Microanalysis of Atmospheric Particles E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Serie: Geophysical Monograph Series
- Sprache: Englisch
Studying atmospheric particles from a microscopic perspective
Most of what is visible in the atmosphere—such as pollution, dust, haze, fog, and clouds—is due to micrometer- and nanometer-sized aerosol particles. It is important to understand the source, characteristics, and behavior of these small particles as they play a fundamental role in large-scale atmospheric processes.
Microanalysis of Atmospheric Particles: Techniques and Applications presents different microscopic techniques for studying aerosols and explores a range of applications in climate studies and air quality studies.
Volume highlights include:
- Overview of different techniques and applications
- In-depth descriptions of scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, Raman microspectroscopy, and atomic force microscopy
- Techniques for studying physical characteristics and chemical composition
- Methods to examine particle transformation
- Examples including soot, organic aerosols, ice crystals, and sea spray
- Applications for global and regional climate change and urban air quality
The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
LIST OF CONTRIBUTORS
PREFACE
Part I: Overview of Techniques and Applications
1 Analysis of Individual Atmospheric Particles: An Overview of Techniques
1.1. INTRODUCTION
1.2. ELECTRON MICROSCOPY
1.3. RAMAN SPECTROSCOPY
1.4. ATOMIC FORCE MICROSCOPY
1.5. SCANNING TRANSMISSION X‐RAY MICROSCOPY
1.6. CONCLUSION
AVAILABILITY STATEMENT
REFERENCES
2 Importance of Microanalysis in Air Quality Studies
2.1. INTRODUCTION OF AIR POLLUTION WORLDWIDE
2.2. IMPORTANCE OF INDIVIDUAL PARTICLE MEASUREMENTS IN POLLUTED AIR
2.3. IMPLICATIONS FOR HEALTH EVALUATION
2.4. ATMOSPHERIC IMPLICATIONS OF MICROANALYSIS IN AIR QUALITY STUDIES
REFERENCES
3 Importance of Microanalysis in Climate Studies of Atmospheric Aerosol Particles
3.1. INTRODUCTION
3.2. CLIMATE STUDIES USING MICROANALYSIS TECHNIQUES
REFERENCES
Part II: Microanalytical Techniques
4 Particle Population Analysis by Automated Scanning Electron Microscopy
4.1. INTRODUCTION
4.2. THE EVOLUTION OF CCSEM
4.3. EARLY APPLICATIONS OF CCSEM IN THE CHARACTERIZATION OF ATMOSPHERIC PARTICLES
4.4. THE IMPACT OF THE PERSONAL COMPUTER
4.5. PARTICLE CLASSIFICATION OF CCSEM DATA
4.6. PRECISION, ACCURACY, AND REPRODUCIBILITY OF CCSEM DATA
4.7. APPLICATIONS TO RECEPTOR MODELING
4.8. CCSEM ANALYSIS OF ATMOSPHERIC PARTICLES IN THE TWENTY‐FIRST CENTURY
4.9. CONCLUSIONS
ACKNOWLEDGMENTS
AVAILABILITY STATEMENT
REFERENCES
5 Characterization of Atmospheric Particles by Electron Energy Loss Spectrometry
5.1. INTRODUCTION
5.2. ELEMENTAL COMPOSITION BY TEM‐EELS
5.3. ELEMENTAL MAPS AND TOMOGRAPHY BY EF‐TEM
5.4. OXIDATION STATE AND BONDING INFORMATION BY EXPLORING NEAR‐EDGE FINE STRUCTURE
5.5. PARTICLE OPTICAL PROPERTIES DEDUCED FROM LOW‐LOSS SPECTROMETRY (PLASMON)
5.6. CONCLUSION
REFERENCES
6 Raman Microspectroscopy of Particles
6.1. INTRODUCTION
6.2. PRINCIPLES OF RAMAN SPECTROSCOPY
6.3. MICROANALYSIS AND IMAGING
6.4. AUTOMATION AND DATA PROCESSING USING STATISTICAL TOOLS
6.5. BEYOND THE DIFFRACTION LIMIT
6.6. COUPLING WITH OTHER SINGLE‐PARTICLE ANALYSIS TECHNIQUES
REFERENCES
7 Atomic Force Microscopy of Individual Particles
7.1. INTRODUCTION
7.2. MIXING STATE OF AEROSOLS
7.3. HYGROSCOPICITY OF AEROSOLS
7.4. VISCOSITY AND PHASE STATE OF AEROSOLS
7.5. SURFACE TENSION OF AEROSOLS
7.6. CONCLUSION
REFERENCES
Part III: Particle Types and Transformations
8 Transmission Electron Microscopy for Studying Atmospheric Soot Particles
8.1. INTRODUCTION
8.2. PRINCIPLES AND LIMITATIONS OF TEM
8.3. TEM TECHNIQUES
8.4. CONCLUSION
REFERENCES
9 Microscopy Methods for Organic Composition of Atmospheric Aerosol Particles
9.1. INTRODUCTION
9.2. SCANNING TRANSMISSION X‐RAY MICROSCOPY WITH NEAR‐EDGE X‐RAY MICROSCOPY (STXM‐NEXAFS)
9.3. SCANNING AND TRANSMISSION ELECTRON MICROSCOPY WITH ENERGY‐DISPERSIVE X‐RAY SPECTROSCOPY (SEM‐EDX, TEM‐EDX, STEM‐EDX)
9.4. RAMAN MICROSPECTROSCOPY (RMS)
9.5. OTHER TECHNIQUES
9.6. SUMMARY
ACKNOWLEDGMENTS
REFERENCES
10 Microanalytical Studies of Multiphase Reactions and Particle Aging
10.1. INTRODUCTION
10.2. SELECTED OFFLINE TECHNIQUES
10.3. IN SITU MICROANALYTICAL METHODS
10.4. IMAGING METHODS AND MODELING IN AEROSOL MULTIPHASE CHEMISTRY
10.5. OUTLOOK
ACKNOWLEDGMENTS
AUTHOR CONTRIBUTIONS
AVAILABILITY STATEMENT
REFERENCES
11 Microanalysis Techniques to Study Atmospheric Ice Nucleation and Ice Crystal Growth
11.1. INTRODUCTION
11.2. APPLICATION OF MICROANALYSIS TECHNIQUES TO STUDY ICE FORMATION
11.3. APPLICATION OF MICROANALYSIS TECHNIQUES TO STUDY ICE CRYSTAL GROWTH AND SUBLIMATION
11.4. EXPERIMENTAL REQUIREMENTS AND CHALLENGES
ACKNOWLEDGMENTS
AVAILABILITY STATEMENT
REFERENCES
12 Studying Aerosol Hygroscopicity Using Environmental Electron Microscopy
12.1. INTRODUCTION
12.2. ENVIRONMENTAL CELLS
12.3. APPLICATION TO MORE COMPLEX AEROSOL–CLOUD INTERACTIONS: ICE NUCLEATION
12.4. COMPLEMENTARY INSTRUMENTS AND TECHNIQUES FOR THE MICROSCOPIC INVESTIGATION OF AEROSOL–CLOUD INTERACTIONS
12.5. CONCLUSION
AVAILABILITY STATEMENT
REFERENCES
AUTHOR INDEX
SUBJECT INDEX
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Characteristics and Properties of the Various Techniques Associat...
Chapter 4
Table 4.1 Summary of Particle Type and Size Distribution Results from CCSEM...
Table 4.2 Comparison of CCSEM Particle Type Results for Diluted Stack, Down...
Table 4.3 Comparison of CCSEM Results Incorporating Shape Classifications f...
Table 4.4 CMB Source Apportionment Results Based on CCSEM Analysis of TEOM ...
Table 4.5 Comparison of CMB and PMF Receptor Model Results Both Applying the...
Chapter 6
Table 6.1 Characteristic Raman Frequencies for the Most‐Encountered Chemica...
Table 6.2 Laser Focal Distance (
h
in μm) into Transparent Solid Compounds w...
Table 6.3 Typical Compounds Identified in Samples Collected During Field Ca...
Table 6.4 SERS Wavenumbers of Inorganic and Organic Compounds of Atmospheri...
Chapter 9
Table 9.1 Organic Functional Groups and Corresponding Energies from STXM‐NE...
List of Illustrations
Chapter 1
Figure 1.1 Number of references in the atmospheric aerosol literature over f...
Figure 1.2 Signals generated when a high‐energy beam of electrons interacts ...
Figure 1.3 Monte Carlo simulation of the interaction volume for glass K412 (...
Figure 1.4 Percent of particles assigned to particle types determined by ope...
Figure 1.5 EDX spectra and SEM (ESEM) images of selected particles from biom...
Figure 1.6 Secondary electron image (a), EDX spectrum (b), and the indexed E...
Figure 1.7 ESEM images of KNO3 particles at the onset of deliquescence at 93...
Figure 1.8 Process of milling and imaging a mineral‐like dust particle (a) w...
Figure 1.9 Schematic showing the flooding of a specimen with different mass ...
Figure 1.10 Bright‐field images (left) and SAED patterns (right) for nanopar...
Figure 1.11 (a) Bright‐field TEM image, (b) SAED pattern, and (c) EDX spectr...
Figure 1.12 (a) Secondary electron and (b) EFTEM false‐color images of a sli...
Figure 1.13 Electron energy‐loss spectra and mathematical curvefits of an ir...
Figure 1.14 TEM images (a, c) of soot from Asian dust and diesel exhaust. Th...
Figure 1.15 Bright‐field TEM images of internally mixed urban particles (Mex...
Figure 1.16 Schematic of the three energy‐relaxation processes for a polariz...
Figure 1.17 (a) Measured Raman spectrum of a 1 : 6 water–glycerol solution....
Figure 1.18 (a) Optical levitation diagram. Ashkin et al. (1971) with permis...
Figure 1.19 (a) Optical tweezers experiment showing O
3
oxidation of droplets...
Figure 1.20 Raman spectra of soots from a graphite spark‐discharge generator...
Figure 1.21 TERS analyses of atmospheric particles <100 nm emitted from a si...
Figure 1.22 Water uptake by an initially dry particle containing NaCl and ma...
Figure 1.23 AFM images and Raman spectra of particles with ammonium sulfate ...
Figure 1.24 Schematic of conventional AFM‐IR instrumentation. IR laser for h...
Figure 1.25 NEXAFS spectra of a tar ball collected during the Yosemite Aeros...
Figure 1.26 NEXAFS spectra from marine organic particle types: (a) polysacch...
Chapter 2
Figure 2.1 Mortality from ambient air pollution in 2015.
Figure 2.2 TEM images of different types of individual particles emitted fro...
Figure 2.3 Heterogeneously internally mixed particles (internal) and externa...
Figure 2.4 TEM images of fresh and aged individual particles. (a) Fresh soot...
Figure 2.5 Typical transmission electron microscopy (TEM) images of organic ...
Figure 2.6 A schematic diagram of air pollutants trans‐regional transport (T...
Chapter 3
Figure 3.1 Direct and indirect aerosol effects and major feedback loops in t...
Figure 3.2 Typical secondary electron images of particles from different gro...
Figure 3.3 Two‐dimensional spectrally resolved maps of organic composition f...
Figure 3.4 ETEM images of an NaCl particle as the RH is increased from 0% to...
Figure 3.5 Raman spectra of six different sea‐spray aerosol particles in the...
Figure 3.6 (a) TEM image showing a bacterium held within a biofilm by a poro...
Chapter 4
Figure 4.1 Early version of a CCSEM system developed at U.S. Steel Research ...
Figure 4.2 Comparison of ambient particle concentrations based on the CCSEM ...
Figure 4.3 Maps of normalized concentration for (a) PM
10–2.5
, (b) Si/A...
Figure 4.4 Comparison of CCSEM results of source samples collected at a stee...
Figure 4.5 CCSEM user‐defined rules (a) for the sinter plant fugitive source...
Figure 4.6 CCSEM data management/review software showing some of the ways th...
Figure 4.7 Particle size distributions measured by CCSEM of NIST nanoparticl...
Chapter 5
Figure 5.1 Schematic view of an electron energy loss spectrum.
Figure 5.2 Terminology for the edges in the EELS spectrum due to core‐shell ...
Figure 5.3 Relative loss of nitrogen, oxygen, and sulfur evaluated from EELS...
Figure 5.4 Detailed analysis of organic droplet residual particle. (a) Annul...
Figure 5.5 Schematic view of the two different ways to experimentally captur...
Figure 5.6 TEM image and EELS maps. (Left) A tar ball particle with a bright...
Figure 5.7 Carbon K‐edge for (left) a Ticonderoga graphite spectrum with a l...
Figure 5.8 Dark‐field picture and EELS Fe spectrum images (Fe SPIM) of a typ...
Figure 5.9 The refractive index (n − ik) calculated from the plasmon peak fo...
Chapter 6
Figure 6.1 Raman band positions related to Raman anti‐Stokes, Rayleigh, and ...
Figure 6.2 Confocal RMS instrument in backscattering configuration. θ
S
, spec...
Figure 6.3 Optical pathways in a transparent sample with
n
refractive index....
Figure 6.4 3D reconstruction of the composition of an aerosol particle by co...
Chapter 7
Figure 7.1 (a) AFM 3D height and phase micrographs of 1 : 8 (M) glucose:NaCl...
Figure 7.2 (a) Histogram of AFM measured GF values of 6 : 1 (M) NaCl:nonanoi...
Figure 7.3 Examples of AFM force plots obtained on individual particles of (...
Figure 7.4 Phase state framework is used to interpret the measured RID and V...
Figure 7.5 Comparison of measured AR versus VRD and RID for multiple model S...
Figure 7.6 Illustrative example force plot obtained on a submicrometer liqui...
Figure 7.7 (a, c) 3D AFM micrographs of glucose and glutaric acid individual...
Figure 7.8 (a) AFM 3D micrographs of nascent sea‐spray aerosol at low (left)...
Chapter 8
Figure 8.1 Schematic ray path in the TEM mode. Various lenses and apertures ...
Figure 8.2 Signals generated by the interactions of electrons with a specime...
Figure 8.3 (a) Bright‐field TEM image, (b) elemental mapping image (carbon) ...
Figure 8.4 Electron detection systems in the STEM mode. BF, bright‐field STE...
Figure 8.5 Beam damage and removal of sulfate. (a) TEM image before the STEM...
Figure 8.6 High‐resolution TEM images of a soot particle. Graphitic structur...
Figure 8.7 (a) A selected area diffraction pattern, (b) an EDS spectrum, and...
Figure 8.8 (a) Three‐dimensional soot particle image with projected images a...
Chapter 9
Figure 9.1 Images, speciated maps of detectable regions, and representative ...
Figure 9.2 Top: Example images for each category; each panel corresponds to ...
Figure 9.3 Illustration of categorized particle types.
Figure 9.4 (a) Characteristic single‐particle carbon K‐edge near‐edge X‐ray ...
Figure 9.5 SEM image together with carbon, sulfur, and potassium line scans ...
Figure 9.6 Particle classification, analyzed particle number and percentage ...
Figure 9.7 TEM images (dark field) and elemental maps of common particle typ...
Figure 9.8 (a) Raman spectral map of an ammonium sulfate particle coated wit...
Figure 9.9 Marine‐derived organic compound types observed in individual wint...
Figure 9.10 Comparison of the chemical specificity and size resolution of RM...
Chapter 10
Figure 10.1 Illustration of multiphase chemistry between aerosol particles a...
Figure 10.2 Time‐dependent morphological and elemental changes of particles ...
Figure 10.3 In situ fluorescence lifetime imaging microscopy (FLIM) analysis...
Figure 10.4 Ozone barrier coefficient (γ/γ0) through diffusive (squalane, ro...
Figure 10.5 Depletion of shikimic acid over time. Normalized optical density...
Figure 10.6 Imaged photochemical reactions of iron(III)‐citrate (Fe
III
Cit) i...
Figure 10.7 A schematic diagram that illustrates the architecture of the art...
Chapter 11
Figure 11.1 Representation of the various possible atmospheric ice nucleatio...
Figure 11.2 The change in Gibbs free energy necessary for the formation of a...
Figure 11.3 Estimates of (a) the critical ice nucleus radius (r
crit
) and (b)...
Figure 11.4 Ice crystal habit diagram derived from laboratory and field camp...
Figure 11.5 Sequences that can lead to the formation of a freeze‐concentrate...
Figure 11.6 Ice crystal residuals (ICRs) collected during the Indirect and S...
Figure 11.7 Multimodal identification of ice‐nucleating particles (INPs) usi...
Figure 11.8 Ice‐nucleating particle (INP) identification and scanning transm...
Figure 11.9 Collocated optical microscopy with atomic force microscopy (AFM)...
Figure 11.10 Ice formation on the feldspar surface at −29.5 °C was observed ...
Figure 11.11 Raman microscope application to study ice nucleation. (a) An en...
Figure 11.12 Immersion freezing from a synthetic sea‐salt (SSS) particle as ...
Figure 11.13 Ice nucleation cell implementation in an environmental scanning...
Figure 11.14 Deposition ice nucleation and immersion freezing by kaolinite c...
Figure 11.15 Application of an environmental scanning electron microscope (E...
Figure 11.16 (a) Schematic configuration of the ice nucleation X‐ray cell (I...
Figure 11.17 Observation of ice nucleation and spectroscopic identification ...
Figure 11.18 Conceptual mechanism to explain the preferential crystallograph...
Figure 11.19 Ice crystal morphology changes upon merging of isolated crystal...
Figure 11.20 Environmental scanning electron microscopy (ESEM) images of mes...
Figure 11.21 Captured cirrus ice crystals for cryo‐scanning electron microsc...
Figure 11.22 Application of standard INPs that have exactly the same surface...
Chapter 12
Figure 12.1 Environmental scanning electron microscopy images of NaCl partic...
Figure 12.2 Environmental transmission electron microscopy images of NaCl pa...
Figure 12.3 Ice nucleation on the face of a feldspar dust particle at differ...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
LIST OF CONTRIBUTORS
PREFACE
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AUTHOR INDEX
SUBJECT INDEX
WILEY END USER LICENSE AGREEMENT
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Microanalysis of Atmospheric Particles
Techniques and Applications
Joseph M. Conny
Peter R. Buseck
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LIST OF CONTRIBUTORS
Kouji AdachiDepartment of Atmosphere, Ocean, and Earth SystemModeling ResearchMeteorological Research InstituteTsukuba, Japan
Peter A. AlpertPSI Center for Energy and Environmental SciencesPaul Scherrer InstituteVilligen, Switzerland
Markus AmmannPSI Center for Energy and Environmental SciencesPaul Scherrer InstituteVilligen, Switzerland
Veronica BertaScripps Institution of OceanographyUniversity of California at San DiegoLa Jolla, CA, USA
Sébastien BonhommeauInstitute of Molecular SciencesUniversity of BordeauxCNRS, UMR 5255Talence, France
Gary S. CasuccioRJ Lee GroupPittsburgh, PA, USA
Joseph M. ConnyMaterials Measurement Science DivisionNational Institute of Standards and Technology (retd.)Gaithersburg, MD, USA
Pablo Corral ArroyoLaboratory for Physical ChemistrySwiss Federal Institute of Technology ZurichZurich, Switzerland
Karine DeboudtLaboratory of Physical Chemistry of the AtmosphereUniversity of the Littoral Opal CoastDunkirk, France
Arnaud DesmedtInstitute of Molecular SciencesUniversity of BordeauxCNRS, UMR 5255Talence, France
Evelyn FreneyLaboratory of Physical MeteorologyUMR 6016 CNRS/UBPUniversity of Clermont AuvergneAubière, France
Daniel A. KnopfSchool of Marine and Atmospheric SciencesStony Brook UniversityStony Brook, NY, USA
Hansol D. LeeDepartment of ChemistryUniversity of IowaIowa City, IA, USA
Weijun LiDepartment of Atmospheric SciencesZhejiang UniversityHangzhou, China
Lei LiuDepartment of Atmospheric SciencesZhejiang UniversityHangzhou, China
Lynn M. RussellScripps Institution of OceanographyUniversity of California at San DiegoLa Jolla, CA, USA
Sophie SobanskaInstitute of Molecular SciencesUniversity of BordeauxCNRS, UMR 5255Talence, France
David TalagaInstitute of Molecular SciencesUniversity of BordeauxCNRS, UMR 5255Talence, France
Alexei V. TivanskiDepartment of ChemistryUniversity of IowaIowa City, IA, USA
Cynthia H. TwohyNorthWest Research AssociatesRedmond, WA, USAandScripps Institution of OceanographyUniversity of California at San DiegoSan Diego, CA, USA
Liang XuDepartment of Atmospheric SciencesZhejiang UniversityHangzhou, China
PREFACE
Most of what is visible in the atmosphere—be it pollution, windblown dust, haze, fog, or clouds—arises from aerosol particles with sizes in the micrometer and nanometer range. These small atmospheric particles, in their solid or liquid states, play a fundamental role in air quality and climate change. Assessing air quality and how climate is changing requires understanding the behavior of both primary particles from terrestrial and anthropogenic sources and secondary particles formed within the atmosphere. This book presents the study of these atmospheric particles from a microscopic perspective.
It may seem a stretch of scientific faith to extrapolate from single particles at the microanalytical scale to a geophysical scale, such as the global atmosphere or a regional airmass. However, the detailed information derived from microscopic techniques can reveal how particles affect various physicochemical phenomena in the atmosphere. For example, together with particle shape and size, microscopy may be used to identify thin hydrophilic coatings on otherwise hydrophobic particles, which allow these particles to take up water and, therefore, act as cloud condensation nuclei. As this book shows, individual‐particle analyses offer unique insights into the behavior of the particulate atmosphere, whether the scientific goal is determining urban air quality, global climate change over time, climate change in specific regions, or cloud behavior with respect to climate change.
This book describes the most relevant microscopic techniques for studying atmospheric particles and presents specific applications of those techniques for understanding particle behavior. Static single‐particle (microscopic) techniques provide uniquely important information about particle origins and atmospheric processes. The methods are categorized as electron microscopies (scanning and transmission), Raman microspectroscopy, atomic force microscopy (AFM), and scanning transmission X‐ray microscopy (STXM).
The book's 12 chapters are divided into three parts. In Part I, “Overview of Techniques and Applications,” Chapters 1–3, respectively, provide general introductions to the techniques (“Analysis of Individual Atmospheric Particles: An Overview of Techniques”), their application to air quality studies (“Importance of Microanalysis in Air Quality Studies”), and their application to climate change studies (“Importance of Microanalysis in Climate Studies of Atmospheric Aerosol Particles”).
In Part II, “Microanalytical Techniques,” four chapters provide an in‐depth treatment of selected techniques. Chapter 4 (“Particle Population Analysis by Automated Scanning Electron Microscopy”) discusses the use of scanning electron microscopy (SEM) for determining the distribution of particle sizes, morphologies, and elements in particle populations. The technique, known as computer‐controlled SEM, automates the analysis of thousands of particles in a sample. Chapter 5 (“Characterization of Atmospheric Particles by Electron Energy Loss Spectroscopy”) shows how inelastic scattering of beam electrons in transmission electron microscopy (TEM) produces a spectrum of absorption edges, the result of electron energy loss spectroscopy (EELS), which elucidates chemical bonding within individual particles. Chapter 6 (“Raman Microspectroscopy of Particles”) demonstrates how laser excitation in Raman spectroscopy, whereby spectra exhibit vibrational states due to light‐scattering rather than light absorption, reveals chemical information about individual particles. Chapter 7 (“Atomic Force Microscopy of Individual Particles”) illustrates how the cantilever probe in AFM is used to determine particle morphology, mixing state, hygroscopicity, phase state, and surface tension. Sea spray aerosol particles serve as examples.
In Part III, “Particle Types and Transformations,” five chapters discuss specific types of particles and atmospheric processes involving particles. Chapter 8 (“Transmission Electron Microscopy for Studying Atmospheric Soot Particles”) shows how imaging and electron diffraction with TEM are used to examine the size, morphology, mixing state, fractal dimensionality, and crystallography of soot particles. In addition, techniques such as energy dispersive X‐ray spectroscopy (EDX) and EELS associated with TEM provide information on soot composition and chemical bonding. Chapter 9 (“Microscopy Methods for Organic Composition of Atmospheric Aerosol Particles”) describes how the organic chemistry of particles is mapped with STXM and near‐edge X‐ray absorption fine structure (NEXAFS), electron microscopy with EDX, and Raman microspectroscopy. Limitations of these techniques are discussed, as are emerging techniques for organic particles, such as tip‐enhanced and surface‐enhanced Raman spectroscopies, AFM with infrared spectroscopy, and optical photothermal infrared spectroscopy.
The remaining chapters in Part III discuss particle transformations. Chapter 10 (“Microanalytical Studies of Multiphase Reactions and Particle Aging”) focuses on the use of microscopic techniques to understand the chemical transformations, including aging, that particles undergo in ambient air with gases and photochemically. The chapter examines in‐situ studies of heterogeneous reactions with various techniques, particularly STXM‐NEXAFS. Chapter 11 (“Microanalysis Techniques to Study Atmospheric Ice Nucleation and Ice Crystal Growth”) illustrates how ice nucleation proceeds, primarily heterogeneously, with a nucleating particle as substrate. The chapter covers ice nucleation pathways and theory as well as the complexities of ice crystals during nucleation, further growth, and sublimation using various microscopic methods. Chapter 12 (“Studying Aerosol Hygroscopicity Using Environmental Electron Microscopy”) shows at the microscopic level how crystalline particles take up water to become droplets (deliquescence) during cloud formation. The chapter focuses primarily on electron microscopies with humidity‐controlled instrumentation that allow for observing particle hygroscopicity using techniques known as environmental SEM (ESEM) and environmental TEM (ETEM).
We thank everyone who contributed chapters and the reviewers who provided much helpful criticism. We are grateful to everyone who ensured the relevance and thoroughness of the scientific content. We thank Ritu Bose, Lesley Fenske, and Noel McGlinchey at Wiley for, respectively, proposing the topic, managing the project, and assisting with chapter submission. We also thank AGU's Publications Director, Jenny Lunn, for advice and encouragement, Geeta Persad of the University of Texas at Austin for her work as subject editor, and copyeditor Tiffany Taylor. In addition, we thank the National Institute of Standards and Technology (NIST) for support.
Joseph M. Conny
National Institute of Standards and Technology USA (retd.)
Peter R. Buseck
Arizona State University, USA
Part IOverview of Techniques and Applications
1Analysis of Individual Atmospheric Particles: An Overview of Techniques
Joseph M. Conny
Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, MD, USA
ABSTRACT
Investigations into atmospheric aerosols are key to understanding how chemical and physical processes within the atmosphere contribute to climate change, affect public health, and impact visibility. Determinations of atmospheric particulate matter at the microscopic, or single‐particle, level offer distinct advantages over determinations of particulate matter in bulk form. These advantages include chemical composition with respect to particle size, shape, and mixing state along with the physical properties of particles, particularly surface properties, as they affect the formation and lifetime of clouds and haze. This chapter is an overview of the various static techniques that allow the detailed physicochemical interrogation of single atmospheric particles and the benefit of these techniques in atmospheric studies. A general description of techniques is followed by their use in studying atmospheric particles. Included are scanning and transmission electron microscopies and associated techniques such as X‐ray spectroscopy, electron diffraction, and electron energy loss spectroscopy; micro‐Raman spectroscopy and associated techniques such as optical tweezers, surface‐enhanced Raman, and tip‐enhanced Raman; atomic force microscopy; and scanning transmission X‐ray microscopy. Dynamic single‐particle techniques such as aerosol time‐of‐flight mass spectrometry, which can be appreciated only from the analysis of particle ensembles, are briefly mentioned.
1.1. INTRODUCTION
We all enjoy a clear, cloudless sky. Yet, more often than not, there are features in the daylight sky that, for better or worse, make it more interesting. These features include clouds, rainbows, colorless and colored haze, contrails, smoke from fires, steam and smoke from stacks, and soot from tailpipes. Virtually all these features are due to microscopic particles, either as solids or liquids, suspended in the gaseous ambient air, which we know as aerosols.
Particulate matter in the atmosphere is of far greater consequence, however, than keeping our interest in sky‐watching. To assess air quality and regulate harmful emissions, breathable particulate matter is commonly dichotomized as particles <2.5 μm (PM2.5) and <10 μm (PM10) in aerodynamical size. Numerous epidemiological studies have revealed the health hazards of PM2.5 and PM10 for vulnerable populations (e.g. Kim et al., 2015; Miller et al., 2007; Pope et al., 2002, 2009). Especially revealing is the rate of mortality from lung cancer and cardiopulmonary disease in cities due to exposure to PM2.5 (Dockery et al., 1993; Hoek et al., 2013). Reduced visibility from particulate matter is also a concern (Hyslop, 2009). More than an annoyance, the impact of particulate matter on visibility can be critically important in road traffic safety (Pauley et al., 1996). Air traffic is also affected by reduced visibility or, worse, jet engine malfunction. Massive injections of particulate matter from volcanic eruptions in 1982 in Indonesia and 1989 in Alaska caused jet engines to fail before being restarted in flight. In 2010 and 2011, emissions from Iceland's Eyjafjalla and Grimsvötn volcanos, respectively, were responsible for closing airspace to busy air traffic (Brooker, 2010). However, dust storms, albeit intense, appear more likely to affect airline on‐time performance than endanger passengers (Baddock et al., 2013).
Aerosols have also been intensely studied for their effect on climate. Particulate matter shifts Earth's radiative balance, measured as radiative forcing in watts per meter squared (J s−1 m−2), either by absorbing or scattering solar and longwave radiation directly or affecting physical processes within clouds and serving by cloud‐condensation or ice nuclei (Hansen et al., 1997; Haywood & Boucher, 2000; Lohmann & Feichter, 2005; Twomey, 1974). Uncertainties in the behavior of particulate matter in the atmosphere, particularly regarding clouds, cause by far the largest gap in our understanding of how atmospheric aerosols affect climate (Boucher et al., 2013).
At the microscopic level, individual atmospheric particles are fascinating in the breadth of variation in their size, morphology, and chemical composition. The tremendous variation in particle properties provides for the rich geographic and temporal diversity of atmospheric aerosols. This diversity is manifested by volcanic forces, wind‐driven processes such as weathering of Earth's crust and release of sea spray, and the chemical processing of natural and anthropogenic emissions within the atmosphere. Variation in cloud formation and precipitation are also closely related to the prevalence and character of aerosols in the atmosphere.
1.1.1. Bulk Analysis Versus Microanalysis
Atmospheric particulate matter is normally analyzed either as a bulk material or at the scale of individual particles. The approaches are often complementary: for example, in studying heterogeneous aerosol chemistry (e.g., Laskin et al., 2005). Inductively coupled plasma mass spectrometry (ICP‐MS) and gas and liquid chromatography with mass spectrometry (GC/MS and LC/MS, respectively), optical techniques such as Fourier transform IR spectroscopy, fluorescence spectrometry, and magnetic resonance techniques such as 13C‐NMR are powerful bulk‐analysis techniques for chemically speciating organic and metallic constituents in particles from emission sources (Noziere et al., 2015). X‐ray diffraction techniques can help identify crystalline constituents in samples (Jeong, 2008). X‐ray fluorescence and inductively coupled plasma mass spectrometry can quickly reveal elemental composition, the latter isotopic composition as well (Majestic et al., 2009; Mazzei et al., 2008; Pekney & Davidson, 2005). However, these techniques cannot reveal chemical composition with respect to the shapes of particles, their size distribution, surface versus subsurface layers, or whether chemical components are mixed within individual particles (internal mixing) or associated with separate particles (external mixing). These inquiries are better addressed with microanalysis.
Because of the importance of understanding the properties of individual particles, microanalytical methods for atmospheric particles have become as numerous as conventional bulk methods, perhaps more so. Over the last decade or so, a number of comprehensive reviews involving individual particle techniques have been published (Ault & Axson, 2017; Bzdek et al., 2012; Elmes & Gasparon, 2017; Fletcher et al., 2011; Laskin et al., 2012, 2019; Li et al., 2016; Ott et al., 2021; Pósfai & Buseck, 2010; Pratt & Prather, 2012b; Tang et al., 2019). Interestingly, advances in the technology of sample introduction can allow a bulk analysis technique to evolve into a microanalysis technique. This transition is often driven by a need for real‐time or near‐real‐time data to indicate particle mixing state – that is, whether particles are internally or externally mixed – and particle surface properties such as the effect of coatings from oxidized gas‐phase species such as sulfate, nitrate, and semivolatile organic carbon on hygroscopicity.
An example of the transition from bulk analysis to microanalysis is the mass spectrometry of aerosols. For decades, mass spectrometry has been an important aerosol methodology, initially with bulk extracts injected into a gas chromatograph followed by electron ionization of the eluate and ions detected by a quadrupole or magnetic sector spectrometer. As aerosol mass spectrometry (AMS) came to be known, aerosols were introduced into a mass spectrometer as a focused particle stream to facilitate real‐time time analysis. However, AMS is still considered a bulk technique (Pratt & Prather, 2012a) because portions of the particle stream are collected on a surface for subsequent vaporization, typically by resistive heating, and ionization, typically by electrons. Conventional AMS with a quadrupole detector has time‐of‐flight (TOF) functionality, but in this case, it is the time of flight of particles to the collection surface (PToF) rather than the time of flight of ions to the detector (Canagaratna et al., 2007). Numerous variations of real‐time bulk mass spectrometric methods for aerosols exist, with the common connection that they employ the TOF spectrometer. However, as instruments were designed to ionize individual particles by laser in the focused aerosol stream, which required an ion TOF detector, the mass spectrometry of aerosols evolved into a single‐particle methodology known now as aerosol TOF mass spectrometry (AToFMS) (McKeown et al., 1991; Pratt & Prather, 2012b). Along with the TOF spectrometer (either linear or reflectron), the key to real‐time single‐particle mass spectrometry is a laser system to detect, size, and pulse ionize individual particles (laser desorption/ionization) in the particle stream.
AMS and aerosol time‐of‐flight techniques constitute a large analytical field that has been reviewed in a number of comprehensive publications (e.g., Canagaratna et al., 2007; Johnston, 2000; Noble & Prather, 2000; Pratt & Prather, 2012a, 2012b; Zelenyuk & Imre, 2009) and therefore is not covered separately in this book.
1.1.2. Dynamic Versus Static Microanalysis
Microanalysis techniques can be categorized as dynamic or static. Dynamic techniques provide momentary snapshots of particles in a stream. Static techniques operate on lengthier time scales and therefore allow for the physicochemical interrogation of individual particles. Data from dynamic techniques such as AToFMS can only be appreciated from an ensemble of particles, much like a movie is appreciated only as a collection of individual frames. A specific analytical example concerns the particle mixing state. AToFMS can inform if a particle ensemble can be categorized as internally or externally mixed. As a static technique, electron microscopy allows us to examine how chemical phases in a single internally mixed particle are arranged from element mapping with energy‐dispersive X‐ray spectroscopy or phase identification with selected‐area electron diffraction (SAED). The mixing state of actual individual particles helps to identify the appropriate particle mixing‐state model used in climate studies, such as the concentric core‐shell configuration for absorbing and nonabsorbing phases (Fuller et al., 1999; Jacobson, 2001). Other configurations might be nonconcentric inclusion‐within‐matrix and adduct‐upon‐matrix configurations (Jeong & Nousiainen, 2014; Kahnert et al., 2014; Muinonen et al., 2009).
1.1.3. Selection of Microanalytical Techniques
In this book, the authors report a host of modern microanalytical techniques and their applications that allow for the detailed physicochemical interrogation of single atmospheric particles. This chapter provides a description of techniques followed by examples of their specific use in studying particles. The various techniques can be categorized into six static “microscopies” involving a variety of analytical probes:
Electrons: scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
Photons: micro‐Raman spectroscopy and scanning transmission X‐ray microscopy (STXM)
Ions: time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS)
Deflection of a mechanical arm: atomic force microscopy (AFM)
In electron microscopy, the specimen is probed by a beam of electrons generated by an electron gun. Electrons are propelled toward the specimen and focused by a series of electromagnetic lenses. Information about particle shape, topography, and composition is provided by the detection of beam electrons that have been elastically or inelastically scattered and by diffraction patterns of the beam electrons. The detection of secondary electrons and X‐rays from the specimen also provides essential information in electron microscopy. In micro‐Raman spectroscopy, photons in the visible region probe the specimen to reveal different chemical species as the polarization of chemical bonds by visible light shifts the energies of the photons as they interact with atoms. In STXM, intense synchrotron‐generated X‐rays probe the specimen to reveal organic functional groups that absorb highly monochromatic photons. The edge of absorption peaks in STXM spectra reveal bonding‐specific details. In TOF‐SIMS, pulsed primary ions, typically oxygen, argon, cesium, or gallium, are used to sputter surface material from a specimen. The ions probe the specimen surface or at depth by dislodging characteristic secondary ions, which are then rapidly mass‐separated and time‐detected in a flight tube by mass spectrometry. In AFM, specimen surfaces are probed by detecting the movement of a high‐resolution micro‐cantilever above the substrate surface. Cantilever movement is due to mechanical contact with the substrate or atomic interactions between cantilever and substrate, such as van der Waals attractions and electrostatic forces.
The choice to focus on the six methodologies listed is based in part on their prevalence in the analytical literature for atmospheric aerosols. Figure 1.1 shows the number of citations for each methodology retrieved from Web of Science over five decades. Before 1971, microscopy of aerosols was mainly limited to light microscopes. In the decade from 1971 to 1980, a few reported studies involved SEM and micro‐Raman spectroscopy. Although TEM had been developed prior to SEM, TEM studies of atmospheric aerosols began to appear between 1981 and 1990, along with TOF‐SIMS. It wasn't until the 1990s that microanalysis of single particles began to take off. In that decade, AFM appeared for the first time and the use of TEM, TOF‐SIMS, and micro‐Raman for atmospheric aerosols increased severalfold, but particularly with SEM. In the early 2000s, the initial application of STXM to atmospheric aerosols appeared in the literature.
Figure 1.1 Number of references in the atmospheric aerosol literature over five decades from Web of Science for the most prominent microanalysis techniques used to investigate individual atmospheric aerosol particles. The inset magnifies the lower end of the number range to better show the prevalence of techniques in the literature over time.
During the decade 2011–2020, the literature shows that all microscopies except one saw increased use for analyzing atmospheric aerosols. The exception is TOF‐SIMS. Nevertheless, it is a useful analytical technique for studying surface chemistry in particular. As the need to understand particle surface properties and heterogeneous reactions increases, the use of TOF‐SIMS for atmospheric aerosols will likely continue.
SEM appears to be the most‐referenced microscopy (possibly excluding light microscopy) in the atmospheric aerosol literature for the past three decades. Since 2001, micro‐Raman spectroscopy and TEM have been the next‐most‐referenced microscopies. Notwithstanding the need to travel to a specialized facility to perform it, STXM will likely see increased use in future years, particularly for studying individual organic aerosol species in particles. AFM use will also likely increase due to the need to understand how particle topography affects hygroscopicity and reactions on particle surfaces.
Associated with each of the six microscopies (and spectroscopies) are various techniques that can be performed. Table 1.1 shows the techniques, the types of signals acquired, and detection properties. What follows is an overview of the microscopic techniques and examples of applications. (TOF‐SIMS is included when combined with other microscopic techniques.) It is not intended that the applications and associated references reported here are all‐inclusive; rather, they are some salient examples of how the techniques are applied in aerosol studies.
1.2. ELECTRON MICROSCOPY
In electron microscopy, beam electrons that probe the specimen are set to a specific acceleration energy ranging from 500 eV to 30 keV in SEM and from 80 to 400 keV in TEM. The beam current can be adjusted to a specific current from 1 pA to 100 nA or more.
In older SEM and TEM instruments, the electron source is a hairpin tungsten filament that serves as the electron gun's cathode. To provide a brighter beam, later instruments utilized a single, sharp‐tipped crystal of lanthanum boride (LaB6) as the electron source. The tungsten filament and LaB6 electron guns required high heating to overcome the work function to release electrons from the gun tip (Goldstein et al., 2018). State‐of‐the‐art instruments commonly have a thermally assisted field‐emission electron gun (tFEG). The cathode emitter, in this case, is typically a sharp‐tipped tungsten wire heated to around 1600 K and subjected to an intense electric field whereby electrons “tunnel” from the emitter tip (Goldstein et al., 2018). A variation on the tFEG is the Schottky emitter, whereby the tungsten tip is thinly coated with zirconium oxide. The tip is heated to around 1800 K.
Table 1.1 Characteristics and Properties of the Various Techniques Associated with the Six Microscopies for Particle Microanalysis
Technique
Probe
Detected signal
Particle property determined
Relative sensitivity (%)
1
Minimum detectable amount
1
Probe lateral resolution
1
SEM EPMA
2
ESEM
3
VPSEM
4
FIB‐SEM
5
Electrons, Ions (FIB‐SEM)
Secondary electrons
Size, morphology from electron image
0.1
10
−16
g
1–3 nm
Backscattered electrons
Size, topography, composition (qualitative) from TOPO/COMP image
Backscatter electron diffraction
Crystallinity
Secondary ions (FIB‐SEM)
Size, morphology from ion image
Characteristic X‐rays with EDX, WDS
Element composition >Beryllium Element spatial distribution (map)
Luminescence
Cathodoluminescence
CTEM
6
STEM
7
Electrons
Elastically scattered electrons
Size, shape from bright‐field image (low‐angle scattering) Size, shape from dark‐field image (high‐angle scattering) Crystallinity, grain morphology, specimen thickness from diffraction
0.1
Atom
0.15–0.3 nm
Inelastically scattered electrons
Oxidation state, chemical bonding from electron energy loss spectroscopy
Secondary electrons (STEM)
Size, morphology from electron image
Selected‐area electron diffraction
Crystallinity, grain structure
Convergent‐beam electron diffraction
Specimen thickness, small area crystallinity, grain structure
Characteristic X‐rays with EDX
Element composition >Beryllium Element spatial distribution (map)
Luminescence
Cathodoluminescence
Micro‐Raman SERS
8
TERS
9
Photons (VIS)
Photons red‐shifted by molecular vibration and bond polarizability
Molecular composition Molecular functional groups
1
10
−16
g
0.8 μm (micro‐Raman)0.5 μm (SERS)
10
0.1 μm (TERS)
10
AFM AFM‐IR
Mechanical cantilever
Force on oscillating cantilever
Size, volume, morphology Surface area, surface tension Hygroscopicity IR absorption (AFM‐IR)
NA
NA
10 nm
TOF‐SIMS NanoSIMS
Pulsed ions (TOF‐SIMS)Ion stream (NanoSIMS)
Secondary ions (m/z)
Surface composition (elemental/isotopic/molecular) Depth profile
3.1 to 0.1% (1–1000 ppm)
10
−16
g
40 nm to 1 μm (TOF‐SIMS)>40 nm (NanoSIMS)
STXM
Monochromatic (synchrotron) X‐rays
Transmitted and secondary X‐rays
Elemental/molecular composition and bonding (NEXAFS)
0.06 μm
1 Fletcher et al., “Microscopy and Microanalysis of Individual Collected Particles,” in Aerosol Measurement: Principles, Techniques, and Applications, P. Kulkarni, P.A. Baron, & K. Willeke, eds., New York: John Wiley & Sons (2011). Relative sensitivity is the fraction of measured mass for a component (element or molecule) measured analytically by a microscopic technique: for example, X‐ray spectroscopy in SEM. Relative sensitivity is based on a flat polished specimen.
2 Electron probe microanalysis.
3 Environmental SEM.
4 Variable‐pressure SEM.
5 Focused ion beam SEM.
6 Conventional TEM.
7 Scanning TEM.
8 Surface‐enhanced Raman spectroscopy.
9 Tip‐enhanced Raman spectroscopy.
10 Ault & Axson (2017), Anal. Chem., 89, 430–452.
Figure 1.2 Signals generated when a high‐energy beam of electrons interacts with a thin specimen.
Williams and Carter, 1996a/with permission of Springer Nature.
Figure 1.2 shows the signals that emanate from a specimen exposed to an electron beam (primary electrons). The figure depicts signals from TEM. However, signals detected from above the plane of the specimen with SEM (as well as technically with TEM) include backscattered primary electrons, secondary electrons, characteristic X‐rays, and visible light (cathodoluminescence). Signals detected below the plane of the specimen with TEM include elastically and inelastically scattered primary electrons.
The principal drawbacks to using electron microscopy to study atmospheric particles relate to the relatively high energies and currents of the incident electron beam and having the specimen under vacuum. When the specimen and its substrate are insufficiently grounded electrically, the incident beam may statically charge a particle and cause it to move from its position on the substrate. The incident beam may also damage a fragile particle before sufficient time exists to acquire analytical signals: for example, characteristic X‐rays. Although particles with mineral phases are typically undamaged, fragile particles such as organics often undergo damage, which may be observed in the microscope as it occurs. With the high vacuum that is required for conventional SEM and TEM, water, volatile organics, and ammonium nitrate are removed under vacuum, and thus SEM and TEM are often unsuitable for particles with these phases.
1.2.1. Scanning Electron Microscopy
In SEM with a field‐emission gun, the electron probe is focused to approximately a 1 nm spot on the specimen (Table 1.1) and then rastered across the specimen. However, the volume within a thick specimen that is sampled by the probe at each point is much larger than the probe itself. Figure 1.3 shows the extent that primary electrons travel and deposit energy within a specimen of the NIST K‐412 glass (former NIST Standard Reference Material No. 470) using a beam energy of 15 keV (Ritchie, 2020). Also shown is the lateral extent for 90% of characteristic X‐rays that are emitted for the various elements in the glass.
Along with higher magnification (as much as 300,000×) and the concomitant increase in resolution, a significant advantage of SEM over light microscopy is a large depth of field. Magnetic lenses are used to first demagnify the electron beam (condenser lens) and, further down the column, focus the beam to converge on a spot (objective lens). Imaging is performed by detecting secondary electrons released from the specimen or primary (beam) electrons that are backscattered. Backscattered electrons typically lose some energy upon entering the specimen, but the process primarily involves elastic scattering at large angles (Goldstein et al., 2018). Secondary electrons are typically counted by a scintillation device, whereby electrons interact with a scintillating material and light is emitted and then detected. Backscattered electrons may also be counted by a scintillation detector, but a small solid‐state detector placed close to the specimen provides much higher collection efficiency. Configuration of the SEM lenses can also affect how secondary and backscattered electrons are detected.
Figure 1.3 Monte Carlo simulation of the interaction volume for glass K412 (SRM 470) containing several elements probed by a 15 keV electron beam in SEM. Element numbers indicate fractional weight.
From NIST DTSA‐II (Ritchie, 2020), public‐domain software for electron‐excited X‐ray energy‐dispersive spectrometry. https://www.nist.gov/services‐resources/software/nist‐dtsa‐ii.
SEM provides fine detail regarding the topography and composition of a specimen from the strength of secondary electron and backscattered electron signals above the specimen, as shown in Figure 1.2. Electron imaging with SEM (as well as TEM) can provide quantitative information regarding the size and morphology (e.g., aspect ratio) of atmospheric particles. However, the value of imaging is largely qualitative.
Secondary electron emission occurs when energy from the inelastic scattering of beam electrons ejects valence electrons from specimen atoms. Because valence atoms are weakly bound, secondary electrons have low kinetic energy, typically less than 50 eV. Both specimen structure and composition affect the emission of secondary electrons. Secondary electrons from a particle near its surface form an image that mainly reveals the particle's two‐dimensional (2D) shape and topographic features. Maximum resolution is on the order of the beam spot size, which is typically around 1 nm.
Backscattered electrons can have a wide range of energies depending on the specimen's elements. As a result, backscatter electron imaging can reveal qualitative element differences in a particle's composition as well as its shape and topography. For heavier elements in the specimen, the energy of backscattered electrons is a sizable fraction of the beam energy. Thus, energies of backscattered electrons are typically much higher than secondary electron energies.
With both secondary and backscattered electrons, detailed imaging depends on the energies of the emitted electrons, the angle of the electron beam entering the specimen, and the angle of the electrons exiting above the plane of the specimen. Other factors include the type, configuration, and placement of the electron detector within the instrument. Backscatter electrons reveal contrast in atomic number because higher‐Z atoms are more likely to scatter back electrons at a larger angle relative to the direction of the beam electrons than will lower‐Z atoms.
Topographic information is more dependent on the angles at which the beam enters the specimen and secondary or backscattered electrons exit. In addition, secondary electron emission is enhanced at the edges of the specimen, where the valence electrons energized by the primary beam can escape the specimen surface. Secondary electrons in direct line of sight to the detector tend to provide greater topographic detail than backscattered electrons because with their low energies, secondary electrons can be drawn by a positive electric potential to the detector with greater efficiency.
X‐ray Spectroscopy with Electron Microscopy
Additional analytical devices associated with SEM instrumentation provide essential compositional information on atmospheric particles. A technique that is typically integral to SEM and commonly associated as well with TEM instrumentation is X‐ray spectroscopy. When energetic electrons enter a material and liberate core‐shell electrons, the ionized states of lower‐Z atoms relax primarily by emitting Auger electrons, whereas ionized states of higher‐Z atoms relax primarily by emitting X‐rays. In addition to emission by target atoms, X‐rays can be absorbed by other atoms within the material without further X‐ray emission, or they can result in X‐ray emission by other atoms: that is, fluorescence.
Both X‐ray absorption and X‐ray fluorescence can be corrected algorithmically for specimens with surfaces that are not perfectly flat. In seminal work reported by Armstrong and Buseck, the theory behind X‐ray absorption and fluorescence in individual particles with surface topography was developed (Armstrong & Buseck, 1975). That and subsequent early work (Bradley et al., 1981) led the way for the quantitative elemental analysis of atmospheric particles by energy‐dispersive X‐ray spectroscopy (EDX or EDS)‐associated SEM and TEM. However, the quantitative analysis of particles in the size range to produce Mie scattering (particle diameters on the order of incident light wavelengths in the actinic region) remains problematic, particularly when measurement reference particles are not used in the calibration process. Studies of pollution, as well as solar radiative forcing by aerosols, focus on particles in the Mie‐scattering size range.
Another historic development in the X‐ray microanalysis of atmospheric aerosols was automated SEM imaging and X‐ray analysis for determining the sizes and compositions of thousands of particles collected on a filter substrate (Byers et al., 1971; Casuccio et al., 1983; Germani & Buseck, 1991; Li et al., 2023). In generating distributions for the sizes and compositions of large numbers of particles, automated particle analysis, or computer‐controlled SEM (CCSEM), links the analysis of an individual particle by SEM‐EDX with the population of aerosols in an airshed.
In newer instruments, EDX with silicon‐drift detectors provides rapid, high‐throughput counting of characteristic X‐rays from elements with atomic numbers greater than that of boron. In analyzing atmospheric particles, electron probe microanalysis (EPMA) is often synonymous with SEM‐EDX (e.g., Ro et al., 1999). However, materials scientists and geochemists typically associate EPMA with wavelength‐dispersive X‐ray spectroscopy (WDS). The technique is important for studying mineral samples and is therefore relevant with respect to atmospheric mineral dust. With WDS, a series of crystals is used to collect the X‐ray photons rather than the silicon‐drift detector. The crystals are mechanically aligned to diffract the emitted photons toward a proportional counter. Due to the mechanical alignment of crystals, WDS is more time consuming than EDX but provides far better resolution of X‐ray energies and quantification (Goldstein et al., 2018).
When the electron‐probe beam enters the specimen, X‐rays produced within the specimen fill a volume (X‐ray interaction volume) that is larger than the volume of the probe itself (probe interaction volume). The size of the X‐ray interaction volume depends in large part on the density and thickness of the material. However, even with the disparity in the X‐ray and probe interaction volumes, rastering of the probe allows for element mapping of a particle to reveal the qualitative compositional heterogeneity of atmospheric particles with rather high resolution. This is because X‐ray emission is greatest from the part of the interaction volume nearest where the beam enters the specimen. A typical minimum resolution for X‐ray mapping is around 20 nm (Fletcher et al., 2011). As an example, the resolution for a 1 μm particle filling a 60‐pixel by 60‐pixel scan frame is 17 nm.
Studies have looked at ways to improve EDX analysis of lighter elements (low‐Z elements) in aerosols and how to improve substrates to avoid inaccuracies. The analysis of carbon, nitrogen, and oxygen by EDX without a conventional beryllium spectrometer window (i.e., an ultrathin window or windowless spectrometer) was demonstrated along with the importance of assessing the absorption of soft X‐rays of light elements by the matrix to interpret spectral intensities (Ro et al., 1999, 2000
