Simplified Analytic Model to Estimate the Ångstrom Exponent in a Junge Aerosol Size Distribution

Abstract. We are reporting the calculated values of columnar aerosol size distribution function for atmosphere of Zanjan, a city in Northwest Iran (36.70° N, 48.51° E). Ground-based measurements of the total optical depth of the Zanjan atmosphere at 440 nm, 670 nm, 870 nm, and 1020 nm are recorded using a Cimel CE318-2 sunphotometer in the period of October 2006 to September 2008. The spectral aerosol optical depth has been obtained by subtraction of molecular optical depth from the total optical depth for each wavelength channel. Also the Ångström exponent is determined by a logarithmic fit to the aerosol optical depth when it is plotted versus the logarithm of the wavelength. Daily averages of the measured aerosol optical depth and Ångström exponent values have been implemented in an inversion algorithm for calculation of the columnar aerosol size distribution function. In this algorithm, the aerosols are considered as spheres of different size and refractive index of 1.45. We found that for 82% of the days, aerosols are in the coarse mode. For these days, more than 50% of the aerosol volume concentration has a radius >1 μm. We believe this is related to the geographical location of Zanjan in a mostly dry area and subject to frequent dust winds.

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Analytic Solution on the Estimation of the Ångstrom Exponent in Log-Normal Aerosol Size Distribution

Particulate Science And Technology ◽

10.1080/02726351.2012.658902 ◽

2013 ◽

Vol 31 (1) ◽

pp. 92-99 ◽

Cited By ~ 5

Author(s):

Chang H. Jung ◽

Yong P. Kim

Keyword(s):

Size Distribution ◽

Analytic Solution ◽

Aerosol Size Distribution ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Log Normal ◽

Ångstrom Exponent

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Measurements of optical properties of atmospheric aerosols in Northern Finland

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-11703-2005 ◽

2005 ◽

Vol 5 (6) ◽

pp. 11703-11728 ◽

Cited By ~ 1

Author(s):

V. Aaltonen ◽

H. Lihavainen ◽

V.-M. Kerminen ◽

M. Komppula ◽

J. Hatakka ◽

...

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Seasonal Pattern ◽

Late Summer ◽

Aerosol Optical Properties ◽

Air Masses ◽

Angstrom Exponent ◽

Northern Border ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. Three years of continuous measurements of aerosol optical properties and simultaneous aerosol number size distribution measurements at Pallas GAW station, a remote subarctic site in the northern border of the boreal forest zone, have been analysed. The scattering coefficient at 550 nm varied from 0.2 to 94.4 Mm−1 with an average of 7.1±8.6 Mm−1. Both the scattering and backscattering coefficients had a clear seasonal cycle with an autumn minimum and a 4–5 times higher summer maximum. The scattering was dominated by submicron aerosols and especially so during late summer and autumn. The Ångström exponent had a clear seasonal pattern with maximum values in late summer and minimum values during wintertime. The highest hemispheric backscattering fraction values were observed in autumn, indicating clean air with few scattering particles and a particle size distribution strongly dominated by ultrafine particles. To analyse the influence of air mass origin on the aerosol optical properties a trajectory climatology was applied to the Pallas aerosol data. The most polluted trajectory patterns represented air masses from the Kola Peninsula, Scandinavia and Russia as well as long-range transport from Britain and Eastern Europe. These air masses had the largest average scattering and backscattering coefficients for all seasons. Higher than average values of the Ångström exponent were also observed in connection with transport from these areas.

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Estimating cloud condensation nuclei number concentrations using aerosol optical properties: role of particle number size distribution and parameterization

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-15483-2019 ◽

2019 ◽

Vol 19 (24) ◽

pp. 15483-15502 ◽

Cited By ~ 1

Author(s):

Yicheng Shen ◽

Aki Virkkula ◽

Aijun Ding ◽

Krista Luoma ◽

Helmi Keskinen ◽

...

Keyword(s):

Optical Properties ◽

Size Distribution ◽

Cloud Condensation Nuclei ◽

Size Distributions ◽

Condensation Nuclei ◽

Aerosol Optical Properties ◽

Average Bias ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. The concentration of cloud condensation nuclei (CCN) is an essential parameter affecting aerosol–cloud interactions within warm clouds. Long-term CCN number concentration (NCCN) data are scarce; there are a lot more data on aerosol optical properties (AOPs). It is therefore valuable to derive parameterizations for estimating NCCN from AOP measurements. Such parameterizations have already been made, and in the present work a new parameterization is presented. The relationships between NCCN, AOPs, and size distributions were investigated based on in situ measurement data from six stations in very different environments around the world. The relationships were used for deriving a parameterization that depends on the scattering Ångström exponent (SAE), backscatter fraction (BSF), and total scattering coefficient (σsp) of PM10 particles. The analysis first showed that the dependence of NCCN on supersaturation (SS) can be described by a logarithmic fit in the range SS <1.1 %, without any theoretical reasoning. The relationship between NCCN and AOPs was parameterized as NCCN≈((286±46)SAE ln(SS/(0.093±0.006))(BSF − BSFmin) + (5.2±3.3))σsp, where BSFmin is the minimum BSF, in practice the 1st percentile of BSF data at a site to be analyzed. At the lowest supersaturations of each site (SS ≈0.1 %), the average bias, defined as the ratio of the AOP-derived and measured NCCN, varied from ∼0.7 to ∼1.9 at most sites except at a Himalayan site where the bias was >4. At SS >0.4 % the average bias ranged from ∼0.7 to ∼1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was higher, ∼1.4–1.9. In other words, at SS >0.4 % NCCN was estimated with an average uncertainty of approximately 30 % by using nephelometer data. The biases were mainly due to the biases in the parameterization related to the scattering Ångström exponent (SAE). The squared correlation coefficients between the AOP-derived and measured NCCN varied from ∼0.5 to ∼0.8. To study the physical explanation of the relationships between NCCN and AOPs, lognormal unimodal particle size distributions were generated and NCCN and AOPs were calculated. The simulation showed that the relationships of NCCN and AOPs are affected by the geometric mean diameter and width of the size distribution and the activation diameter. The relationships of NCCN and AOPs were similar to those of the observed ones.

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Impacts of the Variations of Aerosols Components and Relative Humidity on the Visibility and Particles Size Distribution of the Desert Atmosphere

Asian Journal of Research and Reviews in Physics ◽

10.9734/ajr2p/2021/v4i330146 ◽

2021 ◽

pp. 42-65

Author(s):

S. U. Yerima ◽

U. Y. Abdulkarim ◽

B. I. Tijjani ◽

U. M. Gana ◽

M. Idris ◽

...

Keyword(s):

Size Distribution ◽

Hygroscopic Growth ◽

Angstrom Exponent ◽

Microphysical Properties ◽

Coarse Mode ◽

Ångström Exponent ◽

Particles Size Distribution ◽

Fine Mode ◽

The Mean ◽

Ångstrom Exponent

This paper investigates the Impact of relative humidity, varying the concentrations of water-soluble aerosol particle concentrations (WASO), Mineral Nuclei Mode Aerosols Particle Concentration (MINN), mineral accumulation mode, nonspherical (MIAN) aerosol particles concentrations and Mineral Coarse Mode Aerosols Particle Concentration (MICN) on the visibility and particles size distribution of desert aerosols based on microphysical properties of desert aerosols. The microphysical properties (the extinction coefficients, volume mix ratios, dry mode radii and wet mode radii) were extracted from Optical Properties of Aerosols and Clouds (OPAC 4.0) at eight relative humidities, RHs (00 to 99%) and at the spectral visible range of 0.4-0.8mm, the concentrations were varied to obtain five different models for each above-mentioned component. Regression analysis of some standard equations were used to determine the Angstrom exponent (α), the turbidity coefficient (β), the curvature (α2), humidification factor (), the mean exponent of aerosol growth curve (µ) and the mean exponent of aerosol size distributions (n). The values of angstrom exponent (α) were observed to be less than 1 throughout the five models at all RHs for the four studied components, and this signifies the dominance of coarse mode particles over fine mode particles. But the magnitude of the angstrom exponent (α) fluctuates all through the studied components except for WASO which increased with the increase in RH across the models and this also signifies the dominance of coarse mode particles with some traces of fine mode particles. The investigation also revealed that the curvature (α2) has both monomodal (negative signs) and bimodal (positive signs) types of distributions all through the five models and this also signifies the dominance of coarse mode particles with some traces of fine mode particles across the individual models for all the studied components. it was also found that the visibility decreased with the increase in RH and increased with the increase in wavelength. The investigation further revealed that the turbidity coefficient (β) fluctuates with the increase in RH and the particles concentrations, and this might be due to major coagulation and sedimentation. The analysis further found that there is a direct inverse power relation between the humidification factor and the mean exponent of aerosols size distribution with the mean exponent of aerosols growth curve. It was also found that as the magnitude of µ increased for MIAN, MINN and MICN, the effective hygroscopic growth decreased. For WASO, it was found that as the magnitude of µ decreased, the effective hygroscopic growth increased with the increase in particles concentrations and RH. The decreased in the magnitude of µ for WASO might be due to the fact that as we increase the non-hygroscopic particles, we decrease the deliquescence. The mean exponent of aerosol size distribution (n) being less than 3 shows foggy condition of the desert atmosphere the four investigated components and five studied models.

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Aerosol optical properties derived from POLDER-3/PARASOL (2005–2013) over the western Mediterranean Sea – Part 1: Quality assessment with AERONET and in situ airborne observations

Atmospheric Measurement Techniques ◽

10.5194/amt-11-6761-2018 ◽

2018 ◽

Vol 11 (12) ◽

pp. 6761-6784 ◽

Cited By ~ 5

Author(s):

Paola Formenti ◽

Lydie Mbemba Kabuiku ◽

Isabelle Chiapello ◽

Fabrice Ducos ◽

François Dulac ◽

...

Keyword(s):

Size Distribution ◽

Regional Scale ◽

Coarse Fraction ◽

Western Mediterranean ◽

Angstrom Exponent ◽

Full Dataset ◽

Western Mediterranean Sea ◽

Ångström Exponent ◽

Using Data ◽

Ångstrom Exponent

Abstract. The western Mediterranean atmosphere is impacted by a variety of aerosol sources, producing a complex and variable mixture of natural and anthropogenic particles, with different chemical and physical properties. Satellite sensors provide a useful global coverage of aerosol parameters but through indirect measurements that require careful validation. Here we present the results of a long-term regional scale analysis of the full dataset (March 2005 and October 2013) of POLDER-3/PARASOL ocean operational retrievals of the total, fine, and coarse aerosol optical depth (AOD, AODF, and AODC), Ångström exponent (AE), and the spherical or non-spherical partition of coarse-mode AOD (AODCS and AODCNS), respectively. The evaluation is performed using data from 17 coastal and insular ground-based AERONET sites on one side, and airborne vertical profiles of aerosol extinction and number size distribution obtained by the SAFIRE ATR-42 aircraft operated in the area during summer 2012 and 2013 on the other side. This study provides the first regional evaluation of uncertainties of the POLDER-3 products, and highlights their quality. The POLDER-3 Ångström exponent, representing AOD spectral dependence in link with the aerosol particle size distribution, is biased towards small values. This bias, however, does not prevent using AE for classifying the regional aerosol laden air masses. AODF corresponds to particles smaller than 0.6–0.8 µm in diameter and appears suitable to monitor the aerosol submicron fraction from space. We also provide an original validation of POLDER-3 AODC and its spherical or non-spherical partition, which shows agreement within 25 % with AERONET shape retrievals when the aerosol coarse fraction dominates.

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Study of columnar aerosol size distribution in Hong Kong

Atmospheric Chemistry and Physics ◽

10.5194/acp-9-6175-2009 ◽

2009 ◽

Vol 9 (16) ◽

pp. 6175-6189 ◽

Cited By ~ 19

Author(s):

X. Yang ◽

M. Wenig

Keyword(s):

Size Distribution ◽

Atmospheric Conditions ◽

Hygroscopic Growth ◽

Angstrom Exponent ◽

Coarse Mode ◽

Ångström Exponent ◽

Fine Mode ◽

Fine Mode Aerosol ◽

Ångstrom Exponent ◽

Aerosol Properties

Abstract. This paper presents studies on columnar aerosol optical properties in Hong Kong with focus on aerosol volume size distribution, which helps understand local aerosol properties, variation, hygroscopic growth and coagulation. Long-term ground measurements in the wet season in the years of 2002, 2003, 2004 and 2008 have been performed using a sun-sky radiometer. Data validation made using MODIS and local AERONET shows agreement. A bimodal size distribution is found with the fine mode centering at ~0.2 μm and coarse mode centering at ~3 μm respectively. The fine and coarse mode have close volume concentrations of nearly 50% fraction in composing local aerosols. Intercomparison of different years shows similar aerosol properties while a small increase of fine mode aerosol could be observed. A systematic shift of size distribution parameters is observed with different atmospheric conditions, where higher aerosol loadings and Angstrom exponent correspond to more fine mode aerosols. The fine mode is found to be more closely correlated with this shift than the coarse mode. A higher fine mode volume fraction and smaller median fine radius correspond to a larger Angstrom exponent. The fine mode aerosol hygroscopic growth is one of the main mechanisms for such systematic shifting. A third mode centering at ~1–2 μm could be discovered under high aerosol loading and high fine mode aerosol conditions. It becomes more pronounced with high aerosol optical depth and larger Angstrom exponent. Investigation of its variation with corresponding optical parameters and correlation with atmospheric conditions appears to support the hypothesis that it is mainly due to the fine mode aerosol hygroscopic growth and coagulation rather than the contribution from the coarse mode. While the very humid environment facilitates the aerosol hygroscopic growth, aerosol coagulation might further produce larger aerosols under high fine aerosol conditions. The continental outflow with transported aging aerosols and biomass burning might have also contributed to this additional mode.

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Aerosol optical properties derived from POLDER-3/PARASOL (2005–2013) over the western Mediterranean Sea: I. Quality assessment with AERONET and in situ airborne observations

10.5194/amt-2018-251 ◽

2018 ◽

Cited By ~ 1

Author(s):

Paola Formenti ◽

Lydie Mbemba Kabuiku ◽

Isabelle Chiapello ◽

Fabrice Ducos ◽

François Dulac ◽

...

Keyword(s):

Size Distribution ◽

Regional Scale ◽

Coarse Fraction ◽

Western Mediterranean ◽

Angstrom Exponent ◽

Full Dataset ◽

Western Mediterranean Sea ◽

Ångström Exponent ◽

Using Data ◽

Ångstrom Exponent

Abstract. The western Mediterranean atmosphere is impacted by a variety of aerosol sources, producing a complex and variable mixture of natural and anthropogenic particles, with different chemical and physical properties. Satellite sensors provide a useful global coverage of aerosol parameters but through indirect measurements that request careful validation. Here we present the results of a long-term regional scale analysis of the full dataset (March 2005 and October 2013) of POLDER-3/PARASOL ocean operational retrievals of the total, fine and coarse aerosol optical depth (AOD, AODF and AODC), Angstrom exponent (AE), and the spherical/non-spherical partition of coarse-mode AOD (AODCS and AODCNS), respectively. The evaluation is performed using data from seventeen coastal and insular ground-based AERONET sites on one side, and airborne vertical profiles of aerosol extinction and number size distribution obtained by the SAFIRE ATR 42 aircraft operated in the area during summer 2012 and 2013 on the other side. This study provides the first regional evaluation of uncertainties of the POLDER-3 products, and highlights their quality. The POLDER-3 Ångström exponent, representing AOD spectral dependence in link with the aerosol particle size distribution, is biased towards small values. This bias, however, does not prevent using AE for classifying the regional aerosol laden air masses. AODF corresponds to particle smaller than 0.6–0.8 µm in diameter and appears suitable to monitor the aerosol submicron fraction from space. We also provide an original validation of POLDER-3 AODC and its spherical/non-spherical partition, which shows agreement within 25 % with AERONET shape retrievals when the aerosol coarse fraction dominates.

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The absorption Ångstrom exponent of black carbon with brown coatings: effects of aerosol microphysics and parameterization

Atmospheric Chemistry and Physics ◽

10.5194/acp-20-9701-2020 ◽

2020 ◽

Vol 20 (16) ◽

pp. 9701-9711 ◽

Cited By ~ 5

Author(s):

Xiaolin Zhang ◽

Mao Mao ◽

Yan Yin ◽

Shihao Tang

Keyword(s):

Size Distribution ◽

Black Carbon ◽

Volume Fraction ◽

Optical Parameter ◽

Brown Carbon ◽

Angstrom Exponent ◽

Aerosol Absorption ◽

Ångström Exponent ◽

Bc Aerosols ◽

Ångstrom Exponent

Abstract. The aerosol absorption Ångstrom exponent (AAE) is a crucial optical parameter for apportionment and characterization. Due to considerable inconsistences associated with observations, numerical research is a powerful means to give a better understanding of the AAE of aged black carbon (BC) aerosols. Numerical studies of the AAE of polydisperse BC aggregates with brown coatings using the exact multiple-sphere T-matrix method (MSTM) are performed. The objective of the study is to thoroughly assess the AAE of coated BC influenced by their observation-based detailed microphysics and then provide a new AAE parameterization for application. At odds with our expectations, more large-sized BC particles coated by thin brown carbon can have an AAE smaller than 1.0, indicating that BC aerosols internally mixing with brown carbon can even show lower AAE than pure BC particles. The AAE of BC with brown coatings is highly sensitive to the absorbing volume fraction of the coating, coated volume fraction of BC, shell ∕ core ratio, and particle size distribution with a wide variation, whereas the impacts of BC geometry and BC position within the coating are negligible. The AAE of BC with brown coatings can be larger than 3.0 if there are plenty of small-sized coated BC particles, heavy coating, or a large amount of brown carbon. However, the AAE of BC with non-absorbing coating appears to be weakly sensitive to particle microphysics with values around 1.0 (i.e., 0.7–1.4), suggesting the substantial role of the absorbing volume fraction of the coating in AAE determination. With more realistic BC geometries, our study also indicates that the occurrence of brown carbon may not be confidently determined unless AAE > 1.4. The currently popular core–shell Mie model reasonably approximates the AAE of fully coated BC by brown carbon, whereas it underestimates the AAE of partially coated or externally attached BC and underestimates more for a lower coated volume fraction of BC. In addition, we present a parameterization of the AAE of coated BC with a size distribution on the basis of numerical results, which can act as a guide for the AAE response to the absorbing volume fraction of the coating, coated volume fraction of BC, and shell ∕ core ratio. The proposed parameterization of coated BC AAE generates a decent prediction for moderate BC microphysics, whereas caution should be taken in applying it for extreme cases, such as externally attached coated BC morphology. Our findings could improve the understanding and application of the AAE of BC with brown coatings.

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The absorption Angstrom exponent of black carbon with brown coatings: effects of aerosol microphysics and parameterization

10.5194/acp-2020-224 ◽

2020 ◽

Author(s):

Xiaolin Zhang ◽

Mao Mao ◽

Shihao Tang

Keyword(s):

Size Distribution ◽

Volume Fraction ◽

Optical Parameter ◽

Brown Carbon ◽

Angstrom Exponent ◽

Aerosol Absorption ◽

Highly Sensitive ◽

Ångström Exponent ◽

Bc Aerosols ◽

Ångstrom Exponent

Abstract. Aerosol absorption Angstrom exponent (AAE) is a crucial optical parameter for their apportionment and characterization. Due to considerable inconsistences associated with observations, a numerical research is a powerful means to give better understanding of the AAE of aged BC aerosols. Numerical studies of the AAE of polydisperse BC aggregates with brown coatings using the exact multiple-sphere T-matrix method (MSTM) are performed. The objective of the study is to thoroughly assess the AAE of coated BC influenced by their observation-based detailed microphysics and then provide a new AAE parameterization for application. At odds with our expectations, BC coated by thin brown carbon with more large particles can have an AAE smaller than 1.0, indicating that BC aerosols internally mixing with brown carbon can even show lower AAE than pure BC particles. The AAE of BC with brown coatings is highly sensitive to absorbing volume fraction of coating, coated volume fraction of BC, shell / core ratio, and particle size distribution with a wide variation, whereas the impacts of BC geometry and BC position within coating are trivial. The AAE of BC with brown coatings can be larger than 3.0, if there are more small coated BC particles, heavy coating, or more brown carbon. However, the AAE of BC with non-absorbing coating shows weakly sensitive to particle microphysics with values around 1.0 (i.e., 0.7–1.4), suggesting the substantial role of absorbing volume fraction of coating in the AAE determination. With more realistic BC geometries, our study also indicates that occurrence of brown carbon may be made confidently unless AAE > 1.4. In addition, we present a parameterization of the AAE of coated BC with a size distribution on the basis of numerical results, which can act as a guide for the AAE response to absorbing volume fraction of coating, coated volume fraction of BC, and shell / core ratio. Our findings can improve the understanding and application of the AAE of BC with brown coatings.

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