Cloud processing, cloud evaporation and Angström exponent

Abstract. With a cloud parcel model we investigated how cloud processing and cloud evaporation modify the size distribution and the Angström exponent of an aerosol population. Cloud processing causes a decrease in particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dispersion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is subject to other influences. It is very sensitive for relative humidity, especially between 95% and 100%. In addition, kinetic limitations delay droplet evaporation during cloud dissipation, which hampers a direct relation between the Angström exponent and the relative humidity. Consequently, a direct interpretation of the Angström exponent in terms of aerosol properties that play a role in aerosol-cloud interactions, such as the fine mode fraction, is rather complex.

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Cloud processing, cloud evaporation and Angström exponent

Atmospheric Chemistry and Physics ◽

10.5194/acp-9-71-2009 ◽

2009 ◽

Vol 9 (1) ◽

pp. 71-80 ◽

Cited By ~ 10

Author(s):

G.-J. Roelofs ◽

V. Kamphuis

Keyword(s):

Relative Humidity ◽

Optical Thickness ◽

Aerosol Optical Thickness ◽

Relative Dispersion ◽

Cloud Processing ◽

Angstrom Exponent ◽

Ambient Relative Humidity ◽

Coarse Mode ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. With a cloud parcel model we investigate how cloud processing and cloud evaporation modify the size distribution and the Angström exponent of an aerosol population. Our study provides a new explanation for the observed variability of the aerosol optical thickness and Angström exponent in the vicinity of clouds. Cloud processing causes a decrease of aerosol particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dispersion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is very sensitive for changes in relative humidity during cloud evaporation, especially between 90% and 100%. In addition, kinetic limitations delay evaporation of relatively large cloud drops, especially in clean and mildly polluted environments where the coarse mode fraction is relatively large. This hampers a direct relation between the aerosol optical thickness, the Angström exponent and the ambient relative humidity, which may severely complicate interpretation of these parameters in terms of aerosol properties, such as the fine mode fraction.

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Investigation of the relationship between the fine mode fraction and Ångström exponent: Cases in Korea

Atmospheric Research ◽

10.1016/j.atmosres.2020.105217 ◽

2021 ◽

Vol 248 ◽

pp. 105217

Author(s):

Ja-Ho Koo ◽

Juhee Lee ◽

Jhoon Kim ◽

Thomas F. Eck ◽

David M. Giles ◽

...

Keyword(s):

Angstrom Exponent ◽

Ångström Exponent ◽

Fine Mode ◽

The Relationship ◽

Ångstrom Exponent

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Four-year long-path monitoring of ambient aerosol extinction at a central European urban site: dependence on relative humidity

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-12583-2015 ◽

2015 ◽

Vol 15 (8) ◽

pp. 12583-12616

Author(s):

A. Skupin ◽

A. Ansmann ◽

R. Engelmann ◽

P. Seifert ◽

T. Müller

Keyword(s):

Relative Humidity ◽

Extinction Coefficient ◽

Enhancement Factor ◽

Urban Site ◽

Aerosol Extinction ◽

Ambient Aerosol ◽

Local Pollution ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Ångstrom Exponent

Abstract. The ambient aerosol particle extinction coefficient is measured with the Spectral Aerosol Extinction Monitoring System (SÆMS) along a 2.84 km horizontal path at 30–50 m height above ground in the urban environment of Leipzig (51.3° N, 12.4° E), Germany, since 2009. The dependence of the particle extinction coefficient (wavelength range from 300–1000 nm) on relative humidity up to almost 100% was investigated. The main results are presented. For the wavelength of 550 nm, the mean extinction enhancement factor was found to be 1.75 ± 0.4 for an increase of relative humidity from 40 to 80%. The respective four-year mean extinction enhancement factor is 2.8 ± 0.6 for a relative-humidty increase from 40 to 95%. A parameterization of the dependency of the urban particle extinction coefficient on relative humidity is presented. A mean hygroscopic exponent of 0.463 for the 2009–2012 period was determined. Based on a backward trajectory cluster analysis, the dependence of several aerosol optical properties for eight air flow regimes was investigated. Large differences were not found indicating that local pollution sources widely control the aerosol conditions over the urban site. The comparison of the SÆMS extinction coefficient statistics with respective statistics from ambient AERONET sun photometer observations yield good agreement. Also, time series of the particle extinction coefficient computed from in-situ-measured dry particle size distributions and humidity-corrected SÆMS extinction values (for 40% relative humidity) were found in good overall consistency, which corroborates the applicability of the developed humidity parameterization scheme. The analysis of the spectral dependence of particle extinction (Ångström exponent) revealed an increase of the 390–881 nm Ångström exponent from, on average, 0.3 (at 30% relative humidity) to 1.3 (at 95% relative humidity) for the four-year period.

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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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Day and night columnar aerosol properties at Granada (Spain) retrieved from sun-and star-photometry

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-11941-2012 ◽

2012 ◽

Vol 12 (5) ◽

pp. 11941-11978

Author(s):

D. Pérez-Ramírez ◽

H. Lyamani ◽

F. J. Olmo ◽

D. N. Whiteman ◽

L. Alados-Arboledas

Keyword(s):

Seasonal Pattern ◽

Meteorological Conditions ◽

Spectral Difference ◽

Night Time ◽

Mean Values ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Fine Mode ◽

The Mean ◽

Ångstrom Exponent

Abstract. This works present the first analysis of long-term day- and night-time columnar aerosol optical properties. To this end we have used a combination of sun and star photometer measurements at the city of Granada (37.16° N, 3.60° W, 680 m a.s.l.; South-East of Spain) from 2007 to 2010. For the whole study period, mean aerosol optical depth (AOD) at 440 nm (± standard deviation) is 0.18 ± 0.10 and 0.19 ± 0.11 for day- and night-time, respectively, while the mean Angström exponent (α) is 1.0 ± 0.4 and 0.9 ± 0.4 for day- and night-time. The ANOVA statistical tests reveal that there are no significant differences between the AOD and α obtained at day-time and those obtained at night-time. Additionally, the mean day-time values of AOD and α obtained during this period are within the values obtained in the surrounding AERONET stations. On the other hand, AOD presents an evident seasonal pattern characterized by large values in summer (mean values of 0.20 ± 0.10 both at day- and night-time) and low values in winter (mean values of 0.15 ± 0.09 at day-time and 0.17 ± 0.10 at night-time). The Angström exponent presents clear seasonal pattern with low values in summer (mean values of 0.8 ± 0.4 and 0.9 ± 0.4 at day- and night-time) and relatively large values in winter (mean values of 1.2 ± 0.4 and 1.0 ± 0.3 at day- and night-time). These seasonal patterns are explained by the differences in the meteorological conditions and by the differences in the strength of aerosol sources during day and night. To take more insight about the changes in aerosol particles between day and night, the spectral difference of the Angström exponent (δα) as function of α is also studied. This analysis reveals an increase in the fine mode radius and in the fine mode contribution to AOD during night-time, being more remarkable in summer seasons. These changes are explained by the changes in the local aerosol source emissions and meteorological conditions between day- and night-time, as well as aerosol aging processes.

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Aerosol climatology: dependence of the Angstrom exponent on wavelength over four AERONET sites

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-7347-2007 ◽

2007 ◽

Vol 7 (3) ◽

pp. 7347-7397 ◽

Cited By ~ 103

Author(s):

D. G. Kaskaoutis ◽

H. D. Kambezidis ◽

N. Hatzianastassiou ◽

P. G. Kosmopoulos ◽

K. V. S. Badarinath

Keyword(s):

Second Order ◽

Spectral Variation ◽

Angstrom Exponent ◽

Coarse Mode ◽

Polynomial Fit ◽

Ångström Exponent ◽

Order Polynomial ◽

Fine Mode ◽

Aerosol Types ◽

Ångstrom Exponent

Abstract. The Ångström exponent, α, is often used as a qualitative indicator of aerosol particle size. In this study, aerosol optical depth (AOD) and Ångström exponent (α) data were analyzed to obtain information about the adequacy of the simple use of the Ångström exponent for characterizing aerosols, and for exploring possibilities for a more efficient characterization of aerosols. This was made possible by taking advantage of the spectral variation of α, the so-called curvature. The data were taken from four selected AERONET stations, which are representative of four aerosol types, i.e. biomass burning, pollution, desert dust and maritime. Using the least-squares method, the Ångström-α was calculated in the spectral interval 340–870 nm, along with the coefficients α1 and α2 of the second order polynomial fit to the plotted logarithm of AOD versus the logarithm of wavelength, and the second derivative of α. The results show that the spectral curvature can provide important additional information about the different aerosol types, and can be effectively used to discriminate between them, since the fine-mode particles exhibit negative curvature, while the coarse-mode aerosols positive. In addition, the curvature has always to be taken into account in the computations of Ångström exponent values in the spectral intervals 380–440 nm and 675–870 nm, since fine-mode aerosols exhibit larger α675–870 than α380–440 values, and vice-versa for coarse-mode particles. A second-order polynomial fit simulates the spectral dependence of the AODs very well, while the associated constant term varies proportionally to the aerosol type. The correlation between the coefficients α1 and α2 of the second-order polynomial fit and the Ångström exponent α, and the atmospheric turbidity, is further investigated. The obtained results reveal important features, which can be used for better discriminating between different aerosol types.

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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 and direct radiative forcing based on measurements from the China Aerosol Remote Sensing Network (CARSNET) in eastern China

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-405-2018 ◽

2018 ◽

Vol 18 (1) ◽

pp. 405-425 ◽

Cited By ~ 49

Author(s):

Huizheng Che ◽

Bing Qi ◽

Hujia Zhao ◽

Xiangao Xia ◽

Thomas F. Eck ◽

...

Keyword(s):

Remote Sensing ◽

Optical Properties ◽

Biomass Burning ◽

Radiative Forcing ◽

Eastern China ◽

Size Distributions ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Fine Mode ◽

Ångstrom Exponent

Abstract. Aerosol pollution in eastern China is an unfortunate consequence of the region's rapid economic and industrial growth. Here, sun photometer measurements from seven sites in the Yangtze River Delta (YRD) from 2011 to 2015 were used to characterize the climatology of aerosol microphysical and optical properties, calculate direct aerosol radiative forcing (DARF) and classify the aerosols based on size and absorption. Bimodal size distributions were found throughout the year, but larger volumes and effective radii of fine-mode particles occurred in June and September due to hygroscopic growth and/or cloud processing. Increases in the fine-mode particles in June and September caused AOD440 nm > 1.00 at most sites, and annual mean AOD440 nm values of 0.71–0.76 were found at the urban sites and 0.68 at the rural site. Unlike northern China, the AOD440 nm was lower in July and August (∼ 0.40–0.60) than in January and February (0.71–0.89) due to particle dispersion associated with subtropical anticyclones in summer. Low volumes and large bandwidths of both fine-mode and coarse-mode aerosol size distributions occurred in July and August because of biomass burning. Single-scattering albedos at 440 nm (SSA440 nm) from 0.91 to 0.94 indicated particles with relatively strong to moderate absorption. Strongly absorbing particles from biomass burning with a significant SSA wavelength dependence were found in July and August at most sites, while coarse particles in March to May were mineral dust. Absorbing aerosols were distributed more or less homogeneously throughout the region with absorption aerosol optical depths at 440 nm ∼ 0.04–0.06, but inter-site differences in the absorption Angström exponent indicate a degree of spatial heterogeneity in particle composition. The annual mean DARF was −93 ± 44 to −79 ± 39 W m−2 at the Earth's surface and ∼ −40 W m−2 at the top of the atmosphere (for the solar zenith angle range of 50 to 80∘) under cloud-free conditions. The fine mode composed a major contribution of the absorbing particles in the classification scheme based on SSA, fine-mode fraction and extinction Angström exponent. This study contributes to our understanding of aerosols and regional climate/air quality, and the results will be useful for validating satellite retrievals and for improving climate models and remote sensing algorithms.

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Airborne observation of aerosol optical depth during ARCTAS: vertical profiles, inter-comparison and fine-mode fraction

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-3673-2011 ◽

2011 ◽

Vol 11 (8) ◽

pp. 3673-3688 ◽

Cited By ~ 40

Author(s):

Y. Shinozuka ◽

J. Redemann ◽

J. M. Livingston ◽

P. B. Russell ◽

A. D. Clarke ◽

...

Keyword(s):

Aerosol Optical Depth ◽

Optical Depth ◽

Biomass Burning ◽

The Arctic ◽

Vertical Profiles ◽

Horizontal Structure ◽

Angstrom Exponent ◽

Ångström Exponent ◽

Fine Mode ◽

Ångstrom Exponent

Abstract. We describe aerosol optical depth (AOD) measured during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) experiment, focusing on vertical profiles, inter-comparison with correlative observations and fine-mode fraction. Arctic haze observed in <2 km and 2–4 km over Alaska in April 2008 originated mainly from anthropogenic emission and biomass burning, respectively, according to aerosol mass spectrometry and black carbon incandescence measurements. The Ångström exponent for these air masses is 1.4 ± 0.3 and 1.7 ± 0.1, respectively, when derived at 499 nm from a second-order polynomial fit to the AOD spectra measured with the 14-channel Ames Airborne Tracking Sunphotometer (AATS-14) over 354–2139 nm. We examine 55 vertical profiles selected from all phases of the experiment. For two thirds of them, the AOD spectra are within 3% + 0.02 of the vertical integral of local visible-light scattering and absorption. The horizontal structure of smoke plumes from local biomass burning observed in central Canada in June and July 2008 explains most outliers. The differences in mid-visible Ångström exponent are <0.10 for 63% of the profiles with 499-nm AOD > 0.1. The retrieved fine-mode fraction of AOD is mostly between 0.7 and 1.0, and its root mean square difference (in both directions) from column-integral submicron fraction (measured with nephelometers, absorption photometers and an impactor) is 0.12. These AOD measurements from the NASA P-3 aircraft, after compensation for below-aircraft light attenuation by vertical extrapolation, mostly fall within ±0.02 of AERONET ground-based measurements between 340–1640 nm for five overpass events.

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