Surface Of Aerosol Particles Research Articles

Bridging New Observational Capabilities and Process-Level Simulation: Insights into Aerosol Roles in the Earth System

Abstract The spatial distribution of ambient aerosol particles significantly impacts aerosol–radiation–cloud interactions, which contribute to the largest uncertainty in global anthropogenic radiative forcing estimations. However, the atmospheric boundary layer and lower free troposphere have not been adequately sampled in terms of spatiotemporal resolution, hindering a comprehensive characterization of various atmospheric processes and impeding our understanding of the Earth system. To address this research data gap, we have leveraged the development of uncrewed aerial systems (UAS) and advanced measurement techniques to obtain mesoscale spatial data on aerosol microphysical and optical properties around the U.S. Southern Great Plains (SGP) atmospheric observatory. Our study also benefits from state-of-the-art laboratory facilities that include three-dimensional molecular imaging techniques enabled by secondary ion mass spectrometry and nanogram-level chemical composition analysis via micronebulization aerosol mass spectrometry. Through our study, we have developed a framework for observation–modeling integration, enabling an examination of how various assumptions about the organic–inorganic components mixing state, inferred from chemical analysis, affect clouds and radiation in observation-constrained model simulations. By integrating observational constraints (derived from offline chemical analysis of the aerosol surface using collected samples) with in situ UAS observations, we have identified a prominent role of organic-enriched nanometer layers located at the surface of aerosol particles in determining profiles of aerosol optical and hygroscopic properties over the SGP observatory. Furthermore, we have improved the agreement between predicted clouds and ground-based cloud lidar measurements. This UAS–model–laboratory integration exemplifies how these new advanced capabilities can significantly enhance our understanding of aerosol–radiation–cloud interactions.

The technical innovation of integrating the gold mesh sampling method into FTIR spectroscopy, along with its application in the domain of transportation-related pollution studies, signifies a noteworthy advancement in analytical capabilities

Recently, Fourier transform infrared micro-spectroscopy (FTIR) has played a crucial role in the study of heterogeneous reactions in atmospheric chemistry. This includes research on aerosol hygroscopicity, nucleation kinetics, chemical composition analysis of aerosols, and heterogeneous chemical reactions on aerosol particle surfaces. This paper presents a novel approach that introduces a gold mesh during the sampling process, enabling the true spectrum of liquid water to be measured in transmission mode, thereby addressing spectral distortion issues at 3400 cm-1. Additionally, common inorganic components of atmospheric pollution were measured, providing a feasible method for using FTIR spectroscopy to address atmospheric pollution issues. Finally, the introduction of the gold mesh sampling method was employed to measure the heterogeneous chemical kinetics of road dust surface during traffic pollution processes, offering a viable solution for using FTIR spectroscopy to address atmospheric pollution concerns.

Peroxides on the Surface of Organic Aerosol Particles Using Matrix-Assisted Ionization in Vacuum (MAIV) Mass Spectrometry.

Organic peroxides are key intermediates in the atmosphere but are challenging to detect, especially in the particle phase, due to their instability, which has led to substantial gaps in the understanding of their environmental effects. We demonstrate that matrix-assisted ionization in vacuum (MAIV) mass spectrometry (MS), which does not require an ionization source, enables in situ characterization of peroxides and other products in the surface layers of organic particles. Hydroxyl radical oxidation of glutaric acid particles yields hydroperoxides and organic peroxides, which were detected with signals of the same order of magnitude as the major, more stable products. Product identification is supported by MS/MS analysis, peroxide standards, and offline high-resolution MS. The peroxide signals relative to the stable carbonyl and alcohol products are significantly larger using MAIV compared to those in the offline bulk analysis. This is also the case for analysis using fast, online easy ambient sonic-spray ionization mass spectrometry. These studies demonstrate the advantage of MAIV for the real-time characterization of labile peroxides in the surface layers of solid particles. The presence of peroxides on the surface may be important for surface oxidation processes as well as for the toxicity of inhaled particles.

Heterogeneous Ice Nucleation in Model Crystalline Porous Organic Polymers: Influence of Pore Size on Immersion Freezing.

Heterogeneous ice nucleation activity is affected by aerosol particle composition, crystallinity, pore size, and surface area. However, these surface properties are not well understood, regarding how they act to promote ice nucleation and growth to form ice clouds. Therefore, synthesized materials for which surface properties can be tuned were examined in immersion freezing mode in this study. To establish the relationship between particle surface properties and efficiency of ice nucleation, materials, here, covalent organic frameworks (COFs), with different pore diameters and degrees of crystallinity (ordering), were characterized. Results showed that out of all the highly crystalline COFs, the sample with a pore diameter between 2 and 3 nm exhibited the most efficient ice nucleation activity. We posit that the highly crystalline structures with ordered pores have an optimal pore diameter where the ice nucleation activity is maximized and that the not highly crystalline structures with nonordered pores have more sites for ice nucleation. The results were compared and discussed in the context of other synthesized porous particle systems. Such studies give insight into how material features impact ice nucleation activity.

Unexpected Behavior of Chloride and Sulfate Ions upon Surface Solvation of Martian Salt Analogue

Gas-phase interactions with aerosol particle surfaces are involved in the physicochemical evolution of our atmosphere as well as those of other planets (e.g., Mars). However, our understanding of interfacial properties remains limited, especially in natural systems with complex structures and chemical compositions. In this study, a surface-sensitive technique, ambient pressure X-ray photoelectron spectroscopy, combined with molecular dynamics simulations, were employed to investigate a Martian salt analogue sampled on Earth, including a comparison with a typical sulfate salt (MgSO4) commonly found on both Earth and Mars. For MgSO4, elemental depth profiles show that there always exists residual water on the salt surface, even at very low relative humidity (RH). When RH rises, water is well mixed with the salt within the probed depth of a few nanometers. The Cl–- and SO42–-bearing Martian salt analogue surface is extremely sensitive to water vapor, and the surface layer is already fully solvated at very low RH. Unexpected ion-selective surface behavior are observed as RH rises, where the chloride is depleted, while another major anion, sulfate, is relatively enhanced when the surface becomes solvated. Molecular dynamics simulations suggest that, upon solvation with the formation of an ion-concentrated water layer adsorbed on the crystal substrate, monovalent ions experience a higher degree of dehydration than the divalent ions. Thus, to complete their first solvation shell, monovalent ions are driven away from the surface and move toward the water accumulated at the hydrophilic crystal structure.

Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO2 laser

Understanding the formation and long-timescale evolution of atmospheric plasmas produced by ultrashort, long-wavelength infrared (LWIR) pulses is an important but partially understood problem. Of particular interest are plasmas produced in air with a peak laser intensity ∼1012 W/cm2, the so-called clamping intensity observed in LWIR atmospheric guiding experiments where tunneling and multi-photon ionization operative at near-IR or shorter wavelengths are inoperative. We find that avalanche breakdown on the surface of aerosol (dust) particles can act to seed the breakdown of air observed above the 200 GW/cm2 threshold when a train of 3 ps 10.6 μm laser pulses separated by 18 ps is used. The breakdown first appears at the best focus but propagates backward toward the focusing optic as the plasma density approaches critical density and makes forward propagation impossible. The velocity of the backward propagating breakdown can be as high as 109 cm/s, an order of magnitude greater than measured with ns pulse-produced breakdown, and can be explained rather well by the so-called breakdown wave mechanism. Transverse plasma expansion with a similar velocity is assisted by UV photoionization and is observed as a secondary longitudinal breakdown mechanism in roughly 10% of the shots. When a cm-size, TW power beam is propagated, interception of aerosol particles is guaranteed and several (40 cm−3) breakdown sites appear, each initially producing a near-critical density plasma. On a 10 ns–1 μs timescale, shockwaves from each site expand radially and coalesce to produce a large hot gas channel. The radial velocity of the expansion agrees well with the prediction of the blast wave theory developed for ultrafast atmospheric detonations.

In situ analysis of the bulk and surface chemical compositions of organic aerosol particles

Understanding the chemical and physical properties of particles is an important scientific, engineering, and medical issue that is crucial to air quality, human health, and environmental chemistry. Of special interest are aerosol particles floating in the air for both indoor virus transmission and outdoor atmospheric chemistry. The growth of bio- and organic-aerosol particles in the air is intimately correlated with chemical structures and their reactions in the gas phase at aerosol particle surfaces and in-particle phases. However, direct measurements of chemical structures at aerosol particle surfaces in the air are lacking. Here we demonstrate in situ surface-specific vibrational sum frequency scattering (VSFS) to directly identify chemical structures of molecules at aerosol particle surfaces. Furthermore, our setup allows us to simultaneously probe hyper-Raman scattering (HRS) spectra in the particle phase. We examined polarized VSFS spectra of propionic acid at aerosol particle surfaces and in particle bulk. More importantly, the surface adsorption free energy of propionic acid onto aerosol particles was found to be less negative than that at the air/water interface. These results challenge the long-standing hypothesis that molecular behaviors at the air/water interface are the same as those at aerosol particle surfaces. Our approach opens a new avenue in revealing surface compositions and chemical aging in the formation of secondary organic aerosols in the atmosphere as well as chemical analysis of indoor and outdoor viral aerosol particles.

Sources and processes of iron aerosols in a megacity in Eastern China

Abstract. Iron (Fe) in aerosol particles is a major external source of micronutrients for marine ecosystems and poses a potential threat to human health. To understand the impacts of aerosol Fe, it is essential to quantify the sources of dissolved Fe and total Fe. In this study, we applied receptor modeling for the first time to apportion the sources of dissolved Fe and total Fe in fine particles collected under five different weather conditions in the Hangzhou megacity of Eastern China, which is upwind of the East Asian outflow. Results showed that Fe solubility (dissolved Fe to total Fe) was the largest on fog days (6.7 ± 3.0 %), followed by haze (4.8 ± 1.9 %), dust (2.1 ± 0.7 %), clear (1.9 ± 1.0 %), and rain (0.9 ± 0.5 %) days. Positive matrix factorization (PMF) analysis suggested that industrial emissions were the largest contributor to dissolved Fe (44.5 %–72.4 %) and total Fe (39.1 %–55.0 %, except for dust days) during haze, fog, dust, and clear days. Transmission electron microscopy analysis of individual particles showed that > 75 % of Fe-containing particles were internally mixed with acidic secondary aerosol species on haze, fog, dust, and clear days. Furthermore, Fe solubility showed significant positive correlations with aerosol acidity/total Fe and liquid water content. These results indicated that the wet surface of aerosol particles promotes heterogeneous reactions between acidic species and Fe aerosols, contributing to a high Fe solubility.

An experimental and modelling analysis of cloud droplet growth from vehicular emissions with non‐ideal microphysics over an Asian mega‐city

AbstractLarge cities in the developing world have unregulated traffic with dysfunctional diesel operated vehicles spewing out copious amounts of micron to sub‐micron sized soot and black carbon particles. These particles mix with other aerosol particles as they are lifted by buoyant eddies. It is shown that the particles are rendered partly soluble as they get coated with soluble sulphate to activate into cloud droplets. This multi‐component aerosol mixture thus comprises of fully soluble as well as partially soluble components so that mass accommodation and diffusional mass transfer considerations need to be carefully quantified over small non‐planar surfaces of aerosol particles sourced from such emissions. The emitted soot and black carbon particles have number concentrations comparable to the sulphate mode and are small enough that the assumption of an infinitely dilute solution droplet is untenable. An X‐ray fluorescence spectrometry indicates the presence of inorganic ions confirming the validity of non‐ideal solution effects warranting a closer look at the osmotic coefficients. Additionally, laboratory values of the binary diffusion coefficients of water vapour cannot be applied to processes operating over the cloud base located several kilometres above where the pressure and temperature are different. A full Lennard‐Jones diffusivity model is used with an accounting of the dipole moment of water vapour which most models ignore. This first study, combining these micro‐scale enhancements are included in a Weather Research and Forecasting (WRF) case study over an Asian city. Not only are significant differences obtained in the cloud morphology when compared to a baseline case, but modelled cloud optical depths agree better with observations.

Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles.

Although the multiphase chemistry of SO2 in aerosol particles is of great importance to air quality under polluted haze conditions, a fundamental understanding of the pertinent mechanisms and kinetics is lacking. In particular, there is considerable debate on the importance of NO2 in the oxidation of SO2 in aerosol particles. Here experiments with atmospherically relevant deliquesced particles at buffered pH values of 4-5 show that the effective rate constant for the reaction of NO2 with SO32- ((1.4 ± 0.5) × 1010 M-1 s-1) is more than three orders of magnitude larger than the value in dilute solutions. An interfacial reaction at the surface of aerosol particles probably drives the very fast kinetics. Our results indicate that oxidation of SO2 by NO2 at aerosol surfaces may be an important source of sulfate aerosols under polluted haze conditions.

Quantitative relationship between the structures and properties of VOCs and SOA formation on the surfaces of acidic aerosol particles.

In this research, all the efforts, based on a series of molecular dynamics simulations on the interfacial process between VOC-contaminated air and acidic sulfate, were made to find how the structures and properties of VOCs are related to the formation of SOAs. The experimental fractional aerosol coefficients (FACs) were used to quantify the SOA formation and 14 VOC species were chosen based on the atmosphere inventory and the FAC magnitude. Finally, the quantitative relationship (QR) was found through the FAC as a function of the two variables the total valid interactions (Tg) and the diffusion coefficient (D), with R square 0.94. Meanwhile, the effect of water was explored and the QR was proved to be rational and reliable. The QR not only explained the SOA formation capacity of VOCs, but could also predict the SOA formation of new molecules.

The Impact of Cesium Vapor Bulk Condensation on the Transport of Aerosols in the Reactor Plant’s Primary Circuit during a Severe Accident at a Nuclear Power Plant with VVER

The article presents the results from numerical modeling of aerosol particles' deposition process during their transportation in the horizontal section of the reactor plant’s primary circuit model for the conditions typical for a severe accident at an NPP equipped with VVER reactors. The effect that the bulk condensation of cesium vapors has on the growth and deposition of aerosols during their transportation is analyzed. The influence of the initial size and concentration of aerosol particles on their growth rate and subsequent deposition is considered. Homogeneous condensation occurring in the absence of heterogeneous condensation nuclei (solid aerosol particles) and also heterogeneous condensation under the conditions in which nuclei of the new phase do not emerge according to the homogeneous mechanism are considered as the limit cases. Data on the average size of the produced droplets and aerosol particles along the channel axis in the condensation zone with the varied value of aerosol particles mass fraction in the flow have been obtained, which corresponded to different ratios of the condensate mass fraction in the form of droplets and liquid layer on the surface of solid aerosol particles. It has been established that the general deposition rate decreases if the flow contains droplets generated as a result of homogeneous bulk condensation and having a size smaller than the size of grown aerosol particles. The article describes the analysis scheme of the modeled section whose geometrical shape was specified based on the horizontal section of the experimental installation in the FALCON-ISP1 series of experiments on the deposition and transport of radionuclide aerosols. The cesium vapor homogeneous-heterogeneous condensation process is modeled by numerically solving—using the method of moments—the kinetic equation for the droplet size distribution function with involvement of the authors’ COND-KINET-1 computer program. The transport and deposition of aerosol particles in the primary circuit channel were modeled using the MAVR-TA computer code developed at the National Research Center Kurchatov Institute.

In Situ Spectroscopic Probing of Polarity and Molecular Configuration at Aerosol Particle Surfaces

The growth of aerosol particles in the atmosphere is related to chemical reactions in the gas and particle phases and at aerosol particle surfaces. While research regarding the gas and particle phases of aerosols is well-documented, physical properties and chemical reactivities at aerosol particle surfaces have not been studied extensively but have long been recognized. In particular, in situ measurements of aerosol particle surfaces are just emerging. The main reason is a lack of suitable surface-specific analytical techniques for direct measurements of aerosol particles under ambient conditions. Here we develop in situ surface-specific electronic sum frequency scattering (ESFS) to directly identify spectroscopic behaviors of molecules at aerosol particle surfaces. As an example, we applied an ESFS probe, malachite green (MG). We examined electronic spectra of MG at aerosol particle surfaces and found that the polarity of the surfaces is less polar than that in bulk. Our quantitative orientational analysis shows that MG is orientated with a polar angle of 25°-35° at the spherical particle surfaces of aerosols. The adsorption free energy of MG at the aerosol surfaces was found to be -20.75 ± 0.32 kJ/mol, which is much lower than that at the air/water interface. These results provide new insights into aerosol particle surfaces for further understanding the formation of secondary organic aerosols in the atmosphere.

Lifetime of Triplet Photosensitizers in Aerosol Using Time-Resolved Photoelectric Activity

Time-resolved photoelectric activity of aerosol (TPEAA) experiments measure the kinetics of photochemically initiated reactions near the surface of aerosol particles. In the demonstration presented here, TPEAA experiments monitor the evolution of excited triplet states, which are key intermediates in the photochemical formation of secondary organic aerosol (SOA). One example is the decay of the triplet excited state of imidazole-2-carboxaldehyde (IC) in aqueous NaCl aerosol particles. The decay of the triplet IC shows two distinct timescales, indicative of two populations of IC in the particles: aqueous and pure IC. The lifetime in the pure-IC phase is less than 20 ns, and the lifetime associated with aqueous IC is limited by reaction with chloride ions (k = 5.6 × 105 M–1 s–1). The results demonstrate how phase separation in aerosol particles can control reaction pathways and kinetics. In another example, TPEAA monitors the decay of the triplet anthraquinone-2-sulfonate (AQ2S) anion in aqueous NaCl aerosol. The AQ2S triplet reacts with water (k′ = 8.7 × 106 s–1) to produce adduct species. In this case, tuning the wavelength of the photoemission laser permits the preferential observation of the triplet or a combination of the AQ2S triplet and the water adduct. These results bridge a gap between aerosol phase measurements, which are normally steady-state and bulk-phase transient absorption measurements, which typically probe homogeneous phases.

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Abstract. Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (HO2) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC ∕ CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of HO2 production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with HO2. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the HO2 production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high HO2 turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (O3) at 5.0×10-6 M s−1 assuming O3 was in Henry's law equilibrium with the particle. However, using an O3 diffusion coefficient of 1×10-12 cm2 s−1, iodine activation in an aged, organic-rich sea spray is estimated to be 5.5×10-8 M s−1. The estimated iodine production from BrC photochemistry based on the results reported here amounts to 4.1×10-7 M s−1 and indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Photoemission of Iodide from Aqueous Aerosol Particle Surfaces.

The photoemission of iodide from aqueous aerosol particle surfaces measures the surface concentration of iodide in predominantly supersaturated NaCl aerosol particles. Using the Langmuir model to describe the adsorption to the surface of aqueous iodide anions, the standard Gibbs free energy of adsorption is -15 kJ/mol in these systems. The presence of charged surfactants on the particle surfaces changes the adsorption behavior of iodide. The addition of sodium docecylsulfate (SDS) reduces the coverage of iodide, consistent with a competitive adsorption scenario. For surfaces coated with C12-, C14-, or C16-trimethylammonium chloride, the addition of iodide results in the formation of iodide-surfactant ion pairs at the surface with enhanced photoemission. The adsorption free energy for iodide in these systems is -21 kJ/mol. The results demonstrate the surface enhancement of iodide in supersaturated, atmospherically relevant conditions and demonstrate important differences between single-salt solutions and mixtures in the limit of high concentration.

Insights into submicron particulate evolution, sources and influences on haze pollution in Beijing, China

The real-time continuous measurements of non-refractory submicron aerosol (NR-PM1) species including organics (Org), sulfate (SO42−), nitrate (NO3−), ammonium (NH4+) and chloride (Cl−) in winter from 10 December 2015 to 30 January 2016 in Beijing were performed with an Aerodyne Aerosol Chemical Speciation Monitor (ACSM). The average concentration of NR-PM1 was 81.24 μg m−3, with the mean concentration in heavy haze days being 220.00 μg m−3, ∼3 times higher than that in light haze days and ∼15 times higher than that in clean days. Org was the most significant component of NR-PM1 species, accounting for 53% of the total NR-PM1 for the entire study. SO42− was also a significant component, accounting for 23% of the total NR-PM1. However, NO3−, NH4+ and Cl− composition together accounted for 25% of the total NR-PM1. All NR-PM1 species presented remarkably diurnal cycles in haze days, characterized by the highest concentrations occurring at midnight, and the lowest concentrations occurring at daytime. Note that the sulfur oxidation ratios were higher than the nitrogen oxidation ratios for the entire study, especially during the haze periods. The formation of sulfate was mainly affected by relative humidity (RH), while that of nitrate was more associated with NH3. Heterogeneous oxidation of NO2 on the surfaces of aerosol particles might be a significant pathway of nitrate formations during haze periods. NR-PM1 was mainly from secondary chemical reactions contributing 46.1%, vehicle emissions contributing 22.3%, coal combustion contributing 16.1%, and biomass burning contributing 15.6% in clean days. However, compared to the clean-day source contributions, the haze-day secondary source contribution to NR-PM1 increased to 66.8%, indicating that NR-PM1 in haze days was dramatically dominated by the secondary pollutants.

A theoretical study on the activation of insoluble particles in atmospheric conditions

A theoretical analysis for the description of insoluble particles activation in the atmosphere has been performed using as framework the classical theory of heterogeneous nucleation. In the current work a model for heterogeneous nucleation for one-component and two-component gaseous systems was presented taking into account the process of surface diffusion during nucleating embryo formation. The theory predicts higher nucleation rates compared to the classical theory of heterogeneous nucleation and consequently higher probability of particle activation. The model including the effect of surface diffusion shows a qualitative agreement with experimental results of heterogeneous nucleation of water at different substrates. In addition, the process of heterogeneous nucleation of the sulphuric acid – water system on the surface of insoluble spherical aerosol particles in atmospheric conditions has been examined. Insoluble particle activation is depended on several factors such as the concentration of the participating vapours, the ambient temperature, the chemical composition and particle size. Our results show that activation of insoluble particles is an energetically favourable process under specific conditions in the atmosphere.

NO<sub>2</sub>-initiated multiphase oxidation of SO<sub>2</sub> by O<sub>2</sub> on CaCO<sub>3</sub> particles

Abstract. The reaction of SO2 with NO2 on the surface of aerosol particles has been suggested to be important in sulfate formation during severe air pollution episodes in China. However, we found that the direct oxidation of SO2 by NO2 was slow and might not be the main reason for sulfate formation in ambient air. In this study, we investigated the multiphase reaction of SO2 with an O2 ∕ NO2 mixture on single CaCO3 particles using Micro-Raman spectroscopy. The reaction converted the CaCO3 particle to a Ca(NO3)2 droplet, with CaSO4 ⚫ 2H2O solid particles embedded in it, which constituted a significant fraction of the droplet volume at the end of the reaction. The reactive uptake coefficient of SO2 for sulfate formation was on the order of 10−5, which was higher than that for the multiphase reaction of SO2 directly with NO2 by 2–3 orders of magnitude. According to our observations and the literature, we found that in the multiphase reaction of SO2 with the O2 ∕ NO2 mixture, O2 was the main oxidant of SO2 and was necessary for radical chain propagation. NO2 acted as the initiator of radical formation, but not as the main oxidant. The synergy of NO2 and O2 resulted in much faster sulfate formation than the sum of the reaction rates with NO2 and with O2 alone. We estimated that the multiphase oxidation of SO2 by O2 initiated by NO2 could be an important source of sulfate and a sink of SO2, based on the calculated lifetime of SO2 regarding the loss through the multiphase reaction versus the loss through the gas-phase reaction with OH radicals. Parameterization of the reactive uptake coefficient of the reaction observed in our laboratory for further model simulation is needed, as well as an integrated assessment based on field observations, laboratory study results, and model simulations to evaluate the importance of the reaction in ambient air during severe air pollution episodes, especially in China.

Using X-ray computed tomography and micro-Raman spectrometry to measure individual particle surface area, volume, and morphology towards investigating atmospheric heterogeneous reactions

Heterogeneous reactions on the aerosol particle surface in the atmosphere play important roles in air pollution, climate change, and global biogeochemical cycles. However, the reported uptake coefficients of heterogeneous reactions usually have large variations and may not be relevant to real atmospheric conditions. One of the major reasons for this is the use of bulk samples in laboratory experiments, while particles in the atmosphere are suspended individually. A number of technologies have been developed recently to study heterogeneous reactions on the surfaces of individual particles. Precise measurements on the reactive surface area, volume, and morphology of individual particles are necessary for calculating the uptake coefficient, quantifying reactants and products, and understanding the reaction mechanism better. In this study, for the first time we used synchrotron radiation X-ray computed tomography (XCT) and micro-Raman spectrometry to measure individual CaCO3 particle morphology, with sizes ranging from 3.5–6.5μm. Particle surface area and volume were calculated using a reconstruction method based on software three-dimensional (3-D) rendering. The XCT was first validated with high-resolution field-emission scanning electron microscopy (FE-SEM) to acquire accurate CaCO3 particle surface area and volume estimates. Our results showed an average difference of only 6.1% in surface area and 3.2% in volume measured either by micro-Raman spectrometry or X-ray tomography. X-ray tomography and FE-SEM can provide more morphological details of individual CaCO3 particles than micro-Raman spectrometry. This study demonstrated that X-ray computed tomography and micro-Raman spectrometry can precisely measure the surface area, volume, and morphology of an individual particle.