AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
How Important is Nitrogen Dioxide (NO2) to Sulfur (S(IV)) Oxidation at Air-Water Interface of Aquated Aerosol under Acidic Conditions?
LIJIE LI, Agustin Colussi, Michael Hoffmann, California Institute of Technology
Abstract Number: 660 Working Group: There Must be Something in the Water: Cloud, Fog and Aerosol Aqueous Chemistry for Aerosol Production
Abstract Water plays an essential role in aerosol formation. Understanding the chemical and physical processes at the air-water interface is critical for the development of more accurate heterogeneous reaction mechanisms that are included in aerosol modeling. Experimental field evidence indicates that heterogeneous reactions results in a dramatic enhancement in secondary aerosol formation as observed during mainland Chinese haze episodes. Inorganic sulfate is a major component of secondary aerosol that determines the hygroscopic state of the aerosol. Hence, the mechanisms of sulfate formation in the boundary layer not only affects aerosol nucleation but drives the ensuing chemistry within and on the surface of haze aerosol. Pathways for sulfur oxidation in the aqueous phase include reactions with ozone, hydrogen peroxide, organic peroxides, OH radical, oxides of nitrogen, and O2. The mechanism of S(IV) oxidation during haze episodes is exceptional since thick haze aerosol layers limit the formation of photo-generated oxidants, such as ozone and OH radicals due to significant light attentuation. It has been proposed that NO2(g) is actually a major of S(IV) in haze aerosol. NO2(g) emitted from fossil fuel combustion sources has been identified a primary oxidant based on assumption that haze aerosol over China megacities is neutral or slightly basic. However, haze aerosols are more often acidic (pH= -0.5-6) worldwide. It essential to understand that the reaction rates, gas solubilities, and ionic equilibria involved in sulfur oxidation change as a function of pH. The importance of NO2 as an oxidant of S(IV) oxidation with haze aerosol needs to be revisited. Further, the oxidation of sulfur at the air-water interface remains ambiguous. Our previous studies have shown orders of enhancement in NO2(g) uptake with millimolar concentrations of organic and inorganic anions and an increase in S(IV) oxidation rates under acidic conditions at air-water interface. In this presentation, we investigate the significance of NO2 to S(IV) oxidation at air-water interface over a wide range of acidity, in the presence and absence of transition metal ions. The aqueous aerosol surface is generated by microjets containing S(IV) and is instantaneously exposed to NO2(g) in a reaction chamber. Products and intermediates formed on interfacial layers of the microjets are analyzed via in situ via online electrospray ionization mass spectrometry. The impact of NO2 and its disproportionation products (e.g. HONO) on sulfur oxidation is studied over the pH range of 0-6. The interaction between iron (Fe(II)/Fe(III)) and NO2 and its impact on the oxidation of S(IV) is explored. In addition, we evaluate the importance of direct and Fe(II)-catalyzed oxidation of S(IV) by NO2(g) vs O2(g) at air-aqueous interfaces at pH=5-6.