Nitrate Photochemistry on Laboratory Proxies of Mineral Dust Aerosol: Wavelength Dependence and Action Spectra

Nitrate ion adsorbed on the surface of mineral dust particles from heterogeneous reaction of nitric acid, nitrogen pentoxide, and nitrogen dioxide is thought to be a sink for nitrogen oxides. However, it has the potential to release gas-phase nitrogen oxides back into the atmosphere when irradiated with UV light. In this study, the wavelength dependence of nitrate ion photochemistry when adsorbed onto model laboratory proxies of mineral dust aerosol including Al₂O₃, TiO₂, and NaY zeolite was investigated using FTIR spectroscopy. These proxies represent non-photoactive oxides, photoactive semiconductor oxides, and porous aluminosilicate materials, respectively, present in mineral dust aerosol. Nitrate photochemistry on mineral dust particles is governed by the wavelength of light, physicochemical properties of the dust particles, and the adsorption mode of the nitrate ion. Most interestingly, in some cases, nitrate ion adsorbed on oxide particles can undergo photochemistry over a broader wavelength region of the solar spectrum compared to nitrate ion in solution. As shown here, gas-phase NO₂ is the major photolysis product formed from nitrate adsorbed on the surface of oxide particles under dry conditions. The NO₂ yield and the initial rate of production is highest on TiO₂, indicating that nitrate photochemistry is more efficient on photoactive oxides present in mineral dust. Nitrite ion complexed to Na⁺ sites in aluminosilicate zeolite pores is the major photolysis product found for zeolites. Mechanisms for the formation of gas-phase and surface-adsorbed products and a discussion of the wavelength dependence of nitrate ion photochemistry are presented, as is a discussion of the atmospheric implications

Influence of Ligand Size and Chelation Strength on Zerovalent Iron Nanoparticle Adsorption and Oxidation Behavior in the Presence of Water Vapor and Liquid Water

The effectiveness of zerovalent iron (ZVI) nanoparticles in applications from water remediation to catalysis is intimately tied to adsorption and oxidation processes at the nanoparticle surface. Understanding water sorption and ZVI oxidation as a function of surface-sorbed organic ligand properties can provide new fundamental insights into tuning the reactivity of the nanoparticles. In this work, ZVI nanoparticles were synthesized in the presence of four different organic ligand molecules: two carboxymethyl cellulose polymers of different molecular weights and two phosphonate chelators with different known iron chelation strengths. The resulting ZVI nanoparticles were similar in size (∼100 nm), and adsorption and oxidation behavior are compared on the basis of the properties of the ligand sorbed to the surface of the ZVI nanoparticles. Adsorption and oxidation processes are studied via quartz crystal microbalance (QCM) measurements, where the change in nanoparticle mass is followed over time as the nanoparticles were exposed to varying levels of relative humidity in air or oxygenated water. A clear dependence was shown between measured change in mass and either chelation strength or polymer molecular weight. An increase in either the ligand size or the chelation strength reduced oxidation in oxygenated water. Ligand size resulted in an increase in water vapor adsorption. Reversible mass changes were observed for RH values ≤50% and as a function of ligand, suggesting water sorption, while irreversible mass changes were observed for RH values ≥50% and suggest ZVI oxidation. QCM results were further corroborated with dynamic light scattering, zeta-potential measurements, and scanning electron microscopy. Our results suggest that water adsorption on and oxidation of ZVI nanoparticles may be engineered to a suitable degree through a more thorough understanding of ligand–ZVI interactions

Role of Atmospheric CO₂ and H₂O Adsorption on ZnO and CuO Nanoparticle Aging: Formation of New Surface Phases and the Impact on Nanoparticle Dissolution

Heterogeneous reactions of atmospheric gases with metal oxide nanoparticle surfaces have the potential to cause changes in their physicochemical properties including their dissolution in aqueous media. In this study, gas-phase CO₂ adsorption on ZnO and CuO nanoparticle surfaces was studied as a function of relative humidity to better understand the role of CO₂ and H₂O on nanoparticle aging and the influence of this aging process on metal ion dissolution from nanoparticles. Upon nanoparticle exposure to atmospherically relevant pressures of CO₂ under different relative humidity (RH) conditions, temporal variations of surface-adsorbed species were monitored using Fourier transmission infrared spectroscopy (FTIR). Under dry conditions, gas-phase CO₂ readily reacts with surface hydroxyl groups present on the ZnO and CuO nanoparticle surface to form adsorbed bicarbonate, whereas the interaction of CO₂ with surface defect sites and lattice oxygen gives rise to surface-adsorbed monodentate and bidentate carbonate species as well as adsorbed carboxylate. With increasing relative humidity from 0 to 70%, surface speciation gradually changes to that of water-solvated adsorbed carbonate, which was the only detectable surface species at the highest relative humidity investigated (70% RH). High-resolution TEM analysis of reacted ZnO and CuO nanoparticles revealed considerable surface restructuring consistent with the precipitation of crystalline carbonate phases in the presence of adsorbed water. Furthermore, the restructuring of ZnO and CuO nanoparticles during CO₂ exposure is limited to the near surface region. Importantly, the reacted ZnO nanoparticles also show an increase in the extent of their dissolution when placed in aqueous media. Thus, this work provides valuable insights into reactions of atmospheric gases, CO₂ and H₂O, on ZnO and CuO nanoparticle surfaces and the irreversible changes such reactions can induce on nanoparticle surface chemistry and behavior in aqueous media

Heterogeneous Reactions of Acetic Acid with Oxide Surfaces: Effects of Mineralogy and Relative Humidity

We have investigated the heterogeneous uptake of gaseous acetic acid on different oxides including γ-Al₂O₃, SiO₂, and CaO under a range of relative humidity conditions. Under dry conditions, the uptake of acetic acid leads to the formation of both acetate and molecularly adsorbed acetic acid on γ-Al₂O₃ and CaO and only molecularly adsorbed acetic acid on SiO₂. More importantly, under the conditions of this study, dimers are the major form for molecularly adsorbed acetic acid on all three particle surfaces investigated, even at low acetic acid pressures under which monomers are the dominant species in the gas phase. We have also determined saturation surface coverages for acetic acid adsorption on these three oxides under dry conditions as well as Langmuir adsorption constants in some cases. Kinetic analysis shows that the reaction rate of acetic acid increases by a factor of 3–5 for γ-Al₂O₃ when relative humidity increases from 0% to 15%, whereas for SiO₂ particles, acetic acid and water are found to compete for surface adsorption sites