A novel pathway of atmospheric sulfate formation through carbonate radicals
Yangyang LiuYue DengJiarong LiuXiaozhong FangTao WangKejian LiKedong GongAziz-Ur-Rahim BachaIqra NabiQiuyue GeXiuhui ZhangC. GeorgeLiwu Zhang
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Abstract:
Abstract. Carbon dioxide is considered an inert gas that rarely participates in atmospheric chemical reactions. Nonetheless, we show here that CO2 is involved in some important photo-oxidation reactions in the atmosphere through the formation of carbonate radicals (CO3⚫-). This potentially active intermediate CO3⚫- is routinely overlooked in atmospheric chemistry concerning its effect on sulfate formation. The present work demonstrates that the SO2 uptake coefficient is enhanced by 17 times on mineral dust particles driven by CO3⚫-. Importantly, upon irradiation, mineral dust particles are speculated to produce gas-phase carbonate radical ions when the atmospherically relevant concentration of CO2 presents, thereby potentially promoting external sulfate aerosol formation and oxidative potential in the atmosphere. Employing a suite of laboratory investigations of sulfate formation in the presence of carbonate radicals on the model and authentic dust particles, ground-based field measurements of sulfate and (bi)carbonate ions within ambient PM, together with density functional theory (DFT) calculations for single electron transfer processes in terms of CO3⚫--initiated S(IV) oxidation, a novel role of carbonate radical in atmospheric chemistry is elucidated.Keywords:
Carbonate Ion
Atmospheric chemistry
Inert gas
Mineral dust
Sulfur Cycle
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Inorganic sulfate pools, sulfate sorption characteristics, and Fe and Al fractions were determined on soils at Panola Mountain, a 41-ha forested watershed in the Georgia piedmont. Sulfate adsorption was measured in batch solutions that bracketed the range of ambient sulfate concentrations and soil solution acidity. The slope and intercept of the initial mass (IM) isotherms, formed from plots of sulfate retained against sulfate added, were used to compare sorption behavior among soils. The reversibility of sulfate adsorption was determined by measuring desorption of soluble sulfate from soil before and after equilibration with a concentrated sulfate solution. Sulfate sorption properties of soils at Panola Mountain fall along a continuum between two end members. The “low-adsorbing” end member comprises shallow soils (0–10 cm), with high water-soluble sulfate (Sw), low phosphate-extractable sulfate (Sp-w), high organic matter, low sulfate retention ability (IM isotherm slope near 0.0), and high sulfate adsorption reversibility. The “high-adsorbing” end member comprises deeper soils (>10 cm), with higher total native sulfate (mostly as Sp-w), low organic matter, high sulfate retention ability (isotherm slope near 1.0), and low sulfate adsorption reversibility. Sulfate retention was only weakly related to Fe and Al fractions, possibly because of inhibition of adsorption by organic matter. Sulfate concentrations in surface waters reflect the spatial distribution of soil sulfate retention properties; baseflow, representing water which has equilibrated with the mineral subsoil, has sulfate concentrations near 10 μeq/L, whereas stormflow, which is dominated by water flowing through shallow horizons, has sulfate concentrations near 100 μeq/L.
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