Global long-range transport and lung cancer risk from polycyclic aromatic hydrocarbons shielded by coatings of organic aerosol

Significance Polycyclic aromatic hydrocarbons (PAHs) adversely impact human health and ecosystems and are known to persist in the atmosphere. Despite decades of research, the mechanisms by which these PAHs persist are not well understood. Here, we combine theory and laboratory and field measurements within a global climate model to produce new insights into mechanisms that are responsible for the observed persistence of PAHs. We show that temperature- and humidity-dependent variations in effective viscosity of organic aerosol (OA) shield PAHs from chemical degradation. This OA shielding results in higher PAH concentrations at both near-urban and remote locations, leading to a fourfold increase in global lung cancer risk. Our study represents new research frontiers in terms of connecting climate-relevant OA with health-relevant PAHs. Polycyclic aromatic hydrocarbons (PAHs) have toxic impacts on humans and ecosystems. One of the most carcinogenic PAHs, benzo(a)pyrene (BaP), is efficiently bound to and transported with atmospheric particles. Laboratory measurements show that particle-bound BaP degrades in a few hours by heterogeneous reaction with ozone, yet field observations indicate BaP persists much longer in the atmosphere, and some previous chemical transport modeling studies have ignored heterogeneous oxidation of BaP to bring model predictions into better agreement with field observations. We attribute this unexplained discrepancy to the shielding of BaP from oxidation by coatings of viscous organic aerosol (OA). Accounting for this OA viscosity-dependent shielding, which varies with temperature and humidity, in a global climate/chemistry model brings model predictions into much better agreement with BaP measurements, and demonstrates stronger long-range transport, greater deposition fluxes, and substantially elevated lung cancer risk from PAHs. Model results indicate that the OA coating is more effective in shielding BaP in the middle/high latitudes compared with the tropics because of differences in OA properties (semisolid when cool/dry vs. liquid-like when warm/humid). Faster chemical degradation of BaP in the tropics leads to higher concentrations of BaP oxidation products over the tropics compared with higher latitudes. This study has profound implications demonstrating that OA strongly modulates the atmospheric persistence of PAHs and their cancer risks.