Adsorption onto aerosol soot carbon dominates gas-particle partitioning of polycyclic aromatic hydrocarbons

Gas-particle partitioning has an important influence on the fate of atmospheric polycyclic aromatic hydrocarbons (PAHs) and other semivolatile organic compounds (SOCs). In the present paper, gas- and aerosol-phase PAH concentrations and organic and elemental carbon concentrations in the aerosols measured in the Baltimore atmosphere and over the adjacent Chesapeake Bay in July 1997 were used to assess the mechanisms driving gas-particle partitioning of PAHs. The relative importance of adsorption onto the soot carbon and absorption into aerosol organic matter is evaluated by means of estimated soot/air (K-SA) and octanol/air (K-OA) partition coefficients, respectively. The results show that absorption into organic carbon may account for less than 10% of the total PAHs in the particulate phase. Adsorption onto the soot phase predicts accurately the total suspended particulate matter normalized partition coefficients (K-P) for PAHs. For example, K-SA predicts K-P values for phenanthrene over the Chesapeake Bay within a factor of 3. K-P predictions at the Baltimore atmosphere are within a factor of 5 to 10 of measured K-P values. This is consistent with a lack of equilibrium between the gas and aerosol soot phase.