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

摘要

We present a kinetic double-layer surface model (K2-SURF) that describes thedegradation of polycyclic aromatic hydrocarbons (PAHs) on aerosol particlesexposed to ozone, nitrogen dioxide, water vapor, hydroxyl and nitrateradicals. The model is based on multiple experimental studies of PAHdegradation and on the PRA framework (Pöschl-Rudich-Ammann, 2007) foraerosol and cloud surface chemistry and gas-particle interactions.For a wide range of substrates, including solid and liquid organic andinorganic substances (soot, silica, sodium chloride, octanol/decanol,organic acids, etc.), the concentration- and time-dependence of theheterogeneous reaction between PAHs and O can be efficiently describedwith a Langmuir-Hinshelwood-type mechanism. Depending on the substratematerial, the Langmuir adsorption constants for O vary over threeorders of magnitude ( ≈ 10–10 cm),and the second-order rate coefficients for the surface layerreaction of O with different PAH vary over two orders of magnitude( ≈ 10–10 cm s). Theavailable data indicate that the Langmuir adsorption constants for NOare similar to those of O, while those of HO are several ordersof magnitude smaller ( ≈ 10–10 cm).The desorption lifetimes and adsorption enthalpies inferred fromthe Langmuir adsorption constants suggest chemisorption of NO andO and physisorption of HO. Note, however, that the exactreaction mechanisms, rate limiting steps and possible intermediates stillremain to be resolved (e.g., surface diffusion and formation of O atoms orO ions at the surface).The K2-SURF model enables the calculation of ozone uptake coefficients,γ, and of PAH concentrations in the quasi-static particle surfacelayer. Competitive adsorption and chemical transformation of the surface(aging) lead to a strong non-linear dependence of γ on time and gasphase composition, with different characteristics under dilute atmosphericand concentrated laboratory conditions. Under typical ambient conditions,γ of PAH-coated aerosol particles are expected to be in the range of10–10.At ambient temperatures, NO alone does not efficiently degrade PAHs,but it was found to accelerate the degradation of PAHs exposed to O.The accelerating effect can be attributed to highly reactive NOradicals formed in the gas phase or on the surface. Estimated second-orderrate coefficients for O-NO and PAH-NO surface layerreactions are in the range of 10–10 cm sand 10–10 cm s, respectively.The chemical half-life of PAHs is expected to range from a few minutes onthe surface of soot to multiple hours on organic and inorganic solidparticles and days on liquid particles. On soot, the degradation ofparticle-bound PAHs in the atmosphere appears to be dominated by a surfacelayer reaction with adsorbed ozone. On other substrates, it is likelydominated by gas-surface reactions with OH or NO radicals(Eley-Rideal-type mechanism).To our knowledge, K2-SURF is the first atmospheric process model describingmultiple types of parallel and sequential surface reactions between multiplegaseous and particle-bound chemical species. It illustrates how the generalequations of the PRA framework can be simplified and adapted for specificreaction systems, and we suggest that it may serve as a basis for thedevelopment of a general master mechanism of aerosol and cloud surfacechemistry.