Heterogeneous interactions of calcite aerosol with sulfur dioxide and sulfur dioxide–nitric acid mixtures

Uptake and hydration of sulfur dioxide on dry and wet hydroxylated silica surfaces: a computational study

We present a first-principles molecular dynamics study on the uptake and hydration of sulfur dioxide on the dry and wet fully hydroxylated surfaces of (0001) α-quartz, which are a proxy for suspended silica dust in the atmosphere. The average adsorption energy for SO₂ is about -10 kcal mol^-1 on both dry and wet surfaces. The adsorption is driven by hydrogen bond formation between SO₂ and the interfacial hydroxyl groups (on dry silica), or with water molecules (in the wet case). In the dry system, we report an additional electrostatic interaction between the interfacial hydroxyl oxygen and the sulfur atom, which further stabilizes the adsorbate. On dry silica, the interfacial hydroxyl group coordinates to SO₂ yielding a surface bound bisulfite (Si-SO₃H) complex. On the wet surface, SO₂ reacts with water forming bisulfite (HSO₃^-), and the latter remains solvated inside the adsorbed water layer. The hydration barrier for sulfur dioxide is 1 kcal mol^-1 and 3 kcal mol^-1 on dry and wet silica, respectively, while for the backward reaction (i.e., bisulfite to SO₂) the barrier is 6 kcal mol^-1 on both surfaces. The modest backward barrier rationalizes earlier experimental findings showing no SO₂ uptake on silica. These results underline the importance of the surface hydroxylation and/or adsorbed water layers for the SO₂ uptake and its hydration on silica. Moreover, the hydration to bisulfite may prevent direct SO₂ photochemistry and be an additional source of sulfate; this is especially relevant in atmospheres subject to a high level of suspended mineral dust, intense solar radiation and atmospheric oxidizers.

Impact of adsorbed nitrate on the heterogeneous conversion of SO2 on α-Fe2O3 in the absence and presence of simulated solar irradiation

Adsorbed nitrate is ubiquitous in the atmosphere, and it can undergo photolysis to produce oxidizing active radicals. Nitrate photolysis may be coupled with the oxidation conversions of atmospheric gaseous pollutants. However, the processes involved remain poorly understood. In this study, the impact of adsorbed nitrate on the heterogeneous oxidation of SO₂ on α-Fe₂O₃ was investigated in the absence and presence of simulated solar irradiation by using in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that for α-Fe₂O₃ particles with no adsorbed nitrate, the formation of adsorbed sulfate on humid particles is stronger than that on dry particles. Meanwhile, light can also promote the heterogeneous conversion of SO₂ and the formation of sulfate on dry particles because α-Fe₂O₃ is a typical photocatalyst. However, the heterogeneous conversion of SO₂ on humid α-Fe₂O₃ particles is somewhat suppressed under light, suggesting the occurrence of photoinduced reductive dissolution. For the heterogeneous conversion of SO₂ on α-Fe₂O₃ particles with adsorbed nitrate, the formation of sulfate on humid particles is still higher than that on dry particles. For the dry α-Fe₂O₃ particles with adsorbed nitrate, light promotes the formation of adsorbed sulfate. For the humid α-Fe₂O₃ particles with adsorbed nitrate, the heterogeneous conversion of SO₂ under light is stronger than that under no light, indicating that the photolysis of adsorbed nitrate is coupled with the oxidation of SO₂ and the formation of sulfate. The consumption of adsorbed nitrate and the formation of adsorbed N₂O₄ are observed during the introduction of SO₂. A possible mechanism for the impact of adsorbed nitrate on the heterogeneous conversion of SO₂ on α-Fe₂O₃ particles is proposed, and atmospheric implications based on these results are discussed.

Quantification of SO2 Oxidation on Interfacial Surfaces of Acidic Micro-Droplets: Implication for Ambient Sulfate Formation

Sulfate formation on the surface of aqueous microdroplets was investigated using a spray-chamber reactor coupled to an electrospray ionization mass spectrometer that was calibrated using Na₂SO₄(aq) as a function of pH. The observed formation of SO₃^-•, SO₄^-•, and HSO₄^- at pH < 3.5 without the addition of other oxidants indicates that an efficient oxidation pathway takes place involving direct interfacial electron transfer from SO₂ to O₂ on the surface of aqueous microdroplets. Compared to the well-studied sulfate formation kinetics via oxidation by H₂O₂(aq), the interfacial SO₄^2- formation rate on the surface of microdroplets was estimated to be proportional to the collision frequency of SO₂ with a pH-dependent efficiency factor of 5.6 × 10^-5[H⁺]^3.7/([H⁺]^3.7+10^-13.5). The rate via the acidic surface reactions is approximately 1-2 orders of magnitude higher than that by H₂O₂(aq) for a 1.0 ppbv concentration of H₂O₂( g) interacting with 50 μg/m³ of aerosols. This finding highlights the relative importance of the interfacial SO₂ oxidation in the atmosphere. Chemical reactions on the aquated aerosol surfaces are overlooked in most atmospheric chemistry models. This interfacial reaction pathway may help to explain the observed rapid conversion of SO₂ to sulfate in mega-cities and nearby regions with high PM2.5 haze aerosol loadings.

The influence of relative humidity on the heterogeneous oxidation of sulfur dioxide by ozone on calcium carbonate particles

Heterogeneous reactions of SO₂ and O₃ with CaCO₃ particles were investigated at a series of relative humidity (RH, 1% to 90%) and 298K using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The uptake coefficients of SO₂ on CaCO₃ at different RHs were obtained for the first time. Our results proved that high RH could substantially promote the formation of sulfate, for which the highest concentration (80% RH and reaction time of 200min) and highest formation rate in stable stage (85% RH) were 14 times and 43 times that at 1% RH, respectively. The surface products, increment of concentration and formation rate of sulfate changed with RH which were due to the surface adsorbed water (SAW) on the particles. SAW could increase the reactive sites on the particles and thus accelerate the conversion of SO₂ into sulfite, and sulfite could be oxidized rapidly. Liquid-like water layers formed on the particle surface could enhance the ion mobility and promote the aggregation of CaSO₄ hydrates, which could expose more reactive sites and result in additional adsorption of SO₂. Piecewise equations of uptake coefficient with RH were given and could be referred by model simulation. The results are of importance in understanding the explosive growth of sulfate during severe haze episodes accompanied with high RH.

The formation and growth of calcium sulfate crystals through oxidation of SO2by O3on size-resolved calcium carbonate

Calcium sulfate is a major constituent of atmospheric sulfate, with a typical rod-like morphology ranging from several hundred nanometers to approximately two micrometers observed in field studies. However, the chemical formation mechanism is still not well known. In this study, the kinetics and mechanism for the formation and growth of rod-like calcium sulfate crystals through oxidation of SO₂ by O₃ on size-resolved CaCO₃ at different relative humidity (RH) were investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and scanning electron microscopy (SEM). We found that the concentration and formation rate of sulfate decreased with the increasing diameter of CaCO₃ particles, and thus smaller particles could enhance the formation of sulfate due to more reactive sites on smaller particles. The rod-like calcium sulfate crystals were formed only at RH above 60% and in the presence of reactant gases through the heterogeneous pathway. The liquid-like water layer formed by promotion of high RH in the presence of reactant gases could facilitate the formation and aggregation of calcium sulfate hydrates and thus promote the formation and growth of rod-like calcium sulfate crystals. This study provides a possible mechanism for the formation and growth of rod-like calcium sulfate crystals existing in the atmosphere.

The heterogeneous formation pathway of rod-like calcium sulfate crystals in the atmosphere.

Oxidation of Gas-Phase SO2 on the Surfaces of Acidic Microdroplets: Implications for Sulfate and Sulfate Radical Anion Formation in the Atmospheric Liquid Phase

The oxidation of SO2(g) on the interfacial layers of microdroplet surfaces was investigated using a spray-chamber reactor coupled to an electrospray ionization mass spectrometer. Four major ions, HSO3(-), SO3(•-), SO4(•-) and HSO4(-), were observed as the SO2(g)/N2(g) gas-mixture was passed through a suspended microdroplet flow, where the residence time in the dynamic reaction zone was limited to a few hundred microseconds. The relatively high signal intensities of SO3(•-), SO4(•-), and HSO4(-) compared to those of HSO3(-) as observed at pH < 3 without addition of oxidants other than oxygen suggests an efficient oxidation pathway via sulfite and sulfate radical anions on droplets possibly via the direct interfacial electron transfer from HSO3(-) to O2. The concentrations of HSO3(-) in the aqueous aerosol as a function of pH were controlled by the deprotonation of hydrated sulfur dioxide, SO2·H2O, which is also affected by the pH dependent uptake coefficient. When H2O2(g) was introduced into the spray chamber simultaneously with SO2(g), HSO3(-) is rapidly oxidized to form bisulfate in the pH range of 3 to 5. Conversion to sulfate was less at pH < 3 due to relatively low HSO3(-) concentration caused by the fast interfacial reactions. The rapid oxidation of SO2(g) on the acidic microdroplets was estimated as 1.5 × 10(6) [S(IV)] (M s(-1)) at pH ≤ 3. In the presence of acidic aerosols, this oxidation rate is approximately 2 orders of magnitude higher than the rate of oxidation with H2O2(g) at a typical atmospheric H2O2(g) concentration of 1 ppb. This finding highlights the relative importance of the acidic surfaces for SO2 oxidation in the atmosphere. Surface chemical reactions on aquated aerosol surfaces, as observed in this study, are overlooked in most atmospheric chemistry models. These reaction pathways may contribute to the rapid production of sulfate aerosols that is often observed in regions impacted by acidic haze aerosol such as Beijing and other megacities around the world.

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.

Opposing effects of humidity on rhodochrosite surface oxidation

Rhodochrosite (MnCO3) is a model mineral representing carbonate aerosol particles containing redox-active elements that can influence particle surface reconstruction in humid air, thereby affecting the heterogeneous transformation of important atmospheric constituents such as nitric oxides, sulfur dioxides, and organic acids. Using in situ atomic force microscopy, we show that the surface reconstruction of rhodochrosite in humid oxygen leads to the formation and growth of oxide nanostructures. The oxidative reconstruction consists of two consecutive processes with distinctive time scales, including a long waiting period corresponding to slow nucleation and a rapid expansion phase corresponding to fast growth. By varying the relative humidity from 55 to 78%, we further show that increasing humidity has opposing effects on the two processes, accelerating nucleation from 2.8(±0.2) × 10(-3) to 3.0(±0.2) × 10(-2) h(-1) but decelerating growth from 7.5(±0.3) × 10(-3) to 3.1(±0.1) × 10(-3) μm(2) h(-1). Through quantitative analysis, we propose that nanostructure nucleation is controlled by rhodochrosite surface dissolution, similar to the dissolution-precipitation mechanism proposed for carbonate mineral surface reconstruction in aqueous solution. To explain nanostructure growth in humid oxygen, a new Cabrera-Mott mechanism involving electron tunneling and solid-state diffusion is proposed.

Ab initio and metadynamics studies on the role of essential functional groups in biomineralization of calcium carbonate and environmental situations

The interactions of proteins, polysaccharides and other biomolecules with Ca(2+), CO3(2-), and water are central to the understanding of biomineralization and crystallization of calcium carbonate (CaCO3), and their association with the natural organic matter (NOM) in the environment. A molecular-level investigation of how such interactions and thermodynamic forces drive the nucleation and growth of crystalline CaCO3 in living organisms remains elusive. This paper presents ab initio and metadynamics studies of the interactions of Ca(2+), CO3(2-), and water with the essential amino acids/functional groups, e.g. arginine/NH2(+), aspartate or glutamate/COO(-), aspartic or glutamic acid/COOH, and serine/OH, of protein/organic molecules that are likely to be critical to the biomineralization of CaCO3. These functional groups were modeled as guanidinium (Gdm(+)), acetate (AcO(-)), acetic acid (AcOH), and ethanol (EtOH) molecules, respectively. The Gdm(+)-Ca(2+)-CO3(2-) and AcO(-)-Ca(2+)-CO3(2-) systems were found to form stable ion-complexes irrespective of the presence of near neighbor water molecules. The strong electrostatic interactions of these functional groups with their counter-ions significantly affect the fundamental vibrational frequencies of the functional groups, mainly the NH2 stretching (str.) and degenerate (deg.) scissors modes of Gdm(+) and -C=OO, CC, and CO str. modes of AcO(-). The free-energy calculations reveal that EtOH forms weakly bound molecular complexes with the Ca(2+)-CO3(2-) ion pairs in water. However, the interaction strength of EtOH with crystalline CaCO3 can increase significantly due to combined effect of H-bond and electron donor acceptor (EDA) type of interactions. These results indicate that -NH2(+) and -COO(-) bearing molecules serve as potential nucleation sites promoting crystallization of CaCO3 phases while -OH bearing molecules are likely to control the morphology of the crystalline phases by attaching to the growing crystal surfaces.

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.

Observed chemical characteristics of long-range transported particles at a marine background site in Korea

Deokjeok Island is located off the west coast of the Korean Peninsula and is a suitable place to monitor the long-range transport of air pollutants from the Asian continent. In addition to pollutants, Asian dust particles are also transported to the island during long-range transport events. Episodic transport of dust and secondary particles was observed during intensive measurements in the spring (March 31-April 11) and fall (October 13-26) of 2009. In this study, the chemical characteristics of long-range-transported particles were investigated based on highly time-resolved ionic measurements with a particle-into-liquid system coupled with an online ion chromatograph (PILS-IC) that simultaneously measures concentrations of cations (Li+, Na , NH4+, K+, Ca2+, Mg2+) and anions (F-, C1-, NO3-, SO42-). The aerosol optical thickness (AOT) distribution retrieved by the modified Bremen Aerosol Retrieval (M-BAER) algorithm from moderate resolution imaging spectroradiometer (MODIS) satellite data confirmed the presence of a thick aerosol plume coming from the Asian continent towards the Korean peninsula. Seven distinctive events involving the long-range transport (LRT) of aerosols were identified and studied, the chemical components of which were strongly related to sector sources. Enrichment of acidic secondary aerosols on mineral dust particles, and even of sea-salt components, during transport was observed in this study. Backward trajectory, chemical analyses, and satellite aerosol retrievals identified two distinct events: a distinctively high [Ca2++Mg2]/[Na+] ratio (>2.0), which was indicative of a preprocessed mineral dust transport event, and a low [Ca2++Mg2+]/[Na+] ratio (<2.0), which was indicative of severe aging of sea-salt components on the processed dust particles. Particulate C1- was depleted by up to 85% in spring and 50% in the fall. A consistent fraction of carbonate replacement (FCR) averaged 0.53 in spring and 0.55 in the fall. Supporting evidences of C1- enrichment on the marine boundary layer prior to a dust front were also found. Supplemental materials are available for this article. Go to the publisher's online edition of the Journal of the Air & Waste Management Association for sector and air mass classifications of clean and LRT cases.

Asian dust particles converted into aqueous droplets under remote marine atmospheric conditions

The chemical history of dust particles in the atmosphere is crucial for assessing their impact on both the Earth's climate and ecosystem. So far, a number of studies have shown that, in the vicinity of strong anthropogenic emission sources, Ca-rich dust particles can be converted into aqueous droplets mainly by the reaction with gaseous HNO(3) to form Ca(NO(3))(2). Here we show that other similar processes have the potential to be activated under typical remote marine atmospheric conditions. Based on field measurements at several sites in East Asia and thermodynamic predictions, we examined the possibility for the formation of two highly soluble calcium salts, Ca(NO(3))(2) and CaCl(2), which can deliquesce at low relative humidity. According to the results, the conversion of insoluble CaCO(3) to Ca(NO(3))(2) tends to be dominated over urban and industrialized areas of the Asian continent, where the concentrations of HNO(3) exceed those of HCl ([HNO(3)/HCl] >  ∼ 1). In this regime, CaCl(2) is hardly detected from dust particles. However, the generation of CaCl(2) becomes detectable around the Japan Islands, where the concentrations of HCl are much higher than those of HNO(3) ([HNO(3)/HCl] <  ∼ 0.3). We suggest that elevated concentrations of HCl in the remote marine boundary layer are sufficient to modify Ca-rich particles in dust storms and can play a more important role in forming a deliquescent layer on the particle surfaces as they are transported toward remote ocean regions.

Detection of S(IV) species in aerosol particles using XANES spectroscopy

X-ray absorption near-edge structure spectroscopy (XANES) has been applied to the determination and quantification of S(IV) species in aerosol samples collected at Qingdao in northeastern China. The XANES spectra showed that sulfite was found only in particles with larger diameters (mineral aerosols) collected in August 2001. Two oxidation treatments suggested that calcium sulfite (hannebachite) was the main S(IV) species in aerosols. No S(IV) species, however, were found at the surface of the aerosols as shown by surface-sensitive conversion electron/He ion yield XANES. The presence of hannebachite in the interior of aerosols demonstrates the importance of heterogeneous oxidation of SO2 (adsorption of SO2 at the surface of mineral aerosols such as calcite with subsequent oxidation). The fact that this process is supported from XANES analysis for natural samples is important because sulfite formed by the adsorption of SO2 has only been detected in laboratory studies so far. The contribution of heterogeneous oxidation to the total rate of SO2 oxidation is not clear at present. However, this study suggests that the adsorption of SO2 on mineral aerosols without oxidation can reduce the oxidation of SO2 in the atmosphere, especially in the presence of calcite.