As a novel and unique large-scale instrument, the synchrotron radiation facilities have been progressively applied to the research of atmospheric science. This article introduces the theoretical principles, key technologies, and recent major research results of key synchrotron radiation-based experimental methods. The main synchrotron radiation technologies include the atmospheric pressure X-ray photoelectron spectroscopy (APXPS), the near edge X-ray absorption fine structure (NEXAFS) and the scanning transmission X-ray microscopy (STXM). A key component (environmental cell) commonly used in all three technologies is explained in detail. This article classifies the collected research according to experimental types, i.e., APXPS experiments, liquid jet experiments and STXM experiments. The main topics include: (1) ice surface, (2) salt surface, (3) acidic solution, (4) organic solution, (5) halite solution, (6) ozonolysis, (7) soot, (8) ice nuclei, (9) hygroscopicity and (10) reaction mechanism. The development of synchrotron radiation facilities has provided strong support for the research of aerosol science and atmospheric heterogeneous chemistry, giving atmospheric scientists the ability to explore unknown fields and latitudes. It is foreseeable that more and more important atmospheric processes and mechanisms will be revealed by technologies based on synchrotron radiation, which will also reflect the great potential and value of synchrotron radiation devices in the field of atmospheric and environmental science.
Contents
1 Introduction
2 Synchrotron-based techniques used in atmospheric science
2.1 X-ray Photoelectron Spectroscopy(XPS)
2.2 X-ray Adsorption Spectroscopy(XAS)
3 Key technologies for connecting synchrotron-based techniques and atmospheric science——Environmental cell
The atmosphere, which plays a central role in the environment and climate, is a complex multiphase system formed by the interaction of gases, aerosol particles, and clouds. Aerosol interface (the interface between particle surface and gas) plays an important role in many important atmospheric processes, such as photochemical smog, new particle formation, atmospheric halogen chemistry, greenhouse gas removal, and ozone depletion[1][2][3][4,5][6]. Interfacial processes usually start with the uptake of gas molecules or clusters on the surface of particulate matter. The ingested substance will directly affect the properties of particulate matter, including mass, optical properties, surface chemical activity, hygroscopicity and ice nucleation activity[7]. At the same time, the surface after ingestion may undergo changes in properties, such as viscosity, chemical properties, and changes in the surface and even internal structural properties of particles, which in turn feed back to its uptake capacity[8][9][10]. Therefore, the uptake capacity of particulate matter surface and the corresponding interfacial processes are very sensitive to environmental factors, which requires the study of aerosol interfacial processes to be carried out under relevant environmental conditions.
From a technical point of view, due to the very small mass proportion of surface matter, many interfacial processes have limited impact on the amount of particulate matter, so they are easily ignored by some traditional experimental techniques. At present, the microscopic understanding of the surface properties of particulate matter is mostly focused on the description of the morphology by electron microscopy or atomic force microscopy, while the detection of its chemical properties is still extremely limited[11][12]. Secondly, even homogeneous particles (surfaces not covered by other substances) may have properties very different from those of the interior due to the unique structure of the molecules on the surface.
The development of synchrotron radiation facilities has provided strong support for the exploration of this uncharted territory. As a large instrument, synchrotron radiation is considered to be one of the most advanced technological means for human beings to explore the properties of matter. However, due to the ultra-high vacuum conditions required by synchrotron radiation facilities, the study of gas and condensed phase interfaces is limited, and the aerosol interfaces concerned by atmospheric science are precisely such gas/solid and gas/liquid interfaces. Until recently, several state-of-the-art synchrotron radiation techniques could be used to study scientific problems in the field of atmospheric science due to the introduction of state-of-the-art technical means. These new methods provide new research perspectives, and some superficial phenomena that have not been fully understood or realized before are discovered and studied.
An accurate description of aerosol interfacial properties requires direct access to atmospherically relevant environments using experimental means with a high degree of surface sensitivity. Environmental factors (temperature, humidity, trace gases) are extremely important parameters affecting the properties of particulate matter, so the detection of aerosol interface needs to be carried out under the corresponding environmental factors.
2 Synchrotron radiation techniques used in atmospheric science
2.1 X-ray photoelectron spectroscopy (X-ray ray photo electron spectroscopy; APXPS)[13]
X-ray photoelectron spectroscopy (XPS) is an energy spectroscopy technique based on the photoelectric effect developed mainly by Nobel Prize winner Kai Siegbahn in the mid-1960s. Because of its extreme surface sensitivity, XPS is often used to study the chemical properties of material surfaces. As shown in Figure 1, atoms on the surface of a material produce Photo electrons when irradiated by X-rays. With the known Photon energy (hν) and the kinetic energy of the photoelectron being measured (Kinetic energy,EK), the electron binding energy of the photoelectron in the atom (Binding energy,EB) can be calculated by the following relation.
W is the work function, which is the minimum energy required for an electron to escape immediately from a solid sample, that is, the difference between the Fermi level and the Vacuum level.
Because of the characteristic fingerprint information of electron binding energy, XPS can analyze the important information of chemical elements, functional groups, dissociation state, oxidation state and protonation degree on the surface of samples. In addition, XPS has the following characteristics: extremely high surface sensitivity (often used to detect surface information within 1 nm); Limited damage to the sample; Sensitive to the photoelectron signal depth. Since the Inelastic mean free path (IMFP) of an electron is determined by its kinetic energy, the kinetic energy of the photoelectron can be changed by changing the photon energy of the X-ray, so that the information of different depths below different surfaces (Depth profile) can be obtained.
In the field of atmospheric science, XPS has been used to study the physical and chemical changes of aerosol surface under different environmental conditions and the depth distribution of various chemical forms.
2.2 X-ray adsorption spectroscopy (XAS)
There are two common methods to measure X-ray absorption: first, the X-ray intensity transmitted through the sample can be measured directly after the sample through the photosensitive coupling element, and then the absorption amount can be obtained by comparing with the initial intensity, such as scanning transmission X-ray microscopy, which is one of the main contents of this paper; Second, an electron kinetic energy analyzer can be used to measure the Auger electron signal intensity, which is proportional to the absorbed X-ray intensity. Because this method uses an electron kinetic energy analyzer, it is often used in conjunction with APXPS.
No matter which method, the important application technology is Near edge X-ray absorption fine structure (NEXAFS). The first method is to directly measure the transmission to obtain the absorption rate, thus obtaining the characteristic absorption spectrum. Scanning transmission X-ray microscopy (STXM) is a combination of this method and X-ray microscopy[14]. The principle of NEXAFS experiments by Auger electrons is that when an electron at the core level is excited and emitted, a vacancy is formed in the electron shell. An electron of higher energy level can transit to this vacancy and escape the Auger electron at the same time. Taking the KLL Auger electron as an example (Fig. 2), its measurable electron kinetic energy (EK) is determined by the following factors: (1) the driving potential energy, the difference between the binding energies of K and L2,3 layer electrons; (2) drag potential energy, the binding energy of electrons in the L2,3 layer.
NEXAFS is more sensitive to the local environment of the detected electrons/atoms than XPS. For example, NEXAFS can be used to study the changes of hydrogen bond network structure in water ice and liquid water[15,16].
In addition, Scanning photo electron microscopy (SPEM) combines an X-ray Scanning program similar to that in STXM with an electron analyzer, so it has both high surface sensitivity and high spatial resolution[17].
3 Environmental Cavity: a Key Technology for Connecting Synchrotron Radiation and Atmospheric Science
Conventional XPS experiments require ultra-high vacuum (UHV) conditions because electrons are consumed by background gas molecules in a non-UHV environment. However, atmosphere-related materials are often highly volatile, which means that they cannot remain stable in UHV environments. The current solution is to add an environmental cavity around the sample: in this cavity, the local gas pressure near the sample surface can be increased to keep the solid/gas or liquid/gas interface in equilibrium. At the same time, the attenuation of the signal is suppressed due to the shortening of the high pressure path through which electrons need to pass. The addition of this environmental cavity has been successfully used in non-vacuum experimental environments by several highly sensitive techniques, such as environmental scanning electron microscopy, environmental tunneling electron microscopy, environmental molecular beam, and atmospheric pressure X-ray photoelectron spectroscopy[18][19][20][21⇓~23].
3.1 APXPS environmental cavity
The main obstacle that needs to be overcome for APXPS is the scattering of electrons by gas molecules. The attenuation of the photoelectron signal depends on the type of gas, the gas pressure, and the distance traveled by the electron in the gas-bearing region. Therefore, the environmental cavity is used to minimize the path length of electrons in the atmospheric pressure region. The original APXPS instrument was designed in the early 1970s by the Siegbahn research group at Uppsala University[24]. In the late 1990s, synchrotron-based APXPS instruments began to develop[21]. At present, due to the popularity of commercial instruments (such as SPECS GmbH and Scienta Omicron), more and more APXPS research groups and scientific research projects have emerged[25]. The principle of the APXPS environmental cavity is shown in Fig. 3. In general, current APXPS devices can support tests at pressures up to 20 mbar[23]. Because this pressure is close to the vapor pressure of water at room temperature, APXPS can be used in many research topics related to the atmospheric environment (see Section 4.1).
3.2 Fluid beam environment cavity
For Liquid samples, another way to realize the study of volatile surfaces is to use a liquid jet[26]. This method allows the liquid phase to be directly exposed to the high vacuum while maintaining the high vacuum. The principle is that the solution is pumped into the sample cavity at a certain pressure and flow rate. Because of the small surface area of the fluid stream (tens of microns in diameter), the resulting change in air pressure is small even if the liquid stream remains vaporized until it reaches the trap. The trap is usually cooled by liquid nitrogen to reduce the vapor pressure of the liquid. In addition, the fluid beam method has the characteristics of less pollution and less radiation impact because of its flowing water column, which is continuously renewed. Unlike the sample inside the environmental cavity, the fluid beam is directly exposed to high vacuum. Therefore, in order to study the interaction between the liquid surface and the surrounding gas, a SiNx window can be introduced at the end of the X-ray light source, an additional vacuum pump can be added to form a local relatively low vacuum in the sample cavity, and an additional nozzle can be used at the periphery of the fluid beam nozzle, through which ozone, CO, NO, SO2 and other gases can be introduced, as shown in Figure 4[9].
3.3 STXM environmental cavity
The STXM environmental cavity is also designed to reduce the path length of the signal in the normal pressure region. The signal of STXM is a photon rather than a photoelectron in APXPS. Since photons have a relatively high transmission capability, higher experimental gas pressures can be reached by adding STXM through the environmental cavity. STXM applications at Swiss light source (SLS) have ambient chamber pressures up to 150 mbar. By adjusting the humidity and temperature in the cavity, the physical and chemical mechanisms in different atmospheric environments can be simulated. Moreover, the temperature in the environmental cavity can be as low as -103 ° C and is used to study the ice nucleation mechanism. The STXM environmental cavity structure of SLS is shown in Fig. 5.
4 Research Progress
For the convenience of the reader, synchrotron radiation experiments are classified according to the type of experiment (APXPS, fluid beam, STXM) in order to connect with the familiar field. Note that in the APXPS environmental chamber, although the main research object is the solid/gas interface, more and more work has been done recently to study the solid/liquid phase transition in the presence of the gas phase.
4.1 APXPS environmental cavity
4.1.1 Ice surface
Ice is one of the most important substances in the atmospheric system. However, due to the relatively high vapor pressure of ice, the XPS study of ice has been limited by the experimental conditions of high vacuum. This limitation was first broken through by Bluhm's team in the ALS accelerator at Lorentz National Laboratory in the United States by adding an environmental cavity[21].
The interaction of hydrochloric acid with ice plays an important role in stratospheric ozone depletion[27]. Therefore, the adsorption of hydrochloric acid molecules on ice and whether similar dissociation of hydrochloric acid molecules in the liquid phase occurs after adsorption have been extensively studied for decades. However, before the application of APXPS, the understanding of chemical processes at the ice/air interface (especially on the ice surface with temperature above 230 K) was largely limited. In 2017, Kong et al. Used the NAPP terminal station in the Swiss Light Source to detect the presence of both unionized hydrochloric acid molecules and ionized chloride ions on the ice surface at 253 K based on the chemical shift of chlorine elements after adsorption[15]. This is the first time that undissociated HCl molecules can exist on the surface of water or ice in any temperature range, which provides a direct experimental basis for the debate that has lasted for decades[28⇓⇓~31].
The Depth profile of APXPS can be realized by changing the photon energy of X-ray. Fig. 6 shows the depth profiles of HCl and Cl- on the ice surface thus obtained. In the first layer, which is less than 1 nm thick, it is dominated by unionized HCl and water molecules. In the ∼ 8 nm thick second layer, only the ionic form of Cl- is present. The ratio of chlorine atoms in the first and second layers is about 1: 0.65. It is worth noting that the coverage of HCl at this time is much lower than that of a monolayer, which is similar to the situation in the actual atmosphere, and also excludes the possibility that the non-ionization of hydrochloric acid molecules is caused by the lack of adjacent water molecules.
In addition, chloride ions ionize with surrounding water molecules while diffusing, so the hydrogen-bonding network of the ice is rearranged. This disordering of the ice surface by external impurities is confirmed by the results of the O'K-edge NEXAFS in the same paper[15].
The research group also studied the adsorption of organic acid (formic acid) on ice surface and its effect on ice surface structure in SLS[32,33]. The driving force for the solute to enter the interfacial region is the demand of water molecules by the solute in forming the solvation shell. The flexibility of the hydrogen bond network is one of the prerequisites for solute entry. The degree of ice surface disordering (6 ~ 7 nm) caused by formic acid is also quantitatively reported in this paper[32].
In the early APXPS work of ALS, Krepelov Krepelová et al. Studied the adsorption, dissociation, diffusion and surface disorder effects of nitrate and acetic acid on ice, and the adsorption of non-dissociative organic compounds, acetone and isopropanol, on ice at 218 ~ 245 K and 227 K, respectively[34][35][36][37]. Unlike acidic molecules, acetone does not cause significant disordering of the ice surface. The ratio of carbon intensity to saturated carbon intensity measured by APXPS is the coverage of isopropanol on the ice surface, from which the adsorption isotherm is measured[37].
4.1.2 Surface processes of salts
The crystal structure analogous to the ice surface will be disordered by the interference of other molecules, and the salt crystal surface will also be disordered by the influence of water molecules-deliquescence. Unlike the deliquescence described by conventional thermodynamics, which occurs when a critical RH is reached, pre-deliquescence may occur on the salt surface below this RH. Under this condition, the formation of a water molecular film on the salt surface has been confirmed by other experimental methods, but whether the water molecular film will dissolve the salt molecules on the salt surface to form a salt solution layer has been confirmed by recent APXPS experiments[11,12]. Kong et al. Used APXPS and NEXAFS experiments to prove that not only water molecules are condensed on the surface of the salt in the pre-deliquescence stage, but also reversible dissociation of the salt molecules occurs, as shown by the NEXAFS energy spectrum in Fig. 7[38].
Recently, a novel Surface promoted redox mechanism (SPRO) has been discovered on the Surface of common aerosol particles[39]. Through this mechanism, some thermodynamically impermissible reactions can occur spontaneously during the dissolution of the surface. Specifically, on the surface of ammonium sulfate, ammonium ions () and sulfate ions () react spontaneously to form various species containing nitrogen and sulfur elements in different oxidation States, such as HONO, N2, S0, and HS-).
Phase transition processes (such as deliquescence, weathering, and hydrate formation) in the binary system of salt and water were also studied by earlier APXPS, such as NaCl and KBr/KI[40][41]. Halogen salt aerosols, including sea salt, play an important role in atmospheric chemistry and cloud formation processes in the troposphere[42]. Krepelov Krepelová et al. Studied the NaCl-water phase diagram by controlling the temperature and relative humidity, and showed the change of hydrogen bonding environment in different States by O K-edge NEXAFS[40]. The system was recently re-investigated at the NAPP workstation of the SLS, recording for the first time the NEXAFS spectrum of Hydrohalite, a potential contributor to ice cloud formation[23][43].
Ghosal et al. Used APXPS to study the enrichment of halide anions on the surface of the solution during the deliquescence of KBr/KI, and the concentration of anions within about 0.5 nm of the surface was twice that of cations[41]. In contrast to Br and I, this surface enrichment is more pronounced as the larger and more polarizable I anion: the ratio of the anion-cation abundances of KI is about 4.5, while that of KBr is about 2. In the interior of the solution below the surface, the anion/cation ratio remains unity, which proves that the surface enrichment of the anionic-cationic imbalance does occur at the surface of the solution. This phenomenon is commonly observed on liquid surfaces and will be described in detail later in the progress of fluid beams.
4.2 Fluid beam
4.2.1 Inorganic acid solution
According to the Brnsted-Lowry acid-base theory, the strength of acids and bases is measured by the ability to produce or accept protons. Like other classical theories, the pH it describes depends on the characteristics of the solution as a whole. However, with the development of surface sensitive technology, scientists have found that the behavior of acid on the surface of solution is different from that inside the solution. Since the solution surface is where the liquid/gas heterogeneous chemistry takes place, a correct description of the solution surface properties is extremely important. The fluid beam method provides the feasibility to study the liquid/gas interface.
Similar to the undissociated hydrochloric acid molecule on the ice surface, another strong acid, nitric acid, exhibits a weaker acidity than nitric acid on the surface of its aqueous solution. Lewis et al. Combined XPS and fluid beam methods to find undissociated nitric acid molecules on the surface of concentrated nitric acid solution, but nitric acid still showed its normal acidity inside the solution[44]. Moreover, the ratio of undissociated nitric acid molecules to nitrate increases with increasing nitric acid concentration.
In contrast, for the weak acid carbonic acid (H2CO3), its fully dissociated form () is more concentrated at the interface than its semi-dissociated state ion ()[45]. In this case, the doubly charged strongly hydrated carbonate anion is enriched at the surface relative to the singly charged weakly bicarbonate ion, indicating that the surface of the solution has a higher acidity than the interior.
Ottosson et al. Studied the surface abundance of oxychloride ions (, , , ClO-) at the liquid/gas interface,It is found that the surface abundance increases roughly with the increase of X in the , that is, ≈ > >ClO-[46].
4.2.2 Organic solution
Ottosson et al. Studied the dissociation of small molecular carboxylic acids (formic acid, acetic acid and butyric acid) in solutions of different concentrations, and found that with the increase of concentration, the concentration of molecules and ions increased, and the concentration of molecules was much greater than that of ions, but the rate of increase of molecular concentration was lower than that of ions[47]. At the same time, the degree of dissociation decreases with the number of carbon atoms, that is, formic acid > acetic acid > butyric acid.
Lee et al. Studied C1 ~ C4 alcohol and C1 ~ C4 carboxylic acid solutions[48]. The ratio of the carbon XPS signal intensity of the aliphatic group to the functional group indicates the direction of the molecule relative to the solution surface. With the increase of the chain length, the alcohol and acid molecules tend to be perpendicular to the surface. In comparison, the carboxylate ions are more obviously oriented perpendicular to the surface, which may be due to the stronger hydration of the carboxylate ions.
The results of Öhrwall et al. Showed that the presence of ammonium ion greatly affected the acid-base forms of propionic acid and octanoic acid[49]. In the presence of in solution, more protonated molecular forms can be seen at the interface, and a high concentration of ammonium ions is observed in the surface region. They proposed that this was due to the formation of a double layer of carboxylate ammonium near the surface between the carboxylate and ammonium ions.
Prisle et al. Studied the solution surface/internal partition of sodium caprate in aqueous solution and compared the "salting-out" effect of several inorganic ions[50]. They found that the molecular form of decanoic acid was increased when was present, but this effect did not appear when was replaced by Na+.
Werner et al. Studied the Surface propensity of protonated and deprotonated forms of succinic acid[51]. Similar to strong acids, the succinic acid molecule has a higher surface propensity than its acid radical form.
Walz et al. Compared amphiphilic isomers (1-pentanol and 3-pentanol) with surface active properties on the solution surface by combining XPS and MD results[52]. At lower concentrations, the two isomers begin to form a monolayer with the hydroxyl group pointing toward the interior of the liquid phase and the alkyl chain pointing toward the vacuum. The monolayer packing density of 3-pentanol is about 70% of that of 1-pentanol. The molar concentration of both molecules at the surface is about 90 times the volume molar concentration. Therefore, they concluded that the isomer with smaller molecular area has more surface enrichment than the isomer with larger molecular area.
The same research group studied different concentrations of C4 to C6 alcohols to compare the effects of carbon chain length and isomers on surface coverage, surface concentration, and surface enrichment. For all molecules, with full monolayer coverage, the surface concentration of linear alcohols is about 2.4×1014 molecules per square centimeter, while that of branched alcohols is about 1.6×1014 molecules per square centimeter[53].
4.2.3 Halite solution
Bj Björneholm et al. Combined XPS data and MD simulation results to study the surface tendencies of different halide anions and cations[54]. The surface enhancement of halide anions decreases with increasing concentration, and more polar anions are more sensitive to concentration. When NaCl is added to NaBr solution, although the total anion abundance on the surface decreases, the Br- is selectively enhanced on the solution surface due to the presence of Cl-.
4.2.4 Ozonolysis
Ammann et al. Studied the ozonolysis of NaBr solution and the effect of citric acid[55]. In pure NaBr solution, the rate of ozone absorption is limited by the internal reaction rate. Sodium bromide solution was observed to be more reactive to ozone when citric acid was present. They suggested that the increase in ozone solubility caused by citric acid may be one of the reasons for this. In 2017, the research group first proved the existence of intermediate States of BrOOO- at the interface of sodium bromide solution and ozone with XPS experimental evidence, and obtained the theoretical support of reaction energy spectrum, core electron binding energy and MD model[9]. In addition, the research team also studied the effect of different types of surfactants on the interfacial distribution of halides, such as the effect of n-butanol and butyric acid on the surface distribution of bromide ions in NaBr aqueous solution[56]. When n-butanol was present, the surface concentration of bromide ion was enhanced by nearly 25%, while butyric acid reduced it by 40%. In 2020, the research group also conducted a systematic study on the interfacial properties of bromides with different valence States related to the atmosphere by combining Liquid and MD models[57].
4.3 Scanning transmission X-ray microscopy (STXM)
An important application of STXM/NEXAFS in atmospheric science is the analysis of valence or functional groups of different elements in aerosol particles, including C, O, S, K, Ca, Fe and other elements, among which C is the main element. Moffet et al. Summarized the particulate matter research using STXM technology into the following three categories: (1) black carbon/brown carbon[14]; (2) organic carbon; (3) Inorganic (sulfur, iron, zinc, etc.). This paper highlights the results of the research carried out following the Moffet review[14].
4.3.1 Black carbon
Moffet et al. Proposed the power of the STXM method in characterizing the chemical composition with high spatial resolution[58]. The separation of absorption peaks is the principle of STXM to distinguish different chemical bonds, especially for carbon bonds. Taking advantage of STXM high-resolution imaging, Moffet et al. Studied the morphology, mixing state, and chemical composition of black carbon (BC)[59]. The location of BC in particulate matter and the thickness of organic matter can be determined by STXM chemical imaging analysis. Generally, BC with a thicker organic outer layer has a higher degree of aging. In addition, BC is usually located in the center of the particle, but when other inorganic substances are present, BC may be located at the edge of the particle.
4.3.2 Ice nucleation and hygroscopicity
Particulate matter can act as ice nuclei to form ice crystals in the atmosphere and affect cloud formation and precipitation. Wilson et al. Studied the ice nucleation performance of marine source-related particulate matter: STXM was used to identify organic material for example ice nuclei and compared with diatom leachate[60]. The results show that the organic matter associated with phytoplankton cell exudate may be the main factor affecting the ice nucleation performance of the samples. Combining measurements and global model simulations, marine organic matter may be an important source of ice nucleation particles in the marine environment.
Wang et al. Developed an ice nucleation observation system combined with an environmental scanning electron microscope to directly observe the icing process of particulate matter, and combined with STXM technology to analyze ice nucleation particles[61]. This technique can be used to analyze the spatial distribution of different chemical components of ice nucleation particles, to explore whether there are significant differences in the chemical composition and distribution of ice nucleation particles and non-ice nucleation active particles, and to explore the icing active sites of particles.
In another study by the same research group, Knopf et al. Used STXM to analyze the chemical composition of particulate matter, including BC, organic carbon, and inorganic substances, using an ice nucleation measurement system combined with electron microscopy-energy dispersive X-ray analysis (SEM/EDX)[62]. They did not find a correlation between the chemical composition resolved by STXM and the ability to form ice nuclei in the samples tested, and considered that the ice nuclei in these samples did not have special chemical characteristics, and that the freezing of particulate matter as ice nuclei might be controlled by random processes.
Wang et al. Reported the initial conditions of heterogeneous nucleation and moisture absorption of particles collected in Los Angeles and Mexico City, and performed single particle characterization with STXM/NEXAFS.Four main particle types were identified, including BC coated with organic matter, a mixture of organic, inorganic, and BC, inorganic particles coated with marine organic matter, and Pb/Zn-containing particles from waste incineration emissions[63]. It is found that there are significant differences in the initial conditions of temperature and humidity for different types of particles to absorb moisture and soak and freeze above 230 K. Below 230 K, the particles show a high efficiency of freezing by sublimation. Compared with sublimation freezing, water uptake and infiltration freezing are more sensitive to changes in particle chemical composition.
Ghorai et al. Used in situ STXM to study the hygroscopic characteristics of NaCl, NaBr, and NaI, demonstrating the feasibility of using STXM for salt phase diagram studies[64]. Piens et al. Combined SEM/EDX and in situ STXM to quantify the correlation of hygroscopicity of individual particles with their inorganic atomic fraction and mixing state[65]. P Pöhlker et al. Used STXM to find that particles collected in the Amazon rainforest could undergo reproducible morphological transformation in the RH range of 60% ~ 80%[66]. They believe that this may be dominated by the sulfate component of the particles.
4.3.3 Reaction mechanism
Alpert et al. Used STXM to identify the presence of unsaturated carbon bonds in the aerosol particle phase, complementing other aerosol experimental results[67]. Ghorai et al. Used STXM/NEXAFS to study the tautomerism of malonic acid at different RH, and quantified the C = C bond and — COOH bond from the C K-edge and O K-edge spectra[68].
Steimer et al. First used STXM/NEXAFS to study the ozone degradation rate of shikimic acid at different RH[69]. By quantitatively tracking the C = C bond in the C K-edge spectrum, it was found that the degradation rate of shikimic acid in ozone was particularly affected by RH: when RH decreased from 82% to 12%, its Life time increased by three orders of magnitude. These data were then applied to the KM-SUB model by Berkemeier et al. To determine some important physicochemical parameters, such as the chemical reaction kinetic constants of shikimic acid and reactive oxygen intermediates, the movement of ozone molecules in the particle phase, and the phase transition of particles[70]. The KM-SUB model also simulated that there was a concentration gradient of shikimic acid in the particles during the ozone degradation process. However, this result was not observed in the experiments of Steimer et al[67]. Alpert et al. Used the same experimental setup, using a spatial resolution of 35 nm, to observe different degrees of iron concentration gradients in the reaction system of Xanthan gum, ferric chloride (FeCl2) and ozone when RH increased from 0 to 80%[71]. The advantage of using iron in STXM/NEXAFS is that the X-ray absorption peak of iron is stronger and narrower than that of carbon functional groups (such as C = C bond), and higher contrast can be observed to detect Fe (II) and Fe (III). Using the diffusion coefficients of ozone and iron reported in the literature, as well as the Henry constants of ozone at different RH, the KM-SUB model well reproduced the concentration gradient of iron in xanthan gum particles, demonstrating the importance of the reaction-diffusion kinetic mechanism in the aging process of high-viscosity (low-RH) aerosol particles.
Recently, Alpert et al. Used STXM/NEXAFS to study a new organic particle degradation system, Iron citrate photocatalytic Citric acid[72]. Similarly, at RH = 40%, 50%, 60%, a concentration gradient of iron was observed in particulate matter to varying degrees. Based on this, a new photochemical reaction and diffusion model (PRAD) was developed[73]. By adjusting some reaction equilibrium constants and reaction rate constants (such as the photoreduction rate of ferric compounds and the oxidation rate of ferrous compounds), as well as the diffusion coefficient of molecules including oxygen in organic acid particles, the model can reproduce the experimental results of STXM/NEXAFS and two other techniques (Electrodynamic balance and Coated wall flow tube). The model not only confirms these results, but also predicts that oxygen is difficult to penetrate into the particle phase due to the combined effects of rapid chemical reactions and slow diffusion near the particle surface, so that photochemically generated radicals are effectively confined in the anoxic organic matrix. These particles with high concentrations of free radicals not only change the aging process of aerosols, but also have harmful effects on organisms inhaling the particles.
5 Limitations and Prospects
Most of the experimental systems related to atmospheric science need experimental conditions such as atmospheric pressure and low temperature. At present, a few synchrotron radiation facilities in the world are equipped with experimental devices that can achieve atmospheric pressure and low temperature, but the limited machine time and resources make the research requiring accelerator facilities bear relatively high risks and time costs. This limitation will be alleviated in the future due to the accelerator facilities currently under construction and experimental stations suitable for atmospheric science research. In addition, the current travel restrictions caused by the COVID-19 epidemic have prompted recent attempts by accelerators (such as MAX IV) to allow users to conduct experiments remotely. Preliminary results show that through the cooperation of remote users and local accelerator experts, the experimental efficiency has been greatly improved compared with the traditional experimental methods. Therefore, the optimization of accelerator experimental methods in the future will further alleviate the problem of accelerator machine time shortage.
6 Sum up
This paper introduces the theoretical principles, key technologies and main research results of the detection technology based on synchrotron radiation in the context of atmospheric science. Obviously, the development of synchrotron radiation facilities has provided strong support for the study of aerosol science and atmospheric heterogeneous chemistry, enabling atmospheric scientists to explore unknown areas and latitudes. It can be predicted that more and more important atmospheric processes and mechanisms will be revealed by synchrotron radiation-based technology, which will also reflect the great potential and value of synchrotron radiation devices in the field of atmosphere and environment.
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