Modeling Polycyclic Aromatic Hydrocarbons Emissions and Ambient Concentrations in the United States

Abstract

PAHs (polycyclic aromatic hydrocarbons) in the environment are of significant concern due to their high toxicity. Although PAHs are monitored in the United States (US) at the air toxics monitoring network stations, measurements alone are not sufficient to provide a complete picture of current ambient PAH levels. In this study, speciation profiles for PAHs are prepared and the Sparse Matrix Operator Kernel Emissions (SMOKE) model is used to generate the gridded national emissions of 16 priority PAHs in the US. The estimated emissions are applied in a modified Community Multi-scale Air Quality (CMAQ) model (v5.0.1) to simulate ambient concentrations of PAHs and quantify the contributions of different emission sources to the predicted concentrations. The emission modeling results show that 16-PAH emission in the US is approximately 34.8 Gg in 2011. Residential wood combustion, motor vehicles and industrial point sources are major sources of PAHs. Predicted ambient PAH concentrations by the modified CMAQ model show low biases for most species. Mean fractional bias (MFB) based on daily concentrations are generally less than 0.67, and mean fractional error (MFE) less than 1.0. Averaging the predictions over a month reduces the overall error of the prediction, as indicated by lower MFE values. Heterogeneous reactions of PAHs with O3 on particle surface are needed to reduce the bias of the model results. Source apportionment simulations show that residential wood combustion is the most significant contributor of PAHs concentrations in winter. Motor vehicles and industrial point sources are shown to be major contributors in the US of PAHs throughout of the year.