Identification of Intermediates in the Reaction Pathway of SO2 on the CaO Surface: From Physisorption to Sulfite to Sulfate

The interaction of CaO and Ca(OH)₂ with solvated or gaseous SO₂ plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO₂ from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO₂ with a CaO(001) single crystal. First, the surface chemistry of SO₂ was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO₂ solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivatating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas.

Shining New Light on the Kinetics of Water Uptake by Organic Aerosol Particles

The uptake of water vapor by various organic aerosols is important in a number of applications ranging from medical delivery of pharmaceutical aerosols to cloud formation in the atmosphere. The coefficient that describes the probability that the impinging gas-phase molecule sticks to the surface of interest is called the mass accommodation coefficient, α_M. Despite the importance of this coefficient for the description of water uptake kinetics, accurate values are still lacking for many systems. In this Feature Article, we present various experimental techniques that have been evoked in the literature to study the interfacial transport of water and discuss the corresponding strengths and limitations. This includes our recently developed technique called photothermal single-particle spectroscopy (PSPS). The PSPS technique allows for a retrieval of α_M values from three independent, yet simultaneous measurements operating close to equilibrium, providing a robust assessment of interfacial mass transport. We review the currently available data for α_M for water on various organics and discuss the few studies that address the temperature and relative humidity dependence of α_M for water on organics. The knowledge of the latter, for example, is crucial to assess the water uptake kinetics of organic aerosols in the Earth's atmosphere. Finally, we argue that PSPS might also be a viable method to better restrict the α_M value for water on liquid water.

How Relevant Is It to Use Mineral Proxies to Mimic the Atmospheric Reactivity of Natural Dust Samples? A Reactivity Study Using SO2 as Probe Molecule

The experimental investigation of heterogeneous atmospheric processes involving mineral aerosols is extensively performed in the literature using proxy materials. In this work we questioned the validity of using proxies such as Fe2O3, FeOOH, Al2O3, MgO, CaO, TiO2, MnO2, SiO2, and CaCO3 to represent the behavior of complex mixtures of minerals, such as natural desert and volcanic dusts. Five volcanic dusts and three desert dusts were compared to a number of metal oxides, commonly used in the literature to mimic the behavior of desert dusts in the ability to form sulfites and sulfates on the surface exposed to SO2 gas. First, all samples were aged at room temperature, atmospheric pressure, under controlled experimental conditions of 175 ppm SO2 for 1 h under 30% of relative humidity. Second, they were extracted with 1% formalin and analyzed by High-Performance Liquid Chromatography (HPLC) to quantify and compare the amount of sulfites and sulfates formed on their surfaces. It was evidenced that under the experimental conditions of this study neither one selected pure oxide nor a mixture of oxides can adequately typify the behavior of complex mixtures of natural minerals. Therefore, to evaluate the real-life impact of natural dust on atmospheric processes it is of vital importance to work directly with the natural samples, both to observe the real effects of desert and volcanic dusts and to evaluate the relevancy of proposed proxies.

Method development and validation for the determination of sulfites and sulfates on the surface of mineral atmospheric samples using reverse-phase liquid chromatography

Earlier studies suggest that SO₂ gas reacts at the surface of mineral dust and forms sulfites or bisulfites, which are then converted to sulfates. In order to monitor and quantify the amounts of both sulfites and sulfates formed on the surface of mineral dusts of volcanic and desert origins an accurate and precise reversed-phase liquid chromatography method was developed and validated to extract, stabilize and individually analyze sulfites and sulfates initially present on the surface of dusts exposed to SO₂. The method was developed on a 25 mm Restek Ultra Column C18, Particle size: 5 μm, I.D. 4.60 mm column which was dynamically coated with 1.0 mM cetylpyridinium chloride in 7% acetonitrile solution to produce a charged surface as recommended in the literature. Mobile phase used: 1 mM Potassium Hydrogen Phthalate at pH 6.5 at a flow rate of 1.0 ml/min with negative UV-Vis detection at 255 nm in 15 min. The method was validated for specificity, linearity and range, injection repeatability, stability, robustness, limit of detection and limit of quantitation, and sample preparation and extraction reproducibility. The method was adapted for straight sulfite and sulfate quantification: (i) of environmental samples, and (ii) natural samples additionally exposed to SO₂ gas in a dedicated laboratory setup. The method was then successfully applied to quantify sulfites and sulfates on natural volcanic and a desert dust samples both collected in the environment and additionally exposed to SO₂ gas in the laboratory. The method can be efficiently used to identify sulfites and sulfates on fresh volcanic ash following an eruption, on aeolian desert dust exposed to industrial pollutants, as well as for laboratory investigations of sulfite and sulfate formation on the surface of minerals and natural dusts of different origins.

Effects of NO2 and SO2 on the heterogeneous reaction of acetic acid on α-Al2O3 in the presence and absence of simulated irradiation

The effects of NO2 and SO2 on the atmospheric heterogeneous reaction of acetic acid on α-Al2O3 in the presence and absence of simulated irradiation were investigated at ambient conditions by using the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique. The experiment was divided into two parts: the heterogeneous reaction experiment and the pre-adsorption reaction experiment under light and dark conditions. In the heterogeneous reaction experiment, solar radiation stimulates the formation of more acetate and nitrate. At the same time, it can promote the partial conversion of sulfites to sulfates in the heterogeneous reaction of SO2 on α-Al2O3 particles. It can be seen that solar radiation plays a significant role in the heterogeneous reactions of inorganic and organic gases on mineral particles. In the pre-adsorption reaction experiment, the pre-adsorbed nitrate, sulfite or sulfate have conspicuous inhibition influence on the formation of acetate in the presence and absence of simulated irradiation. This indicates that the role of pre-adsorbed species should be given more attention for the heterogeneous reaction of acetic acid on the surface of α-Al2O3 particles. When α-Al2O3 particles were pre-adsorbed by different species, simulated irradiation could facilitate the growth of different amounts of acetate. It was found that the extent to which solar radiation contributes to heterogeneous reactions of different kinds of gases on different mineral particles is different. This further emphasizes the complexities of the heterogeneous conversion processes of atmospheric trace gases on the surface of mineral aerosols, promoting a better understanding of the effects of solar radiation and pre-adsorption on the heterogeneous reaction in the atmosphere.

Competitive reactions of SO2 and acetic acid on α-Al2O3 and CaCO3 particles

Heterogeneous reactions between gaseous pollutants and mineral particles have gradually become a research hotspot in the field of atmospheric chemistry. In this paper, competitive reactions between SO₂ and acetic acid on the surface of α-Al₂O₃ and CaCO₃ particles were studied by the diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) technique in dark and dry conditions. At the same time, the temporary evolution of the integrated absorbance of acetate and sulfite was investigated to further understand the interaction of SO₂ and acetic acid on the mineral particles. On the surface of α-Al₂O₃ particles, acetate and sulfite can compete for surface-active sites, resulting in a decrease in the total amount of acetates. In dark and dry conditions, the effect of acetic acid on SO₂ cannot be obtained by the DRIFTS method. On the surface of CaCO₃ particles, SO₂ can have a competitive impact on acetic acid by grabbing active sites, leading to a slight decrease of the amount of acetates. The heterogeneous reaction of SO₂ can be impeded by coexisting acetic acid, resulting in a drastic reduction of the number of sulfites. It can be seen that the formation mechanisms of acetate and sulfite on the surface of different mineral particles in the atmosphere are different, which provides a variety of ideas and possibilities for the formation of related inorganic and organic salts in the atmosphere.

Gas-Phase Reactivity of Carbonate Ions with Sulfur Dioxide: an Experimental Study of Clusters Reactions

The reactivity of carbonate cluster ions with sulfur dioxide has been investigated in the gas phase by mass spectrometric techniques. SO₂ promotes the displacement of carbon dioxide from carbonate clusters through a stepwise mechanism, leading to the quantitative conversion of the carbonate aggregates into the corresponding sulfite cluster ions. The kinetic study of the reactions of positive, negative, singly, and doubly charged ions reveals very fast and efficient processes for all the carbonate ions.

Nitrate-Enhanced Oxidation of SO2 on Mineral Dust: A Vital Role of a Proton

Heterogeneous oxidation of SO₂ on mineral dust is a significant source of sulfate in the atmosphere. Given that a large fraction of nitrate is deposited on the mineral aerosols, the determination of the effect of nitrate on the SO₂ oxidation on mineral dust and its in-depth mechanism are much desired. In this work, we report nitrate-enhanced SO₂ oxidation on authentic mineral dust. By comparing the SO₂ uptake behaviors on Arizona test dust (ATD, a typical proxy of mineral dust) with or without nitrate, we found that although nitrate hinders the initial SO₂ uptake, it substantially accelerates SO₂ uptake and oxidation after a pronounced induction period. In other words, a hindering-then-accelerating feature in the SO₂ uptake profile was observed on nitrate-containing ATD (N-ATD) particles. In addition, HONO was released in the accelerating period as the reduction product of nitrate. The accumulation of protons (H⁺) from SO₂ oxidation during the induction period plays a key role in the acceleration of SO₂ oxidation. Our work suggests that the nitrate-participating SO₂ oxidation on mineral dust can be one of the important contributions of the sulfate source in the atmosphere.

Parameterization of heterogeneous reaction of SO2 to sulfate on dust with coexistence of NH3 and NO2 under different humidity conditions

Sulfate plays an important role in atmospheric haze in China, which has received considerable attention in recent years. Various types of parameterization methods and heterogeneous oxidation rates of SO₂ have been used in previous studies. However, properly representing heterogeneous sulfate formation in air quality models remains a big challenge. In this study, we quantified the heterogeneous oxidation reaction using experimental results that approximate the haze conditions in China. Firstly, a series of experiments were conducted to investigate the heterogeneous uptake of SO₂ with different relative humidity (RH) levels and the presence of NH₃ and NO₂ on natural dust surfaces. Then the uptake coefficients for heterogeneous oxidation of SO₂ to sulfate at different RH under NH₃ and NO₂coexistence were parameterized based on the experimental results and implemented in the Community Multiscale Air Quality modeling system (CMAQ). Simulation results suggested that this new parameterization improved model performance by 6.6% in the simulation of wintertime sulfate concentrations for Beijing. The simulated maximum growth rate of SO₄ ^2- during a heavy pollution period increased from 0.97 μg m^-3 h^-1 to 10.11 μg m^-3 h^-1. The heterogeneous oxidation of SO₂ in the presence of NH₃ contributed up to 23% of the sulfate concentration during heavy pollution periods.

Complex to simple: In vitro exposure of particulate matter simulated at the air-liquid interface discloses the health impacts of major air pollutants

Particulate matter (PM) exposure poses many adverse effects on human health. However, it is challenging to clearly differentiate between the contributions of individual pollutants on toxicity from complex mixtures of ambient air pollutants. The aim of this study is to generate aerosols constituted by silica nanoparticles (NPs) and bisulfate to serve as simulators of particle-associated high-sulfur air pollution. Then, the health impacts of sulfur dioxide were evaluated at the cellular level using an air-liquid interface (ALI) exposure chamber. BEAS-2B cells were exposed to either nano-silica or bisulfite aerosol individually or bisulfate-coated silica (SiO₂-NH₂@HSO₃) for 3 h using the ALI. The cellular toxicities were carefully compared based on the exposure dosages. The ALI exposure of SiO₂ NPs alone did not produce any apparent cytotoxicity in cells, but the aerosol exposure of SiO₂-NH₂@HSO₃ significantly decreased the cell viability and enhanced the production of cellular reactive oxygen species in a dose-dependent manner. Consequently, the excessive oxidative stress resulted in mitochondrial damage as well as cellular apoptosis. ALI exposure can possibly reflect the realistic physiological exposure condition of the human respiratory system. As a derivative of the sulfur dioxide component of air pollution, sulfate exacerbates the toxic effects of inhalable PMs. This result may be due to the large surface area of the nanoparticles, with the possibility of carrying more sulfite to the target cells during aerosol exposure. The sulfate levels offer a meaningful complement to the present PM_2.5 index of air pollution for achieving better human health protection.

Sources of pollution and interrelationships between aerosol and precipitation chemistry at a central California site

This study examines co-located aerosol and precipitation chemistry data between 2010 and 2016 at Pinnacles National Monument ~65 km east of the coastline in central California. Positive matrix factorization analysis of the aerosol composition data revealed seven distinct pollutant sources: aged sea salt (25.7% of PM2.5), biomass burning (24.2% of PM2.5), fresh sea salt (15.0% of PM2.5), secondary sulfate (11.7% of PM2.5), dust (10.0% of PM2.5), vehicle emissions (8.2% of PM2.5), and secondary nitrate (5.2% of PM2.5). The influence of meteorology and transport on monthly patterns of PM2.5 composition is discussed. Only secondary sulfate exhibited a statistically significant change (a reduction) over time among the PM2.5 source factors. In contrast, PMcoarse exhibited a significant increase most likely due to dust influence. Monthly profiles of precipitation chemistry are summarized showing that the most abundant species in each month was either SO42-, NO3-, or Cl-. Intercomparisons between the precipitation and aerosol data revealed several features: (i) precipitation pH was inversely related to factors associated with more acidic aerosol constituents such as secondary sulfate and aged sea salt, in addition to being reduced by uptake of HNO3 in the liquid phase; (ii) two aerosol source factors (dust and aged sea salt) and PMcoarse exhibited a positive association with Ca2+ in precipitation, suggestive of directly emitted aerosol types with larger sizes promoting precipitation; and (iii) sulfate levels in both the aerosol and precipitation samples analyzed were significantly correlated with dust and aged sea salt PMF factors, pointing to the partitioning of secondary sulfate to dust and sea salt particles. The results of this work have implications for the region's air quality and hydrological cycle, in addition to demonstrating that the use of co-located aerosol and precipitation chemistry data can provide insights relevant to aerosol-precipitation interactions.

Effect of Acid on Surface Hydroxyl Groups on Kaolinite and Montmorillonite

Mineral dust aerosol participates in heterogeneous chemistry in the atmosphere. In particular, the hydroxyl groups on the surface of aluminosilicate clay minerals are important for heterogeneous atmospheric processes. These functional groups may be altered by acidic processing during atmospheric transport. In this study, we exposed kaolinite (KGa-1b) and montmorillonite (STx-1b) to aqueous sulfuric acid and then rinsed the soluble reactants and products off in order to explore changes to functional groups on the mineral surface. To quantify the changes due to acid treatment of edge hydroxyl groups, we use 19F magic angle spinning nuclear magnetic resonance spectroscopy and a probe molecule, 3,3,3-trifluoropropyldimethylchlorosilane. We find that the edge hydroxyl groups (OH) increase in both number and density with acid treatment. Chemical reactions in the atmosphere may be impacted by the increase in OH at the mineral edge.

Heterogeneous Photo-oxidation of SO2 in the Presence of Two Different Mineral Dust Particles: Gobi and Arizona Dust

The impact of authentic mineral dust particles sourced from the Gobi Desert (GDD) on the kinetic uptake coefficient of SO₂ was studied under varying environments (humidity, O₃, and NO_x) using both an indoor chamber and an outdoor chamber. There was a significant increase in the kinetic uptake coefficient of SO₂ (γ_SO4^2-_,light) for GDD particles under UV light compared to the value (γ_SO4^2-_,dark) under dark conditions at various relative humidities (RH) ranging from 20% to 80%. In both the presence and the absence of O₃ and NO_x, γ_SO4^2-_,light and γ_SO4^2-_,dark greatly increased with increasing RH. The resulting γ_SO4^2-_,light of GDD particles was also compared to that of Arizona Test Dust (ATD) particles. The γ_SO4^2-_,light values of GDD were 2 to 2.5 times greater than those of ATD for all RH levels. To understand the photocatalytic act of dust particles, both GDD and ATD were characterized for the metal element composition of fresh particles, the aerosol acidity of aged particles, and the hygroscopic properties of both fresh and aged particles. We conclude that the difference in the formation of sulfate between GDD and ATD particles is regulated mainly by the quantity of the semiconductive metals in dust particles and partially by hygroscopic properties.

Heterogeneous Reaction of SO2 on Manganese Oxides: the Effect of Crystal Structure and Relative Humidity

Manganese oxides from anthropogenic sources can promote the formation of sulfate through catalytic oxidation of SO₂. In this study, the kinetics of SO₂ reactions on MnO₂ with different morphologies (α, β, γ and δ) was investigated using flow tube reactor and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Under dry conditions, the reactivity towards SO₂ uptake was highest on δ-MnO₂ but lowest on β-MnO₂, with a geometric uptake coefficient (γ_obs) of (2.42 ± 0.13) ×10^–2 and a corrected uptake coefficient (γ_c) of (1.48 ± 0.21) ×10⁻⁶ for the former while γ_obs of (3.35 ± 0.43) ×10⁻³ and γ_c of (7.46 ± 2.97) ×10⁻⁷ for the latter. Under wet conditions, the presence of water altered the chemical form of sulfate and was in favor for the heterogeneous oxidation of SO₂. The maximum sulfate formation rate was reached at 25% RH and 45% for δ-MnO₂ and γ-MnO₂, respectively, possibly due to their different crystal structures. The results suggest that morphologies and RH are important factors influencing the heterogeneous reaction of SO₂ on mineral aerosols, and that aqueous oxidation process involving transition metals of Mn might be a potential important pathway for SO₂ oxidation in the atmosphere.

The effects of coexisting Na2SO4 on heterogeneous uptake of NO2 on CaCO3 particles at various RHs

Atmospheric particles can undergo nucleation, coagulation, chemical-aging, dissolution-precipitation or other atmospheric processes, resulting in complex multicomponent aerosols. The coexisting species have potentially important consequences in the heterogeneous reactions of multicomponent aerosol particles with polluted gases, which are still poorly understood. The effect of coexisting Na₂SO₄ on heterogeneous uptake of NO₂ on CaCO₃ particles is investigated in a broad RH range. The combination of DRIFTS, Raman, SEM and IC provides qualitative and quantitative information about the formation of nitrate and other surface species. Ca(NO₃)₂ and NaNO₃ are generated on mixed CaCO₃-Na₂SO₄ particles under dry condition. Both the amount of NO₃^- formed and the NO₃^- formation rates for the mixtures can be predicted based on the linear addition of those for pure CaCO₃ and Na₂SO₄ particles under dry condition. The further reaction of Ca(NO₃)₂ with Na₂SO₄ could lead to the formation of crystal NaNO₃ and CaSO₄·0.5H₂O at 30% RH. Coagulation between Ca²⁺ and SO₄^2- in surface adsorbed water is observed after part conversion of CaCO₃ to Ca(NO₃)₂, resulting in the formation of CaSO₄·2H₂O at 80% RH. The amount of NO₃^- formed on the mixtures is dramatically enhanced relative to the predictions at 30% and 80% RH. The findings presented here highlight the role of coexisting species in the heterogeneous reactions of trace gases with multicomponent aerosols due to the complexity of atmospheric particles.

SO2 Emissions in China - Their Network and Hierarchical Structures

SO₂ emissions lead to various harmful effects on environment and human health. The SO₂ emission in China has significant contribution to the global SO₂ emission, so it is necessary to employ various methods to study SO₂ emissions in China with great details in order to lay the foundation for policymaking to improve environmental conditions in China. Network analysis is used to analyze the SO₂ emissions from power generation, industrial, residential and transportation sectors in China for 2008 and 2010, which are recently available from 1744 ground surface monitoring stations. The results show that the SO₂ emissions from power generation sector were highly individualized as small-sized clusters, the SO₂ emissions from industrial sector underwent an integration process with a large cluster contained 1674 places covering all industrial areas in China, the SO₂ emissions from residential sector was not impacted by time, and the SO₂ emissions from transportation sector underwent significant integration. Hierarchical structure is obtained by further combining SO₂ emissions from all four sectors and is potentially useful to find out similar patterns of SO₂ emissions, which can provide information on understanding the mechanisms of SO₂ pollution and on designing different environmental measure to combat SO₂ emissions.

SO2 Initiates the Efficient Conversion of NO2 to HONO on MgO Surface

Nitrous acid (HONO) is an important source of hydroxyl radical (OH) that determines the fate of many chemically active and climate relevant trace gases. However, the sources and the formation mechanisms of HONO remain poorly understood. In this study, the effect of SO₂ on the heterogeneous reactions of NO₂ on MgO as a mineral dust surrogate was investigated. The reactivity of MgO to NO₂ is weak, while coexisting SO₂ can increase the uptake coefficients of NO₂ on MgO by 2-3 orders of magnitude. The uptake coefficients of NO₂ on SO₂-aged MgO are independent of NO₂ concentrations in the range of 20-160 ppbv and relative humidity (0-70%RH). The reaction mechanism was demonstrated to be a redox reaction between NO₂ and surface sulfite. In the presence of SO₂, NO₂ was reduced to nitrite under dry conditions, which could be further converted to gas-phase HONO in humid conditions. These results suggest that the reductive effect of SO₂ on the heterogeneous conversion of NO₂ to HONO may have a significant contribution to the unknown sources of HONO observed in polluted areas (for example, in China).

Aerosol composition and properties variation at the ground and over the column under different air masses advection in South Italy

Aerosol composition and properties variation under the advection of different air masses were investigated, as case studies, by contemporary measurements over the atmospheric column and at the ground in a semi-rural site in South Italy. The absence of local strong sources in this area allowed to characterize background aerosol and to compare particle mixing effects under various atmospheric circulation conditions. Aerosol optical depth (AOD) and Ǻngström parameters from radiometric measurements allowed the detection and identification of polluted, dust, and volcanic atmospheric conditions. AODs were the input for a suitable model to evaluate the columnar aerosol composition, according to six main atmospheric components (water-soluble, soot, sea salt accumulation, sea salt coarse, mineral dus,t and biological). Scanning electron microscope (SEM) analysis of particulate sampled with a 13-stage impactor at the ground showed not only fingerprints typical of the different air masses but also the effects of transport and aging on atmospheric particles, suggesting processes that changed their chemical and optical properties. Background columnar aerosol was characterized by 72% of water-soluble and soot, in agreement with ground-based findings that highlighted 60% of contribution from anthropogenic carbonate particles and soot. In general, a good agreement between ground-based and columnar results was observed. Under the advection of trans-boundary air masses, water-soluble and soot were always present in columnar aerosol, whereas, in variable percentages, sea salt and mineral particles characterized both dust and volcanic conditions. At the ground, sulfates characterized the amorphous matrix produced in finer stages by the evaporation of solutions of organic and inorganic aerosols. Sulfates were also one of the key players involved in heterogeneous chemical reactions, producing complex secondary aerosol, as such clay-sulfate internally mixed particle externally mixed with soot chains.

Heterogeneous uptake of gaseous hydrogen peroxide on mineral dust

The heterogeneous uptake processes of hydrogen peroxide on Arizona test dust and two types of authentic Chinese mineral dusts, i.e., Inner Mongolia desert dust and Xinjiang calciferous dust, were investigated using a Knudsen cell reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of H2O2 from 2.6 × 10(11) to 1.2 × 10(12)molecules/cm(3), and the temperature dependence of the uptake coefficients was investigated over a range from 253 to 313K. The concentration of H2O2 showed little effect on the uptake coefficients of these heterogeneous processes. As a function of temperature, the initial uptake coefficients decrease with increasing temperature, whereas the steady state uptake coefficients of Arizona test dust and Inner Mongolia desert dust increase with increasing temperature. Implications for the understanding of the uptake processes onto mineral dust samples were also discussed.

Synergistic formation of sulfate and ammonium resulting from reaction between SO2 and NH3 on typical mineral dust

A synergistic effect between SO₂ and NH₃ on typical mineral dust.

Heterogeneous photooxidation of sulfur dioxide in the presence of airborne mineral dust particles

Heterogeneous photocatalytic oxidation of SO₂on the surface of Arizona dust particles was investigated in the absence and the presence of NOxand O₃under varying humidity using a 2-m³indoor photoirradiation chamber.

Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide

Heterogeneous reaction of SO2 on mineral dust seems to be an important sink for SO2. However, kinetic data about this reaction on authentic mineral dust are scarce and are mainly limited to low relative humidity (RH) conditions. In addition, little is known about the role of hydrogen peroxide (H2O2) in this reaction. Here, we investigated the uptake kinetics of SO2 on three authentic mineral dusts (i.e., Asian mineral dust (AMD), Tengger desert dust (TDD), and Arizona test dust (ATD)) in the absence and presence of H2O2 at different RHs using a filter-based flow reactor, and applied a parameter (effectiveness factor) to the estimation of the effective surface area of particles for the calculation of the corrected uptake coefficient (γc). We found that with increasing RH, the γc decreases on AMD particles, but increases on ATD and TDD particles. This discrepancy is probably due to the different mineralogy compositions and aging extents of these dust samples. Furthermore, the presence of H2O2 can promote the uptake of SO2 on mineral dust at different RHs. The probable explanations are that H2O2 rapidly reacts with SO2 on mineral dust in the presence of adsorbed water, and OH radicals, which can be produced from the heterogeneous decomposition of H2O2 on the mineral dust, immediately react with adsorbed SO2 as well. Our results suggest that the removal of SO2 via the heterogeneous reaction on mineral dust is an important sink for SO2 and has the potential to alter the physicochemical properties (e.g., ice nucleation ability) of mineral dust particles in the atmosphere.

Knudsen cell and smog chamber study of the heterogeneous uptake of sulfur dioxide on Chinese mineral dust

The heterogeneous uptake processes of sulfur dioxide on two types of Chinese mineral dust (Inner Mongolia desert dust and Xinjiang sierozem) were investigated using both Knudsen cell and smog chamber system. The temperature dependence of the uptake coefficients was studied over a range from 253 to 313 K using the Knudsen cell reactor, the initial uptake coefficients decreased with the increasing of temperature for these two mineral dust samples, whereas the steady state uptake coefficients of the Xinjiang sierozem increased with the temperature increasing, and these temperature dependence functions were obtained for the first time. In the smog chamber experiments at room temperature, the steady state uptake coefficients of SO2 decreased evidently with the increasing of sulfur dioxide initial concentration from 1.72 × 10¹² to 6.15 × 10¹² mol/cm³. Humid air had effect on the steady state uptake coefficients of SO₂onto Inner Mongolia desert dust. Consequences about the understanding of the uptake processes onto mineral dust samples and the environmental implication were also discussed.

Hygroscopic properties of internally mixed particles composed of NaCl and water-soluble organic acids

Atmospheric aging of naturally emitted marine aerosol often leads to formation of internally mixed particles composed of sea salts and water-soluble organic compounds of anthropogenic origin. Mixing of sea salt and organic components has profound effects on the evolving chemical composition and hygroscopic properties of the resulted particles, which are poorly understood. Here, we have studied chemical composition and hygroscopic properties of laboratory generated NaCl particles mixed with malonic acid (MA) and glutaric acid (GA) at different molar ratios using micro-FTIR spectroscopy, atomic force microscopy, and X-ray elemental microanalysis. Hygroscopic properties of internally mixed NaCl and organic acid particles were distinctly different from pure components and varied significantly with the type and amount of organic compound present. Experimental results were in a good agreement with the AIM modeling calculations of gas/liquid/solid partitioning in studied systems. X-ray elemental microanalysis of particles showed that Cl/Na ratio decreased with increasing organic acid component in the particles with MA yielding lower ratios relative to GA. We attribute the depletion of chloride to the formation of sodium malonate and sodium glutarate salts resulted by HCl evaporation from dehydrating particles.

Aerosol effects on ozone concentrations in Beijing: a model sensitivity study

Most previous O3 simulations were based only on gaseous phase photochemistry. However, some aerosol-related processes, namely, heterogeneous reactions occurring on the aerosol surface and photolysis rate alternated by aerosol radiative influence, may affect O3 photochemistry under high aerosol loads. A three-dimensional air quality model, Models-3/Community Multi-scale Air Quality-Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution, was employed to simulate the effects of the above-mentioned processes on O3 formation under typical high O3 episodes in Beijing during summer. Five heterogeneous reactions, i.e., NO2, NO3, N2O5, HO2, and O3, were individually investigated to elucidate their effects on 03 formation. The results showed that the heterogeneous reactions significantly affected O3 formation in the urban plume. NO2 heterogeneous reaction increased O3 to 90 ppb, while HO2 heterogeneous reaction decreased O3 to 33 ppb. In addition, O3 heterogeneous loss decreased O3 to 31 ppb. The effects of NO2, NO3, and N2O5 heterogeneous reactions showed opposite O3 concentration changes between the urban and extra-urban areas because of the response of the reactions to the two types of O3 formation regimes. When the aerosol radiative influence was included, the photolysis rate decreased and O3 decreased significantly to 73 ppb O3. The two aerosol-related processes should be considered in the study of O3 formation because high aerosol concentration is a ubiquitous phenomenon that affects the urban- and regional air quality in China.

A case study of Asian dust storm particles: chemical composition, reactivity to SO2 and hygroscopic properties

Mineral dust comprises a great fraction of the global aerosol loading, but remains the largest uncertainty in predictions of the future climate due to its complexity in composition and physico-chemical properties. In this work, a case study characterizing Asian dust storm particles was conducted by multiple analysis methods, including SEM-EDS, XPS, FT-IR, BET, TPD/mass and Knudsen cell/mass. The morphology, elemental fraction, source distribution, true uptake coefficient for SO2, and hygroscopic behavior were studied. The major components of Asian dust storm particles are aluminosilicate, SiO2 and CaCO3, with organic compounds and inorganic nitrate coated on the surface. It has a low reactivity towards SO2 with a true uptake coefficient, 5.767 x 10(-6), which limits the conversion of SO2 to sulfate during dust storm periods. The low reactivity also means that the heterogeneous reactions of SO2 in both dry and humid air conditions have little effect on the hygroscopic behavior of the dust particles.

Mixing of dust with pollution on the transport path of Asian dust--revealed from the aerosol over Yulin, the north edge of Loess Plateau

Both PM(2.5) and TSP were monitored in the spring from 2006 to 2008 in an intensive ground monitoring network of five sites (Tazhong, Yulin, Duolun, Beijing, and Shanghai) along the pathway of Asian dust storm across China to investigate the mixing of dust with pollution on the pathway of the long-range transport of Asian dust. Mineral was found to be the most loading component of aerosols both in dust event days and non-dust days. The concentrations of those pollution elements, As, Cd, Pb, Zn, and S in aerosol were much higher than their mean abundances in the crust even in dust event days. The high concentration of SO(4)(2-) could be from both sources: one from the transformation of the local emitted SO(2) and the other from the sulfate that existed in primary dust, which was transported to Yulin. Na(+), Ca(2+), and Mg(2+) were mainly from the crustal source, while NO(3)(-) and NH(4)(+) were from the local pollution sources. The mixing of dust with pollution aerosol over Yulin in dust event day was found to be ubiquitous, and the mixing extent could be expressed by the ratio of NO(3)(-)/Al in dust aerosol. The ratio of Ca/Al was used as a tracer to study the dust source. The comparison of the ratios of Ca/Al together with back trajectory analysis indicated that the sources of the dust aerosol that invaded Yulin could be from the northwestern desert in China and Mongolia Gobi.

The Uptake of SO2 on α-Fe2O3 and Mineral Dust Surfaces in the Temperature Range 250 K to 600 K

The uptake of SO2 on α-Fe2O3 and Saharan dust has been studied in the temperature range 250 K to 600 K using a Knudsen cell reactor. SO2 adsorbs readily and irreversibly on both mineral oxides with a mean initial uptake coefficient of γ ini = (5.9±0.3) · 10−2 on dry surfaces, independent of the surface temperature. In the presence of adsorbed water the initial uptake coefficients at T = 300 K are slightly higher with values of γ ini = (8.4±0.2) · 10−2 for Fe2O3 and γ ini = (7.3±0.4) · 10−2 for mineral dust. The uptake of SO2 is time-dependent and influenced by diffusion into the bulk of the sample at longer timescales. The adsorption capacity has been determined to be (3.0±0.4) · 1018 molecules g−1 for Fe2O3 and (2.6±1) · 1019 molecules g−1 for mineral dust. Sulphite has been identified as the primary reaction product on both surfaces which is readily oxidized to sulphate under atmospheric conditions. Oxidation of sulphite to sulphate only takes place at elevated temperature. From these results a mechanism for the uptake of SO2 onto mineral oxides is inferred, which has been used to compare the experimental concentration-time-profiles with simulated ones.

Reactive collisions of sulfur dioxide with molten carbonates

Molecular beam scattering experiments are used to investigate reactions of SO(2) at the surface of a molten alkali carbonate eutectic at 683 K. We find that two-thirds of the SO(2) molecules that thermalize at the surface of the melt are converted to gaseous CO(2) via the reaction SO(2)(g) + CO(3)(2-) --> CO(2)(g) + SO(3)(-2). The CO(2) product is formed from SO(2) in less than 10(-6) s, implying that the reaction takes place in a shallow liquid region less than 100 A deep. The reaction probability does not vary between 683 and 883 K, further implying a compensation between decreasing SO(2) residence time in the near-interfacial region and increasing reactivity at higher temperatures. These results demonstrate the remarkable efficiency of SO(2) --> CO(2) conversion by molten carbonates, which appear to be much more reactive than dry calcium carbonate or wet slurries commonly used for flue gas desulfurization in coal-burning power plants.

Chemistry and Microphysics of Atmospheric Aerosol Surfaces: Laboratory Techniques and Applications

A number of current techniques are presented by which the chemistry of interaction of selected gas phase species with atmospheric surfaces as well as the microphysical behaviour of such surfaces can be investigated. The techniques discussed include (i) the coated wall flow tube reactor, (ii) the Knudsen-cell / DRIFT spectroscopy, (iii) the surface aerosol microscopy and (iv) the molecular beam scattering technique. In each of these methods specific and robust information is deduced on the kinetics and thermodynamics of gas adsorption and reaction on surfaces. Specific examples include the adsorption of acetone on ice surfaces, the adsorption and reaction of SO2 on iron oxides, the hygroscopic and phase behaviour of binary and ternary salt solution droplets (ammonium sulphate and ammonium sulphate / dicarboxylic acids solutions) as well as on the dynamics of inelastic collisions of noble gases on super-cooled sulphuric acid surfaces. In addition we also show how quantum chemistry can be utilized to assist in interpreting absorption energies on structurally different ice surfaces. Whilst each example represents different aspects of heterogenous atmospheric interactions, they jointly represent significant progress in laboratory investigations of multi-phase atmospheric chemistry with substantial potential for application to other systems and/or problems.

Heterogeneous uptake of carbonyl sulfide on hematite and hematite-NaCl mixtures

Heterogeneous uptake of carbonyl sulfide (COS) on hematite, NaCl, and a series of hematite-NaCl mixtures was investigated in a static reaction chamber at 298 K using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The adsorbed COS was oxidized on the surface of hematite and hematite-NaClmixtures, forming surface hydrogen thiocarbonate (HSCO2-), carbonate (CO3(2-)), and sulfate (SO4(2-)) species as well as gaseous CO2. The reactivity of hematite-NaCI mixtures was lower than that of hematite alone. No uptake of COS was observed on the pure NaCl sample. For mixtures, the 40% hematite + 60% NaCl sample exhibited the highest reactivity. Preadsorption experiments using CO2 as a probe molecule indicated that about 70% of adsorbed COS was oxidized by surface oxygen ions on hematite. In contrast, the Fe-CIO species formed during the sample preparation procedure is proposed to be the active site on the hematite-NaCI mixtures. A plausible reaction mechanism of the heterogeneous oxidation of COS is proposed, and atmospheric implications based on these results are discussed.