Size Distributions of Polycyclic Aromatic Hydrocarbons at Two Receptor Sites in the Pearl River Delta Region, China: Implications of a Dominant Droplet Mode

Abstract

Size distributions of twelve polycyclic aromatic hydrocarbons (PAHs), elemental carbon (EC), organic carbon (OC) and major inorganic ions were measured at two receptor locations in the Pearl River Delta (PRD) Region in southern China during 2006–2008. The size distributions of three- and four-ring PAHs were characterized with three modes, a condensation mode with a mass median aerodynamic diameter (MMAD) in the range of 0.2–0.3 μm, a droplet mode (MMAD: .7–1.0 μm), and a coarse mode (MMAD: 3–5 μm). The less volatile five- to six-ring PAHs were mainly distributed in the condensation mode and droplet mode, with little presence in the coarse mode. The droplet mode was the most prominent mode for all PAHs at both sites, accounting for more than 70% of the total particle-phase five- and six-ring PAHs and more than 60% of the total particle-phase three- and four-ring PAHs. The dominant droplet mode could be attributed to a result of in-cloud processing of vehicular soot particles and biomass burning particles during the transport of source aerosols to the receptor sites. Presence of sulfate aqueous coating on the droplet-mode particles was hypothesized to make PAHs inaccessible to volatilization from particle phase and to oxidation by gaseous oxidants. Further implications of a dominant droplet-mode PAHs were discussed.