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Abstract
Based on actual atmospheric observations of internal mixing of light-absorbing brown carbon (BrC)-coated black carbon (BC), the optical properties of mixed black and brown carbon aerosols (BBC) were calculated using four mixing models (external mixing, core–shell, Bruggeman, and Maxwell-Garnett models), and changes in their optical properties with wavelength were compared and analyzed. Under the assumption of different volumetric mixing ratios (VR=VBC/VBBC), there is little difference in volumetric absorptive coefficient (Kab) of BBC in these models in the ultraviolet band where both BC and BrC have strong absorption, particularly in the ultraviolet A band. In visible and near-infrared bands, the three internal mixing models significantly reduce the single scattering albedo compared to the external mixing scenario. In addition, the widely used core–shell model was used to evaluate the effects of BrC shell thickness and environmental relative humidity (RH) on the optical properties of BBC. The impacts of these factors are mainly seen in ultraviolet and visible bands. The volumetric extinction coefficient (Kex) decreases with BrC shell thickness under a fixed BC core radius (0.12 µm) in these bands. This is because the radiation reaching the surface of BC particles is reduced under the absorption of less efficient BrC shells (known as the blocking effect), which is different from the BC and light-scattering aerosols internal mixing scenario. Moreover, the Kex and Kab of BBC decrease with RH, which is mainly due to both the increasing thickness of the BrC shell and the change in the complex refractive index of the BrC shell. Based on the assumptions of unchanged BC core parameters and actual observations, the extinction ability increases when BrC particles are more light-scattering in the ultraviolet and visible bands. The increase in extinction is mainly from scattering, rather than absorption. However, the situation is the opposite in the near-infrared band.
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1. Introduction
Carbon aerosols, including organic carbon (OC) and black carbon (BC), play important roles in the radiation balance of the Earth’s climate system, and BC has a strong heating effect on global and regional climates [1–4]. Its positive radiative forcing is only second to carbon dioxide [4]. In most climate models, BC is considered the only carbon aerosol that absorbs solar radiation, while OC only has a scattering effect [5]. However, recent experimental observations and laboratory studies show that some OC particles can also absorb solar radiation [6–8]. Absorption by these OC particles is mainly in the near ultraviolet and blue bands, and is strongly correlated with wavelength, which is different from BC’s absorption in the ultraviolet and visible bands [9–17].
In 2000, the British Meteorological Agency carried out a preliminary regional scientific observational experiment during the Southern African Regional Science Initiative (SAFARI 2000) using a C-130 aircraft [18]. This was the first observation of the existence of the colored organic compound called brown carbon (BrC) in smoke from biomass combustion. Subsequently, Andreae and Gelencser (2006) identified several mechanisms of BrC production and proposed that it would interfere with the measurement of BC light absorption, which would lead to significant deviations [19]. They also proposed that there existed a significant error in retrieving the solar radiation absorption of tropospheric BC using the monochromatic light absorption in the visible band without considering the absorptive effects of BrC in the ultraviolet band. Since then, BrC has attracted widespread attention in atmospheric science research. Lukacs et al. (2007) used simplified spectroscopy to analyze the BrC concentration from aerosol water extracts collected at six European stations during the CARBOSOL observational experiment [20]. The results showed that the water-soluble BrC concentration has obvious seasonal variation at all sites with higher values in winter and lower values in summer. Alexander et al. (2008) measured the optical properties of a single submicron amorphous carbon sphere collected during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) using electron energy loss spectra in transmission electron microscopy [21]. They found that the ubiquitous carbon spheres are brown with an average refractive index of 1.67–0.27i $1$ at 550 nm. In addition to the above results from experimental observations, model studies have also found that atmospheric heating due to radiative absorption from BrC in the ultraviolet/visible band is obvious [15,16,22–25]. Therefore, BrC is widely considered an important contributor to aerosol–radiation interactions [26–28].
As a complex mixture of various organic compounds with different absorption rates, BrC can be produced in the combustion processes of many different types of carbon fuel. Its sources are very complex, with biomass combustion being the main contributor [9,10,15,24,29–31]. The combustion of biomass and biofuels can produce a large amount of mixed organic and BC aerosols [21]. Most previous studies have focused on the internal mixing of BC and scattering aerosols, and have found that, acting as a prism, the shell of scattering aerosols can make more photons reach the BC core, which results in an increase in radiation absorption [2,32–34]. This effect is theoretically demonstrated to increase the absorption of a single BC particle by 50–100% [2]. However, recent observations have shown that the probability of organic particles (including primary and secondary aerosols) appearing in internally mixed aerosols is equal to or greater than that of inorganic scattering particles (such as sulfate and nitrate) [35]. Aerosol particles emitted during a fire in a canyon 4 miles from Boulder, Colorado, were collected and analyzed in the laboratory by Lack et al.; they found that the mixing of BC and BrC increased light absorption by 70% [36]. Lack and Cappa used Mie scattering theory to calculate and investigate the effects of a BrC shell on the absorptivity of a BC core. They found that the absorptive improvement of the BC core with the BrC shell was smaller than that with non-absorptive aerosols [37].
There are two main ways to study the optical properties of BBC aerosol mixtures: experimental observations [36,38–40] and theoretical model simulations [37,41–44]. The experimental observations often use different solvents (such as water and methane) to dissolve and extract samples, whereas the theoretical calculations are mainly based on the complex refractive indices obtained from experimental observations using different internal mixing models and theoretical methods. In addition, the absorption of BrC can be inversely correlated with the wavelength dependence of BC absorption [45,46], and be used to further study the contribution of BrC mixing to light absorption [47]. Both methods have advantages and disadvantages. Field experiments can observe the increase in the optical absorption of the internally mixed particles. However, the results have spatial and temporal limitations due to finite and sparse observation sites. Although theoretical models lack some experimental bases for observation, the effects of different factors on the optical properties of the internally mixed particles can be simulated. However, both the two methods have proved that the absorptive enhancement for BC mixture is obviously different between coated with BrC and other pure scattering materials [36]. It is mainly caused by the refractive indices of coating materials. Therefore, this work uses the numerical models which focuses on the refractive indices of mixtures.
In this work, the optical properties of mixed BBC aerosols are calculated using the external mixing model, core–shell model, Maxwell-Garnett model, and Bruggeman model. It is a first step work based on spherical particles assumption as the fact that real carbonaceous aerosol particles are non-spherical [48–50]. The descriptions of these models are presented in Section 2. Section 3.1 evaluates the optical properties of BBC using the different mixing models. Then the influences of BrC shell thickness and humidity are analyzed using the core–shell model, and the results are presented in Section 3.2. Based on the above, the impacts of different polluted conditions on the optical properties of BBC are investigated and presented in Section 3.3. Finally, the main conclusions are presented in Section 4.
2. Model and method
2.1. Model description
There are four mixing models used to calculate the optical properties of differently mixed aerosols in this study, of which the diagrams shown in Fig. 1 are the same as Zhang et al. [51]. The external mixing model(a) assumes that spherical particles exist independently without secondary scattering. In the core–shell model(b), a homogeneous sphere core is surrounded by a concentric homogeneous shell; this model is widely used for internal mixing [52]. The Maxwell model(c) assumes that the mixed aerosols are spherical as a whole, but the nuclei are randomly distributed within the particles. It is usually used to describe cases where the shell volume is significantly larger than the core, such as the internal mixing of BC particles and cloud droplets [53]. In the Bruggeman model(d), the portions of the particles are adjacent but not completely encapsulated [54,55].
Fig. 1. Diagrams of aerosol mixing models [51] (a) External mixing model (b) Core–shell model (c) Maxwell-Garnett model (d) Bruggeman model
The optical properties of externally mixed aerosols (β) are the sum of the optical properties of each aerosol type (αi) using the volume fraction as the weight function (fi):
While originally proposed for homogeneous spheres Mie theory [56] has been extended to other geometries, including core-shell geometries, in which core and shell consist of materials of different refractive index. In this work, the algorithm of the core–shell model (BHCOAT code) is based on the layered spherical Mie scattering theory proposed by Bohren and Huffman [52]. . Both the Maxwell-Garnett and Bruggeman models are based on the equivalent complex refractive index ($m$), which regard the mixture as a homogeneous sphere. The equivalent complex refractive index of the mixture is input to a Mie scattering algorithm as it now comprised of a single material exhibiting single complex refractive index [56]. The equivalent complex refractive index of the Maxwell-Garnett and Bruggeman models can be calculated using Eqs. (2) and (3), respectively. Here ${m_1}$ and ${m_2}$ are the complex refractive indices of the core and shell substances, and ${f_1}$ and ${f_2}$ are the volume ratio of the core and shell, respectively.2.2. Experimental design
The complex refractive index of BC is derived from the HITRAN 2004 dataset [58], and the BrC complex refractive index is from the ACE-Asia results in the HITRAN 2016 dataset [59]. Here, we set up four numerical experiments. Test 1 is under the assumption of a BBC radius of 0.13 µm. The optical properties of BBC in shortwave band are calculated using four mixing models with volumetric mixing ratios (VRs) of 0.1, 0.3, 0.5, 0.7, and 0.9. The VR involved in the study is the volumetric fraction of the BC core in the mixed particles (VR = VBC/VBBC). In test 2, we assume that the radius of the BC core is always 0.12 µm. The optical properties of BBC are calculated using the core–shell model with VRs of 0.1, 0.3, 0.5, 0.7 and 0.9. In contrast to test 1, its purpose is to study the effects of BrC shell thickness on the optical properties of BBC particles.
In test 3, we calculate the optical properties of BBC under different environmental relative humidity (RH) conditions, with RH set to 0.2, 0.5, 0.7, 0.9, 0.95, and 0.99. As a soluble substance, BrC shell will absorb moisture when RH reaches the deliquescence point. This will not only enlarge the volume of the BBC but also affect the complex refractive indices, which can be calculated using Eq. (4) according to Liou [60].
Here, ${m_d}$ and ${m_w}$ are the complex refractive indies of dry soluble substance and water, respectively; ${r_d}$ and ${r_w}$ are the equivalent radii of the BrC shell before and after hygroscopic growth, respectively. ${r_w}$ can be obtained using the Köhler equation. BC core is treated as an insoluble substance in this study.
Due to the complex chemical composition of BrC, the physical and chemical properties of BrC particles are affected by emission sources and polluted conditions, which result in great differences in the BrC complex refractive index. In test 4, both the complex refractive index of the BC core and its radius (0.12 µm) remain conserved, with a VR of 0.3. Based on the results of Cheng et al. [61], four environmental conditions are assumed to evaluate the effects of the BrC shell complex refractive index on the optical properties of BBC particles. We assumed the variation of the complex refractive indices with wavelength in Beijing provided by Cheng et al. are the same with those observed in ACE-Asia. Thus, the interpolated real part of the complex refractive index at 365, 500, and 870 nm is 1.58, 1.65, and 1.75, respectively, and the imaginary part (mi) is shown in Table 1. These environmental conditions represent various shortwave radiation absorptive ability of BrC shell. In the first group of test 4, the imaginary part of the complex refractive index of BrC particles is assumed to be 0 in the shortwave band, which means that BrC is pure scattering. Previous studies from observations have found that there are no pure scattering OC particles in the atmosphere [30], thus this group shows an extreme case not existing in the atmosphere. The second and third groups represent the conditions of summer and winter observations in Beijing, China, respectively. Thirty groups of 24 h averaged PM2.5 samples were taken from the observation site in Tsinghua University, and BrC was extracted using methane solvent [61]. Here, we convert its mass absorption cross-section (MAE) to mi values using the method of Bohren and Huffman [52]. In the fourth group, the maximum MAE value of BrC extracted by methane solvent is converted into the mi value to represent the upper absorption limit of BrC particles observed in Beijing, China.
Table 1. Imaginary part (mi) of the complex refractive indices of BrC particles under different environmental conditions
3. Results
3.1. Volumetric optical properties of BBC in different models
With the radius of the BBC particle set to 0.13 µm, the changes in the volumetric extinction coefficient (Kex), volumetric absorption coefficient (Kab), single scattering albedo (ω), and asymmetric factor (g) of BBC as a function of wavelength were calculated under different VRs (0.1, 0.3, 0.5, 0.7, and 0.9) using the external mixing model, core–shell model, Maxwell-Garnett model, and Bruggeman model. The results are shown in Fig. 2. What is noted that the discontinuity around 1.2 µm in all Figures shown below is due to interpolation error as the complex refractive indices of BrC in HITRAN 2016 dataset range from 0.2 to 1.2 µm.
Fig. 2. Volumetric optical properties of aerosols (Kex, Kab, ω and g) in external mixing, core–shell, Bruggeman, and Maxwell-Garnett models.
In the external mixing model (first column of Fig. 1), there is a slight difference in Kex among various VRs in the ultraviolet and visible bands. In the near-infrared band, Kex decreases with VR. However, Kex increases with VR when the wavelength is larger than 1.87 µm. As for Kab, there is no significant difference under different VR assumptions in the ultraviolet band, indicating that both BC and BrC have strong absorption of ultraviolet light, and the absorption in the ultraviolet A band is the strongest. The Kab decreases gradually with wavelength in the near-infrared band, whereas the difference in Kab at different VRs increases with wavelength. The Kab is larger at greater VRs. This shows that the light absorption of BC is larger than that of BrC in the visible and near infrared bands, which is consistent with the conclusions of Lack and Cappa [37]. The ω values show that in the ultraviolet band, the differences in ω among different VRs are very small, particularly in the ultraviolet A band. In the ultraviolet-B and ultraviolet-C bands, ω increases with VR, whereas the opposite occurs in the visible and near-infrared bands. The increased Kex of mixed particles in the near-infrared band is produced by the scattering effects of BrC. The differences in g factors among various VRs are very small.
The second column of Fig. 2 shows volumetric optical properties of BBC calculated using the core–shell model. The Kex values vary slightly under different VR assumptions, particularly in the ultraviolet and visible bands. The Kab values are similar to those of external mixing, and there is no obvious difference in the ultraviolet band among different VRs. With increasing wavelength, the Kab increases with VR. ω is proportional to VR in the ultraviolet band. The larger the volumetric ratio of the BC nucleus to the whole BBC particle, the larger the ω value of these internally mixed particles. In the visible and near infrared bands, the ω value decreases with the increase in VR, mainly due to the strong absorption of BC particles in these bands. The above analyses show that the influence of BrC on radiation is mainly caused by absorption in the UV band and scattering in the near-infrared band.
In the Maxwell-Garnett model, the Kex and Kab of BBC particles increase with VR in almost the entire shortwave band. In addition, there are obvious differences in Kex and Kab in the ultraviolet and visible bands, which is different from the results in the core–shell and external mixing models. In the Bruggeman model, Kex and Kab are similar to those in the core–shell model, but ω is smaller under all given VRs.
To evaluate optical properties from these four models directly, we compare the results under an assumed VR of 0.3, as shown in Fig. 3. In the external mixing model, core–shell model, and Bruggeman model, the Kex values are similar in the shortwave band, while the Kex calculated by the Maxwell-Garnett model is relatively small. For Kab, the value calculated by the Bruggeman model is the largest, followed by the core–shell model, and the result in the Maxwell-Garnett model is the smallest. Figure 3(c) shows the variation in ω. The values in the Bruggeman and core–shell models are larger than those of other two models in the ultraviolet band. In the visible band, ω in the core–shell and external mixing models increases with wavelength, while the trends in the Bruggeman and Maxwell-Garnett models are the opposite. In the near-infrared band, ω from all four models decreases with wavelength, with the value in the external mixing model being the largest. The g values in all four models decrease with wavelength (Fig. 2(d)).
Fig. 3. Volumetric optical properties of aerosols ((a) Kex, (b) Kab, (c) ω and (d) g) with a volumetric mixing ratio of 0.3, calculated using external mixing, core–shell, Bruggeman, and Maxwell-Garnett models.
3.2 Effects of different parameters on optical properties of BBC
In this section, taking the core–shell model as an example, the influences of VR and RH on the optical properties of BBC particles are investigated. To evaluate the effects of different BrC shells thicknesses, we assume the radius parameters of BC nuclei are fixed, and VRs of 0.1, 0.3, 0.5, 0.7, and 0.9. In addition, we calculate the optical properties of BBC under RH conditions of 0.2, 0.5, 0.7, 0.9, 0.95, and 0.99.
3.2.1. BrC shell thickness
The radius of the BC core in the BBC mixed particle is fixed at 0.12 µm. Figure 4 shows the optical properties of BBC mixed particles under different VR assumptions. It should be noted that the VR represents the volumetric fraction of the BC core in the entire mixed particle. Therefore, VR_0.1 in Fig. 4 is the internally mixed particle with the thickest BrC shell, while VR_0.9 represents particle with the thinnest shell. There are obvious differences in Kex and Kab among the various VR schemes, particularly in the ultraviolet and visible bands. Kex increases with the decrease in BrC shell thickness at the same wavelength because the absorption of BrC in the band results in a decrease in the radiation energy reaching the BC surface (called the blocking effect), while ω values in the ultraviolet and visible bands have little difference, resulting in a decrease in the absorption of BC nuclei and a gradual decrease in the extinction effect. The absorption of the BC nucleus is not improved by the encapsulating BrC shell, which is contrary to the situation when BC and other scattering aerosols are mixed.
Fig. 4. (a) Kex, (b) Kab, (c) ω and (d) g of BBC with various BrC shell thicknesses (BC core is fixed at 0.12 µm, the volumetric fraction of the BC core in the whole mixed BBC particle is 0.1, 0.3, 0.5, 0.7 and 0.9, respectively).
The scattering hydrophilic component encapsulating the BC core surface acts as a lens and converges the radiation energy toward the center, thus it can increase the radiation flux on the BC surface and enhance the absorption of particles (see Zhang et al. [51]). However, when BC is encapsulated by BrC, the absorption of the BrC shell leads to a decrease in flux to the BC core, which has a certain blocking effect. Compared to the optical properties of BC and BrC, the absorption of BC is greater than that of BrC, and as a result, the BrC shell does not absorb enough light to compensate for the decrease in light flux to the BC nucleus. According to the reports about the scattering directionality of particles with various radii [62,63], both the scattering cross-section and the proportion of forward scattering increase when particle size grows. In Fig. 3(c) and (d), the ω and g of the particles are relatively large when the thickness of the BrC shell increases. This is consistent with the above conclusion.
3.2.2. Relative humidity
Figure 5 shows BBC optical properties under environmental RH conditions of 0.2, 0.5, 0.7, 0.9, 0.95, and 0.99. The influence of RH on Kex and Kab is mainly concentrated in the ultraviolet and visible bands. According to Eq. (4), the imaginary part of the complex refractive index of particles is closer to that of water vapor in the process of hygroscopic growth due to increased atmospheric RH. Water absorption of BrC leads to the decrease in mi due to the small absorption of water vapor in the shortwave band, resulting in the decrease in Kab with the increase in RH, particularly in the ultraviolet and visible bands. In addition, the increasing thickness of the BrC shell due to absorbing water vapor makes the size of the aerosol particles larger and affects their optical properties. The influence contains the changes in both the photon propagation direction (lens effect) and photon energy absorption (blocking effect).
Fig. 5. (a) Kex, (b) Kab, (c) ω and (d) g of BBC under various relative humidity conditions from 0.2 to 0.99 (BC core is fixed at 0.12 µm).
Comparing and analyzing the trends of Kex, Kab, and ω reveals that the changes in Kex and Kab calculated under different RH conditions are basically the same in the ultraviolet and visible bands, but the change in ω is very small, which indicates that the main effect of RH on Kex is the change in Kab. It also shows that ω under 0.99 RH fluctuates in the near-infrared band. This is because the complex refractive index of BrC is very close to that of water vapor when RH is 0.99. Water vapor has strong absorption at 1.4, 1.9, and 2.7 µm, which results in strong absorption of BBC in these three bands.
3.3. Optical properties of BBC under different polluted conditions
According to the hypothesis in test 4 described in Section 2, the complex refractive index of the BC core is fixed, and the BC core radius remains at 0.12 µm. In addition, the volumetric fraction of the BC core in the whole mixed BBC particle is 0.3. Based on the assumption of the BrC complex refractive index mi values in the four environmental conditions shown in Table 1 [61], the optical properties of BBC particles were calculated using the core–shell model and are shown in Fig. 6. Kex is inversely proportional to the imaginary part of the complex refractive index of BrC in the ultraviolet and visible bands, but the situation is the opposite in the near-infrared band. The main reason is that more photons are scattered when BrC is pure scattering in the ultraviolet and visible bands. In terms of Kab, the value of pure scattering BrC is obviously smaller than the other three results. In addition, there is little difference in absorption among the situations of summer (mi_1), winter (mi_2), and the extreme upper limit of BrC (mi_3) observed in Beijing in ultraviolet band. Moreover, the increase in mi results in the increase in absorption in the visible and near-infrared bands. As for ω in Fig. 6(c), the absorption of the BrC shell significantly reduces its single scattering albedo, particularly in the ultraviolet band. In addition, the increase in mi makes the proportion of forward scattering light (g) increase at the same wavelength. Denjean et al. [64] evaluated radiative effects of biomass burning aerosols BBA from southern and central African fires and found coating enhance light absorption though the absorption of brown carbon itself is negligible compared to well-aged black carbon. In summary, the different composition of the BrC shell not only changes the absorption but also changes the scattering direction. The lens effect of the BrC shell increases the extinction but decreases the absorption in the ultraviolet and visible bands, whereas the blocking effect of the BrC shell decreases the radiation extinction and has little impact on absorption. Moreover, the proportion of forward scattering increases when BrC becomes more absorptive.
Fig. 6. (a) Kex, (b) Kab, (c) ω and (d) g of BBC under different polluted conditions (BC core radius is 0.12 µm, and the volumetric fraction of the BC core in the whole mixed BBC particle is 0.3. The BrC complex refractive indices mi is shown in Table 1)
4. Conclusions
We used four numerical models (external mixing, core–shell, Bruggeman, and Maxwell-Garnett models) to calculate optical properties of BBC mixed particles. Furthermore, we evaluated the influence of different parameters such as BrC shell thickness, environmental RH, and polluted conditions on the optical properties. Our conclusions are summarized as below.
The Kex value calculated by the Maxwell-Garnett model is relatively small compared to the other three models. There is little difference in the Kab of BBC under different VR assumptions in the four models in the ultraviolet band where both BC and BrC have strong absorption, particularly in the ultraviolet A band. As for ω, the three internal mixing models significantly reduce the single scattering albedo compared to the external mixing scenario.
When the radius of the BC core in the BBC mixed particle is 0.12 µm, Kex decreases with BrC shell thickness at the same wavelength in the ultraviolet and visible bands, which is contrary to the results of BC mixing with other scattering aerosols. This is because the absorption of BrC in the bands (Kab, ω) results in a decrease in radiation energy reaching the BC surface (blocking effect), and thus a decrease in the absorption and extinction of the BC core. The influences of environmental RH on the properties of BBC are due to both the change in the mi of the BrC shell and the increase in BrC shell thickness by absorbing water vapor. The combined influence of these two factors makes Kex and Kab decrease with the increase in RH, particularly in the ultraviolet and visible bands.
With a fixed BC core, we evaluated the influence of the BrC shell in four environmental conditions based on ground observations. The extinction ability (Kex) increases when the BrC shell is more scattering in the ultraviolet and visible bands, but the opposite occurs in the near-infrared band. The Kab of pure scattering BrC (mi_0 in Fig. 5) is obviously smaller than those of the other three situations, which represent the conditions of summer, winter, and the upper absorption limit observed in Beijing, China, respectively. Furthermore, there is little difference among Kab values in the ultraviolet band in these three conditions. It can be concluded that the increased extinction of internally mixed aerosols in the ultraviolet band is mainly from scattering, rather than absorption.
As the current parameterization schemes of aerosol optical properties in most radiative transfer models and climate models do not take into account internal mixing of black carbon and brown carbon, this work provides the volumetric optical properties of BBC particles using different mixing models and detailed explanation about the influences of various factors (such as shell thickness and RH) as well as the BrC shell itself on the calculated optical properties of internally mixed BBC. The basis and possibility are provided for a more comprehensive understanding of aerosol radiative forcing. It should be noted that the concentration and size of BBC and its mixing-state diversity in the atmosphere [64] requires more experimental observations. Even though soot-containing aerosol comes in various complex non-spherical shapes, this study aims to provide a first set of optical properties of BBC for use in radiative transfer models and climate models. In the future, we should further evaluate the different radiation effects and climatic effects compared to those of BC.
Funding
S&T Development Fund of CAMS (2021KJ004, 2021KJ019); External Cooperation Program of CAS (131211KYSB20180002); National Key Research and Development Program of China (2017YFA0603502).
Acknowledgments
We thank Jingyi He for her writing suggestions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. I. H. Rehman, T. Ahmed, P. S. Praveen, A. Kar, and V. Ramanathan, “Black carbon emissions from biomass and fossil fuels in rural India,” Atmos. Chem. Phys. 11(14), 7289–7299 (2011). [CrossRef]
2. T. C. Bond and R. W. Bergstrom, “Light absorption by carbonaceous particles: An investigative review,” Aerosol Sci. Technol. 40(1), 27–67 (2006). [CrossRef]
3. V. Ramanathan and G. Carmichael, “Global and regional climate changes due to black carbon,” Nat. Geosci. 1(4), 221–227 (2008). [CrossRef]
4. T. C. Bond, S. J. Doherty, D. W. Fahey, P. M. Forster, T. Berntsen, B. J. DeAngelo, M. G. Flanner, S. Ghan, B. Kärcher, and D. Koch, “Bounding the role of black carbon in the climate system: A scientific assessment,” J. Geophys. Res.: Atmos. 118(11), 5380–5552 (2013). [CrossRef]
5. T. Bond, C. Zarzycki, M. Flanner, and D. Koch, “Quantifying immediate radiative forcing by black carbon and organic matter with the Specific Forcing Pulse,” Atmos. Chem. Phys. 11(4), 1505–1525 (2011). [CrossRef]
6. H. Sun, L. Biedermann, and T. C. Bond, “Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol,” Geophys. Res. Lett. 34, 1 (2007). [CrossRef]
7. T. Kirchstetter and T. Thatcher, “Contribution of organic carbon to wood smoke particulate matter absorption of solar radiation,” Atmos. Chem. Phys. 12(14), 6067–6072 (2012). [CrossRef]
8. E. N. Kirillova, A. Andersson, J. Han, M. Lee, and O. Gustafsson, “Sources and light absorption of water-soluble organic carbon aerosols in the outflow from northern China,” Atmos. Chem. Phys. 14(3), 1413–1422 (2014). [CrossRef]
9. T. W. Kirchstetter, T. Novakov, and P. V. Hobbs, “Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon,” J. Geophys. Res.: Atmos. 109(D21), 21208 (2004). [CrossRef]
10. Y. Chen and T. Bond, “Light absorption by organic carbon from wood combustion,” Atmos. Chem. Phys. 10(4), 1773–1787 (2010). [CrossRef]
11. R. Chakrabarty, H. Moosmüller, L.-W. Chen, K. Lewis, W. Arnott, C. Mazzoleni, M. Dubey, C. Wold, W. Hao, and S. Kreidenweis, “Brown carbon in tar balls from smoldering biomass combustion,” Atmos. Chem. Phys. 10(13), 6363–6370 (2010). [CrossRef]
12. A. Hecobian, X. Zhang, M. Zheng, N. Frank, E. S. Edgerton, and R. J. Weber, “Water-Soluble Organic Aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States,” Atmos. Chem. Phys. 10(13), 5965–5977 (2010). [CrossRef]
13. A. Arola, G. L. Schuster, M. R. A. Pitkanen, O. Dubovik, H. Kokkola, A. V. Lindfors, T. Mielonen, T. Raatikainen, S. Romakkaniemi, S. N. Tripathi, and H. Lihavainen, “Direct radiative effect by brown carbon over the Indo-Gangetic Plain,” Atmos. Chem. Phys. 15(22), 12731–12740 (2015). [CrossRef]
14. R. Bahadur, P. S. Praveen, Y. Xu, and V. Ramanathan, “Solar absorption by elemental and brown carbon determined from spectral observations,” Proc. Natl. Acad. Sci. 109(43), 17366–17371 (2012). [CrossRef]
15. C. E. Chung, V. Ramanathan, and D. Decremer, “Observationally constrained estimates of carbonaceous aerosol radiative forcing,” Proc. Natl. Acad. Sci. 109(29), 11624–11629 (2012). [CrossRef]
16. Y. Feng, V. Ramanathan, and V. R. Kotamarthi, “Brown carbon: a significant atmospheric absorber of solar radiation?” Atmos. Chem. Phys. 13(17), 8607–8621 (2013). [CrossRef]
17. J. Liu, E. Scheuer, J. Dibb, G. S. Diskin, L. D. Ziemba, K. L. Thornhill, B. E. Anderson, A. Wisthaler, T. Mikoviny, J. J. Devi, M. Bergin, A. E. Perring, M. Z. Markovic, J. P. Schwarz, P. Campuzano-Jost, D. A. Day, J. L. Jimenez, and R. J. Weber, “Brown carbon aerosol in the North American continental troposphere: sources, abundance, and radiative forcing,” Atmos. Chem. Phys. 15(14), 7841–7858 (2015). [CrossRef]
18. P. Formenti, W. Elbert, W. Maenhaut, J. Haywood, S. Osborne, and M. O. Andreae, “Inorganic and carbonaceous aerosols during the Southern African Regional Science Initiative (SAFARI 2000) experiment: Chemical characteristics, physical properties, and emission data for smoke from African biomass burning,” J. Geophys. Res.: Atmos. 108(D18), 8576 (2003). [CrossRef]
19. M. O. Andreae and A. Gelencser, “Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols,” Atmos. Chem. Phys. 6(10), 3131–3148 (2006). [CrossRef]
20. H. Lukacs, A. Gelencser, S. Hammer, H. Puxbaum, C. Pio, M. Legrand, A. Kasper-Giebl, M. Handler, A. Limbeck, D. Simpson, and S. Preunkert, “Seasonal trends and possible sources of brown carbon based on 2-year aerosol measurements at six sites in Europe,” J. Geophys. Res. 112, 1 (2007). [CrossRef]
21. D. T. L. Alexander, P. A. Crozier, and J. R. Anderson, “Brown carbon spheres in East Asian outflow and their optical properties,” Science 321(5890), 833–836 (2008). [CrossRef]
22. D. S. Jo, R. J. Park, S. Lee, S.-W. Kim, and X. Zhang, “A global simulation of brown carbon: implications for photochemistry and direct radiative effect,” Atmos. Chem. Phys. 16(5), 3413–3432 (2016). [CrossRef]
23. J. Liu, E. Scheuer, J. Dibb, L. D. Ziemba, K. L. Thornhill, B. E. Anderson, A. Wisthaler, T. Mikoviny, J. J. Devi, M. Bergin, and R. J. Weber, “Brown carbon in the continental troposphere,” Geophys. Res. Lett. 41(6), 2191–2195 (2014). [CrossRef]
24. R. Saleh, E. S. Robinson, D. S. Tkacik, A. T. Ahern, S. Liu, A. C. Aiken, R. C. Sullivan, A. A. Presto, M. K. Dubey, R. J. Yokelson, N. M. Donahue, and A. L. Robinson, “Brownness of organics in aerosols from biomass burning linked to their black carbon content,” Nat. Geosci. 7(9), 647–650 (2014). [CrossRef]
25. X. Wang, C. L. Heald, D. A. Ridley, J. P. Schwarz, J. R. Spackman, A. E. Perring, H. Coe, D. Liu, and A. D. Clarke, “Exploiting simultaneous observational constraints on mass and absorption to estimate the global direct radiative forcing of black carbon and brown carbon,” Atmos. Chem. Phys. 14(20), 10989–11010 (2014). [CrossRef]
26. M. Z. Jacobson, “Effects of biomass burning on climate, accounting for heat and moisture fluxes, black and brown carbon, and cloud absorption effects,” J. Geophys. Res. 119(14), 8980–9002 (2014). [CrossRef]
27. G. Lin, J. E. Penner, M. G. Flanner, S. Sillman, L. Xu, and C. Zhou, “Radiative forcing of organic aerosol in the atmosphere and on snow: Effects of SOA and brown carbon,” J. Geophys. Res.: Atmos. 119(12), 7453–7476 (2014). [CrossRef]
28. J. K. Kodros, C. E. Scott, S. C. Farina, Y. H. Lee, C. L’Orange, J. Volckens, and J. R. Pierce, “Uncertainties in global aerosols and climate effects due to biofuel emissions,” Atmos. Chem. Phys. 15(15), 8577–8596 (2015). [CrossRef]
29. A. Clarke, C. McNaughton, V. Kapustin, Y. Shinozuka, S. Howell, J. Dibb, J. Zhou, B. Anderson, V. Brekhovskikh, and H. Turner, “Biomass burning and pollution aerosol over North America: Organic components and their influence on spectral optical properties and humidification response,” J. Geophys. Res.: Atmos. 112(D12), 12–18 (2007). [CrossRef]
30. A. Laskin, J. Laskin, and S. A. Nizkorodov, “Chemistry of Atmospheric Brown Carbon,” Chem. Rev. 115(10), 4335–4382 (2015). [CrossRef]
31. R. A. Washenfelder, A. R. Attwood, C. A. Brock, et al., “Biomass burning dominates brown carbon absorption in the rural southeastern United States,” Geophys. Res. Lett. 42(2), 653–664 (2015). [CrossRef]
32. M. Z. Jacobson, “Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols,” Nature 409(6821), 695–697 (2001). [CrossRef]
33. J. C. Barnard, R. Volkamer, and E. I. Kassianov, “Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area,” Atmos. Chem. Phys. 8(22), 6665–6679 (2008). [CrossRef]
34. C. Erlick and D. Schlesinger, “Another look at the influence of absorbing aerosols in drops on cloud absorption: Large aerosols,” J. Atmos. Sci. 65(2), 661–669 (2008). [CrossRef]
35. Y. Zhang, H. Forrister, J. Liu, J. Dibb, B. Anderson, J. P. Schwarz, A. E. Perring, J. L. Jimenez, P. Campuzano-Jost, Y. Wang, A. Nenes, and R. J. Weber, “Top-of-atmosphere radiative forcing affected by brown carbon in the upper troposphere,” Nat. Geosci. 10(7), 486–489 (2017). [CrossRef]
36. D. A. Lack, J. M. Langridge, R. Bahreini, C. D. Cappa, A. M. Middlebrook, and J. P. Schwarz, “Brown carbon and internal mixing in biomass burning particles,” Proc. Natl. Acad. Sci. 109(37), 14802–14807 (2012). [CrossRef]
37. D. A. Lack and C. D. Cappa, “Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon,” Atmos. Chem. Phys. 10(9), 4207–4220 (2010). [CrossRef]
38. D. A. Lack, M. S. Richardson, D. Law, J. M. Langridge, C. D. Cappa, R. J. McLaughlin, and D. M. Murphy, “Aircraft Instrument for Comprehensive Characterization of Aerosol Optical Properties, Part 2: Black and Brown Carbon Absorption and Absorption Enhancement Measured with Photo Acoustic Spectroscopy,” Aerosol Sci. Technol. 46(5), 555–568 (2012). [CrossRef]
39. Q. Wang, R. J. Huang, J. Cao, Y. Han, G. Wang, G. Li, Y. Wang, W. Dai, R. Zhang, and Y. Zhou, “Mixing State of Black Carbon Aerosol in a Heavily Polluted Urban Area of China: Implications for Light Absorption Enhancement,” Aerosol Sci. Technol. 48(7), 689–697 (2014). [CrossRef]
40. S. Liu, A. C. Aiken, K. Gorkowski, et al., “Enhanced light absorption by mixed source black and brown carbon particles in UK winter,” Nat. Commun. 6, 8435 (2015). [CrossRef]
41. Y. Xie, Z. Li, L. Li, L. Wang, D. Li, C. Chen, K. Li, and H. Xu, “Study on influence of different mixing rules on the aerosol components retrieval from ground-based remote sensing measurements,” Atmos. Res. 145-146, 267–278 (2014). [CrossRef]
42. P. M. Shamjad, R. V. Satish, N. M. Thamban, N. Rastogi, and S. N. Tripathi, “Absorbing Refractive Index and Direct Radiative Forcing of Atmospheric Brown Carbon over Gangetic Plain,” ACS Earth Space Chem. 2(1), 31–37 (2018). [CrossRef]
43. J. Luo, Y. Zhang, F. Wang, and Q. Zhang, “Effects of brown coatings on the absorption enhancement of black carbon: a numerical investigation,” Atmos. Chem. Phys. 18(23), 16897–16914 (2018). [CrossRef]
44. G. Curci, U. Alyuz, R. Barò, et al., “Modelling black carbon absorption of solar radiation: combining external and internal mixing assumptions,” Atmos. Chem. Phys. 19(1), 181–204 (2019). [CrossRef]
45. O. Favez, H. Cachier, J. Sciare, R. Sarda-Estève, and L. Martinon, “Evidence for a significant contribution of wood burning aerosols to PM2. 5 during the winter season in Paris, France,” Atmos. Environ. 43(22-23), 3640–3644 (2009). [CrossRef]
46. M. Yang, S. Howell, J. Zhuang, and B. Huebert, “Attribution of aerosol light absorption to black carbon, brown carbon, and dust in China–interpretations of atmospheric measurements during EAST-AIRE,” Atmos. Chem. Phys. 9(6), 2035–2050 (2009). [CrossRef]
47. D. A. Lack and J. M. Langridge, “On the attribution of black and brown carbon light absorption using the Angstrom exponent,” Atmos. Chem. Phys. 13(20), 10535–10543 (2013). [CrossRef]
48. Z. Wang, H. Zhang, X. Jing, and X. Wei, “Effect of non-spherical dust aerosol on its direct radiative forcing,” Atmos. Res. 120-121, 112–126 (2013). [CrossRef]
49. C. C. Womack, K. M. Manfred, N. L. Wagner, G. Adler, A. Franchin, K. D. Lamb, A. M. Middlebrook, J. P. Schwarz, C. A. Brock, and S. S. Brown, “Complex refractive indices in the ultraviolet and visible spectral region for highly absorbing non-spherical biomass burning aerosol,” Atmos. Chem. Phys. 21(9), 7235–7252 (2021). [CrossRef]
50. J. Luo, Q. Zhang, Y. Zhang, and Z. Li, “Radiative Properties of Non-spherical Black Carbon Aerosols,” in Springer Series in Light Scattering (Springer, 2021), pp. 69–124.
51. H. Zhang, C. Zhou, Z. Wang, S. Zhao, and J. Li, “The influence of different black carbon and sulfate mixing methods on their optical and radiative properties,” J. Quant. Spectrosc. Radiat. Transfer 161, 105–116 (2015). [CrossRef]
52. C. F. Bohren and D. R. Huffman, Absorption and scattering of light by small particles (John Wiley & Sons, 2008).
53. J. C. M. Garnett, “Colours in metal glasses and in metallic films,” Phil. Trans. R. Soc. Lond. A 203(359-371), 385–420 (1904). [CrossRef]
54. G. A. Niklasson, C. Granqvist, and O. Hunderi, “Effective medium models for the optical properties of inhomogeneous materials,” Appl. Opt. 20(1), 26–30 (1981). [CrossRef]
55. C. Erlick, “Effective refractive indices of water and sulfate drops containing absorbing inclusions,” J. Atmos. Sci. 63(2), 754–763 (2006). [CrossRef]
56. W. Hergert and T. Wriedt, The Mie theory: basics and applications (Springer, 2012).
57. H. Zhang, S. Zhu, S. Zhao, and X. Wei, “Establishment of high-resolution aerosol parameterization and its influence on radiation calculations,” J. Quant. Spectrosc. Radiat. Transfer 243, 106802 (2020). [CrossRef]
58. L. S. Rothman, D. Jacquemart, A. Barbe, et al., “The HITRAN 2004 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer 96(2), 139–204 (2005). [CrossRef]
59. C. Hill, I. E. Gordon, R. V. Kochanov, L. Barrett, J. S. Wilzewski, and L. S. Rothman, “HITRANonline: An online interface and the flexible representation of spectroscopic data in the HITRAN database,” J. Quant. Spectrosc. Radiat. Transfer 177, 4–14 (2016). [CrossRef]
60. K. N. Liou, Radiation and cloud processes in the atmosphere. Theory, observation, and modeling (Oxford University Press, 1992).
61. Y. Cheng, K. He, Z. Du, G. Engling, J. Liu, Y. Ma, M. Zheng, and R. J. Weber, “The characteristics of brown carbon aerosol during winter in Beijing,” Atmos. Environ. 127, 355–364 (2016). [CrossRef]
62. I. Cantor and A. Petriw, “Atmospheric light transmission in a Wisconsin area,” Appl. Opt. 7(7), 1365–1381 (1968). [CrossRef]
63. Y. Kaufman, A. Gitelson, A. Karnieli, E. Ganor, R. Fraser, T. Nakajima, S. Mattoo, and B. Holben, “Size distribution and scattering phase function of aerosol particles retrieved from sky brightness measurements,” J. Geophys. Res.: Atmos. 99(D5), 10341–10356 (1994). [CrossRef]
64. C. Denjean, J. Brito, Q. Libois, M. Mallet, T. Bourrianne, F. Burnet, R. Dupuy, C. Flamant, and P. Knippertz, “Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State,” Geophys. Res. Lett. 47(19), e2020GL089055 (2020). [CrossRef]
