Uncertainties in the effects of organic aerosol coatings on polycyclic aromatic hydrocarbon concentrations and their estimated health effects

Abstract. We utilized the CAM5 model to examine how different degradation approaches for particle-bound Polycyclic Aromatic Hydrocarbons (PAHs) affect the spatial distribution of benzo(a)pyrene (BaP). Three approaches were evaluated: NOA (no OA coatings), Shielded (where viscous OA coatings protect PAHs from oxidation), and ROI-T (where OA coatings influence PAHs through reactive oxygen intermediates related to temperature). Our findings indicate that the seasonal variation of BaP is influenced by emissions, deposition, and degradation approaches. All simulations predict higher population-weighted global average (PWGA) fresh BaP concentrations during December-January-February (DJF) compared to June-July-August (JJA), primarily due to increased emissions from household activities, less efficient wet removal, and unfavourable winter conditions. The Shielded and ROI-T approaches show that viscous OA coatings significantly inhibit BaP oxidation, leading to PWGA fresh BaP concentrations two to six times higher in DJF than in NOA. The Shielded approach predicts the highest PWGA fresh BaP concentration of 1.3 ng m^-3 in DJF, with 90 % of BaP protected from oxidation. In contrast, the ROI-T approach forecasts lower concentrations in mid-to-low latitudes. Model evaluations indicate the Shielded method performs best, with a normalized mean bias within ±20 %. The incremental lifetime cancer risk ranges from 0.6 to 2 deaths per 100,000 persons based on fresh BaP exposure. Overall, the human health risks from fresh and oxidized PAHs are comparable, underscoring the importance of including both forms in risk assessments and highlighting the critical role of accurate degradation approaches in PAH modelling.