Mineral Aerosol Surfaces Research Articles

Key factors influencing the formation of sulfate aerosol on the surface of mineral aerosols: Insights from laboratory simulations and ACSM measurements

A significant technique of combining simulation experiments with a long-term real-time field measurement was first deployed to explore impacts of various environmental factors on formations of sulfate aerosol on the surface of mineral aerosols as MgO in the different reaction systems, and synergistic impacts of these environmental factors on the formation of sulfate aerosols in the different seasonal systems, especially in haze days. Meanwhile, a new correction technique was developed to estimate the effective reaction area and initial uptake coefficient of SO2 on the MgO aerosol surfaces. The significant results showed that the reactions of SO2 on the MgO aerosol surfaces in the system of SO2-MgO-dark could produce a small amount of sulfate and a large amount of sulfite and bisulfite, while in the systems of SO2-MgO-hν and SO2-MgO-O3 produce a large amount of sulfate and a small amount of sulfite and bisulfite. The impacts of temperature T and relative humidity RH on the sulfate formation on MgO aerosol surfaces showed a single-peak mode under the UV light condition, respectively. The sensitivities of these environmental factors to the sulfate aerosol formation were found to follow the order of RH > UV >T > O3 > SO2. The revised initial uptake coefficient γ0.REV of SO2 on the MgO aerosols was always between γ0.BET and γ0.GEO, and much closer to actual value. The heterogeneous reaction mechanisms of SO2 on the MgO aerosol surfaces in the different reaction systems were obviously different. In the presence of water, light irradiation and O3 exerted crucially significant roles in promoting sulfate heterogeneous formation. The various environmental factors in the different seasonal systems, especially in haze days, showed different synergistic effects on sulfate aerosol formations. The sulfate aerosol formations in haze days were mostly impacted by RH, T, O3, and SO2 in spring, RH in summer, RH, T, and SO2 in autumn, and RH and SO2 in winter. The study will provide significant scientific bases for understanding and controlling haze pollution formation.

Dry-deposition of inorganic and organic nitrogen aerosols to the Arabian Sea: Sources, transport and biogeochemical significance in surface waters

Air-to-sea deposition of water-soluble total nitrogen (WSTN) can influence the primary productivity in the coastal oceans. Here, we assessed the concentrations of aerosol inorganic (WSIN: NH4+ + NO3−) and WSTN over the Arabian Sea during winter season (SS379:6–24 December 2018). The mean concentrations of NH4+ (109 ± 83 nmol m−3) overwhelm that of NO3− (32 ± 13 nmol m−3) and water-soluble organic nitrogen (WSON: WSTN-WSIN: 86 ± 81 nmol m−3), and contributing to ~50 ± 31% of WSTN mass. Significant linear relationships of WSON with water-soluble organic carbon and NH4+ with non-sea-salt (nss)-K+ is observed suggesting their common origin from biomass burning and fertilizers. Backward air mass trajectories and satellite-based fire counts further revealed their provenance in the Indo-Gangetic Plain and southern India. The concentration of NO3− moderately correlated with nssCa2+ (dust tracer), indicating heterogeneous reactive uptake on mineral aerosol surface. Despite high concentrations, the deposition fluxes of NH4+ (~9.4 ± 7.1 μmol m−2 d−1) and WSON (7.4 ± 7.0 μmol m−2 d−1) are lower than NO3− (27 ± 11 μmol m−2 d−1) because of their predominant fine nature (i.e., strong correlation with nss-SO42−). We also constrained the total annual atmospheric deposition rates of WSIN (0.94 Tg yr−1) and WSON (0.08 Tg yr−1) to the Arabian Sea during the continental outflow (November-April). The maximum dry-deposition of WSON to the Arabian Sea (0.24 Tg yr−1) is twice that of the riverine supply (0.11 Tg yr−1), highlighting the significance of aeolian sources. By using Redfield Stoichiometry, the WSTN deposition (21–73 μmol m−2 d−1) can account for <5.3% of fixed‑carbon by a primary production in the Arabian Sea.

Heterogeneous reactions of isoprene and ozone on α-Al2O3: The suppression effect of relative humidity

The heterogeneous reactions of α-Al2O3 particles with a mixture of ozone (∼50 ppm) and isoprene (∼50 ppm) were studied as a function of relative humidities (RHs). The reactions were monitored in real time through the microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the presence of ozone leads to the rapid conversion of isoprene to carboxylate (COO−) ions on the surfaces of α-Al2O3 particles in the initial stage. The water significantly suppresses the formation of the carboxylate ions. For the isoprene ozonolysis reaction on the α-Al2O3 particles, the reactive uptake coefficient is strongly suppressed by over a factor of 8 when the RH increases from 8% to 89%. The negative correlation between RH with the secondary organic aerosol (SOA) produced by isoprene ozonolysis plays a key role in the actual atmospheric environment under high humidity. Our results may provide insight into the ozonolysis process of biogenic alkenes over mineral aerosol surfaces with the influence of RHs.

DFT study of the carbonation on mineral aerosol surface models of olivine: effect of water

Mineral aerosols play a significant role in gas–solid interfacial and atmospheric chemistry. Carbonation of olivine aerosol, which takes place in a multiphase reaction processes, can be an effective means to reduce the concentration of atmospheric carbon dioxide. Due to the presence of a huge reserve of silicate minerals in nature, olivine aerosol could be an ideal potential raw material for mineral carbonation for its higher reactivity with H2O and CO2. However, quantitative information about the carbonation process on the surface of natural olivine aerosol is not available. In this paper, calculations on the carbonation reaction processes with and without a H2O molecule using a periodic olivine model has been carried out via the density functional theory. The pathways and their corresponding energies and structures in the carbonation reactions have been established, and the effect of water as means to reduce the energy barriers and stabilize the carbonated structures by forming hydrogen bonds has been confirmed.

Effect of water on carbonation of mineral aerosol surface models of kaolinite: a density functional theory study

Gas–solid interfacial phenomena always play a significant role in the multiphase process in atmospheric chemistry. The mineral aerosols have desirable interfacial reactivity on the carbonation of kaolinite. In addition, carbonation of kaolinite may play a role in carbon dioxide capture. It is not well-known about the mechanisms of this reaction. In this paper, the carbonation of kaolinite cluster models with or without water on the atomic scale is studied. We simulate the corresponding reaction paths and accurately calculate the transition states with the homologous enthalpies via using the density functional theory (DFT) method. The study shows that the reaction barriers are lowered down seriously with the existence of water. In addition, water can help stabilize the reaction regions thereby firming the structure of carbonated product.

Secondary organic aerosol over an urban environment in a semi–arid region of western India

Carbonaceous species in PM10 and PM2.5 samples, collected from an urban location at Ahmedabad in India during summer, were analyzed to study variability in water-soluble organic carbon (WSOC) and secondary organic carbon (SOC) along with atmospheric water vapor content. A significant correlation between WSOC and SOC was found indicating major contribution of soluble organic compounds by secondary organic aerosol formation. A strong inverse dependence of WSOC and SOC on atmospheric water vapor content is observed in both PM10 and PM2.5 (at <45% relative humidity, RH) during daytime; whereas data collected during monsoon season at higher RH conditions do not exhibit such relation. Aerosol liquid water content (LWC) calculated from thermodynamic equilibrium model suggests that the decrease in secondary organic aerosol (SOA) with increase in RH occurs when LWC is absent or insignificant amount. The inverse correlation in summertime indicates possible decrease in the extent of heterogeneous photochemical oxidation of precursor volatile organic compounds on mineral aerosol surface with increase in ambient water vapor. These results have implications for SOA estimations on regional scales especially in arid and semi-arid regions where significant amount of fine mineral dust is present.

Pathways of sulfate enhancement by natural and anthropogenic mineral aerosols in China

AbstractChina, the world's largest consumer of coal, emits approximately 30 million tons of sulfur dioxide (SO2) per year. SO2 is subsequently oxidized to sulfate in the atmosphere. However, large gaps exist between model‐predicted and measured sulfate levels in China. Long‐term field observations and numerical simulations were integrated to investigate the effect of mineral aerosols on sulfate formation. We found that mineral aerosols contributed a nationwide average of approximately 22% to sulfate production in 2006. The increased sulfate concentration was approximately 2 µg m−3 in the entire China. In East China and the Sichuan Basin, the increments reached 6.3 µg m−3 and 7.3 µg m−3, respectively. Mineral aerosols led to faster SO2 oxidation through three pathways. First, more SO2 was dissolved as cloud water alkalinity increased due to water‐soluble mineral cations. Sulfate production was then enhanced through the aqueous‐phase oxidation of S(IV) (dissolved sulfur in oxidation state +4). The contribution to the national sulfate production was 5%. Second, sulfate was enhanced through S(IV) catalyzed oxidation by transition metals. The contribution to the annual sulfate production was 8%, with 19% during the winter that decreased to 2% during the summer. Third, SO2 reacts on the surface of mineral aerosols to produce sulfate. The contribution to the national average sulfate concentration was 9% with 16% during the winter and a negligible effect during the summer. The inclusion of mineral aerosols does resolve model discrepancies with sulfate observations in China, especially during the winter. These three pathways, which are not fully considered in most current chemistry‐climate models, will significantly impact assessments regarding the effects of aerosol on climate change in China.

Periodic DFT study of acidic trace atmospheric gas molecule adsorption on Ca- and Fe-doped MgO(001) surface basic sites.

The electronic properties of undoped and Ca- or Fe-doped MgO(001) surfaces, as well as their propensity toward atmospheric acidic gas (CO2, SO2, and NO2) uptake was investigated with an emphasis on gas adsorption on the basic MgO oxygen surface sites, O(surf), using periodic density functional theory (DFT) calculations. Adsorption energy calculations show that MgO doping will provide stronger interactions of the adsorbate with the O(surf) sites than the undoped MgO for a given adsorbate molecule. Charge transfer from the iron atom in Fe-doped MgO(001) to NO2 was shown to increase the binding interaction between adsorbate by an order of magnitude, when compared to that of undoped and Ca-doped MgO(001) surfaces. Secondary binding interactions of adsorbate oxygen atoms were observed with surface magnesium sites at distances close to those of the Mg-O bond within the crystal. These interactions may serve as a preliminary step for adsorption and facilitate further adsorbate transformations into other binding configurations. Impacts on global atmospheric chemistry are discussed as these adsorption phenomena can affect atmospheric gas budgets via altered partitioning and retention on mineral aerosol surfaces.

Interaction Energies and Spectroscopic Effects in the Adsorption of Formic Acid on Mineral Aerosol Surface Models

Heterogeneous reactions of atmospheric volatile organic compounds (VOCs) on aerosol particles may play an important role in atmospheric chemistry. Silicate particles are present in airborne mineral dust in aerosols, and the atmospheric chemistry in general can be modified by their presence. In this work, the adsorption of a single formic acid molecule on different silicate surface models has been studied using quantum-mechanical methods. Both molecular clusters and a periodic crystal model of the (001) pyrophyllite surface have been employed, and all possible adsorption geometries have been considered. We find that silanol groups are always the most reactive formic acid adsorption sites. In the case of a periodic system, silanol groups at the crystal edges are favored. However, OH groups on the phyllosilicate octahedral sheet are also reactive sites through the tetrahedral cavities. The effect of formic acid adsorption on the spectroscopic properties of the whole system is also analyzed. Significant frequ...

The atmospheric lifetimes and concentrations of cyclic methylsiloxanes octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) and the influence of heterogeneous uptake

Abstract The environmental fate of the cyclic methylsiloxanes (cVMS), octamethylcyclotetrasiloxane (D 4 ) and decamethylcyclopentasiloxane (D 5 ), is important because of the potential for long range transport. Previous studies of environmental loss pathways for D 4 and D 5 focused on gas phase oxidation, establishing OH as the main gas phase reactant, with OH kinetics consistent with atmospheric lifetimes of approximately 7 days (D 5 ) and 11 days (D 4 ) at an OH concentration of 10 6 molec cm −3 . In this work, we assess the sensitivity of atmospheric lifetime to other factors using an atmospheric compartment model. D 4 and D 5 were modeled using an compartment model to simulate emission, decay, and atmospheric transport across a range of three levels of population density corresponding to urban, transition, and rural areas of a developed mid-latitude location. The model was used to examine sensitivities of lifetime and concentration of D 4 and D 5 to spatial variation in OH, variation in the time-of-day for cVMS emissions, and to relative humidity dependent heterogeneous uptake and/or reactions on mineral dusts. cVMS lifetime was found to be insensitive to urban OH concentrations because of limited residence time, and somewhat sensitive to enhanced OH levels in the transition area between the urban and rural compartments. Realistic aerosol loadings and heterogeneous kinetics consistent with recent laboratory results (uptake coefficients of 1.2 × 10 −5 and 1.6 × 10 −5 for D 4 and D 5 , respectively) were calculated to reduce the cVMS lifetime by ∼3% under dry conditions but have negligible effects at RH values above 50%. The heterogeneous kinetics were based on experimental observations of uptake to mineral aerosol surfaces at cVMS loadings well above ambient levels. The compartment model, after tuning of residence times and ceiling heights to reproduce benzene observations, predicted D 5 at 6, 28, and 50 ng m −3 in areas with low, medium, and high population densities, respectively. Corresponding D 4 concentration predictions were 5, 20, and 33 ng m −3 . These values were compared with available measurements.

A modeling study of the impact of heterogeneous reactions on mineral aerosol surfaces on tropospheric chemistry over East Asia

Nine heterogeneous reactions (uptake of H 2O 2, HNO 3, HO 2, N 2O 5, NO 2, NO 3, O 3, OH and SO 2 on mineral aerosol surfaces) are incorporated into a Regional Air Quality Model System (RAQMS) to investigate their impacts on tropospheric chemistry in East Asia during the dust storm period in March 2006. Comparison with observations shows the model system well represents the behaviors of the gaseous and aerosol species. Most of the reaction probability γ values used for this study are the best estimation specifically for dust samples from deserts of China, derived from analysis of a number of recent laboratory studies. There are large variations in gas and aerosol concentrations while taking heterogeneous reactions on mineral aerosol surface into account, especially during dust storm events. The domain-averaged monthly mean percentage changes in SO 2, NO 2, O 3, HNO 3, NH 3, total sulfate, total nitrate and total ammonium concentrations are −4.4%, −3.8%, −2.1%, −22.0%, 12.7%, 6.6%, 26.1%, and −9.5%, respectively below 3 km. These changes indicate the considerable perturbation of heterogeneous reactions on mineral aerosol surface to tropospheric chemical system and components. The strength of heterogeneous reactions is determined by both reaction probability and gas precursor concentration. Among the nine reactions, dust uptakes of HNO 3, SO 2, and N 2O 5 exert relatively large influences on the other chemical components, whereas the reactions regarding H 2O 2, HO 2, and OH have little impacts.

Impact of dust on tropospheric chemistry over polluted regions: a case study of the Beijing megacity

Abstract. The box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere) is extended by incorporating detailed heterogeneous chemistry occurring on mineral aerosol surfaces. The model is used to investigate the impact of dust on tropospheric photochemistry, when the dust is transported to a polluted region, focusing on the example of Beijing. The impacts of dust via heterogeneous removal of gases are analyzed for different hypothetical transport rates, which are described by four different exchange rate coefficients Kt in the model. Along with the dust, airmasses with trace gas levels characteristic for regions upwind of Beijing are transported with the same rate (Kt). Substantial impacts are found for many gases, including Ox (O3+O(3P)), NOx (NO+NO2) and OH. The Ox daily average mixing ratio decreases due to heterogeneous reactions on dust. The change ranges from −2.5 to −18.4 nmol mol−1, and is larger for faster mixing with upwind air masses (i.e. greater Kt). This translates into a large relative change in Ox, ranging from −44% to −55%, depending on Kt. By assuming an artificial 50% decrease of all photolysis rates, the impacts of dust via perturbation of the photolysis rates in the polluted region are also estimated. Furthermore, the uncertainties in the results due to the uncertainties in the uptake coefficients are evaluated. It is found that for all gases which are heterogeneously removed, the self-removal results in the largest uncertainty (e.g. −49% for O3, −76% for NO2, −47% for HNO3, −92% for HCHO, −64% for CH3OH and −93% for SO2). The heterogeneous removal of NO2 is found to be particularly important, because it results in significant levels of uncertainty not only for itself, but also for OH (340%) and HO2 (365%). Moreover, the heterogeneous removal rates of HCHO and O3 also have farther-reaching effects on the OH concentration (resulting in changes of −55% and 45%, respectively), and the heterogeneous removal of HCHO results in an uncertainty of −38% in the HO2 concentration. The limitations of MECCA due to its missing oxidation mechanism for aromatics and other higher VOC species has also been considered, and shown to be potentially important in the quantitative results, though not likely to change the qualitative results of this study.

Neutralization of Calcite in Mineral Aerosols by Acidic Sulfur Species Collected in China and Japan Studied by Ca K-edge X-ray Absorption Near-Edge Structure

Calcium species in mineral aerosols collected simultaneously in Aksu (near the Taklimakan Desert), Qingdao (eastern China), and Tsukuba (Japan) during dust and nondust periods were determined using Ca K-edge X-ray absorption near-edge structure (XANES). From the fitting of XANES spectra, it was found that (i) calcite and gypsum were the main Ca species in the aerosol samples, and (ii) the gypsum fraction versus total Ca minerals [Gyp]/[Ca2+]t increased progressively in the order Aksu < Qingdao < Tsukuba. Surface-sensitive XANES in the conversion electron yield mode (CEY) showed that the gypsum is formed selectively at the surface of mineral aerosols for all the samples except for that taken in Aksu during the dust period. The decrease of the [Gyp]/[Ca2+]t ratio with an increase in particle size showed that the neutralization effect proceeds from the particle surface. For the Aksu sample in the dust period, however, (i) the [Gyp]/[Ca2+]t ratios obtained by XANES measured in the fluorescence (FL; regarded as bulk analysis) and CEY modes were similar and (ii) size dependence was not found, showing that neutralization is not important for the sample because of the large supply of mineral aerosol with little neutralization effect in Aksu. It was also found that the pH of the aerosol and the ratio of (NH4)2SO4 to gypsum were positively and negatively correlated with the Ca (or calcite) content, respectively. The speciation of Ca by XANES revealed the neutralization processes of acidic sulfur species by calcite during the long-range transport of mineral aerosols.

Heterogeneous Ozone Oxidation Reactions of 1-Pentene, Cyclopentene, Cyclohexene, and a Menthenol Derivative Studied by Sum Frequency Generation

We report vibrational sum frequency generation (SFG) spectra of glass surfaces functionalized with 1-pentene, 2-hexene, cyclopentene, cyclohexene, and a menthenol derivative. The heterogeneous reactions of ozone with hydrocarbons covalently linked to oxide surfaces serve as models for studying heterogeneous oxidation of biogenic terpenes adsorbed to mineral aerosol surfaces commonly found in the troposphere. Vibrational SFG is also used to track the C=C double bond oxidation reactions initiated by ozone in real time and to characterize the surface-bound product species. Combined with contact angle measurements carried out before and after ozonolysis, the kinetic and spectroscopic studies presented here suggest reaction pathways involving vibrationally hot Criegee intermediates that compete with pathways that involve thermalized surface species. Kinetic measurements suggest that the rate limiting step in the heterogeneous C=C double bond oxidation reactions is likely to be the formation of the primary ozonide. From the determination of the reactive uptake coefficients, we find that ozone molecules undergo between 100 and 10000 unsuccessful collisions with C=C double bonds before the reaction occurs. The magnitude of the reactive uptake coefficients for the cyclic and linear olefins studied here does not follow the corresponding gas-phase reactivities but rather correlates with the accessibility of the C=C double bonds at the surface.

Speciation of Sulfate in Size-Fractionated Aerosol Particles Using Sulfur K-Edge X-ray Absorption Near-Edge Structure

X-ray absorption near-edge structure (XANES) at sulfur K-edge was used to determine the sulfate species present in size-fractionated aerosol particles based on the postedge structure after the main absorption peak in the XANES region. A comparison of the XANES spectra of reference sulfate materials and aerosol samples collected in Tsukuba in Japan clearly showed that (NH4)2SO4 was the main sulfur species in particles with a smaller diameter and CaSO4 x 2H2O (gypsum) was the main sulfur species in particles with a larger diameter. A simulation of the XANES spectra by reference materials allows us to obtain the quantitative mixing ratios of the different sulfate species present in the aerosol samples. The presence of minor sulfur species other than (NH4)2SO4 and gypsum at the surface of mineral aerosols is suggested in our simulations and by a surface-sensitive conversion electron/He-ion yield XANES. In the absence of a contribution from a large dust event, the mole concentration of gypsum in the mineral aerosol fraction (particle diameter > 1 microm) determined by XANES is similar to that of Ca which is determined independently using ion chromatography. This shows that the Ca and sulfate in the mineral aerosols are present only as gypsum. Considering that calcite is the main Ca mineral in the original material arising from an arid and semiarid area in China, it is strongly suggested that gypsum is formed in aerosol during its long-range transportation by a reaction between calcite and sulfate ions.

Global modeling of heterogeneous chemistry on mineral aerosol surfaces: Influence on tropospheric ozone chemistry and comparison to observations

Mineral aerosols can affect gas phase chemistry in the troposphere by providing reactive sites for heterogeneous reactions. We present here a global modeling study of the influence of mineral dust on the tropospheric photochemical cycle. This work is part of the Mineral Dust and Tropospheric Chemistry (MINATROC) project, which focussed on measurement campaigns, laboratory experiments, and integrative modeling. The laboratory experiments provide uptake coefficients for chemical species on mineral aerosol surfaces, which are used to compute the heterogeneous reaction rates in the model. The field measurements at Mount Cimone, northern Italy, provide trace gas and aerosol measurements during a Saharan dust episode and are used to evaluate the model. The simulations include the reactions between mineral dust aerosols and the gas‐phase species O3, HNO3, NO3, and N2O5. Under the conditions for the year 2000 the model simulates a decrease in global tropospheric ozone mass by about 5% due to the heterogeneous reactions on dust aerosols. The most important heterogeneous reaction is the uptake of HNO3 on the dust surface, whereby the direct uptake of ozone on dust is not important in atmospheric chemistry. The comparison of the model results to observations indicates that the model simulates well the aerosol mass transported into the Mediterranean during the dust events and the arrival of all major dust events that were observed during a 7 month period. The decrease in ozone concentration during dust events is better simulated by the model when the heterogeneous reactions are included.

Size distributions of ionic aerosols measured at Waliguan Observatory: Implication for nitrate gas‐to‐particle transfer processes in the free troposphere

Waliguan Observatory (WO) is a land‐based Global Atmosphere Watch baseline station on the Tibetan Plateau. Size‐resolved ionic aerosols (NH4+, Na+, K+, Ca2+, Mg2+, SO42−, Cl−, NO3− CO32−, formate, acetate and oxalate), organic aerosols, black carbon and gaseous HNO3 and SO2 were measured during an intensive fall‐winter field experiment. The observational data were analyzed with a focus on the partitioning of nitrate between the gas and particle phases. Nitrate was found to exist mainly in the particle phase with a typical particulate‐to‐total nitrate ratio, i.e., NO3−(p)/(NO3−(p) + HNO3(g)), of about 0.9. It was also found that the size distribution pattern of particulate nitrate at WO varied in different samples and the amount of particulate nitrate residing in the fine mode (Dp &lt; 2.0 μm) was typically larger than or comparable with that in the coarse mode. A gas‐particle chemical equilibrium model was used to predict these particulate nitrate size distributions. The size distributions of particulate nitrate were reasonably reproduced with the model within the uncertainties caused by the detection limits. The chemical pathways for the formation of particulate nitrate at WO were analyzed with the size distributions of measured ionic aerosols. It was demonstrated that fine nitrate particles may have been produced by the reaction of gaseous nitric acid with gaseous ammonia, while coarse nitrate particles may have been generated via the condensation of nitric acid on the surface of mineral aerosols. The signature of biomass burning at WO was found to be associated with black carbon as well as the accumulation of potassium and oxalate in the fine particles.