An uncertainty-focused approach to modeling the atmospheric chemistry of persistent organic pollutants

Abstract

In this thesis, I study polycyclic aromatic hydrocarbons (PAHs) and perfluorocarboxylic acids (PFCAs). PAHs are by-products of burning and therefore have important anthropogenic sources in the combustion of fuels, biomass, etc. PFCAs and their atmospheric precursors are used in making firefighting foams, non-stick coatings, and other surfactant applications. I quantitatively examine the relative importance of uncertainty in emissions and physicochemical properties (including reaction rate constants) to Northern Hemisphere (NH) and Arctic PAH concentrations. NH average concentrations are more sensitive to uncertainty in the atmospheric lifetime than to emissions rate. The largest uncertainty reductions would come from precise experimental determination of PHE, PYR and BaP rate constants for the reaction with OH. I calculate long-chain PFCA formation theoretical maximum yields for the degradation of precursor species at a representative sample of atmospheric conditions from a three dimensional chemical transport model, finding that atmospheric conditions farther from pollution sources have both higher capacities to form long chain PFCAs and higher uncertainties in those capacities. I present results from newly developed simulations of atmospheric PFCA formation and fate using the chemical transport model GEOS-Chem, simulating the degradation of fluorotelomer precursors, as well as deposition and transport of the precursors, intermediates and end-products of the PFCA formation chemistry. I compare the model results to remote deposition measurements and find that it reproduces Arctic deposition of PFOA effectively. Given the most recent precursor emission inventory, the atmospheric indirect source of PFOA and PFNA is 10-45 t/yr globally and 0.2-0.7 t/yr to the Arctic.