Kinetic double-layer model of aerosol surface chemistry and gas-particle interactions (K2-SURF): Degradation of polycyclic aromatic hydrocarbons exposed to O3, NO2, H2O, OH and NO3

Abstract

We present a kinetic double-layer surface model (K2-SURF) that describes the degradation of polycyclic aromatic hydrocarbons (PAHs) on aerosol particles exposed to ozone, nitrogen dioxide, water vapor, hydroxyl and nitrate radicals. The model is based on multiple experimental studies of PAH degradation and on the PRA framework (Pöschl-Rudich-Ammann, 2007) for aerosol and cloud surface chemistry and gas-particle interactions. For a wide range of substrates, including solid and liquid organic and inorganic substances (soot, silica, sodium chloride, octanol/decanol, organic acids, etc.), the concentration- and time-dependence of the heterogeneous reaction between PAHs and O 3 can be efficiently described with a Langmuir-Hinshelwood-type mechanism. Depending on the substrate material, the Langmuir adsorption constants for O 3 vary over three orders of magnitude (K ads,O3 ≈ 10−15- 10−13 cm3), and the second-order rate coefficients for the surface layer reaction of O 3 with different PAH vary over two orders of magnitude (k SLR,PAH,O3 ≈ 10−18-10−17 cm2 s−1). The available data indicate that the Langmuir adsorption constants for NO 2 are similar to those of O 3, while those of H 2 O are several orders of magnitude smaller (K ads,H2O ≈ 10−18-10−17 cm3). The desorption lifetimes and adsorption enthalpies inferred from the Langmuir adsorption constants suggest chemisorption of NO 2 and O 3 and physisorption of H 2 O. Note, however, that the exact reaction mechanisms, rate limiting steps and possible intermediates still remain to be resolved (e.g., surface diffusion and formation of O atoms or O 3 − ions at the surface). The K2-SURF model enables the calculation of ozone uptake coefficients, γ O3, and of PAH concentrations in the quasi-static particle surface layer. Competitive adsorption and chemical transformation of the surface (aging) lead to a strong non-linear dependence of γ O3 on time and gas phase composition, with different characteristics under dilute atmospheri and concentrated laboratory conditions. Under typical ambient conditions, γ O3 of PAH-coated aerosol particles are expected to be in the range of 10−6-10−5. At ambient temperatures, NO 2 alone does not efficiently degrade PAHs, but it was found to accelerate the degradation of PAHs exposed to O 3. The accelerating effect can be attributed to highly reactive NO 3 radicals formed in the gas phase or on the surface. Estimated second-order rate coefficients for O 3 -NO 2 and PAH-NO 3 surface layer reactions are in the range of 10−17-10−16 cm2 s−1 and 10−15-10−12 cm2 s−1, respectively. The chemical half-life of PAHs is expected to range from a few minutes on the surface of soot to multiple hours on organic and inorganic solid particles and days on liquid particles. On soot, the degradation of particle-bound PAHs in the atmosphere appears to be dominated by a surface layer reaction with adsorbed ozone. On other substrates, it is likely dominated by gas-surface reactions with OH or NO 3 radicals (Eley-Rideal-type mechanism). To our knowledge, K2-SURF is the first atmospheric process model describing multiple types of parallel and sequential surface reactions between multiple gaseous and particle-bound chemical species. It illustrates how the general equations of the PRA framework can be simplified and adapted for specific reaction systems, and we suggest that it may serve as a basis for the development of a general master mechanism of aerosol and cloud surface chemistry.