Multiphase chemistry and partitioning of PAHs: numerical modeling from molecular to global scales

Abstract

To predict the atmospheric concentrations of polycyclic aromatic hydrocarbons (PAHs) and their oxidation products, chemical reactions, partitioning between the gas and particle phases, as well as atmospheric transport must be resolved and integrated into mathematical models. In this study, a kinetic model is used to calculate the equilibration time of PAHs between the gas phase and the surface of soot particles. The model shows that these timescales can be controlled by the processes of desorption or adsorption, depending on temperature and the number concentration of particles. The interplay between the chemical loss and partitioning of PAHs is found to perturb the gas-particle distribution from the state expected at equilibrium. For large-scale atmospheric models that assume to reach partitioning equilibrium instantaneously, these non-equilibrium effects are shown to produce significant errors. The global spatial distribution of two nitrated PAHs (NPAHs), 2-nitrofluoranthene (2-NFLT) and 2-nitropyrene (2-NPYR), are predicted with a chemical transport model. The model predicts that due to atmospheric transport of both products and precursors, 2-NFLT and 2-NPYR are spread across the globe. A visualization tool (KinViz) is developed to analyze systems of chemical reactions in the form of a chemical network and provide an alternative to time-concentration profiles.