PAHs and SOA versus primary carbonaceous aerosols

We use the chemical transport model GEOS-
Chem to evaluate the hypothesis that atmospheric polycyclic
aromatic hydrocarbons (PAHs) are trapped in secondary
organic aerosol (SOA) as it forms. We test the ability of three
different partitioning configurations within the model to
reproduce observed total concentrations in the midlatitudes
and the Arctic as well as midlatitude gas-particle phase
distributions. The configurations tested are (1) the GEOS-
Chem default configuration, which uses instantaneous
equilibrium partitioning to divide PAHs among the gas
phase, a primary organic matter (OM) phase (absorptive),
and a black carbon (BC) phase (adsorptive), (2) an SOA configuration in which PAHs are trapped in SOA when emitted and slowly evaporate from SOA thereafter, and (3) a configuration in which PAHs are trapped in primary OM/BC upon emission and subsequently slowly evaporate. We also test the influence of changing the fraction of PAHs available for particle-phase oxidation. Trapping PAHs in SOA particles upon formation and protecting against particle-phase oxidation (2) better simulates observed remote concentrations compared to our default configuration (1). However, simulating adsorptive partitioning to BC is required to reproduce the magnitude and seasonal pattern of gasâ particle phase distributions. Thus, the last configuration (3) results in the best agreement between observed and simulated concentration/phase distribution data. The importance of BC rather than SOA to PAH transport is consistent with strong observational evidence that PAHs and BC are coemitted.