Investigation of Optical Properties of Size-Selected Black Carbon Under Controlled Laboratory Conditions Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Ziying Lei, B.S. Graduate Program in Civil Engineering The Ohio State University 2016 Thesis Committee: Andrew May, Advisor John Lenhart Barbara Wyslouzil
Copyright by Ziying Lei 2016
Abstract Air pollution, one of the most concerning and widespread environmental issues has grown in importance in the world. The carbonaceous aerosols significantly contribute to air pollution, which not only causes public health concern, but also impacts climate change. The aerosol particles have properties to absorb or scatter solar radiation and thermal radiation; therefore, they play an important role in climate change. In order to improve the understanding and control the aerosol particles, it is crucial to study aerosol size distribution and chemical compounds. Black carbon is the strongest radiative absorber suspends in the atmosphere, and can have great influence on climate change. This study investigates the size-selected black carbon optical properties under laboratory conditions with varied relative humidity referred to dry, humid, and wet respectively. The single scattering albedo that measures the relative amount of aerosol light extinction due to scattering, and the absorption enhancement due to lensing effect are measured in this study, and ii
compared with the modeling results based on Mie theory, which is used to predict the absorption and scattering of light by a spherical particle. The results show that the single scattering albedo under the dry, humid, and wet conditions are similar, while the single scattering albedo for black carbon particles that have undergone heating to 160 o C is slightly greater than other three conditions. In general, with respect to particle size, single scattering albedo increases with smaller particle size and then levels off at larger size diameters. In addition, the absorption enhancement for black carbon particles is estimated in this study, and it ranges from 1 to 2.5. The values based on observations generally follow the predicted trend from Mie theory. Comparing the observed and modeled values suggests that 25%-50% of the total particle diameter is attributable to coating material. Dry black carbon particles do not have high absorption enhancement because they likely have no or thin coating materials. However, the absorption enhancement for humid particles and wet particles are higher than the dry particles, likely due to an enhanced lensing effect due to water uptake by the black carbon particles. iii
Future study will focus on improving the understanding of black carbon optical properties and accuracy of experimental results. Further research is recommended to focus on ranges of BC particles with the diameters smaller than 100 nm and larger than 650 nm, which are not included in this study and constrain the particle charge units. iv
Dedication Dedicated to the students at The Ohio State University v
Acknowledgments This thesis represents not only my hard work, but this thesis is also the result of many experiences I have encountered at The Ohio State University from lots of remarkable individuals who I wish to acknowledge. First, my deep gratitude goes to my advisor, professor Andrew May, who expertly guided me through my Master s study. He is the person who stimulates my interest and enthusiasm on air pollution and aerosol research. Whenever I ran into a trouble or was confused by a question, he was always happy to help and solve my confusion. His enthusiasm for research and conscientious attitude to work encouraged me that never give up when I face challenges, and his personal generosity and humor make my time very enjoyable. Second, I would like to thank Dr. John Lenhart and Dr. Barbara Wyslouzil for being my committees, a thesis reader. They improved my background of understanding the environmental engineering and air pollution, and gave me inspiration in many ways. vi
Special appreciation goes to Dr. Christopher Cappa of the University of California at Davis for sharing his version of Mie scattering and absorption code and his previous study. My appreciation also extends to my laboratory colleagues. Simon Bartos shared lots of lab experiments with me and his mentoring and encouragement have been very valuable. Thanks also go to Gustavo Acra de Oliveira, who helped with lab work with his dutiful and precise attitude. Thank my friend Mariantonieta Gutierrez Soto encouraged and helped me during the time I was working on my thesis. Above ground, I am indebted to my parents and friends, who consistently encouraged me and enlivened my life. vii
Vita The Ohio State University Columbus, OH M.S. Environmental Engineering Dec. 2016 Guilin University of Technology Guilin, China B. S. Resource Environment and Urban Planning Management June. 2015 Publications Change Analysis of Soil Organic Carbon and Aggregate Stability of Newly Reclaimed Field of Salinization Abandoned Farmland in the Manas River Valley Lei Jun, Ziying Lei, Lin Hairong, Han Chunli and Zhang Fenghua Journal of Anhui Agricultural Sciences 2014, 42 (28) 9755-9757,9859 Change Analysis of Organic Carbon, Nitrogen Content and Carbon Reserve of Oasis Cotton Soil in the Manas River Valley Lei Jun, Ziying Lei, Lin Hairong and Zhao Ruihai Journal of Anhui Agricultural Sciences, 2013, 41 (28) 11347-11349 Fields of Study Major Field: Civil Engineering viii
Table of Contents Abstract... ii Dedication...v Acknowledgments... vi Vita... viii List of Tables... xii List of Figures... xiii List of symbols...xv Chapter1: Introduction...1 1.1 Air Pollution and Climate Change...1 1.2 Aerosol...2 1.3 Black Carbon...5 1.4 Mie Theory...8 1.5 Project Overview...10 Chapter 2 Materials and Methods...13 2.1 BC Generation...13 2.2 Instrumentation...14 ix
2.2.1 Photoacoustic Extinctiometer...15 2.2.2 Differential Mobility Analyzer...16 2.2.3 Condensation Particle Counter...20 2.3 Experiment Process...22 2.4 Data Description...25 2.5 Potential Limitation...25 Chapter 3 Optical Properties of BC...29 3.1 Single Scattering Albedo of BC...29 3.1.1 Single Scattering Albedo of dry soot...30 3.1.2 Single Scattering Albedo of humid soot...31 3.1.3 Single Scattering Albedo of wet soot...33 3.1.4 Single Scattering Albedo of heated soot...34 3.1.5 Comparison of different treatments...36 3.2 Single Scattering Albedo Modeling...38 Chapter 4 Absorption Enhancement...40 4.1 Absorption Enhancement...40 4.2 Absorption Enhancement Modeling...41 x
4.3 Result...44 Chapter 5 Summary and Conclusion...48 5.1 Summary...48 5.2 Conclusion...49 5.3. Future work...51 References...54 Appendix A-Mie theory equations...63 Appendix B-Probability distribution of particle mobility multiple charge units...66 Appendix C-Single scattering albedo data... Appendix D-Matlab code for Mie theory...67 xi
List of Tables Table 1. Particle size ranges in the DMA estimated using Equations 5-8...28 Table 2. T-test result of different selected-sizes SSA under the same treatment...47 Table 3. Previous study for absorption enhancement of BC particles with different coating materials...47 Table 4. Fraction of total particle concentration with multiple charges...64 Table 5. SSA mean and standard deviation under different laboratory conditions with selected size...65 xii
List of Figures Figure 1 Data provided from SMPS scanning through all size distribution a) Geometric mean diameter for all size particles. b) Total particle number concentration across all sizes...14 Figure 2. Diagram of the Photoacoustic Extinctiometer (PAX), based on the DMT manual...15 Figure 3 Principle of a general DMA (Intra and Tippayawong, 2008)...17 Figure 4. Cunningham slip correction for different diameters...19 Figure 5. Basic Principle of CPC...21 Figure 6. Experiment setting under dry condition...22 Figure 7. Experiment settings under wet condition...23 Figure 8 Experiment settings under heated condition...24 Figure 9. Mobility diameter versus electrical mobility for three different numbers of charge. See text for more details....26 Figure 10. a) Transfer functions with different particle sizes. b) Transfer functions with different electric mobility...27 xiii
Figure 11. Single Scattering Albedo in different size of dry BC particles with 20.5% RH...30 Figure 12. Single Scattering Albedo in different sizes of humid BC particles with 42% RH...31 Figure 13. Single Scattering Albedo in different size of wet BC particles with 80% RH...33 Figure 14. Single Scattering Albedo in different size of heated BC particles with 20.5% RH...34 Figure 15. Single scattering albedo of particles under different treatment...36 Figure 16 Single Scattering Albedo in different size of BC particles...38 Figure 17. Absorption enhancement as a function of particle size for both experimental observations (markers) and model predictions (lines)...44 xiv
List of symbols BC- Black Carbon BrC-Brown Carbon CPC-Condensation Particle Counter C c -Cunningham slip correction factor DMA-Differential Mobility Analyzer E abs -absorption enhancement e-charge of an electron Kn-Knudsen number L-effective electrode length m-refractive index n-number of charge units n real term in refractive index ik-imaginary term in refractive index OC-Organic Carbon PAX-Photoacoustic Extinctiometer PM-Particulate Matter Q ext -extinction efficiency xv
Q a -polydisperse charged aerosol Q s -sheath flow Q scat -scattering efficiency R 1 -radium of outer electrodes R 2 -radius of inner electrodes RH-relative humidity V-applied voltage Z p -target electrical mobility Z! - electrical mobility β abs -absorption coefficient β scat -scattering coefficient β ext -extinction coefficient λ-wavelength µ-gas viscosity ω-single scattering albedo Ω-transfer function xvi
Chapter1: Introduction 1.1 Air Pollution and Climate Change With the rapid development of technology and economic growth, an increasing number of environmental issues have come into our life. Air pollution has become a common issue, which attracts a lot of attention in the world. Over the last century, the emission rates of carbonaceous aerosols have increased dramatically (Ito and Penner, 2005) with a variety of human activities, such as industrial and agricultural activities, biomass burning and deforestation (Seinfeld et al., 2016). The air pollution will not only adversely affect public health, but also have a significant impact on climate change. From the view of outer space, the Earth is like a colorful marble, which has white clouds, blue oceans and brown continents. In the white areas, about 30% solar radiation is reflected back to space, and 70% solar energy will be absorbed by the atmosphere surface. This solar energy warms the planet and atmosphere, but some solar energy will escape to reach an energy balance. Overall, the solar radiation provides a constraint for global temperature (Ramanathan & Carmichael, 2008). 1
However, a variety of air pollutants change Earth s climate. For instance, greenhouse gasses as a common exhaust from mobile vehicles. Greenhouse gas will trap heat from solar energy and suspend in the atmosphere, which will cause global warming. Additional, there are other tiny air pollutant particles in the atmosphere, which are aerosols cause different impacts on climate change. This will be discussed in the next section. 1.2 Aerosol Aerosols are fine particles suspended in the atmosphere. Aerosols include both particles and the suspending gas, which is usually air. Aerosols of various sizes can be generated by human activities such as in the form of industrial particulate, dust, and soot. It also can form from nature such as volcanic dust, desert dust, sea salt, and biological debris. For atmospheric aerosols, the size distribution and chemical composition are fundamental properties relevant to environmental impact and studying them is crucial in understanding and managing regional aerosol effects on public health, visibility and climate change. Aerosol particles play an important role in global climate and climate change (Buseck and Pósfai, 1999) by scattering or absorbing solar radiation and thermal radiation. The size of the atmospheric aerosols is an essential parameter that affects climate. The 2
typical aerosol particles size varies from roughly 1 nm to 100 µm, which affects both the lifetime and the physical and chemical properties of aerosols. The size distribution can be helpful in understanding the atmospheric aerosols influence on the processes of coagulation and condensational growth, which can have impact on aerosol optical properties. Most studies used size distributions to present the characteristics and radiative properties of the aerosol (Tegen and Lacis, 1996; Whitby et al., 1972). In addition, the size of aerosol particles also has great influence on cloud nucleating ability and cloud formations (Dusek et al., 2006). Numerous aerosol particles of various sizes share condensed water during cloud formation, which can reduce the size of cloud droplets if a larger number of particles are present. This can increase the cloud reflectance of sunlight and cool the Earth s surface (Kaufman et al., 2002). Therefore, size-selected studies should be conducted to more deeply explore the relationship between aerosols size and various properties. The composition of aerosols is another important parameter to assess the aerosols effect on climate change. In general, the predominant chemical components of aerosol particles are ammonium, nitrate, sulfate, organic compounds, sea salt, mineral dust and black carbon (BC). The different chemical compounds can have either warming or cooling effects on climate 3
change due to their scattering or absorption properties. For instance, BC, a particulate pollutant from incomplete combustion, has a strong ability for absorbing light and contributes to the warming of the Earth, while particulate sulfates, nitrates and salts scatter light and cool the Earth's atmosphere. Organic compounds predominantly scatter light (but brown carbon does absorb at shorter wavelengths) and dust impacts radiation to varying degrees. The optical properties of aerosol particles suspended in the atmosphere show a great spatial and temporal variability, and they are determined based on their chemical composition, size, shape, concentration and mixing state (Kokhanovsky, 2008). A lot of studies measured aerosol light absorption to improve understanding of atmospheric aerosols direct radiative impacts. For mixed aerosols in different air masses, the single scattering albedos commonly are 0.7 to 0.9 (Bergstrom et al., 2007). Black carbon is the strongest radiative absorber in the atmosphere, which can absorb up to 90% of the radiation that it interacts with certain wavelengths (Zhang et al., 2008). Aerosol particles are usually mixed in the atmosphere; that is, an aerosol population will contain more than one chemical compound. For example, BC can form an internal mixture with non-refractory materials, where the non-refractory material forms a shell around the BC core. This shell 4
material may bend light towards the particles cores, enhancing light absorption (Mackey et al., 2012; Cappa et al., 2012; Lack et al., 2009). This is referred to as the lensing effect. The lensing effect enhances the light absorption so that particles can have strong warming effects and potentially play a role in climate change. Therefore, in order to control global warming due to air pollution, it is important to understand the influence of fine particulate matter and other air pollution on climate change, and understand the interactions between natural chemical compounds and man-made pollutants in the atmosphere. The study of absorption enhancement of BC particles can improve the description of BC absorption of solar radiation, and also have implications for climate effects. The results of such studies can help policy makers to control air pollution and improve our environment. 1.3 Black Carbon BC is a common carbonaceous component of particulate matter (PM) that can interact with solar and terrestrial radiation. BC is a byproduct of incomplete combustion of fossil fuels, biofuels, and biomass. It can be found in outdoor environments as a result of wildfires, diesel and coal combustion, deforestation and crop residue burning. BC is typically comprised of small 5
spherules with sizes ranging from 1 to 5 nanometers (nm). Some of them aggregate to form larger size particles, which have 0.1 to 1 micrometers (µm) diameters. The larger size particles scatter or absorb the wavelengths more effectively due to the range of the size being similar to the sun s wavelengths (Horvath, 1993) and the pure BC particles are likely a spherical. All wavelengths of solar radiation are absorbed by BC when suspended in the atmosphere. In fact, BC absorbs energy a million times more efficiently than CO 2 (Bond and Sun, 2005). The energy will be released as heat and warms up the atmosphere, which might contribute to acceleration of increased ice and snow melting. This will make a significant impact on global warming and climate change. Over the past decade, BC and other aerosols have been recognized as contributors to climate change based on advanced scientific understanding of BC and study results from satellite observation and modeling (Bond and Sun, 2005; Ramanathan and Carmichael, 2008). BC, as the main light-absorbing aerosol in the atmosphere, has been deeply studied due to its strong positive forcing effect on climate change (Ramanathan & Carmichael, 2008; Jacobson, 2001; Bond et al., 2013). The recent studies show that BC has a significant regional effect in many countries due to the short atmospheric residence time of BC, which is about 6
1-4 weeks (Ramanathan et al., 2007; Quinn et al., 2007; Flanner et al., 2007). BC can also undergo long-range transport to other areas; therefore it can have great influence on global climate change. BC is frequently co-emitted with gases and other particles formed during combustion processes. The composition of the mixture particles depends on the different types of fuel and the combustion conditions. When BC is emitted into the atmosphere, it is often combined with organic carbon, which includes clear carbon and brown carbon (BrC). While clear carbon will only scatter light, BrC will absorb light following an inverse wavelength dependency (Novakov and Corrigan, 1991; Alexander et al., 2008; Posfai et al., 2004; Andreae and Gelencsér, 2006). Additionally, emissions also contain other light-scattering compounds, such as aerosol nitrate and sulfate, which, when present in the atmosphere, may interact with BC. The nonabsorbing compounds that are co-emitted with BC or exist in the atmosphere may comprise the coatings that surround the BC core. Also, aerosol particles may uptake water or semi-volatile organics, which can change particle size, morphology, chemical composition and refractive index. All these changes have impacts on global climate change (Lee et al., 2008). Some previous studies examined the optical properties of aerosol with relatively hygroscopic growth; and the study shows that hygroscopic growth enhances 7
the optical properties of soot aerosols (Shiraiwa et al., 2010, (Zhang et al., 2008). Thus, it is important to consider the impact of co-emitted compounds when estimating the climate impact of BC. 1.4 Mie Theory The interactions of particles and light can be described authentically Mie theory. Mie theory is used to predict the absorption and scattering of light by a spherical particle based on the size and chemical composition of the particles. There are several important parameters: (1) The wavelength λ of the incident radiation; (2) The size of the particle, usually expressed as a dimensionless size parameter (that also accounts for the wavelength); α= πd! λ (1) (3) The particle optical property relative to the surrounding medium, the complex refractive index of a material describes how light propagates through that medium: m = n + ik (2) 8
The real part n is a measure of the refraction of light entering a material and the imaginary part k represents the amount of absorption of light by the material. Both real part and imaginary part of the refractive index are functions of λ. The real (n) and imaginary (k) parts of the refractive index represent the non-absorbing and absorbing components, respectively. The formulas for scattering efficiency (Q scat ) and extinction efficiency (Q ext ) of a particle are:! Q!"#$ m, α = 2 α! 2k + 1 [ a!! + b!! ]!!!! Q!"# m, α = 2 α! 2k + 1 Re[a! + b! ]!!! (3) (4) where a k and b k are parameters for Q scat and Q ext, which can be calculated by formulas is giving in the Appendix A. Mie theory has been widely used in previous studies. The most relevant use to this thesis is the modeling of absorption enhancement (Bond et al., 2006; Cappa et al., 2012; Lack et al., 2009). Other authenticable formulations also exist such as the T-matrix approach (Mishchenko, 1996) and the discrete dipole approximation (Yang et al., 1995; Draine and Flatau, 1994), but none of these are considered in this work. 9
1.5 Project Overview BC has been indicated to be the second most important contributor to climate change after CO 2 (Ramanathan and Carmichael, 2008). However, there are many factors that can influence study results, such as the complicated optical properties of BC (Andreae and Gelencsér, 2006) and the uncertainties of different technical measurements (Arnott, 2003; Petzold et al., 2005; Collaud Coen et al., 2010). Therefore, this study helps to improve the understanding of BC particles optical properties under different laboratory conditions. Also, it is essential to quantify the forcing effect of BC on global climate change. The goal of this study is to investigate the optical properties of BC with specified diameters under controlled laboratory conditions. The extinction coefficient (β scat ) and the absorption coefficient (β abs ) are measured directly, and they can be used to derive other properties including single scattering albedo and absorption enhancement. From this research we investigate: (1) the variability of single scattering albedo under varying humidity levels; (2) the impact on the lensing effect under different relative humidities; (3) and how well the absorption enhancement model based on the Mie theory fits the measurements of propane-torch-generated BC particles with assumed coating thickness. 10
Four different experimental conditions are used in the laboratory for this study. For the first treatment, the BC particles go through the diffusion dryer to eliminate the water vapor from the atmosphere. For the second treatment, we remove the dryer and expose the BC to moderately humid laboratory air (~42% humidity). For the third treatment, we increase the humidity to 80% by using a humidifier and let the BC particles mix with the water vapor. For the fourth treatment, after BC particles go through the dryer, we heat the particles to 160 C by using a heated tube to evaporate organics, water vapor, ammonium nitrate, and ammonium sulfate. During this entire process, we measure the absorption of BC particles under these four laboratory conditions, and the absorption enhancements are calculated by using the measured data. Based on the Mie theory, the measured absorption enhancements of BC particles with selected diameters are compared with the estimated absorption enhancements. In this chapter, we introduced background material for air pollution, aerosol, and black carbon. A brief review of Mie theory and an overview of the project are presented in Chapter 1. Chapter 2 provides a detailed description of size selected atmospheric BC particles measurements and methods. The results of single scattering albedo obtained from the measurements are represented and discussed in Chapter 3. In Chapter 4, the absorption 11
enhancements of BC particles are calculated from measurements and compared with the absorption enhancement model based on Mie theory. Finally, in Chapter 5, conclusions drawn from this study are summarized and recommendations are provided for future work. 12
Chapter 2 Materials and Methods 2.1 BC Generation In this study, BC soot was generated by using a propane torch. In a number of studies, propane flames were used to generate and analyze the properties of BC (Smith et al., 2015; Schnaiter et al., 2006). Aluminum foil was used to reduce the air intake thus, provide an incomplete combustion condition to generate BC particles. The combustion products are collected into an aerosol chamber for optical measurements. In order to ensure the generation process was consistent between different experiments and under different environmental conditions in this study, the repeatability of the particle generation experiment has been verified. Figure 1 represents the data from Scanning Mobility Particle Sizer (SMPS), which is widely used for airborne particle size distributions. All size distributions from 10nm to 700nm was obtained through the SMPS. An SMPS is a combination of a Differential Mobility Analyzer (Section 2.2.2) with a Condensation Particle Counter (Section 2.2.3). 13
In Figure 1, each dot represents the mean geometric mean for five experiments and the error bar represents the standard deviation. Based on the results in panel a), the geometric mean diameters do not have much variance throughout these particle generation consistency experiments. Figure 1 Data provided from SMPS scanning through all size distribution a) Geometric mean diameter for all size particles. b) Total particle number concentration across all sizes 14
Panel b) represents the total particle number concentration distribution under humid conditions to testify the repeatability of the samples measured. From panel b), the standard deviations for five experiments are very small. The data can be considered repeatable since all five experiments generated independently are giving consistent values. Number decreases are likely due to well lose in the chamber, coagulation, and infiltration of lab air into the chamber. Despite all of the external factors that could influence the particle generation parameters (i.e., geomantic mean diameter and total particle number concentration), all repeated tests are similar, which means the particle generation method should not bias the outcome of the experiments. 2.2 Instrumentation In this study, three main laboratory instruments used to run the experiments were 870nm Photoacoustic Extinctiometer (PAX) (Droplet Measurement Technologies; Boulder, CO), the Differential Mobility Analyzer (DMA) Model 3082L (TSI, Inc.; Shoreview, MN), and Condensation Particle Counter (CPC) Model 3775 (TSI). 14
2.2.1 Photoacoustic Extinctiometer The PAX is a sensitive, high-resolution, fast-response instrument for measuring aerosol optical properties relevant for climate change and BC particle sensing. The PAX uses an 870nm laser to simultaneously measure light scattering and absorption. The scattered light is measured by using a photodiode (shown in Figure 2). The scattering measurement responds to all particle types regardless of chemical makeup, mixing state, or morphology. For absorption, particles that absorb light of 870 nm heat up and transfer heat to the surrounding air, releasing sound, which is detected by a sensitive microphone. The standard 870 nm wavelength is especially sensitive to black carbon particles, since there is relatively little absorption from gases and non-bc aerosol species at this wavelength (Droplet Measurement Technology, 2014). Figure 2. Diagram of the Photoacoustic Extinctiometer (PAX), based on the DMT manual 15
2.2.2 Differential Mobility Analyzer The Differential Mobility Analyzer (DMA) is a device that can select submicrometer aerosol particles based on size while keeping them suspended in air. And can measure the diameter of aerosols, ranging from 3nm to 800nm depending on operating conditions. A DMA consists of two concentric metal electrodes as shown in Figure 3. The inner electrode maintains negative voltage (0-10kV), while the outer electrode is electrically grounded. The polydisperse, charged aerosols that enter the DMA are introduced into particle-free sheath air flow and flow towards the outlet. The positively charged particles will be attracted to the negatively charged collector rod. The particles are spread out radially through the sheath air due to the particles varied electrical mobility and the instrument flow rate (Intra and Tippayawong, 2008; Knutson and Whitby, 1975). Typically, a DMA is scanned over a range of particle mobilities by the applied field to measure a size distribution. The number concentrations of a monodisperse aerosol corresponding to a given voltage that edit through the monodisperse slit are measured for each applied voltage. These measurements can then be converted to a size distribution by using the 16
distribution of charges produced by the aerosol neutralizer and the known relation between mobility and size. Figure 3 Principle of a general DMA (Intra and Tippayawong, 2008) The target electrical mobility (Z p *) (or, the target size for the monodisperse aerosol) is a function of DMA, the applied voltage and flow rate (Knutson and Whitby, 1975). Z! = (Q! + Q! )ln R! /R! 2πLV (5) 17
where Z p is electrical mobility (Cs k -1 ), Q s is sheath flow (Lmp), Q a is the aerosol sampling flow rate (Lmp), R 1 and R 2 are the radii of the outer and inner electrodes (m), L is the effective electrode length (m), and V is the applied voltage (kv). This form of the equation assumes that sheath flow equally excess flow and the polydisperse aerosol flow (Q a ) equal the monodisperse aerosol flow. The electrical mobility Z p can be calculated and related to selected particle diameter through the following equation: Z! = nec! 3πμd! (6) where n is the number of elementary charge units on the particle, e is the charge of an electron (1.602 10-19 Coulombs), µ is the gas viscosity, and C c is the Cunningham slip correction factor (Cunningham, 1910): C! = 1 + λ d! [2.34 + 1.05exp ( 0.39 d! λ )] (7) where λ is the mean free path of a gas molecule (assumed to be 65.2 nm). 18
Figure 4 shows that the Cunningham slip correction factor varies with the different diameters; that is, smaller particles require a larger correction factor since they are more likely to be influenced by collisions with air molecules. Figure 4. Cunningham slip correction for different diameters In order to estimate the probility that a particle of a given electrical mobility will exit the DMA through the slit in the monodisperse flow: Ω Z!, β = 1 2β Z! 1 + β + Z! 1 β 2 Z (8) 19
where Z! is the ratio of Z p and Z!, β is the ratio of aerosol flow to sheath flow. In this study, proper conditions can help investigate a broad range of particle sizes, which are 100 nm, 150nm, 200nm, 300nm, 400nm, 500nm, 600nm and 650 nm. Therefore, the sheath air flow is set as 3.9 Lpm based on the 3:1 DMA resolution and aerosol flow is 1.3 Lpm, which is fixed based on PAX and CPC flow rates. 2.2.3 Condensation Particle Counter The Condensation Particle Counter (CPC) is an instrument that detects and counts airborne particles as small as 4 nm in diameter. CPC mainly consists of three parts: the saturator, the condenser and the optical detector, as shown in Figure 5. The aerosol samples will go through a saturator, where the butanol liquid is vaporized and mixed with aerosol samples, and the butanol vapor achieve a supersaturated state. Then, the aerosol butanol mixture passes into a condenser to grow into droplets, which are larger than the threshold for detection and counters. 20
Figure 5. Basic Principle of CPC The aerosol sample is drawn into the CPC inlet by an internal vacuum pump. The inlet flow can be configured for either a 1.5 Lmp high-flow mode operation to improve response time and minimize particle transport loss, or a 0.3 Lpm low-flow mode operation. The inlet flow is set 0.3 Lpm in this study. A CPC can be combined with DMA to form a SMPS, which can provide a measurement of size and number concentration of an aerosol particle with the diameter ranges from 2 nm to 1000 nm. In this study, the SMPS was 21
used to help understand the optical properties of all size BC particles (i.e., not size-selected). 2.3 Experiment Process In this work, the optical properties of size selected BC are measured under several controlled laboratory conditions. First, the generated BC particles were measured under dry conditions. The sample particles went through a diffusion dryer filled with calcium sulfate desiccant, which was connected right after the sample tube from the aerosol chamber. The diffusion dryer removes the water vapor presents in the air and keep the relative humidity low compared with ambient relative humidity. The average relative humidity under dry conditions is 20.5%. The experiment setup is shown in Figure 6. Figure 6. Experiment setting under dry condition 22
Second, the humid BC particles generated in the chamber were measured under normal relative humidity conditions in the laboratory, i.e., there was no treatment to the BC sample particles after generation. The BC particles were directly sent to the instruments and were measured. On this stage, the average relative humidity is 42.2% and the aerosol flow is 1.3 Lpm. Third, the BC particles were measured under the wet conditions shown in Figure 7. A humidifier was added inside of the aerosol chamber to increase the relative humidity, which remained nearly constant. Before the BC sample particles were generated, the relative humidity in the aerosol chamber can reach about 80%. The aerosol flow is 1.3 Lmp. Figure 7. Experiment settings under wet condition Fourth, the BC particles were measured under the heated conditions as shown in Figure 8. The diffusion dryer was connected to the sample tube to 23
remove the water vapor from the air. After the particles are dried, they went through a heated tube. The air temperature inside the tube was measured by a thermocouple. The highest temperature can reach 160 C from the temperature controller. Most of the semi-volatile material is to evaporate at this temperature as it is expected. Some evaporated material may re-condense onto the particles or on the wall of the cooling tube after exiting the heated tube (Saleh et al., 2011); this was not directly quantified, but based on the Saleh et al. paper, it was estimated that a maximum of 50% of the material re-condenses. Under this treatment, it is expected to obtain relatively pure BC particles. Figure 8 Experiment settings under heated condition For each of these four different laboratory conditions, the DMA is used to select the specific diameters of the particles. For each diameter, the propane flame lasted about 10 seconds to generate BC sample particles inside the 24
aerosol chamber and was repeated for five experiments with unique flaming samples. Each experiment ended when the BC mass concentration decreased to an endpoint threshold value of 5µg/m 3, which was defined arbitrarily. 2.4 Data Description The most data used in this study is collected by the PAX, namely scattering and absorption coefficients (β scat and β abs, respectively). The PAX also provides single scattering albedo (ω) calculated by: ω = β!"#$ β!"#$ + β!"# (9) In this study, bulk values (i.e., non-size-selected) of optical properties were also investigated. The particle sizes ranged roughly from 10nm to 700nm, according to the SMPS. Five replicate experiments were conducted with a fixed duration rather than endpoint threshold concentration to assess the bulk values. 2.5 Potential Limitation This section discusses potential limitations in this study for selected sizes. The first issue is that the particles are usually have multiple charge units. However, in this research we assume each particle is single charged. From Figure 9, the same electrical mobility with different numbers of charges 25
relate to different particle diameters. For instance, Z p = 10 7 C-s kg -1 corresponds to particle diameters of 48nm, 71nm, 90nm for the singlycharged, doubly-charged and triply-charged particles, respectively. Therefore, considering the different multiple charge units the diameters of the particles can be different, which may influence the experiment results. Thus, the DMA cannot truly classify the aerosol particles with a monodisperse size due to multiple charges. Probability distributions are shown in Appendix B. Figure 9. Mobility diameter versus electrical mobility for three different numbers of charge. See text for more details. 26
Figure 10. a) Transfer functions with different particle sizes. b) Transfer functions with different electric mobility Another issue is related to the transfer functions. In order to see how well the diameters of particles are selected, the transfer functions are calculated by equations (5), (6), (7), and (8) based on singly charged particle and fixed voltage assumptions. The transfer function has been calculated and represented in Figure 10. In panel a), the red line represents the target diameter, based on the set voltage in the DMA. The top of the black 27
rectangle shows a 50% probability that singly charged particles around 147 nm and 160nm in diameter will be sampled. Panel b) represents the probability that a particle with a given electric mobility will exit from the sample flow. In this study, based on the transfer function, the percentage for the particles with selected diameters can be calculated. The results show that the 75% of total particles transmitted through DMA outlet have > 50% transmission efficiency. The ranges of the diameters are calculated based on assuming particles are singly charged without diffusion impact. If we let the transfer function equal zero and solve the equations (5), (6), (7), and (8), then we can get the particle size ranges, the calculation result shown in Table 1. This range represents when the transfer function is zero, the particles with smallest and biggest diameters go through the DMA (shown in Figure 10 a). The particle size range varies and the larger size particles have wider ranges, which could influence the measurements of particles. Table 1. Particle size ranges in the DMA estimated using Equations 5-8 D p,mean (nm) 100 150 200 300 400 500 600 650 D p,min (nm) 90 138 179 267 355 443 530 574 D p,max (nm) 135 214 286 446 612 781 951 1037 28
Chapter 3 Optical Properties of BC 3.1 Single Scattering Albedo of BC Single scattering albedo (SSA) is a measure of the amount of aerosol light extinction due to scattering. It is defined as the ratio of scattering efficiency to extinction efficiency (Eq 9). In this study, SSA for laboratory BC was measured under different laboratory conditions. The results from the experiments are presented as follows. Each colored dot represents one experiment with specific selected BC diameters, and there are a total of five replicable experiments per size selected. The red line is the best-fit based on the measured SSA data. The two blue dotted lines represent the standard deviation in the best-fit line for SSA in each size selected. The gray line represents the bulk value of SSA, a single value representative of the entire size distribution. Data are presented in Appendix C (mean and standard deviation of SSA) 29
3.1.1 Single Scattering Albedo of dry soot Figure 11. Single Scattering Albedo in different size of dry BC particles with 20.5% RH Figure 11 shows the SSA of BC particles under dry condition, which has 20.5% relative humidity. It increases from 0.12 for the BC particles with 100nm diameter to roughly 0.2 after 200nm and remains roughly constant up to 650nm. All these values are relatively low, which means that the BC particles are barely scattering the light and, instead, are highly absorbing the light. Particles with small sizes from 100nm to 300nm can absorb more light than they scatter since SSA < 0.5. However, for the larger particles, the more 30
light they absorb, the more light they scatter. Thus, the SSA stays constant for larger particles. The bulk value of SSA is 0.215, which is close to the value of the BC particles with diameters from 300nm to 500nm. 3.1.2 Single Scattering Albedo of humid soot Figure 12. Single Scattering Albedo in different sizes of humid BC particles with 42% RH Figure 12 shows the SSA of BC particles that received no treatment upstream. The relative humidity under these laboratory conditions is around 42%. Compare with the dry BC particles, the SSA values of humid BC 31
particles have decreased. Especially for the BC particles with smaller diameters, such as 100nm and 150nm, the SSA decreased 26% and 11% respectively. For the particles with diameters from 300nm to 600nm, the SSA values stay constant at 0.21. However, the bulk value for non-sizeselected BC particles in an ambient relative humidity is 0.185, which is decreased 14% in comparison to dry particles. The t-test is conducted to show the differences of SSA between dry particles and humid particles. The P-value is 0.57, which means there is no significant difference. Thus, the increase in water vapor from 21% RH to 42% RH does not significantly influence the SSA of BC particles with different sizes between these two laboratory conditions. However, there is an extremely statistically significant difference in bulk values when comparing dry particles and humid particles at a given selected size (P-value is less than 0.0001). 32
3.1.3 Single Scattering Albedo of wet soot Figure 13. Single Scattering Albedo in different size of wet BC particles with 80% RH In Figure 13, the single scattering albedo of BC particles is measured under 80% relative humidity conditions to observe the influence of increased water vapor. Compared with the dry BC particles, the SSA of wet BC particles has decreased. However, there isn't much difference compared to the SSA under 42% relative humidity conditions. The SSA values of the small size particles are the same as the humid small size BC particles, which range from 100nm 33
to 200nm. For the particles of diameters ranging from 300nm to 650nm, the SSA values are roughly constant at 0.21. The standard deviation for all the bulk value is relatively large, implying high variability in the aerosol chamber under these conditions. For instance, the SSA of particles with 200nm diameter varies from 0.16 to 0.20. The variability of SSA in specific sizes might be related with the increase of water vapor in the sample inlet. 3.1.4 Single Scattering Albedo of heated soot Figure 14. Single Scattering Albedo in different size of heated BC particles with 20.5% RH 34
The SSA of BC particles under the heating conditions to obtain relatively pure BC is shown in Figure 14. The results show that, when comparing with the dry and wet conditions, the SSA values increase for all sizes. Especially for particles with diameter ranging from 100nm to 300nm, the SSA is increased. The relatively larger particles with diameter ranging from 400nm to 650nm show a very consistent albedo at around 0.25. Based on a previous study, the mass-absorption cross-section (MAC) of BC is 7.5 m 2 g -1 (Bond and Bergstrom, 2006), and the mass-scattering cross-section (MSC) of BC is 2.5 m 2 g -1. The SSA of BC can be calculated based on mass-absorption cross-section and mass-scattering cross-section (assuming no particle size or light wavelength dependence). SSA = MSC C!" MAC C!" + MSC C!" (10) The black carbon concentration (C BC ) can be cancelled out and the calculated SSA is 0.25. The observed SSA for heated particles is consistent with calculations using optical properties from prior work. This shows that the heating treatment removed most of the coating materials, and left relatively pure BC particles. The bulk value for heating BC particles 35
is 0.26, where the standard deviation is particularly small. This shows the SSA of dry relative pure BC particles in all sizes is constant. 3.1.5 Comparison of different treatments In order to compare the SSA of particles with four different treatments, we plotted the experiments data together as shown in Figure 15. The SSA of BC particles with different treatments are, in descending order, heated particles, dry particles, humid particles, and wet particles. However, when intercomparing the SSA of particles under all treatments using a t-test, there are no significant differences. Figure 15. Single scattering albedo of particles under different treatment 36
The majority of the variability in these experiments was observed when comparing different selected-sizes under the same treatment. We conducted a t-test between SSA of selected-sizes for each different condition and the result is shown in Table 2. The P-value smaller than 0.05 shows there is a significant difference between the SSA of selected-size and SSA of the BC particles with 650 nm. All the SSA of BC particles with the 100 nm, 150 nm and 200 nm diameters have extremely significant difference compared with SSA of the BC particles with 650 nm diameter. Based on the experimental results, the particle sizes are more important than relative humidity when considering SSA. Table 2 T-test result of different selected-sizes SSA under the same treatment Dp (nm) P-value of Dry particles P-value of Humid particles P-value of Wet particles P-value of Heated particles 100 <.0001 <.0001 <.0001 <.0001 150 <.0001 <.0001 <.0001 <.0001 200 <.0001 <.0001 0.00077 <.0001 300 0.00047 <.0001 0.06579 0.00808 400 0.00844 0.00436 0.95363 0.38942 500 0.00401 0.00265 0.11823 0.23641 600 0.66465 0.01242 0.01340 0.08835 650 1.00000 1.00000 1.00000 1.00000 37
3.2 Single Scattering Albedo Modeling The SSA of BC particles can also be calculated by using Mie scattering theory when the BC particles are assumed to be spherical: SSA = Q!"#$ Q!"# (11) This is a more sophisticated approach to calculate SSA than Eq 10. The Matlab code is provided in Appendix D. Figure 16 Single Scattering Albedo in different size of BC particles 38
Figure 16 shows the comparison between the SSA calculated with Mie theory and the SSA of heated BC particles measurements, which assume the SSA is for relative pure BC. Comparing the theory and the measurements, for the BC particles with small diameters ranging from 100nm to 200nm, the experimental results are a little higher than the model predicted. However, for the larger size BC particles with diameters ranging from 300nm to 650nm, the calculated SSA values are higher than the measurements, which is a 44% increase compared to experimental results. There could be various causes for this difference. For instance, it is difficult to obtain pure BC particles. During the heating treatment most coating materials surrounding the BC particles are likely removed, but there are likely a few other chemical compounds remaining as coating materials on the BC particles that have very low volatility, such as organic compounds with a boiling point > 160. Additionally, Mie theory assumes all particles are spherical, but in reality the morphology of particles are different (e.g. fractal aggregate comprised of small spherules (Sorensen, 2001)). The optical behavior of the particles with complex morphologies is imperfectly predicted by Mie theory and bias is introduced in absorptivity (Saleh et al., 2016). This study does not expect all the particles are special, and morphology of the BC particles was not verified. 39
Chapter 4 Absorption Enhancement 4.1 Absorption Enhancement BC aerosols do not exist as pure BC in the atmosphere; instead they interact with other chemical compounds and gain the coating materials. The previous study demonstrated that the coating materials of BC aerosols are mainly sulfate, nitrate, ammonium and organic carbon (Moffet and Prather, 2009). The absorption enhancement happens after the BC aerosols mix with non- BC aerosol components in the atmosphere (Bond et al., 2013; Schulz et al., 2006). These non-absorbing coatings of BC act as a lens, and focuses more light onto the core potentially causing the absorption enhancement of BC aerosols (Bond et al., 2006; Jacobson, 2001). The lensing effect has been shown to increase the absorption of single BC particle by 50%-100% based on the theory (Bond et al., 2006). There have been pioneering studies that have attempted to quantify absorption enhancement due to lensing effects. Jacobson (2001) used the theory to estimate the absorption enhancement of BC aerosols in the atmosphere and the results show an enhancement factor of 2. However, there 40
is another study that suggests the absorption enhancement of BC aerosols in North America is negligible (Cappa et al., 2012). A recent study observed an absorption enhancement factor of 1.4 in ambient BC aerosols in U.K (Liu et al., 2015). The discrepancies between the studies imply that the absorption enhancement of BC varies due to different regional conditions, coating materials, coating thickness, and lensing effects. 4.2 Absorption Enhancement Modeling In order to address the issue of the absorption enhancement of BC aerosols, we present a series of calculations using a core-shell Mie theory model (Bohren and Huffman, 1983). The core is assumed to be BC, while the coating is some material that is non-absorbing at 870nm. The core diameter is defined as the diameter of the BC, while the coating diameter is the diameter of the entire particle (including the BC core and the coating materials). We investigate the impact of coating materials on the absorption enhancement factor (E Abs ), which is defined as a ratio of the absorption coefficient of coated BC aerosol (β abs, c ) to an equivalent uncoated BC aerosol (β abs, unc ): 41
E!"# = β!"#,! β!"#,!"# (12) To model the absorption enhancement impact of BC aerosol, we follow the study of Cappa et al. (2012). We assume the refractive index for BC to be n = 1.88 + 0.8i and for the non-bc material to be n = 1.5 + 10-4 i at 870 nm wavelength. The refractive index used in this model has been tested and compared with other referred refractive indices, and there is not much difference between the results. We assumed a range of coating thickness as model inputs since this cannot be measured with our experimental setup, thus the inferred diameter of the core will be the diameter of the entire particle subtracted by the coating thickness. Additionally, it is has been noted that the use of Mie theory assumes the particles are spherical. Some studies demonstrate that as the coating mass increases, the coated particles collapse and become spherical particles (Lewis et al., 2009; Ghazi and Olfert, 2012). However, the BC particles are not always spherical and there is sufficient evidence that BC particles usually fractal when emitted from efficient combustion, and it can become more compact and the overall particle spherical when coated in other inorganic and organic material (Alexander et al., 2008; Lewis et al., 2009). 42