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Analysis of factors affecting the broadband extinction performance of bioaerosol
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Analysis of factors affecting the broadband extinction performance of bioaerosol
T
Xinyu Wanga,b, Yihua Hua,b,⁎, Youlin Gu (doctor)a,b,⁎, Xinying Zhaoa,b, Xi Chena,b a b
State Key Laboratory of Pulsed Power Laser Technology, National University of Defense Technology, Hefei 230037, China Anhui Province Key Laboratory of Electronic Restriction, National University of Defense Technology, Hefei 230037, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Bioaerosol Transmittance Extinction cross section Porosity
Bioaerosol shows significant broadband extinction performance and is a new direction for the development of broadband light attenuation materials. However, factors such as material, original particle size, the number of original particles and the structure of agglomerated particles would have impacts on the broadband extinction performance of bioaerosol. In this paper, cluster–cluster aggregation model was used to construct agglomerated particle of bioaerosol, discrete dipole approximation method and Monte Carlo algorithm were performed to calculate the broadband extinction abilities of bioaerosol. The results demonstrate that material selection affects the extinction performance of bioaerosol in different wavebands. Increasing the radius of original particles and decreasing the porosity of agglomerated particle can enhance the broadband extinction abilities of bioaerosol. The structure details and the number of original particles in an agglomerated particle have little impacts on the broadband extinction performance of bioaerosol directly. These conclusions are crucial for the further research of bioaerosol as a new light attenuation material and are applicable to all optical materials with fractal structure.
1. Introduction Bioaerosol, a significant constituent of atmosphere, is emitted from marine and terrestrial ecosystems or produced and released into the atmosphere by human production and life. It is an important part of the natural environment and has some significant functional properties that can be used by humans. Hence, bioaerosol has attracted increasing attention in the fields of atmospheric science, environmental science and electromagnetic field, et al. On the one hand, bioaerosol particles absorb and scatter solar radiation, which directly changes the energy budget of the groundatmosphere system and affects climate change [1]. It is necessary to master the factors which affect the broadband extinction performance of bioaerosol. On the other hand, bioaerosol is an effective absorbent and scatterer of electromagnetic radiation energy so that they have impacts on people's use of electromagnetic waves. Hu et al. [2] found that bioaerosol has significant broadband extinction capability in ultraviolet to infrared bands, providing a new direction for the development of broadband light attenuation materials. Thus, it is of vital importance to study how the factors affect the broadband extinction performance of bioaerosol and how to improve the broadband extinction abilities. This work will be a key step in obtaining a new optical material. Voitsekhovskaya et al. [3] studied the influence of microphysical parameters of atmospheric aerosol particles on IR radiation extinction. Huang et al. [4] studied the effect of porosity on optical properties of aerosol aggregate particles. Zhao et al. [5] found the aggregation-driven reductions in the mass extinction coefficient of bioaerosols. Liu et al. [6] found that the shape and aggregation of particle have a
⁎
Corresponding authors. E-mail addresses: [email protected] (Y. Hu), [email protected] (Y. Gu).
https://doi.org/10.1016/j.ijleo.2019.163527 Received 2 July 2019; Accepted 2 October 2019 0030-4026/ © 2019 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
X. Wang, et al.
strong influence on the light scattering property. However, the impacts of material, original particle size, the number of original particle and the spatial structure on broadband extinction performance of bioaerosol are not clear. How to improve the broadband extinction abilities of bioaerosol with these factors is an urgent problem to be solved. 2. Materials and methods 2.1. Materials preparation As one of the common microbial materials, AN02 spores are easy to be prepared and can be produced in large quantities. In addition, AN02 spores can be easily sprayed in the air and form bioaerosol quickly. The AN02 bioaerosol shows significant broadband extinction performance [2]. Thus, AN02 spores are suitable materials to analyze the factors affecting the broadband extinction performance of bioaerosol. The bioaerosol materials studied in this paper were provided by the Key Laboratory of Ion Beam Bioengineering, Chinese Academy of Sciences [7,8]. The complex refractive index (CRI) of AN02 spores were calculated with the spectral reflectance of materials and the Kramers-Kronig (K-K) algorithm [9–11], this method has been used several times in our research [2,12–16]. 2.2. Bioaerosol spatial structure model and extinction coefficient calculation model Bioaerosol is a colloidal dispersion system formed by dispersing and suspending biological particles in gaseous media. The bioaerosol particles are often not monomers, but random amorphous aggregated particle systems with complex spatial structures formed by small unit particles due to static electricity, collision, adhesion, and the like. Electron micrographs show that AN02 spores are approximately spherical in shape, with a concentrated particle size distribution, a radius distribution of 1∼2.5 μm, and an average radius of 1.5 μm [2]. In this paper, agglomerated particle simulated by Cluster–Cluster aggregation (CCA) model is used to calculate the extinction coefficients of AN02 spores. The discrete dipole approximation (DDA) method is an approximation method for solving the volume integral equation of electromagnetic scattering. The basic principle of it is to approximate the actual particles with an array of finite discrete, interacting small dipoles. As long as the condition |m − 1| ≤ 3 is satisfied, the DDA method can be applied to scatterer of any geometric shape, and the scatterer can be anisotropic and non-uniform [17]. According to the Fig. 1(a) and (b), the complex refractive index of AN02 meets the required conditions of DDA method. The theoretical principle and calculation formula of DDA method are given in
Fig. 1. The complex refractive index and simulation results of AN02 bioaerosol. (a) Real part of complex refractive index of AN02 spores in the waveband of 0.2–14 μm; (b) Imaginary part of complex refractive index of AN02 spores in the waveband of 0.2–14 μm; (c) The scattered fraction of incident light through AN02 bioaerosol in the waveband of 0.2–14 μm; (d) The absorbed fraction of incident light through AN02 bioaerosol in the waveband of 0.2–14 μm. 2
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
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literature [18–21]. And with this method, the extinction cross sectionCext , the scattering cross section Csca and absorption cross section Cabs of bioaerosol agglomerated particle can be calculated. 2.3. Transmittance simulation model With the extinction coefficients of AN02 agglomerated particle, AN02 agglomerated particle swarm was used to simulate the transmittance of incident light through AN02 bioaerosol. The density of the bioaerosol agglomerated particles in the particle swarm and the optical path through the swarm were set to be ρ and L , respectively. All agglomerated particles were assumed to be evenly distributed in the particle swarm and randomly oriented. Taking all these parameters, the transmittance, the absorbed fraction and the scattered fraction of incident light through the bioaerosol agglomerated particle swarm can be simulated with the Monte Carlo algorithm [22]. 3. Results and discussion 3.1. The impacts of material on broadband extinction performance In order to study the impacts of material on broadband extinction performance of bioaerosol, the extinction coefficients of AN02 agglomerated particle were calculated by DDA method. With these coefficients, assuming that ρ = 800cm−3 andL = 4m , the broadband transmittance through AN02 agglomerated particle swarm was calculated by Monte Carlo algorithm to simulate the broadband transmittance through bioaerosol. As shown in the Fig. 1, the scattered and absorbed fractions of the incident light were compared with the real part and the imaginary part of the CRI of AN02 spores in the waveband of 0.2–14 μm. It was easy to find that with the change of incident wavelength, the scattering fraction and the real part, the absorbed fraction and the imaginary part have the same trend. In other words, the material (mainly complex refractive index) determines the extinction performance of bioaerosol in different wavebands. 3.2. The impacts of original particle radius on broadband extinction performance The spatial structure of the agglomerated particle of AN02 spores was built with the CCA model. In order to study the impacts of original particle radius on broadband extinction performance, the extinction coefficients of agglomerated particle of AN02 spores were calculated six times and the original particle radius were set to be 1 μm, 1.3 μm, 1.6 μm, 1.9 μm, 2.2 μm, 2.5 μm, respectively. The parameter ρ and L were set to be 800 cm−3 and 4 m and the transmittances through the six agglomerated particle swarms were
Fig. 2. Simulation results of AN02 spores with different original particle radiuses. (a)Light transmittance through AN02 agglomerated particle swarm; (b) Extinction cross section of AN02 agglomerated particle; (c) Absorption cross section of AN02 agglomerated particle; (d) Scattering cross section of AN02 agglomerated particle (The four subgraphs share the same legend in the upper right). 3
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
X. Wang, et al.
Fig. 3. Simulation results of AN02 spores with two different structures but same porosity. (a) and (b) Agglomerated particle models with two different structures but same porosity: (a) corresponds to Structure 1 while (b) corresponds to Structure 2 ; (c) Light transmittance through AN02 agglomerated particle swarm; (d) Extinction cross section of AN02 agglomerated particle.
simulated respectively. As we can see from Fig. 2(b)–(d), the absorption cross section Cabs and the scattering cross section Csca of bioaerosol agglomerated particle increase with the growth of original particle radius. The peak of scattering cross section Csca in the wide waveband moves to the wavelength which matches the original particle radius. As a result, the extinction cross section Cext increases with the growth of original particle radius and the peak of the extinction cross section moves to the wavelength which matches the original particle radius. As shown in Fig. 2(a), the transmittance decreases with the original particle radius of bioaerosol agglomerate particle rises. Take the transmittance through bioaerosol agglomerated particle swarm when the wavelength of incident light is 10.6 μm as an example, the simulation result of transmittance decreases more than 90% when the original particle radius increases from 1 μm to 2.5 μm. These results indicated that the broadband extinction performance will be enhanced as the original particle radius grows. 3.3. The impacts of the spatial structure of agglomerated particle on broadband extinction performance The spatial structure of bioaerosol agglomerated particle was simulated by CCA model. In this work, several sets of CCA model parameters were set to control the aggregation process of the model and 120 kinds of various spatial structures were obtained. When the original particle number N was set to be 10, there are two different structures which share the same porosity (As shown in Fig. 3(a) and (b)). Take these two structures to perform the extinction performance simulation and the results are shown in Fig. 3(c) and (d). It shows that when the porosity of the bioaerosol agglomerated particle is a constant number, the broadband extinction performance of bioaerosol is fixed and will not change with the structure details. However, the bioaerosol agglomerated particle swarm shows quite diverse structures when the porosity of it is different. When the original particle number N was set to be 50, five simulation structures of agglomerated particle were chosen to calculate the broadband extinction performance of bioaerosol (As shown in Fig. 4(a)). The results in Fig. 4(b) and (c) show that as the porosity increases, the broadband extinction performance of bioaerosol will decline. Hence, for the bioaerosol agglomerated particles formed by the same number of original particles, the larger the porosity is, the more internal pores in the agglomerated structure are, the smaller the absorption and scattering of light is, and the greater the transmittance of light through the condensed particle swarm is. 3.4. The impacts of number of original particle in an agglomerated particle swarm on broadband extinction performance As we have known that the porosity of bioaerosol agglomerated particle would affect the broadband extinction performance of bioaerosol, we simulated four spatial structures with different original particle numbers but similar porosity (As shown in Fig. 5(a). These four structures were used to calculate the extinction coefficients of the bioaerosol agglomerated particle. When simulating the broadband transmittance through the bioaerosol agglomerated particle swarm, the density of the bioaerosol agglomerated particle in 4
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
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Fig. 4. Simulation results of AN02 bioaerosol with six different structures of agglomerated particles. (a) Agglomerated particle models with five different porosities; (b) Light transmittance through AN02 agglomerated particle swarm; (c) Extinction cross section of AN02 agglomerated particle. ((b) and (c) share the same legend in the upper right).
Fig. 5. Simulation results of AN02 spores with different numbers of original particle in an agglomerated particle. (a) Agglomerated particle models with different numbers of original particle; (b) Light transmittance through AN02 agglomerated particle swarm; (c) Extinction cross section of AN02 agglomerated particle.
the particle swarm ρ were set to be 800 cm−3, 400 cm−3, 200 cm−3, 160 cm−3 respectively so that the mass of bioaerosol were consistent in the four situations. As can be seen in Fig. 5(b) and (c), when the porosity of the bioaerosol agglomerated particle is constant, the extinction coefficients of a single agglomerated particle will be enhanced with the increase of original particle number. However, if the mass of bioaerosol is constant, the broadband transmittance through bioaerosol agglomerated particle swarm will hardly change with the increase of original particle number of a single agglomerated particle. In other words, the number of original particle in an agglomerated particle has little effect on the broadband extinction performance of bioaerosol.
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Optik - International Journal for Light and Electron Optics 201 (2020) 163527
X. Wang, et al.
Fig. 6. The corresponding relationship between the original particle number and the porosity of agglomerated particle.
3.5. The impacts of number of original particle on porosity in an agglomerated particle It is known from the foregoing study that the number of original particle in an agglomerated particle swarm has little effect on the broadband extinction performance of bioaerosol but the porosity of the agglomerated particle swarm has decisive influence. However, it is found from the 120 agglomerated models we simulated by CCA model that there is some relationship between the original particle number and the porosity of agglomerated particle swarm. The corresponding relationship between the original particle number and the porosity of agglomerated particle is depicted in Fig. 6. As shown in the figure, with the increase of the number of original particles, the average value of porosities of the agglomerated particles gradually increased and the distribution range gradually narrowed. That means the number of original particle would influence the distribution of porosity and affect the broadband extinction performance indirectly. 4. Conclusion In conclusion, material selection determines the extinction performance of bioaerosol in different bands. Increasing the radius of original particles and decreasing the porosity of agglomerated particle can enhance the broadband extinction performance of bioaerosol. Take the transmittance through bioaerosol when the wavelength of incident light is 10.6 μm as an example, the simulation result of transmittance decreases more than 90% when the original particle radius increases from 1 μm to 2.5 μm and decreases about 50% when the porosity decreases from 0.9717 to 0.8526. However, the structure details of an agglomerated particle have little effects on its extinction coefficients if the porosity is constant. The number of original particles in a single agglomerated particle has no effects on the extinction performance directly but it can influence the distribution of porosity and affect the broadband extinction performance indirectly. These conclusions are crucial for the further research of bioaerosol as a new optical attenuation material and are applicable to all optical materials with fractal structure. Acknowledgements This work was supported by the National Natural Science Foundation of China (60908033, 61271353, 61871389); Natural Science Foundation of Anhui Province (1408085MKL47); National University of Defense Technology (ZK18-01-02). References [1] K. Adachi, S.H. Chung, P.R. Buseck, Shapes of soot aerosol particles and implications for their effects on climate, J. Geophys. Res. Atmos. 115 (2010). [2] Y. Hu, X. Zhao, Y. Gu, X. Chen, X. Wang, P. Wang, Z. Zheng, X.J.S.C.M. Dong, Significant broadband extinction abilities of bioaerosols, Sci. China Mater. (2019) 1–13. [3] O.K. Voitsekhovskaya, I.V. Golub’, A.Y. Zapryagaev, O.V. Shefer, The influence of the microphysical parameters of atmospheric aerosol particles on IR radiation extinction, Izvestiya, Atmos. Ocean. Phys. 46 (2010) 55–59. [4] C. Huang, Z. Wu, Effect of porosity on optical properties of aerosol aggregate particles, Acta Opt. Sin. 33 (2013) 0129001. [5] X. Zhao, Y. Hu, Y. Gu, X. Chen, X. Wang, P. Wang, X. Dong, Aggregation-driven Reductions in the Mass Extinction Coefficient of Bioaerosols, (2019). [6] H. Liu, J. Ma, Z. Song, D. Liu, Light scattering properties of fractal aggregates, Acta Opt. Sin. 31 (2011) 284–289. [7] P. Wang, H. Liu, Y. Zhao, Y. Gu, W. Chen, L. Wang, L. Li, X. Zhao, W. Lei, Y. Hu, Z. Zheng, Electromagnetic attenuation characteristics of microbial materials in the infrared band, Appl. Spectrosc. 70 (2016) 1456–1463. [8] H. Liu, X. Zhao, M. Guo, H. Liu, Z. Zheng, Growth and metabolism of Beauveria bassiana spores and mycelia, BMC Microbiol. 15 (2015) 267. [9] H.C. Booij, G.P.J.M. Thoone, Generalization of Kramers-Kronig transforms and some approximations of relations between viscoelastic quantities, Rheol. Acta 21 (1982) 15–24. [10] P. Grosse, V. Offermann, Analysis of reflectance data using the Kramers-Kronig Relations, Appl. Phys. A 52 (1991) 138–144. [11] M. Segal-Rosenheimer, R. Linker, Impact of the non-measured infrared spectral range of the imaginary refractive index on the derivation of the real refractive index using the Kramers-Kronig transform, J. Quant. Spectros. Radiat. Transfer 110 (2009) 1147–1161.
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[12] Y. Gu, Y. Hu, X. Zhao, X. Chen, Determination of infrared complex refractive index of microbial materials, J. Quant. Spectrosc. Radiat. Transf. 217 (2018) 305–314. [13] Y. Gu, Y. Hu, X. Zhao, C. Xi, W. Peng, Z. Zheng, Discrimination of viable and dead microbial materials with Fourier transform infrared spectroscopy in 3–5 micrometers, Opt. Express 26 (2018) 15842-. [14] X. Zhao, Y. Hu, Y. Gu, X. Chen, X. Wang, P. Wang, X. Dong, Analysis of optical properties of bio-smoke materials in the 0.25-14 μm band, Chin. Phys. B 28 (2019) 34201. [15] X. Zhao, Y. Hu, Y. Gu, X. Chen, X. Wang, P. Wang, X. Dong, A Comparison of Infrared Extinction Performances of Bioaerosols and Traditional Smoke Materials, (2018). [16] L. Li, Y. Hu, Y. Gu, W. Chen, Y. Zhao, Measurement and analysis on complex refraction indices of pear pollen in infrared band, Spectrosc. Spectr. Anal. 35 (2015) 89. [17] B.T. Draine, P.J. Flatau, User Guide for the Discrete Dipole Approximation Code DDSCAT 7.3, (2013). [18] T. Kozasa, J. Blum, T. Mukai, Optical Properties of Dust Aggregates, Bull. Am. Astron. Soc. (1992). [19] M. Lattuada, W. Hua, M. Morbidelli, Radial density distribution of fractal clusters, Chem. Eng. Sci. 59 (2004) 4401–4413. [20] M. Min, C. Dominik, J.W. Hovenier, A.D. Koter, L.B.F.M. Waters, The 10 mu m amorphous silicate feature of fractal aggregates and compact particles with complex shapes, Astron. Astrophys. 445 (2005) 1005–1014. [21] L. Li, Y. Hu, Y. Gu, X. Zhao, S. Xu, L. Yu, Z.M. Zheng, P. Wang, Infrared extinction performance of randomly oriented microbial-clustered agglomerate materials, Appl. Spectrosc. 71 (2017) 2555–2562. [22] S.L. Jacques, Modeling tissue optics using Monte Carlo modeling: a tutorial, Proc. SPIE. Int. Soc. Opt. Eng. 6854 (2008) 68540T-68540T-68549.
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Analysis of factors affecting the broadband extinction performance of bioaerosol
T
Xinyu Wanga,b, Yihua Hua,b,⁎, Youlin Gu (doctor)a,b,⁎, Xinying Zhaoa,b, Xi Chena,b a b
State Key Laboratory of Pulsed Power Laser Technology, National University of Defense Technology, Hefei 230037, China Anhui Province Key Laboratory of Electronic Restriction, National University of Defense Technology, Hefei 230037, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Bioaerosol Transmittance Extinction cross section Porosity
Bioaerosol shows significant broadband extinction performance and is a new direction for the development of broadband light attenuation materials. However, factors such as material, original particle size, the number of original particles and the structure of agglomerated particles would have impacts on the broadband extinction performance of bioaerosol. In this paper, cluster–cluster aggregation model was used to construct agglomerated particle of bioaerosol, discrete dipole approximation method and Monte Carlo algorithm were performed to calculate the broadband extinction abilities of bioaerosol. The results demonstrate that material selection affects the extinction performance of bioaerosol in different wavebands. Increasing the radius of original particles and decreasing the porosity of agglomerated particle can enhance the broadband extinction abilities of bioaerosol. The structure details and the number of original particles in an agglomerated particle have little impacts on the broadband extinction performance of bioaerosol directly. These conclusions are crucial for the further research of bioaerosol as a new light attenuation material and are applicable to all optical materials with fractal structure.
1. Introduction Bioaerosol, a significant constituent of atmosphere, is emitted from marine and terrestrial ecosystems or produced and released into the atmosphere by human production and life. It is an important part of the natural environment and has some significant functional properties that can be used by humans. Hence, bioaerosol has attracted increasing attention in the fields of atmospheric science, environmental science and electromagnetic field, et al. On the one hand, bioaerosol particles absorb and scatter solar radiation, which directly changes the energy budget of the groundatmosphere system and affects climate change [1]. It is necessary to master the factors which affect the broadband extinction performance of bioaerosol. On the other hand, bioaerosol is an effective absorbent and scatterer of electromagnetic radiation energy so that they have impacts on people's use of electromagnetic waves. Hu et al. [2] found that bioaerosol has significant broadband extinction capability in ultraviolet to infrared bands, providing a new direction for the development of broadband light attenuation materials. Thus, it is of vital importance to study how the factors affect the broadband extinction performance of bioaerosol and how to improve the broadband extinction abilities. This work will be a key step in obtaining a new optical material. Voitsekhovskaya et al. [3] studied the influence of microphysical parameters of atmospheric aerosol particles on IR radiation extinction. Huang et al. [4] studied the effect of porosity on optical properties of aerosol aggregate particles. Zhao et al. [5] found the aggregation-driven reductions in the mass extinction coefficient of bioaerosols. Liu et al. [6] found that the shape and aggregation of particle have a
⁎
Corresponding authors. E-mail addresses: [email protected] (Y. Hu), [email protected] (Y. Gu).
https://doi.org/10.1016/j.ijleo.2019.163527 Received 2 July 2019; Accepted 2 October 2019 0030-4026/ © 2019 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
X. Wang, et al.
strong influence on the light scattering property. However, the impacts of material, original particle size, the number of original particle and the spatial structure on broadband extinction performance of bioaerosol are not clear. How to improve the broadband extinction abilities of bioaerosol with these factors is an urgent problem to be solved. 2. Materials and methods 2.1. Materials preparation As one of the common microbial materials, AN02 spores are easy to be prepared and can be produced in large quantities. In addition, AN02 spores can be easily sprayed in the air and form bioaerosol quickly. The AN02 bioaerosol shows significant broadband extinction performance [2]. Thus, AN02 spores are suitable materials to analyze the factors affecting the broadband extinction performance of bioaerosol. The bioaerosol materials studied in this paper were provided by the Key Laboratory of Ion Beam Bioengineering, Chinese Academy of Sciences [7,8]. The complex refractive index (CRI) of AN02 spores were calculated with the spectral reflectance of materials and the Kramers-Kronig (K-K) algorithm [9–11], this method has been used several times in our research [2,12–16]. 2.2. Bioaerosol spatial structure model and extinction coefficient calculation model Bioaerosol is a colloidal dispersion system formed by dispersing and suspending biological particles in gaseous media. The bioaerosol particles are often not monomers, but random amorphous aggregated particle systems with complex spatial structures formed by small unit particles due to static electricity, collision, adhesion, and the like. Electron micrographs show that AN02 spores are approximately spherical in shape, with a concentrated particle size distribution, a radius distribution of 1∼2.5 μm, and an average radius of 1.5 μm [2]. In this paper, agglomerated particle simulated by Cluster–Cluster aggregation (CCA) model is used to calculate the extinction coefficients of AN02 spores. The discrete dipole approximation (DDA) method is an approximation method for solving the volume integral equation of electromagnetic scattering. The basic principle of it is to approximate the actual particles with an array of finite discrete, interacting small dipoles. As long as the condition |m − 1| ≤ 3 is satisfied, the DDA method can be applied to scatterer of any geometric shape, and the scatterer can be anisotropic and non-uniform [17]. According to the Fig. 1(a) and (b), the complex refractive index of AN02 meets the required conditions of DDA method. The theoretical principle and calculation formula of DDA method are given in
Fig. 1. The complex refractive index and simulation results of AN02 bioaerosol. (a) Real part of complex refractive index of AN02 spores in the waveband of 0.2–14 μm; (b) Imaginary part of complex refractive index of AN02 spores in the waveband of 0.2–14 μm; (c) The scattered fraction of incident light through AN02 bioaerosol in the waveband of 0.2–14 μm; (d) The absorbed fraction of incident light through AN02 bioaerosol in the waveband of 0.2–14 μm. 2
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
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literature [18–21]. And with this method, the extinction cross sectionCext , the scattering cross section Csca and absorption cross section Cabs of bioaerosol agglomerated particle can be calculated. 2.3. Transmittance simulation model With the extinction coefficients of AN02 agglomerated particle, AN02 agglomerated particle swarm was used to simulate the transmittance of incident light through AN02 bioaerosol. The density of the bioaerosol agglomerated particles in the particle swarm and the optical path through the swarm were set to be ρ and L , respectively. All agglomerated particles were assumed to be evenly distributed in the particle swarm and randomly oriented. Taking all these parameters, the transmittance, the absorbed fraction and the scattered fraction of incident light through the bioaerosol agglomerated particle swarm can be simulated with the Monte Carlo algorithm [22]. 3. Results and discussion 3.1. The impacts of material on broadband extinction performance In order to study the impacts of material on broadband extinction performance of bioaerosol, the extinction coefficients of AN02 agglomerated particle were calculated by DDA method. With these coefficients, assuming that ρ = 800cm−3 andL = 4m , the broadband transmittance through AN02 agglomerated particle swarm was calculated by Monte Carlo algorithm to simulate the broadband transmittance through bioaerosol. As shown in the Fig. 1, the scattered and absorbed fractions of the incident light were compared with the real part and the imaginary part of the CRI of AN02 spores in the waveband of 0.2–14 μm. It was easy to find that with the change of incident wavelength, the scattering fraction and the real part, the absorbed fraction and the imaginary part have the same trend. In other words, the material (mainly complex refractive index) determines the extinction performance of bioaerosol in different wavebands. 3.2. The impacts of original particle radius on broadband extinction performance The spatial structure of the agglomerated particle of AN02 spores was built with the CCA model. In order to study the impacts of original particle radius on broadband extinction performance, the extinction coefficients of agglomerated particle of AN02 spores were calculated six times and the original particle radius were set to be 1 μm, 1.3 μm, 1.6 μm, 1.9 μm, 2.2 μm, 2.5 μm, respectively. The parameter ρ and L were set to be 800 cm−3 and 4 m and the transmittances through the six agglomerated particle swarms were
Fig. 2. Simulation results of AN02 spores with different original particle radiuses. (a)Light transmittance through AN02 agglomerated particle swarm; (b) Extinction cross section of AN02 agglomerated particle; (c) Absorption cross section of AN02 agglomerated particle; (d) Scattering cross section of AN02 agglomerated particle (The four subgraphs share the same legend in the upper right). 3
Optik - International Journal for Light and Electron Optics 201 (2020) 163527
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Fig. 3. Simulation results of AN02 spores with two different structures but same porosity. (a) and (b) Agglomerated particle models with two different structures but same porosity: (a) corresponds to Structure 1 while (b) corresponds to Structure 2 ; (c) Light transmittance through AN02 agglomerated particle swarm; (d) Extinction cross section of AN02 agglomerated particle.
simulated respectively. As we can see from Fig. 2(b)–(d), the absorption cross section Cabs and the scattering cross section Csca of bioaerosol agglomerated particle increase with the growth of original particle radius. The peak of scattering cross section Csca in the wide waveband moves to the wavelength which matches the original particle radius. As a result, the extinction cross section Cext increases with the growth of original particle radius and the peak of the extinction cross section moves to the wavelength which matches the original particle radius. As shown in Fig. 2(a), the transmittance decreases with the original particle radius of bioaerosol agglomerate particle rises. Take the transmittance through bioaerosol agglomerated particle swarm when the wavelength of incident light is 10.6 μm as an example, the simulation result of transmittance decreases more than 90% when the original particle radius increases from 1 μm to 2.5 μm. These results indicated that the broadband extinction performance will be enhanced as the original particle radius grows. 3.3. The impacts of the spatial structure of agglomerated particle on broadband extinction performance The spatial structure of bioaerosol agglomerated particle was simulated by CCA model. In this work, several sets of CCA model parameters were set to control the aggregation process of the model and 120 kinds of various spatial structures were obtained. When the original particle number N was set to be 10, there are two different structures which share the same porosity (As shown in Fig. 3(a) and (b)). Take these two structures to perform the extinction performance simulation and the results are shown in Fig. 3(c) and (d). It shows that when the porosity of the bioaerosol agglomerated particle is a constant number, the broadband extinction performance of bioaerosol is fixed and will not change with the structure details. However, the bioaerosol agglomerated particle swarm shows quite diverse structures when the porosity of it is different. When the original particle number N was set to be 50, five simulation structures of agglomerated particle were chosen to calculate the broadband extinction performance of bioaerosol (As shown in Fig. 4(a)). The results in Fig. 4(b) and (c) show that as the porosity increases, the broadband extinction performance of bioaerosol will decline. Hence, for the bioaerosol agglomerated particles formed by the same number of original particles, the larger the porosity is, the more internal pores in the agglomerated structure are, the smaller the absorption and scattering of light is, and the greater the transmittance of light through the condensed particle swarm is. 3.4. The impacts of number of original particle in an agglomerated particle swarm on broadband extinction performance As we have known that the porosity of bioaerosol agglomerated particle would affect the broadband extinction performance of bioaerosol, we simulated four spatial structures with different original particle numbers but similar porosity (As shown in Fig. 5(a). These four structures were used to calculate the extinction coefficients of the bioaerosol agglomerated particle. When simulating the broadband transmittance through the bioaerosol agglomerated particle swarm, the density of the bioaerosol agglomerated particle in 4
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Fig. 4. Simulation results of AN02 bioaerosol with six different structures of agglomerated particles. (a) Agglomerated particle models with five different porosities; (b) Light transmittance through AN02 agglomerated particle swarm; (c) Extinction cross section of AN02 agglomerated particle. ((b) and (c) share the same legend in the upper right).
Fig. 5. Simulation results of AN02 spores with different numbers of original particle in an agglomerated particle. (a) Agglomerated particle models with different numbers of original particle; (b) Light transmittance through AN02 agglomerated particle swarm; (c) Extinction cross section of AN02 agglomerated particle.
the particle swarm ρ were set to be 800 cm−3, 400 cm−3, 200 cm−3, 160 cm−3 respectively so that the mass of bioaerosol were consistent in the four situations. As can be seen in Fig. 5(b) and (c), when the porosity of the bioaerosol agglomerated particle is constant, the extinction coefficients of a single agglomerated particle will be enhanced with the increase of original particle number. However, if the mass of bioaerosol is constant, the broadband transmittance through bioaerosol agglomerated particle swarm will hardly change with the increase of original particle number of a single agglomerated particle. In other words, the number of original particle in an agglomerated particle has little effect on the broadband extinction performance of bioaerosol.
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Fig. 6. The corresponding relationship between the original particle number and the porosity of agglomerated particle.
3.5. The impacts of number of original particle on porosity in an agglomerated particle It is known from the foregoing study that the number of original particle in an agglomerated particle swarm has little effect on the broadband extinction performance of bioaerosol but the porosity of the agglomerated particle swarm has decisive influence. However, it is found from the 120 agglomerated models we simulated by CCA model that there is some relationship between the original particle number and the porosity of agglomerated particle swarm. The corresponding relationship between the original particle number and the porosity of agglomerated particle is depicted in Fig. 6. As shown in the figure, with the increase of the number of original particles, the average value of porosities of the agglomerated particles gradually increased and the distribution range gradually narrowed. That means the number of original particle would influence the distribution of porosity and affect the broadband extinction performance indirectly. 4. Conclusion In conclusion, material selection determines the extinction performance of bioaerosol in different bands. Increasing the radius of original particles and decreasing the porosity of agglomerated particle can enhance the broadband extinction performance of bioaerosol. Take the transmittance through bioaerosol when the wavelength of incident light is 10.6 μm as an example, the simulation result of transmittance decreases more than 90% when the original particle radius increases from 1 μm to 2.5 μm and decreases about 50% when the porosity decreases from 0.9717 to 0.8526. However, the structure details of an agglomerated particle have little effects on its extinction coefficients if the porosity is constant. The number of original particles in a single agglomerated particle has no effects on the extinction performance directly but it can influence the distribution of porosity and affect the broadband extinction performance indirectly. These conclusions are crucial for the further research of bioaerosol as a new optical attenuation material and are applicable to all optical materials with fractal structure. Acknowledgements This work was supported by the National Natural Science Foundation of China (60908033, 61271353, 61871389); Natural Science Foundation of Anhui Province (1408085MKL47); National University of Defense Technology (ZK18-01-02). References [1] K. Adachi, S.H. Chung, P.R. Buseck, Shapes of soot aerosol particles and implications for their effects on climate, J. Geophys. Res. Atmos. 115 (2010). [2] Y. Hu, X. Zhao, Y. Gu, X. Chen, X. Wang, P. Wang, Z. Zheng, X.J.S.C.M. Dong, Significant broadband extinction abilities of bioaerosols, Sci. China Mater. (2019) 1–13. [3] O.K. Voitsekhovskaya, I.V. Golub’, A.Y. Zapryagaev, O.V. Shefer, The influence of the microphysical parameters of atmospheric aerosol particles on IR radiation extinction, Izvestiya, Atmos. Ocean. Phys. 46 (2010) 55–59. [4] C. Huang, Z. Wu, Effect of porosity on optical properties of aerosol aggregate particles, Acta Opt. Sin. 33 (2013) 0129001. [5] X. Zhao, Y. Hu, Y. Gu, X. Chen, X. Wang, P. Wang, X. Dong, Aggregation-driven Reductions in the Mass Extinction Coefficient of Bioaerosols, (2019). [6] H. Liu, J. Ma, Z. Song, D. Liu, Light scattering properties of fractal aggregates, Acta Opt. Sin. 31 (2011) 284–289. [7] P. Wang, H. Liu, Y. Zhao, Y. Gu, W. Chen, L. Wang, L. Li, X. Zhao, W. Lei, Y. Hu, Z. Zheng, Electromagnetic attenuation characteristics of microbial materials in the infrared band, Appl. Spectrosc. 70 (2016) 1456–1463. [8] H. Liu, X. Zhao, M. Guo, H. Liu, Z. Zheng, Growth and metabolism of Beauveria bassiana spores and mycelia, BMC Microbiol. 15 (2015) 267. [9] H.C. Booij, G.P.J.M. Thoone, Generalization of Kramers-Kronig transforms and some approximations of relations between viscoelastic quantities, Rheol. Acta 21 (1982) 15–24. [10] P. Grosse, V. Offermann, Analysis of reflectance data using the Kramers-Kronig Relations, Appl. Phys. A 52 (1991) 138–144. [11] M. Segal-Rosenheimer, R. Linker, Impact of the non-measured infrared spectral range of the imaginary refractive index on the derivation of the real refractive index using the Kramers-Kronig transform, J. Quant. Spectros. Radiat. Transfer 110 (2009) 1147–1161.
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