Search

Organicacids in the atmosphere are ubiquitous and are often correlatedwith mineral dust aerosol. Heterogeneous chemistry and the uptakeof organic acids on mineral dust particles can potentially alter theproperties of the particle. In this study, heterogeneous uptake andreaction of formic acid, HCOOH, the most abundant carboxylic acidpresent in the atmosphere, on oxide and clays of the most abundantelements, Si and Al, present in the Earth’s crust are investigatedunder dry and humid conditions. In particular, quantitative adsorptionmeasurements using a Quartz Crystal Microbalance (QCM) coupled withspectroscopic studies using Attenuated Total Reflection Fourier TransformInfrared (ATR-FTIR) spectroscopy are combined to allow for both quantificationof the amount of uptake and identification of distinct adsorbed speciesformed on silica, alumina, and kaolinite particle surfaces at 298K. These oxides and clay particles show significant differences inthe extent and speciation of adsorbed HCOOH due to inherent differencesin surface −OH group reactivity. Adsorbed water, controlledby relative humidity, can increase the irreversible uptake of formicacid. Interestingly, the resulting layer of adsorbed formate on theparticle surface decreases the particle hydrophilicity thereby decreasingthe amount of water taken up by the surface as measured by QCM. Atmosphericimplications of this study are discussed. [ABSTRACT FROM AUTHOR]

It is well-known that chlorine active species (e.g.,Cl2, ClONO2, ClONO) can form from heterogeneousreactions between nitrogen oxides and hydrogen chloride on aerosolparticle surfaces in the stratosphere. However, less is known about these reactions in the troposphere. In this study, a potentialnew heterogeneous pathway involving reaction of gaseous HCl and HNO3on aluminum oxide particle surfaces, a proxy for mineral dust in thetroposphere, is proposed. We combine transmission Fourier transforminfrared spectroscopy with X-ray photoelectron spectroscopy to investigatechanges in the composition of both gas-phase and surface-bound speciesduring the reaction under different environmental conditions of relativehumidity and simulated solar radiation. Exposure of surface nitrate-coatedaluminum oxide particles, from prereaction with nitric acid, to gaseousHCl yields several gas-phase products, including ClNO, NO2, and HNO3, under dry (RH < 1%) conditions. Under humidmore conditions (RH > 20%), NO and N2O are the only gas productsobserved. The experimental data suggest that, in the presence of adsorbedwater, ClNO is hydrolyzed on the particle surface to yieldNO and NO2, potentially via a HONO intermediate. NO2undergoes further hydrolysis via a surface-mediated process,resulting in N2O as an additional nitrogen-containing product.In the presence of broad-band irradiation (λ > 300 nm) gas-phaseproducts can undergo photochemistry, e.g., ClNO photodissociates to NO and chlorine atoms. The gas-phase product distribution also depends on particlemineralogy (Al2O3vs CaCO3) and the presence of other coadsorbed gases (e.g., NH3). These newly identified reaction pathways discussedhere involve continuous production of active ozone-depleting chlorineand nitrogen species from stable sinks such as gas-phase HCl and HNO3as a result of heterogeneous surface reactions. Given thataluminosilicates represent a major fraction of mineral dust aerosol,aluminum oxide can be used as a model system to begin to understandvarious aspects of possible reactions on mineral dust aerosol surfaces. [ABSTRACT FROM AUTHOR]

The recent increase in cardiovascular and metabolic disease in the Navajo population residing close to the Grants Mining District (GMD) in New Mexico is suggested to be due to exposure to environmental contaminants, in particular uranium in respirable dusts (fine dust small enough to reach gas exchanging/ alveolar region of lungs). However, the chemistry of uranium-containing-dust dissolution in lung fluids and the role of mineralogy are poorly understood, as is their impact on toxic effects. The current study is focused on the dissolution of respirable-sized U-containing-dust, collected from several sites near Jackpile and St. Anthony mines in the GMD, in two simulated lung fluids (SLFs): Gambel's solution (GS) and Artificial Lysosomal Fluid (ALF). We observe that the respirable dust includes uranium minerals that yield the uranyl cation, UO2 2+, as the primary dissolved species in these fluids. Dust rich with minerals uraninite and carnotite is more soluble in GS, which mimics interstitial conditions of the lungs. In contrast, dust with low uraninite and high kaolinite is more soluble in ALF, which simulates the alveolar macrophage environment during phagocytosis. Moreover, geochemical modeling, performed using PHREEQC, is in good agreement with our experimental results. Thus, the current study highlights the importance of site-specific toxicological assessments across mining districts with the focus on their mineralogical differences. [ABSTRACT FROM AUTHOR]

The surface photochemistry of nitrate, formed from nitric acid adsorption, on hematite (α-Fe2O3) particle surfaces under different environmental conditions is investigated using X-ray photoelectron spectroscopy (XPS). Following exposure of α-Fe2O3 particle surfaces to gas-phase nitric acid, a peak in the N1s region is seen at 407.4 eV; this binding energy is indicative of adsorbed nitrate. Upon broadband irradiation with light (λ > 300 nm), the nitrate peak decreases in intensity as a result of a decrease in adsorbed nitrate on the surface. Concomitant with this decrease in the nitrate coverage, there is the appearance of two lower binding energy peaks in the N1s region at 401.7 and 400.3 eV, due to reduced nitrogen species. The formation as well as the stability of these reduced nitrogen species, identified as NO(-) and N(-), are further investigated as a function of water vapor pressure. Additionally, irradiation of adsorbed nitrate on α-Fe2O3 generates three nitrogen gas-phase products including NO2, NO, and N2O. As shown here, different environmental conditions of water vapor pressure and the presence of molecular oxygen greatly influence the relative photoproduct distribution from nitrate surface photochemistry. The atmospheric implications of these results are discussed.