scholarly journals Effects of brown coatings on the absorption enhancement of black carbon: a numerical investigation

2018 ◽  
Vol 18 (23) ◽  
pp. 16897-16914 ◽  
Author(s):  
Jie Luo ◽  
Yongming Zhang ◽  
Feng Wang ◽  
Qixing Zhang
Keyword(s):  
Size Distribution ◽  
Black Carbon ◽  
Volume Fraction ◽  
Absorption Enhancement ◽  
Core Shell ◽  
Sphere Model ◽  
Brown Carbon ◽  
Blocking Effects ◽  
Ultraviolet Spectral Region ◽  
Composition Ratios

Abstract. Using the numerically exact multiple sphere T-matrix (MSTM) method, we explored the effects of brown coatings on absorption enhancement (Eabs) of black carbon (BC) at different wavelengths (λ). In addition, the ratio of the absorption of BC coated by brown carbon (BrC) to an external mixture of BrC and BC (Eabs_internal) is also investigated. In this work, thinly coated BC is defined as that with a BC volume fraction over 20 %, and other BC is considered to be thickly coated. Eabs increases with the absorption of coatings, while an opposite trend is observed for Eabs_internal. A much wider range of Eabs is observed for BC with brown coatings compared to that with non-absorbing coatings. As the mass ratio of BrC to BC (MR) is over 13.9, Eabs can exceed 5.4 for BC with brown coatings at λ=0.35 µm under a typical size distribution. Specifically, as MR increases to approximately 13.9, Eabs values of larger than 3.96 can be observed at 0.532 µm, which is a little higher than the commonly measured Eabs of 1.05–3.5 at this wavelength. Previous studies have focused on the lensing effects of coatings but neglected the blocking effects of absorbing coatings. Eabs_internal can be below 1 at an ultraviolet spectral region for BC with brown coatings, which indicates that the absorption of internally mixed BC is less than that of an external mixture of BrC and BC due to the blocking effects of outer coatings, and we named the blocking effect of absorbing coatings the “sunglasses effect”. In addition, the applicability of a core–shell sphere model is also evaluated for BC with brown coatings. The absorption cross section (Cabs) of thickly coated BC is underestimated by the core–shell sphere model for all wavelengths while the underestimation becomes negligible as the imaginary part of the refractive index of brown carbon (kBrC) becomes very large. The lensing effect and the sunglasses effect are clearly defined. Moreover, the effects of composition ratios and the size distribution are explored at different wavelengths. Our findings can improve the understanding of the absorption enhancement of BC with brown coatings.

scholarly journals Effects of brown coatings on the absorption enhancement of black carbon: a numerical investigation

2018 ◽  
Author(s):  
Jie Luo ◽  
Yongming Zhang ◽  
Feng Wang ◽  
Qixing Zhang
Keyword(s):  
Size Distribution ◽  
Black Carbon ◽  
Blocking Effect ◽  
Absorption Enhancement ◽  
Sphere Model ◽  
Aerosol Absorption ◽  
Opposite Trend ◽  
Blocking Effects ◽  
Ultraviolet Spectral Region ◽  
T Matrix

Abstract. Using the numerically exact multiple sphere T-matrix (MSTM) method, we explored the effects of brown coatings on absorption enhancement (Eabs) and the lensing effects (Eabs_lensing) of black carbon (BC) at different wavelengths (λ). Eabs increases with the absorption of coatings, while an opposite trend is observed for Eabs_lensing. A much wider range of Eabs is observed for BC with brown coatings compared to that with non-absorbing coatings. Eabs can reach approximately 5.4 for λ = 0.35 μm given a typical size distribution. In addition, previous studies have focused on the lensing effects of coatings but neglected the blocking effects of absorbing coatings. Eabs_lensing can be below 0.5 at ultraviolet spectral region for BC with brown coatings, which indicates that the absorption of internal BC is diluted by approximately 0.5 times due to the blocking effects of outer coatings, and we named the blocking effect of absorbing coatings "sunglasses effect". Hence, it is imperative to take the blocking effects of outer coatings into consideration for estimation of total aerosol absorption. Cabs of thickly-coated BC is underestimated by core-shell sphere model for all wavelengths while the underestimation becomes negligible as kBrC turns very large. Eabs and Eabs_lensing are insensitive to the size distribution for thinly-coated BC, and the uncertainties caused by the size distribution have differences of less than 2.6 % and 6 %, respectively. However, the effects of size distribution on Eabs and Eabs_lensing are rather obvious for thickly-coated BC. In addition, the uncertainties in Eabs and Eabs_lensing caused by Df are less than 9 %. Eabs seems to be essentially wavelength-independent for thickly-coated BC with non-absorbing coatings but rather wavelength-dependent for thickly-coated BC with brown coatings.

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

2020 ◽  
Vol 20 (16) ◽  
pp. 9701-9711 ◽  
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.

scholarly journals Variation of the radiative properties during black carbon aging: theoretical and experimental intercomparison

2015 ◽  
Vol 15 (14) ◽  
pp. 19835-19872 ◽  
Author(s):  
C. He ◽  
K.-N. Liou ◽  
Y. Takano ◽  
R. Zhang ◽  
M. L. Zamora ◽  
...  
Keyword(s):  
Black Carbon ◽  
Cross Sections ◽  
Theoretical Calculations ◽  
Absorption Enhancement ◽  
Core Shell ◽  
Hygroscopic Growth ◽  
Aging Model ◽  
Scattering Cross ◽  
Coating Morphology ◽  
Scattering Cross Sections

Abstract. A theoretical black carbon (BC) aging model is developed to account for three typical evolution stages, namely, freshly emitted aggregates, coated BC by soluble material, and BC particles undergoing further hygroscopic growth. The geometric-optics surface-wave (GOS) approach is employed to compute the BC single-scattering properties at each aging stage, which are subsequently compared with laboratory measurements. Theoretical calculations are consistent with measurements in extinction and absorption cross sections for fresh BC aggregates, but overestimate the scattering cross sections for BC mobility diameters of 155, 245, and 320 nm, because of uncertainties associated with theoretical calculations for small particles as well as laboratory scattering measurements. The measured optical cross sections for coated BC by sulfuric acid and for those undergoing further hygroscopic growth are captured by theoretical calculations using a concentric core-shell structure, with differences of less than 20 %. This suggests that the core-shell shape represents the realistic BC coating morphology reasonably well in this case, which is consistent with the observed strong structure compaction during aging. We find that the absorption and scattering properties of fresh BC aggregates vary by up to 60 % due to uncertainty in the BC refractive index, which, however, is a factor of two smaller in the case of coated BC particles. Sensitivity analyses on the BC morphology show that the optical properties of fresh BC aggregates are more sensitive to fractal dimension than primary spherule size. The absorption and scattering cross sections of coated BC particles vary by more than a factor of two due to different coating structures. We find an increase of 20–250 % in absorption and a factor of 3–15 in scattering during aging, significantly depending on coating morphology and aging stages. Applying the aging model to CalNex 2010 field measurements, we show that the resulting BC direct radiative forcing (DRF) first increases from 1.5 to 1.7 W m-2 and subsequently decreases to 1.0 W m-2 during the transport from the Los Angeles Basin to downwind regions, as a result of the competition between absorption enhancement due to coating and dilution of BC concentration. The BC DRF can vary by up to a factor of two due to differences in BC coating morphology. Thus, an accurate estimate of BC DRF requires the incorporation of a dynamic BC aging process that accounts for realistic morphology in climate models, particularly for the regional analysis with high atmospheric heterogeneity.

scholarly journals Mixing characteristics of refractory black carbon aerosols determined by a tandem CPMA-SP2 system at an urban site in Beijing

2019 ◽  
Author(s):  
Hang Liu ◽  
Xiaole Pan ◽  
Dantong Liu ◽  
Xiaoyong Liu ◽  
Xueshun Chen ◽  
...  
Keyword(s):  
Black Carbon ◽  
Fractal Structure ◽  
Particle Mass ◽  
Shell Structures ◽  
Absorption Enhancement ◽  
Urban Site ◽  
Core Shell ◽  
Mixing State ◽  
Carbon Aerosols ◽  
Black Carbon Aerosols

Abstract. Black carbon aerosols play an important role in climate change by absorbing solar radiation and degrading visibility. In this study, the mixing state of refractory black carbon (rBC) at an urban site in Beijing was studied with a single particle soot photometer (SP2), as well as a tandem observation system with a centrifugal particle mass analyzer (CPMA) and a differential mobility analyzer (DMA), in early summer of 2018. The results demonstrated that the mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 171.2 nm. When the site experienced prevailing southerly winds, the MMD of rBC increased notably by 19 %. During the observational period, the ratio of the diameter of rBC-containing particles (Dp) to the rBC core (Dc) was 1.20 on average for Dc = 180 nm, indicating that the majority of rBC particles were thinly coated. The Dp / Dc value exhibited a clear diurnal pattern, with a maximum at 1400 LST and an enhancing rate of 0.013/h; higher Ox conditions increased the coating enhancing rate. Bare rBC particles were primarily in a fractal structure with a mass fractal dimension (Dfm) of 2.35, with limited variation during both clean and pollution periods, indicating significant impacts from on-road vehicle emissions. The morphology of rBC-containing particles vairied with aging processes. The mixing state of rBC particles could be indicated by the mass ratio of non-refractory matter to rBC (MR). In the present study, rBC-containing particles were primarily found in an external fractal structure when MR  6, at which the measured scattering cross section of rBC-containing particles was consistent with that based on the Mie-scattering simulation. We found only 9 % of the rBC-containing particles were in core-shell structures on clean days with a particle mass of 10 fg, and the number fraction of core-shell structures increased considerably to 32 % on pollution days. Considering the morphology change, the absorption enhancement (Eabs) was 11.7 % higher based on core-shell structures. This study highlights the combined effects of morphology and coating thickness on the Eabs of rBC-containing particles, which will be helpful for determining the climatic effects of BC.

scholarly journals Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon

2010 ◽  
Vol 10 (9) ◽  
pp. 4207-4220 ◽  
Author(s):  
D. A. Lack ◽  
C. D. Cappa
Keyword(s):  
Black Carbon ◽  
Light Absorption ◽  
Radiative Forcing ◽  
Large Scale ◽  
Wavelength Dependence ◽  
Absorption Enhancement ◽  
Core Shell ◽  
Enhancement Effect ◽  
Visible Radiation ◽  
Single Scatter

Abstract. The presence of clear coatings on atmospheric black carbon (BC) particles is known to enhance the magnitude of light absorption by the BC cores. Based on calculations using core/shell Mie theory, we demonstrate that the enhancement of light absorption (EAbs) by atmospheric black carbon (BC) when it is coated in mildly absorbing material (CBrown) is reduced relative to the enhancement induced by non-absorbing coatings (CClear). This reduction, sensitive to both the CBrown coating thickness and imaginary refractive index (RI), can be up to 50% for 400 nm radiation and 25% averaged across the visible radiation spectrum for reasonable core/shell diameters. The enhanced direct radiative forcing possible due to the enhancement effect of CClear is therefore reduced if the coating is absorbing. Additionally, the need to explicitly treat BC as an internal, as opposed to external, mixture with CBrown is shown to be important to the calculated single scatter albedo only when models treat BC as large spherical cores (>50 nm). For smaller BC cores (or fractal agglomerates) consideration of the BC and CBrown as an external mixture leads to relatively small errors in the particle single scatter albedo of <0.03. It has often been assumed that observation of an absorption Angström exponent (AAE)>1 indicates absorption by a non-BC aerosol. Here, it is shown that BC cores coated in CClear can reasonably have an AAE of up to 1.6, a result that complicates the attribution of observed light absorption to CBrown within ambient particles. However, an AAE<1.6 does not exclude the possibility of CBrown; rather CBrown cannot be confidently assigned unless AAE>1.6. Comparison of these model results to various ambient AAE measurements demonstrates that large-scale attribution of CBrown is a challenging task using current in-situ measurement methods. We suggest that coincident measurements of particle core and shell sizes along with the AAE may be necessary to distinguish absorbing and non-absorbing OC.

Experimental evidence of the lensing effect suppression for atmospheric black carbon containing brown coatings

2020 ◽  
Author(s):  
Vaios Moschos ◽  
Martin Gysel-Beer ◽  
Robin L. Modini ◽  
Joel C. Corbin ◽  
Dario Massabò ◽  
...  
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Wavelength Dependence ◽  
Chem Phys ◽  
Absorption Enhancement ◽  
Visible Spectroscopy ◽  
Scanning Mobility Particle Sizer ◽  
Absorption Data ◽  
Brown Carbon ◽  
Aerosol Radiative Forcing

&lt;p&gt;Accounting for the wavelength- and source-dependent optical absorption properties of the abundant light-absorbing organic (brown) carbon (BrC) and the mixing state of atmospheric black carbon (BC) are essential to reduce the large uncertainty in aerosol radiative forcing.&amp;#160;Estimation of BrC absorption online by subtraction is highly uncertain and may be biased if not decoupled from the potential BC absorption enhancement (lensing) due to non-refractory (organic and inorganic) coating acquisition.&lt;/p&gt;&lt;p&gt;Here, the reported total particulate absorption is based on long-term, filter-based seven-wavelength Aethalometer (AE33 model) data, corrected for multiple scattering effects with Multi-Wavelength Absorbance Analyzer (5&amp;#955; MWAA) measurements. Using ultraviolet-visible spectroscopy absorbance measurements along with particle size distributions obtained by a scanning mobility particle sizer, we have conducted Mie calculations to assess the importance of source-specific extractable particulate BrC (Moschos et al., 2018) versus BC absorption.&lt;/p&gt;&lt;p&gt;For the species-specific optical closure, the wavelength dependence of bare BC absorption is estimated using MWAA measurements upon successive filter extractions to remove the influence of BrC/coatings. The lensing contribution, supported by observations from field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, is estimated at longer wavelengths using a refined proxy for the BC coating thickness. The approach is validated independently by applying a novel positive matrix factorization-based approach on the calibrated total AE33 absorption data.&lt;/p&gt;&lt;p&gt;Based on the observational constraints established in this study, we demonstrate for various distinct case studies that the interplay between lensing and BrC absorption results in lower than expected BC absorption at shorter wavelengths. This indicates that the volume additivity assumption is not valid for particulate absorption by internally mixed heterogeneous atmospheric aerosol populations. These comprehensive experimental analyses verify the BC lensing suppression predicted for simplified core-shell structures containing moderately absorbing BrC (Lack &amp; Cappa, 2010). The implications discussed in this work are relevant for co-emitted species from biomass burning or aged plumes with high BrC to BC mass/absorption ratio.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Moschos, V., Kumar, N. K., Daellenbach, K. R., Baltensperger, U., Pr&amp;#233;v&amp;#244;t, A. S. H., and El Haddad, I.: Source apportionment of brown carbon absorption by coupling ultraviolet-visible spectroscopy with aerosol mass spectrometry, Environ. Sci. Tech. Lett., 5, 302-308, https://doi.org/10.1021/acs.estlett.8b00118, 2018.&lt;/p&gt;&lt;p&gt;Lack, D. A. and Cappa, C. D.: Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon, Atmos. Chem. Phys., 10, 4207&amp;#8211;4220, https://doi.org/10.5194/acp-10-4207-2010, 2010.&lt;/p&gt;

scholarly journals Enhanced solar energy absorption by internally-mixed black carbon in snow grains

2012 ◽  
Vol 12 (1) ◽  
pp. 2057-2113 ◽  
Author(s):  
M. G. Flanner ◽  
X. Liu ◽  
C. Zhou ◽  
J. E. Penner
Keyword(s):  
Sea Ice ◽  
Black Carbon ◽  
Radiative Forcing ◽  
Climate Model ◽  
Volume Fraction ◽  
Global Climate ◽  
Global Climate Model ◽  
Absorption Enhancement ◽  
Liquid Droplets ◽  
Vapor Transfer

Abstract. Here we explore light absorption by snowpack containing black carbon (BC) particles residing within ice grains. Basic considerations of particle volumes and BC/snow mass concentrations show that there are generally 0.05–109 BC particles for each ice grain. This suggests that internal BC is likely distributed as multiple inclusions within ice grains, and thus the dynamic effective medium approximation (DEMA) (Chýlek and Srivastava, 1983) is a more appropriate optical representation for BC/ice composites than coated-sphere or standard mixing approximations. DEMA calculations show that the 460 nm absorption cross-section of BC/ice composites, normalized to the mass of BC, is typically enhanced by factors of 1.8–2.1 relative to interstitial BC. BC effective radius is the dominant cause of variation in this enhancement, compared with ice grain size and BC volume fraction. We apply two atmospheric aerosol models that simulate interstitial and within-hydrometeor BC lifecycles. Although only ~2% of the atmospheric BC burden is cloud-borne, 71–83% of the BC deposited to global snow and sea-ice surfaces occurs within hydrometeors. Key processes responsible for within-snow BC deposition are development of hydrophilic coatings on BC, activation of liquid droplets, and subsequent snow formation through riming or ice nucleation by other species and aggregation/accretion of ice particles. Applying deposition fields from these aerosol models in offline snow and sea-ice simulations, we calculate that 32–73% of BC in global surface snow resides within ice grains. This fraction is smaller than the within-hydrometeor deposition fraction because meltwater flux preferentially removes internal BC, while sublimation and freezing within snowpack expose internal BC. Incorporating the DEMA into a global climate model, we simulate increases in BC/snow radiative forcing of 43–86%, relative to scenarios that apply external optical properties to all BC. We show that snow metamorphism driven by diffusive vapor transfer likely proceeds too slowly to alter the mass of internal BC while it is radiatively active, but neglected processes like wind pumping and convection may play much larger roles. These results suggest that a large portion of BC in surface snowpack may reside within ice grains and increase BC/snow radiative forcing, although measurements to evaluate this are lacking. Finally, previous studies of BC/snow forcing that neglected this absorption enhancement are not necessarily biased low, because of application of absorption-enhancing sulfate coatings to hydrophilic BC, neglect of coincident absorption by dust in snow, and implicit treatment of cloud-borne BC resulting in longer-range transport.

scholarly journals Aerosol light absorption and the role of extremely low volatility organic compounds

2020 ◽  
Vol 20 (19) ◽  
pp. 11625-11637
Author(s):  
Antonios Tasoglou ◽  
Evangelos Louvaris ◽  
Kalliopi Florou ◽  
Aikaterini Liangou ◽  
Eleni Karnezi ◽  
...  
Keyword(s):  
Black Carbon ◽  
Organic Compounds ◽  
Light Absorption ◽  
Absorption Enhancement ◽  
Scanning Mobility Particle Sizer ◽  
Brown Carbon ◽  
Aerosol Absorption ◽  
Aerosol Mass ◽  
Particle Sizer

Abstract. A month-long set of summertime measurements in a remote area in the Mediterranean is used to quantify aerosol absorption and the role of black and brown carbon. The suite of instruments included a high-resolution aerosol mass spectrometer (HR-ToF-AMS) and a scanning mobility particle sizer (SMPS), both coupled to a thermodenuder and an Aethalometer, a photoacoustic extinctiometer (PAX405), and a single particle soot photometer (SP2). The average refractory black carbon (rBC) concentration during the campaign was 0.14 µg m−3, representing 3 % of the fine aerosol mass. The measured light absorption was two or more times higher than that of fresh black carbon (BC). Mie theory indicated that the absorption enhancement due to the coating of BC cores by nonrefractory material could explain only part of this absorption enhancement. The role of brown carbon (BrC) and other non-BC light-absorbing material was then investigated. A good correlation (R2=0.76) between the unexplained absorption and the concentration of extremely low volatility organic compounds (ELVOCs) mass was found.

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