Hygroscopic Coating of Sulfuric Acid Shields Oxidant Attack on the Atmospheric Pollutant Benzo(a)pyrene Bound to Model Soot Particles.

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.