Synergy between Secondary Organic Aerosols and Long-Range Transport of Polycyclic Aromatic Hydrocarbons

Abstract

Polycyclic aromatic hydrocarbons (PAHs), known for their harmful health effects, undergo long-range transport (LRT) when adsorbed on and/or absorbed in atmospheric particles. The association between atmospheric particles, PAHs, and their LRT has been the subject of many studies yet remains poorly understood. Current models assume PAHs instantaneously attain reversible gas-particle equilibrium. In this paradigm, as gas-phase PAH concentrations are depleted due to oxidation and dilution during LRT, particle-bound PAHs rapidly evaporate to re-establish equilibrium leading to severe underpredictions of LRT potential of particle-bound PAHs. Here we present a new, experimentally based picture in which PAHs trapped inside highly viscous semisolid secondary organic aerosol (SOA) particles, during particle formation, are prevented from evaporation and shielded from oxidation. In contrast, surface-adsorbed PAHs rapidly evaporate leaving no trace. We find synergetic effects between hydrophobic organics and SOA - the presence of hydrophobic organics inside SOA particles drastically slows SOA evaporation to the point that it can almost be ignored, and the highly viscous SOA prevents PAH evaporation ensuring efficient LRT. The data show the assumptions of instantaneous reversible gas-particle equilibrium for PAHs and SOA are fundamentally flawed, providing an explanation for the persistent discrepancy between observed and predicted particle-bound PAHs.

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

“…Previous modeling studies have primarily assumed that PAHs that undergo heterogeneous oxidation in the atmosphere are completely degraded (11,21,38). However, recent experimental studies show that several oxidized PAHs could remain particle-bound, and often appear as higher molecular weight peaks in particle mass spectra (25,39). Some oxidized BaP species have been shown to be toxic (40), and some are direct-acting mutagens (39); therefore, it is important to quantify their atmospheric exposure.…”

“…Highly viscous SOA coatings limit the diffusion of PAH molecules from particle bulk to particle surface (24,25), effectively shielding PAHs from chemical degradation (24). In addition, OA is more viscous in cool/dry conditions (24,26), so the effectiveness of shielding by OA is likely higher at cool locations in the atmosphere compared with laboratory measurements, which are made at room temperature [i.e., around 296°K (18,19,25)]. Thus, temperature-and RH-dependent changes in the viscosity of OA coatings can strongly affect heterogeneous oxidation kinetics of BaP.…”

Gas–particle partitioning of atmospheric aerosols: interplay of physical state, non-ideal mixing and morphology

“…16,52 Therefore, the presence of a semi-solid matrix may effectively shield reactive organic compounds from chemical degradation in long-range transport. 53 The multi-generation process of SOA formation involves reaction pathways of functionalization, fragmentation, and oligomerization. 3,54 Physical state and morphology may influence the competition among these pathways, as fragmentation has been observed to be enhanced for solid particles compared to liquid particles.…”

Relative humidity-dependent viscosities of isoprene-derived secondary organic material and atmospheric implications for isoprene-dominant forests

“…This is because knowledge of these physical properties is needed for modeling the environmental impacts of SOM particles Pfrang et al, 2011;Riipinen et al, 2011Riipinen et al, , 2012Shiraiwa et al, 2011Shiraiwa et al, , 2013Perraud et al, 2012;Shiraiwa and Seinfeld, 2012;Zelenyuk et al, 2012;Zhou et al, 2013). For example, when SOM particles are solid they may participate in heterogeneous ice nucleation in the atmosphere (Murray et al, 2010;Wang et al, 2012).…”