Polycyclic aromatic hydrocarbons in urban atmosphere of Guangzhou, China: Size distribution characteristics and size-resolved gas-particle partitioning

Size distributions of thirteen polycyclic aromatic hydrocarbons (PAHs), elemental carbon (EC), and organic carbon (OC) in the range of 0.01-18 mu m were measured using a nano Micro-Orifice Uniform Deposit Impactor (nano-MOUDI) in an urban location in Guangzhou, China in July 2006. PAH size distributions were fit with five modes and the respective mass median aerodynamic diameters (MMAD) are: Aitken mode (MMAD: similar to 0.05 pm), three accumulation modes AM(I), AM(II), AM(III) (MMAD: 0.13-0.17 mu m, 0.4-0.45 mu m, and 0.9-1.2 mu m, respectively), and coarse mode (MMAD: 4-6 mu m). Seven-ring PAH was mainly in AM(II) and AM(III). Five- and six-ring PAHs were found to be abundant in all the three AM. Three- and four-ring PAHs had a significant presence in the coarse mode in addition to the three AM. Size-resolved gas-particle partition coefficients of PAHs (K-p) were estimated using measured EC and OC data. The K-p values of a given PAH could differ by a factor of up to 7 on particles in different size modes, with the highest K-p associated with the AM(I) particles and the lowest K-p associated with the coarse mode particles. Comparison of calculated overall K-p with measured K-p values in Guangzhou by Yang et al. (2010) shows that adsorption on EC appeared to be the dominant mechanism driving the gas-particle partitioning of three- and four-ring PAHs while absorption in OM played a dominant role for five- and six-ring PAHs. The calculated equilibrium timescales of repartitioning indicate that five- to seven-ring PAHs could not achieve equilibrium partitioning within their typical residence time in urban atmospheres, while three- and four-ring PAHs could readily reach new equilibrium states in particles of all sizes. A partitioning flux is therefore proposed to replace the equilibrium assumption in modeling PAH transport and fate. (C) 2012 Elsevier Ltd. All rights reserved.