Abstract
Substantial impacts on climate have been documented for soot‒sulfuric acid (H2SO4) interactions in terms of optical and hygroscopic properties of soot aerosols. However, the influence of H2SO4 on heterogeneous chemistry on soot remains unexplored. Additionally, oxidation rate coefficients for polycyclic aromatic hydrocarbons intrinsic to the atmospheric particles evaluated in laboratory experiments seem to overestimate their degradation in ambient atmosphere, possibly due to matrix effects which are hitherto not mimicked in laboratory experiments. For the first time, our kinetics study reports significant influence of H2SO4 coating on heterogeneous ozonation of benzo(a)pyrene (BaP) deposited on model soot, representative to atmospheric particles. The approximate specific surface area of model soot (5 m2g−1) was estimated as a measure of the availability of surface molecules to a typical gaseous atmospheric oxidant. Heterogeneous bimolecular reaction kinetics and Raman spectroscopy studies suggested plausible reasons for decreased BaP ozonation rate in presence of H2SO4: 1. decreased partitioning of O3 on soot surface and 2. shielding of BaP molecules to gaseous O3 by acid-BaP reaction or O3 oxidation products.
Highlights
Physicochemical properties such as morphology, hygroscopicity and optical properties, kinetic studies on heterogeneous oxidation of organics on soot surface are limited and the subject is still poorly understood[11]
Studies involving 2–5 ring PAHs associated with wide range of substrates including solid carboxylic acid aerosols[13], dry sodium chloride aerosols[13], ammonium sulfate particles[14], pyrex glass[15], thin film of carboxylic acids[16], water[17] and 1-octanol[17] reported substrate specific but similar non-linear relationships between ozonation rate constant and gaseous ozone concentration, which is consistent with the Langmuir-Hinshelwood model involving an initial rapid equilibration of gaseous ozone on substrate surface, followed by heterogeneous oxidation of PAHs13
From the mechanistic consideration of the Langmuir-Hinshelwood kinetic model, we investigated whether a hygroscopic coating of H2SO4 can influence the ozonation kinetics of BaP deposited on laboratory generated soot particles
Summary
Physicochemical properties such as morphology, hygroscopicity and optical properties, kinetic studies on heterogeneous oxidation of organics on soot surface are limited and the subject is still poorly understood[11]. From the mechanistic consideration of the Langmuir-Hinshelwood kinetic model, we investigated whether a hygroscopic coating of H2SO4 can influence the ozonation kinetics of BaP deposited on laboratory generated soot particles. The approximate specific surface area of the soot particles was estimated not by the accessibility of soot surface but by the availability of surface-bound BaP molecules to gaseous ozone.
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