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.

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 CO 2 adsorption on ZnO and CuO nanoparticle surfaces was studied as a function of relative humidity to better understand the role of CO 2 and H 2 O on nanoparticle aging and the influence of this aging process on metal ion dissolution from nanoparticles. Upon nanoparticle exposure to atmospherically relevant pressures of CO 2 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 CO 2 readily reacts with surface hydroxyl groups present on the ZnO and CuO nanoparticle surface to form adsorbed bicarbonate, whereas the interaction of CO 2 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 watersolvated 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 CO 2 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, CO 2 and H 2 O, on ZnO and CuO nanoparticle surfaces and the irreversible changes such reactions can induce on nanoparticle surface chemistry and behavior in aqueous media.

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.

In the atmosphere, mineral dust particles are often associated with adsorbed nitrate from heterogeneous reactions with nitrogen oxides (N2O5, HNO3, NO3, and NO2). Nitrate ions associated with mineral dust particles can undergo further reactions including those initiated by solar radiation. Although nitrate photochemistry in aqueous media is fairly well studied, much less is known about the photochemistry of nitrate adsorbed on mineral dust particles. In this study, the photochemistry of nitrate from HNO3 adsorption in NaY zeolite under different environmental conditions has been investigated using transmission FTIR spectroscopy. NaY zeolite is used as a model zeolite for studying reactions that can occur in confined space such as those found in porous materials including naturally occurring zeolites and clays. Upon nitrate photolysis under dry conditions (relative humidity, RH, < 1%), surface nitrite is formed as the major adsorbed product. Although nitrite has been proposed as a product in the photochemistry of nitrate adsorbed on metal oxide particle surfaces, such as on alumina, it has not been previously detected. The stability of adsorbed nitrite in NaY is attributed to the confined three-dimensional structure of the porous zeolite, which contains a charge compensating cation that can stabilize the nitrite ion product. Besides adsorbed nitrite, small amounts of gas phase nitrogen-containing products are observed as well including NO2, NO, and N2O at long irradiation times. The amount of nitrite formed via nitrate photochemistry decreases with increasing relative humidity, whereas gas phase NO and N2O become the only detectable products. Gas-phase NO2 does not observe at RH > 1%. In the presence of gas phase ammonia, ammonium nitrate is formed in NaY zeolite. Photochemistry of ammonium nitrate yields gas phase N2O as the sole gas phase product. Evidence for an NH2 intermediate in the formation of N2O is identified with FTIR spectroscopy for HNO3 adsorption and photochemistry in NH4Y zeolite. Here, we discuss mechanisms for the formation of these intermediates from nitrate photochemistry as well as possible atmospheric implications.

Recent atmospheric field and modeling studies have highlighted a lack of understanding of the processes responsible for high levels of sulfate aerosol in the atmosphere, ultimately arising from a dearth of experimental data on such processes. Here we investigated the effect of temperature and simulated solar radiation on the catalytic oxidation of S(iv) to S(vi) (i.e., sulfite to sulfate) in aqueous suspensions of several metal-containing, atmospherically relevant particles including coal fly ash (FA), Arizona test dust (ATD) and an iron oxide (γ-FeO). The effect of temperature and light on S(iv) oxidation was found to be very different for these three samples. For example, in the presence of FA and γ-FeO the temporal evolution of dissolved Fe(ii) (formed via reductive particle dissolution) correlated with S(iv) oxidation. Accordingly, we propose that S(iv) oxidation in most of these systems initially occurs primarily at the particle surface (i.e., a heterogeneous reaction pathway), although a solution-phase (i.e., homogeneous) catalytic pathway also contributes over later timescales due to the formation and accumulation of dissolved Fe(iii) (generated via oxidation of dissolved Fe(ii) by O). It is likely that the homogeneous reaction pathway is operative at initial times in the presence of γ-FeO at 25 °C. In contrast, S(iv) oxidation in the presence of ATD appears to proceed entirely via a heterogeneous reaction, which notably does not lead to any iron dissolution. In fact, the greater overall rate of S(iv) loss in the presence of ATD compared to FA and γ-FeO suggests that other factors, including greater adsorption of sulfite, transition metal ion (TMI) catalysis by other metal ions (e.g., Ti), or different species of iron in ATD, play a role. Overall these studies suggest that the rate, extent and products of atmospheric S(iv) oxidation can be highly variable and dependent upon the nature of aerosol sources and ambient conditions (e.g., temperature and irradiance). Ultimately, such complexity precludes simple, broadly generalized schemes for this reaction when modeling atmospheric processes involving diverse components of different mineral dust aerosol as well as other metal-containing aerosol.