Advanced micro- and nanoscale characterization techniques for carbonaceous aerosols

Advanced micro- and nanoscale characterization techniques for carbonaceous aerosols

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Shahadev Rabha1 and Binoy K. Saikia2 1 Polymer Petroleum and Coal Chemistry Group, Materials Science and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat, India, 2Academy of Scientific and Innovative Research, CSIR-NEIST Campus, Jorhat, India

18.1

Introduction

It is attributed that atmospheric aerosol is the second largest climate forcing agent after carbon dioxide and causes various health problems including cardiovascular and pulmonary diseases depicting about 7.6% of the total global deaths in 2015 [1,2]. Atmospheric aerosol with diverse physical and chemical properties is a great challenge in atmospheric science because of its ability to cause a negative impact on human health and the global climate. Furthermore, the level of scientific understanding about their atmospheric source and sinks and physicochemical properties such as chemical constituents, surface chemistry, optical properties, cloud-forming properties, atmospheric life, and mixing condition is still very low, regardless of having tremendous scientific attention during the last 15 20 years (Fig. 18.1). Major scantiness in this area can be endorsed to the carbonaceous aerosol (CA) fraction that contributes about 20% 50% of the total atmospheric aerosol mass, but their formation and transformation processes, radiative forcing, health impact, and numbers of organic species are yet to be fully explained and identified due to the insufficient analytical facility and less number of research in this specific area [4,5]. However, the scientific endeavors are increasing to tackle the challenge with the recent development of modern analytical instruments and methodologies [6 14]. This chapter focuses on the recent advances in microscopic and spectroscopic methods for micro- and nanoscale characterization of atmospheric CA.

18.1.1 Carbonaceous aerosol and their source Atmospheric aerosol, also known as particulate matter (PM), is a suspension of fine solid particles or liquid droplets in the air. They may originate from both natural sources such as sea salt, desert dust, volcanic eruption, and forest fire and anthropogenic sources such as fossil fuels and biomass burning. Atmospheric aerosols are generally classified based on their aerodynamic sizes such as (1) PM10 that are inhalable coarse particles with aerodynamic diameter between 2.5 and 10 μm, (2) Handbook of Nanomaterials in Analytical Chemistry. DOI: https://doi.org/10.1016/B978-0-12-816699-4.00018-9 Copyright © 2020 Elsevier Inc. All rights reserved.

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Figure 18.1 Radiative forcing by various atmospheric compositions (global mean for 2000, relative to 1750). Carbonaceous aerosol such as black carbon and organic carbon shows both direct cooling and warming and indirect cooling effects. Source: Reproduced from J.E. Penner, M. Andreae, H. Annegarn, L. Barrie, J. Feichter, D. Hegg, et al., Chapter 5—Aerosols, their direct and indirect effects. Climate Change 2001: the scientific basis, in: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, USA, 2001 [3].

PM2.5 that are fine particles with aerodynamic diameter of 2.5 μm or less, and (3) ultrafine or nanoparticles with diameter of 100 nm or less. They are also named on the basis of their chemical composition such as sulfate aerosol that contains sulfate, nitrate aerosol that contains nitrate, and CA that contains carbon. CA is an unwanted by-product of incomplete combustion of fossil and biomass-based fuels comprising various forms of carbon such as black carbon (BC), elemental carbon (EC), and organic carbon (OC). Depending upon their source of emission, the total carbon content varies from 45% to 90% of mass concentration with particle size range 15 600 nm [15]. CA makes up the major but most unpredictable portion of the atmospheric aerosols that are mostly fine particles, less than 1 μm in aerodynamic diameter [16]. They do not well mixed in the atmosphere because of their particulate form but remain suspended in the air until they settle down or washed out by rain or contribute to cloud formation. They comprise a range of carbonaceous materials from char to highly graphitized BC, and a various complex mixture of OC containing carbon carbon bonds is produced from incomplete burning of biomass and fossil fuels, and atmospheric oxidation of biogenic and anthropogenic volatile organic compounds. It was estimated that, in the year 2000, the global emission of BC from fossil fuel and biomass burning was about 6 8 and 6 9 Tg year21, respectively, and that of OC was about 10 30 and 45 80 Tg year21, respectively [3,17]. Streets et al.

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[16] represented a model-based estimate of future emissions of CAs determined by four intergovernmental panel on climate change (IPCC) scenarios A1B, A2, B1, and B2, out to 2030 and 2050 incorporating the fuel use and technology development (Table 18.1). Fig. 18.2A and B shows the global annual average source distribution for anthropogenic BC and OC. The global atmospheric concentration of aerosols and their source location can be obtained through satellite that measures the aerosol optical depth (AOD). AOD is a measure of extinction of solar radiation due to scattering and absorption by aerosols. An AOD of less than 0.1 specifies a clear sky, whereas a value of 1 specifies the very hazy sky due to the presence of dense aerosols. Fig. 18.2C E shows the global average AOD of CA, BC, and organic matter, respectively. Fig. 18.2F indicates that aerosol is mainly concentrated in the tropics, where biomass burning is the major emission source of CA [19].

18.1.2 Importance of nanoscale characterization It is well established that CA plays a major role in global climate forcing [1,8,14]. Fig. 18.3 illustrates a general model of direct and indirect effects of CA on climate. The health problems due to PM are directly linked to the particles size, composition, and concentration. Fig. 18.4 shows how different size fraction of aerosol has a different level of impacts on human health. Knowledge about some of the key aerosol properties has been greatly improved in laboratory and field experiments using recently advanced instrumentation, but how much further information need to acquire to develop accurate and predictive models of their impacts on climate and human health is more important. For example, it is reported that aerosols are associated with cardiovascular diseases such as atherosclerosis, ischemic heart disease, and chronic obstructive pulmonary disease; however, their possible molecular mechanism still unrevealed [20,21]. Also, CAs act as condensation nuclei but what chemical interactions affect the ability of the aerosol particle to nucleate cloud is yet to describe [22]. Generally, aerosol particles are characterized in two size ranges—PM10 (particle , 10 μm) and PM2.5 (particle , 2.5 μm); however, particles smaller than PM2.5 have more peculiar characteristics and are more harmful to the human health. Nanoscale characterization may provide a wider range of information to describe the physical and chemical properties of the CA particle and their role in the atmospheric chemistry.

18.2

Analytical characterization techniques

Although atmospheric aerosol is not a new subject in the current world, with a large number of researches have been performed or going on, there is a lot of uncertainties in the area that encourage the development of methods and techniques for their measurement. Numerous analytical techniques have been developed in recent years that facilitate nanoscale, physicochemical characterization of various engineered nanomaterials [23,24], and also the aerosol particles in the laboratory as well as

Table 18.1 Emission factors (g kg21) for the major black carbon (BC) source types in 1996, 2030, and 2050. Sector

Fuel

Combustor type

Sharea (%)

1996

2030 A1B

b

b

2050 A2

B1 b

B2 b

A1B b

b

A2

B1 b

B2

N/A

b

N/A

N/Ab

Biomass burning Biomass burning Residential

Grassland

N/A

21.2

N/A

N/A

N/A

N/A

N/A

N/A

Tropical forest

N/A

12.4

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

Wood

6.2

3.9

3.9

3.9

3.9

3.9

3.9

3.9

3.9

3.9

Residential

Agricultural waste Crop residues

Traditional cookstove All

4.8

6.5

6.5

5.7

3.3

3.3

6.5

4.8

3.3

3.3

N/A

4.0

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

Superemitting vehicles Vehicles/Opacity regs. Off-road equipment Uncontrolled

3.9

12.0

12.0

12.0

12.0

12.0

12.0

12.0

12.0

12.0

3.3

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.5

3.2

5.5

3.1

3.2

3.1

3.1

3.0

3.0

3.0

3.0

3.1

20.0

20.0

20.0

20.0

20.0

20.0

20.0

20.0

20.0

N/A

2.9

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

N/Ab

Brick kiln All Vehicles/Euro regs. Tractors Open fire

2.6 2.6 2.5

10.0 3.7 1.5

10.0 1.9 0.2

10.0 3.3 0.2

10.0 1.9 0.2

10.0 1.9 0.2

10.0 1.9 0.2

10.0 2.7 0.2

10.0 1.9 0.2

10.0 1.9 0.2

2.5 2.2

4.0 7.7

2.1 7.7

2.1 7.7

2.1 7.7

2.1 7.7

2.0 7.7

2.0 7.7

2.0 7.7

2.0 7.7

Biomass burning Transport

Diesel

Transport

Diesel

Industry

Diesel

Industry Biomass burning Industry Residential Transport

Cooking process Extratropical forest Coal Dung cake Diesel

Transport Residential

Diesel Coal

Residential

Coal

Industry Residential Transport

Cooking process Wood Heavy fuel oil

Industry

Diesel

a

Traditional cookstove Controlled

2.1

7.7

6.2

6.2

6.2

6.2

4.5

4.5

4.5

4.5

1.8

5.8

2.3

2.7

2.0

2.4

1.8

1.8

1.7

1.8

Heating stove International shipping Off-road superemitter

1.7 1.5

15.0 1.8

8.8 1.2

9.5 1.2

7.7 1.2

8.8 1.2

7.6 1.0

8.0 1.0

7.5 1.0

7.6 1.0

1.3

17.0

17.0

17.0

17.0

17.0

17.0

17.0

17.0

17.0

Percentage contribution of this sector/fuel/technology combination to total BC emissions in the base-year 1996 inventory. For biomass burning, BC and organic carbon emission factors are used directly from the work of Andreae and Merlet [18], instead of being derived from PM emission factors, and are constant over time. Source: Reproduced from D.G. Streets, T.C. Bond, T. Lee, C. Jang, On the future of carbonaceous aerosol emissions, J. Geophys. Res. 109 (2004) (D24212) 1 19. doi:10.1029/2004JD004902.

b

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Figure 18.2 AOD of CAs. Panels (A) and (B) show the annual average source strength in kg km22 h21 for anthropogenic sources of BC and OM, respectively. (C), (D), and (E) depict the AOD of CA, BC, and OM, respectively. (F) MODIS AOD of February 2018. AOD, Aerosol optical depth; CA, carbonaceous aerosol; BC, Black carbon; OM, organic matter. Source: (A and B) Reproduced from J.E. Penner, M. Andreae, H. Annegarn, L. Barrie, J. Feichter, D. Hegg, et al., Chapter 5—Aerosols, their direct and indirect effects. Climate Change 2001: the scientific basis, in: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, USA, 2001; (C E) Reproduced from C.E. Chung, V. Ramanathan, D. Decremer, Observationally constrained estimates of carbonaceous aerosol radiative forcing, PNAS 109 (29) (2012) 11624 11629. doi:10.1073/pnas.1203707109; (F) Retrieved from NASA Earth Observatory.

field studies. Microscopy and spectroscopy are the two important techniques used by researchers to study CA, which are broadly discussed in the following sections.

18.2.1 Microscopic techniques Microscopy, especially electron microscopy, either in its transmission or scanning mode with nanometer resolution, is a promising technique for morphological characterization of aerosol particle [25]. Electron microscopy with high spatial

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Figure 18.3 Schematic diagram of the direct and indirect climate effect of CA. BC causes direct heating by absorbing incoming solar radiation and outgoing IR radiation, and OC causes direct cooling by scattering incoming radiation. Indirectly, CA acts as CCN or IN affecting the cloud formation processes. BC, Black carbon; CA, carbonaceous aerosol; CCN, cloud condensation nuclei; IR, infrared; IN, ice nuclei; OC, organic carbon. Source: Modified from http://www.isac.cnr.it/cimone/aerosol_properties (accessed 25.09.18).

resolution (,1 5 nm) allows more detail observation and detection of smaller particles (20 30 nm) and is capable of identifying three-dimensional structures. The use of electron-beam techniques for analysis of CA has largely increased in the last 10 15 years due to their improvement in revealing chemical information of single particles [26]. Particularly, the coupling of spectroscopy with microscopy has facilitated a more detailed understanding of physicochemical mixing state of aerosol. However, first, its high vacuum environment is supposed to cause loss of semivolatile components and second, the high-energy electrons can cause damage to soft materials such as OC in particles. Some useful techniques for studying CA are described in the following sections.

18.2.1.1 Scanning electron microscopy with energy-dispersive X-ray spectroscopy Scanning electron microscopy (SEM) technique has been used for last more than 40 years to study the atmospheric aerosol, initially the coarse particles ( . 2.5 μm) and

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Figure 18.4 Different size fractions of aerosols (PM) and their various human health effects. Smaller particles have more penetration capacity deep into the cardiovascular system leading to cardiovascular diseases. PM, Particulate matter.

recently the fine (,2.5 μm) and even ultrafine particles (0.1 μm) [26]. SEM scans samples with a focused high-energy electron beam and delivers images with highresolution surface information. The schematic diagram of an SEM is illustrated in Fig. 18.5. The electron source is thermoelectron emitted from a filament made of a thin tungsten wire (0.1 mm) by heating the filament at high temperature (about 2800K). The high-energy electrons upon interaction with sample materials generate a variety of signals such as secondary electron (SE), backscattered electron (BSE), and X-rays revealing information about the external morphology and chemical composition of the materials in the sample. SE produces an image with sharpness and depth of focus resulting in a three-dimensional view. Image due to BSE depends on the number of BSEs produced due to the interaction between the electron beam and the sample, which depends on the atomic number (Z) of the materials in the sample and illustrates contrasting brightness [27]. SEM requires microscopically smooth and conductive materials such as polycarbonate filters, because the high accelerating voltage (5 25 kV) that interacts with the sample may build up charge from the electron bombardment if the material is nonconducting and cannot dissipate, and thus degrading image quality. SEM uses a BSE detector and SE detector to produce an image; however, the addition of another detector, the high-angle annular dark field detector for transmitted electron, improves the detection of submicron particles far better. Scanning transmission electron microscopy (TEM) (STEM) requires a

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Figure 18.5 Schematic diagrams of SEM and TEM with their core components. SEM, Scanning electron microscopy; TEM, transmission electron microscopy. Source: Reproduced from B.J. Inkson, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) for Materials Characterization, Woodhead Publishing, 2016 (Chapter 2). https://doi.org/10.1016/B978-0-08-100040-3.00002-X.

thin electron transparent sample that is generally prepared by using TEM grid. STEM image also depends on the atomic composition of the materials in the sample (Z-contrast image) and thickness, giving bright-field image and compositional distinctions with higher spatial resolution up to 0.2 nm at 200 kV accelerating voltage [28]. Field-emission scanning electron microscope (FESEM) is another advanced SEM technique with ultrahigh-resolution imaging (,2 nm spatial resolution) at a low accelerating voltage (0.5 30 kV) and high vacuum (B10 6 Pa). The electron emission is caused by field emitter gun by a strong electric field from the surface of a thin tungsten wire (10 100 nm) that is about 1000 times smaller than that of traditional SEM with a thermal gun. FESEM is suitable for imaging surface sensitive, nonconductive, and carbonaceous nanoparticles where high-energy beam is not always required [29]. These SEM techniques are functional for determining the size distribution and composition of CA [30]. CA from fossil fuel and biomass combustion that is revealed in various SEM analyses, such as BC or soot particles, is in the submicrometer to nanometer size range [31 33]. Fig. 18.6A shows the soot particle with irregular shapes forming chain-like agglomeration revealed in high-resolution FESEM analysis. Energy-dispersive (ED) X-ray (EDX) spectroscopy is generally coupled with SEMs, which allows chemical characterization of the sample. It measures the X-rays emitted from the specific element when core electrons are ejected from atoms of the materials in the sample and from higher energy to lower energy orbital. EDX provides information about the elemental composition of the material in the specific

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Figure 18.6 Electron and atomic force microscopic images of soot aggregates. (A) FESEM image depicts different shape and size distribution. (B) TEM image showing chain-like structure. (C) High-resolution TEM image of soot particle illustrates onion-like curved, graphitic structures. (D) Topographic AFM image displays the surface roughness of soot aggregates. AFM, Atomic force microscopy; FESEM, field-emission scanning electron microscope; TEM, transmission electron microscopy. Source: (A C) Reproduced from J. Li, M. Posfai, P.V. Hobbs, P.R. Buseck, Individual aerosol particles from biomass burning in southern Africa: 2. Compositions and aging of inorganic particles, J. Geophys. Res. 108 (D13) (2003) 201 212. doi:10.1029/2002JD002310 [31]; (D) Reproduced from Y. Shi, Y. Ji, H. Sun, F. Hui, J. Hu, Y. Wu, et al., Nanoscale characterization of PM2.5 airborne pollutants reveals high adhesiveness and aggregation capability of soot particles, Sci. Rep. 5 (2015) 11232. doi:10.1038/srep11232 [34].

focused point, but information such as oxidation state, covalent bonding, and elements that are lighter than sodium cannot be revealed from EDX [31,35]. Another important successor of SEM is an environmental scanning electron microscope (ESEM) that allows imaging of wet and insulating samples without prior sample preparation. ESEM maintains a gaseous environment instead of a high vacuum, which allows imaging of the hydrated sample in their native environment, and also the charge build-up by the incident electron beam is dissipated by the gas [36]. ESEM is suitable for determination of hygroscopic behavior, cloud, and ice nucleation properties of the aerosol single particle and other properties that are not suitable in high vacuum environment [37 39].

18.2.1.2 Transmission electron microscopy with energy-dispersive X-ray spectroscopy TEM is another advanced and highly sophisticated analytical technique effective for characterization of CA, which has been using in aerosol research as long as

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since 40 45 years, but increased in use from the late 1990s and early 2000s [26]. In TEM, high-energy electron beam (150 300 kV) is transmitted through ultrathin (,100 nm) sample at high vacuum (1024 1029 Pa) environment to form an image. A sophisticated system of electromagnetic lenses is used to focus the scattered electrons into an image or a diffraction pattern depending on the mode of operation. Fig. 18.5 shows the schematic diagram of a TEM. It has a high spatial resolution less than 0.1 nm, which allows imaging of smaller particles that are not accessible by SEM [40]. Though, high-resolution TEM (HRTEM) has a resolution of around 0.05 nm that allows determining of even individual atoms of a crystal and its defects [41]. HRTEM uses both the scattered and transmitted electrons to produce an interference image. It is a powerful technique to study the properties of materials on the atomic scale, such as CA nanoparticles. The disadvantage of TEM/HRTEM is the high vacuum and high-energy electron beam that can more easily damage sample and thus limiting its universal applicability. However, to avoid this limit, environmental cells have been added in the recent past for environmental TEM, which allow the analysis of a sample at around ambient pressures and since then it received increasing attention from biological as well as materials scientists [42]. In TEM, chemical characterization can be achieved using EDX or electron energy loss spectroscopy (EELS). EELS uses inelastically scattered electrons, which can give more detail chemical information than the X-rays in EDX. TEM/HRTEM analysis of CA by various researchers has revealed lots of information about their morphological and chemical characteristics. TEM image easily distinguishes soot particles from their unique morphology (Fig. 18.6B). HRTEM image of soot spheres shows a discontinuous onion-like structure of graphitic layer (Fig. 18.6C) [43 46]. It is also revealed that soot also contains aggregates of various carbon nanocrystalline forms such as multiwalled, concentric tubes, shells, spheres, and other structures [9]. Irregular geometry and complex microstructure of soot aggregates may provide active sites for chemical species and hygroscopic properties [45]. HRTEM EELS shows the chemical heterogeneity of soot aggregates [31,46].

18.2.1.3 Atomic force microscopy Atomic force microscopy (AFM) is a versatile and very high-resolution scanning probe microscopy technique suitable for studying materials sample at nanoscale resolution. It provides various types of surface measurements with three-dimensional topography at atomic resolution. An AFM consists of a cantilever with a sharp tip (probe) of the radius of curvature on the order of nanometers. When the probe is brought into the proximity of a sample surface, forces between the probe and sample lead to a deflection of the cantilever. The force imposed by the sample on the probe is used to form a three-dimensional tomographic image of a sample surface. AFM can also measure the mechanical properties of the sample such as adhesive force and stiffness by measuring the forces between the probe and the sample as a function of their mutual separation. The advantage of the AFM over electron microscopy techniques is that it is executed under ambient environment, so there is

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no possibility of changes of sample property by an electron beam or vacuum system, but the lack of chemical information limits its widespread uses. However, the addition of infrared (IR) spectroscopy to AFM can give chemical information to the detailed physical properties [47]. The AFM technique has been used for the characterization of particle morphology, topography, surface tension, hygroscopicity, and mechanical properties of atmospheric aerosol particles since the late 1990s [34,48 50]. Fig. 18.6D is an AFM image that shows the topography with the high surface roughness of soot aggregate.

18.2.2 Spectroscopy techniques For the last two decades, spectroscopy has been used as a powerful analytical technique for chemical characterization of atmospheric aerosol because of its detecting capability up to picogram quantities of materials for both the organic and inorganic compounds [51,52]. Moreover, the recent advances in this technique with improvements in the mass detection limit and spectral quality have made spectroscopy even more powerful tool for analyzing CA, and thus its application in the aerosol research remains significant. In addition, many of these spectroscopy techniques avoid the potential loss of water or semivolatile compounds as they can work at ambient pressure and relative humidity, unlike electron microscope that requires high vacuum condition. Spectroscopy is a broad field with many subdisciplines (e.g., electronic, vibrational) executed with specific spectroscopic techniques (e.g., electron beam, X-ray photon, IR photons, UV visible) and provides specific chemical information (e.g., elemental composition, oxidation state, functional group, surface activity). Some recent techniques that are used by various researchers for CA are discussed briefly in the following sections.

18.2.2.1 Raman spectroscopy Raman spectroscopy is a vibrational spectroscopic technique used to observe molecular vibrations and crystal structures. It provides a characteristic fingerprinting pattern by which substances can be identified. Unlike electron-beam techniques where electronic transitions are used, Raman spectroscopy probes molecular vibrations. It relies on the inelastic scattering or Raman scattering of the monochromatic light, usually from a lesser in the visible, near-IR or near-UV range. The laser light interacts with molecular vibrations, resulting in the shifting of the energy of photons up or down, which gives information about the vibrational mode in the system. Fig. 18.7 shows a schematic diagram of the different energy states involved in Raman spectroscopy technique. The main advantages of Raman spectroscopy that makes its widespread use are that it’s a noncontact and nondestructive analysis technique with high spatial resolution up to submicron scale. Both the organic and inorganic samples in various states such as gas, liquid, solution, solid, crystal, and the emulsion can be measured without any special sample preparation. Raman spectroscopy is a useful technique for characterization of atmospheric aerosol particles. It has high sensitivity and selectivity for chemical speciation [6].

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Figure 18.7 Schematic diagram showing the different energy states involved in Raman spectra: an electron is excited from the ground level and falls to the original ground level (Rayleigh scattering), excited from the ground level and falls to a vibrational level (Stokes Raman scattering) and excited from a vibrational level and falls to the ground level (antistokes Raman scattering).

Its uses in the field of aerosol research were started in the late 1970s, but not widely used until the past decade [53]. Raman spectroscopy has been used for investigating the processes where physicochemical mixing state is important such as heterogeneous reactivity, hygroscopicity, and ice nucleation [54 56]. Raman is sensitive to the structural order of carbon atoms that make it a suitable technique for characterization of CA particles [6,57,58]. It shows the graphite-like carbon structure of CA —soot particles (Fig. 18.8) and can derive information about their origin and evolution processes [6,59,60]. Another significance of Raman spectroscopy is the measurement of pH in individual particles, the study of which began very recently. Rindelaub et al. [61] used Raman microspectroscopy technique to determine the pH in individual particle and reported the potential of Raman for direct measurement of aerosol acidity. Although there are some disadvantages associated with Raman such as the particle size that is mostly limited to around 1 3 μm due to the diffraction limit and the autofluorescent emission from the particle itself on excitation with Raman laser, which can be avoided by photo-bleaching or baseline correction. However, the recent development of the technique such as surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy can probe individual submicron to nanoparticles with a diameter below 100 nm [62,63].

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Figure 18.8 Raman spectrum (λ0 5 632.8 nm) of soot particle with band deconvolution revealing graphitic band G. Source: Reproduced from T. Catelani, G. Pratesi, M. Zoppi, Raman characterization of ambient airborne soot and associated mineral phases, Aerosol Sci. Technol. 48 (2014) 13 21. doi:10.1080/02786826.2013.847270.

18.2.2.2 Fourier-transforms infrared spectroscopy Although Fourier-transform IR spectroscopy (FTIR) technique is limited to greater than 5 μm of particle size due to the diffraction limit, it has significance in aerosol studies for understanding the physical and chemical processes involved in heterogeneous reactions, functional group and organic compositions in aerosols, and source apportionment of aerosol particles [64,65]. FTIR measures the absorption of light in the IR region of the electromagnetic spectrum by the molecules, and the absorption relates to the bonds present in the molecules. The frequency ranges are measured as wave numbers usually more than the range 4000 600 cm21. The emission spectrums of the IR source without (background) and with the sample are measured and the ratio of which is directly related to the absorption spectrum of the sample. The presence of various chemical bonds and functional groups in the sample is identified from the resultant absorption spectrum. The advantage of FTIR is that it doesn’t require any special sample preparation, and the sample is not mounted in a vacuum, so both high and low vapor pressure materials can be examined. Recently, the use of attenuated total reflection FTIR (ATR FTIR) has increased because of its enhanced sensitivity and low interference from Teflon filter and the lower size limit is B1 μm of particles. ATR FTIR in combination with quantitative ED electron probe X-ray microanalysis (EPMA), which is also known as low-Z particle

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EPMA, can be used for single particle analysis [66 68]. ATR FTIR analysis of carbonaceous particle revealed the presence of various types of functional groups, such as carbonyl carbon (CQO), carbon carbon double bond (CQC), carboxylate ion (COO 2 ), organic nitrates (CONO2) or C O H bending, C O, ammonium ion (NH41 ), and sulfate (SO22 4 ) [68].

18.2.2.3 X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) is a powerful technique for the revelation of the surface chemical composition of atmospheric particles. XPS can identify surface elements and their bonding states. The kinetic energy of core shell electrons ejected from elements by monochromatic X-rays is resolved using an electron monochromator. The difference between the energy of ejected electron and incident X-ray (photon) is the electron-binding energy that is specific to the quantum shell energy level and hence the element. XPS has been using for the identification of surface chemical speciation of atmospheric particles since its inception [69 72]. Recently, it has been used to probe size-resolved surface composition, more detailed high-resolution spectra, and heterogeneous reactivity of complex aerosol particles [13,73,74]. Elemental compositions in CA particles may be identified in an XPS survey scan because it spans a broad range of ejected core shell electron energies. Surface chemistry and carbon nanostructure of CA may be quantified by deconvolution of high-resolution scans over the C1s region (Fig. 18.9). It provides identification of the carbon-bound, surface oxygen functional groups, such as hydroxyl, carbonyl, and carboxylic groups and estimates of the relative fractions of

Figure 18.9 XPS analysis of particulate sample (A) survey spectrum and (B) high-resolution C1s spectrum resolved into signals assigned as—(1) elemental; (2) aromatic; (3) aliphatic; (4) C O, C N, C S (5) R2CQO; (6) carbonate; (7) π π elemental; (8) π π aromatic; (9) ( CF2)n PTFE. PTFE, Polytetrafluoroethylene, XPS, X-ray photoelectron spectroscopy. Source: Reproduced from D. Atzei, M. Fantauzzi, A. Rossi, P. Fermo, A. Piazzalunga, G. Valli, et al. Surface chemical characterization of PM10 samples by XPS. Appl. Surf. Sci. 307 (2014) 120 128. doi:10.1016/j.apsusc.2014.03.178.

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sp2 versus sp3 carbon [13,75]. XPS technique has many advantages that are nondestructive and surface sensitive, with the chemical shift. Chemical shifts of the elements allow the qualitative analysis of the different elements while the quantitative analysis leads to the determination of elemental and species concentrations.

18.2.2.4 Time-of-flight secondary ion mass spectrometry Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) allows imaging with high surface sensitivity, with both elemental and molecular information and can be used to explore the surface chemistry of PM [76,77]. ToF-SIMS uses a focused, pulsed particle beam to remove molecules from the very outermost surface of the sample. Particles produced closer to the site of impact tend to be dissociated ions (positive or negative). Secondary particles produced farther from the impact site tend to be molecular compounds, usual fragments of much larger organic macromolecules. The particles are then accelerated into a flight path on their way toward a detector. ToF-SIMS can be performed in either static or dynamic mode. In the static mode the detection limit is in the order of 109 atom cm22, and the mass spectra will contain molecular information of compounds in a few outer monolayers of the surface [78]. In the dynamic mode a much higher ion dose is used, allowing researchers to measure the concentration of different chemical species as a function of depth in a sample. Surface imaging and depth profiling by ToF-SIMS offer both spatial distributions of chemical species on the surface of a sample and depth distribution information. Cheng et al. [79] used ToF-SIMS to differentiate among the surface chemical compositions of the coarse-mode (5.6 10 μm), accumulationmode (0.56 1 μm), and nucleation-mode (0.056 0.1 μm) particles and reported that inorganic salts were the major components on the surfaces of the coarse-mode particles, while long-chain saturated hydrocarbon ions occur in the accumulationmode particles suggesting an aliphatic hydrocarbon-dominated surface, and the nucleation-mode particles contains unsaturated hydrocarbons and aromatic hydrocarbons revealing the possible existence of polycyclic aromatic hydrocarbons or fullerene-like structures on the surfaces. The analysis of aerosols by ToF-SIMS technique can generate a large amount of chemical information, processing of which requires principal component analysis [80].

18.2.2.5 Cavity ringdown spectroscopy Cavity ringdown spectroscopy (CRDS) is a highly sensitive absorption spectroscopy technique with pulsed lasers, which measures absolute optical extinction by samples that scatter and absorb light. CRDS uses a monochromatic laser light source that is focused into a cavity created by the highly reflective mirrors, and the signal is sensed using light transmitted from the mirrors. The advantage of CRDS is the long path length created by the reflective mirrors that increase the sensitivity of the technique and the amount of time that the sample interacts with light. The applicability of CRDS in the measurement of aerosol properties was first demonstrated in 1998 [81]. It is increasingly being applied to study the absorption and scattering

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properties of atmospheric aerosols, the contribution of absorption to the total optical extinction coefficient, arising from the complex component of the aerosol refractive index [82 85]. CRDS and its successors have more potential in the study of aerosol optical properties and their spatial and temporal variation, and also it may have possible applications in the studies of visibility, climate forcing by aerosol, and the validation of aerosol retrieval schemes from satellite data [86].

18.2.2.6 Single Particle Soot Photometer Atmosphere BC particles cause adverse health effects and impact the climate, unfortunately, the accurate measurement of their properties and mass concentrations remains difficult. The Single Particle Soot Photometer (SP2) can help in improving this situation by measuring the mass of refractory BC in individual particles as well as its mixing state [87]. The SP2 technique with high sensitivity, fast response, and specificity to EC can directly quantify the BC in individual aerosol particles [88]. It can detect individual BC particles in the mass range of B3 300 fg (B0.15 0.7 μm volume equivalent diameter) [89]. It is used to quantify the size resolute mass concentration of BC from measurements of individual soot particles. It uses laserinduced incandescence and is based on the light-absorbing property of soot. The laser light is absorbed by the particle causing it to incandesce, which is detected by selected optical filters in two different wavelength regions (350 800 and 630 800 nm) and the intensity of the incandescence in these two windows is converted through a series of calculations to BC particle mass. Besides, BC mass determination SP2 can be used to study the soot particle coatings, hygroscopicity, morphology, and mixing state [90 94].

18.3

Summary

The level of scientific understanding as well as advanced level characterization of CAs is essential in order to evaluate their impacts on both human health and climate. Micro- and nanoscale characterizations by using advanced techniques including microscopy and spectroscopy play key role for determining the detailed physical chemical properties and mixing state of CAs in the atmosphere. This chapter discussed the recent micro- and nanolevel advances for characterization of atmospheric CA. This is observed that each technique has their capabilities to measure aerosol properties and can provide information for future research.

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