström exponent, SSAAE: single scattering albedo Ångström exponent and g: asymmetry parameter) were derived from multi-wavelength aerosol light scattering, hemispheric backscattering and absorption measurements performed at regional (Montseny; MSY, 720 m a.s.l.) and continental (Montsec; MSA, 1570 m a.s.l.) background sites in the WMB. A sensitivity study aiming at calibrating the measured intensive optical properties for SDEs and BB detection is presented and discussed. The detection of SDEs by means of the SSAAE parameter and Ångström matrix (made up by SAE and AAE) depended on the altitude of the measurement station and on SDE intensity. At MSA (mountain-top site) SSAAE detected around 85 % of SDEs compared with 50 % at the MSY station, where pollution episodes dominated by fine anthropogenic particles frequently masked the effect of mineral dust on optical properties during less intense SDEs. Furthermore, an interesting feature of SSAAE was its capability to detect the presence of mineral dust after the end of SDEs. Thus, resuspension processes driven by summer regional atmospheric circulations and dry conditions after SDEs favoured the accumulation of mineral dust at regional level having important consequences for air quality. On average, SAE, AAE and g ranged between −0.7 and 1, 1.3 and 2.5 and 0.5 and 0.75 respectively during SDEs. Based on the aethalometer model, BB contribution to equivalent black carbon (BC) accounted for 36 and 40 % at MSY and MSA respectively. Linear relationships were found between AAE and %BCbb, with AAE values reaching around 1.5 when %BCbb was higher than 50 %. BB contribution to organic matter (OM) at MSY was around 30 %. Thus fossil fuel (FF) combustion sources showed important contributions to both BC and OM in the region under study. Results for OM source apportionment showed good agreement with simultaneous biomass burning organic aerosol (BBOA) and hydrocarbon-like organic aerosol (HOA) obtained by applying a positive matrix factorization model (PMF) to simultaneous Aerosol Chemical Speciation Monitor (ACSM) measurements. A wildfire episode was identified at MSY, showing AAE values up to 2 when daily BB contributions to BC and OM were 73 and 78 % respectively.

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