Optical properties of size-resolved particles at a Hong Kong urban site during winter

Visibility degradation in Hong Kong is related to the city's serious air pollution problems. To investigate the aerosols' optical properties and their relationship with the chemical composition and size distribution of the particles, a monitoring campaign was conducted at an urban site in the early winter period (from October to December, 2010). The particle light scattering coefficient (B_sp) and absorption coefficient (B_ap) were measured. Two collocated Micro-Orifice Uniform Deposit Impactor samplers (MOUDI110, MSP, USA) with nominal 50% cut-off aerodynamic diameters of 18, 10, 5.6, 3.2, 1.8, 1, 0.56, 0.32, 0.18, 0.1, and 0.056μm were used to collect size-resolved particle samples. The average B_sp and B_ap were 201.96±105.82Mm^-1 and 39.91±19.16Mm^-1, with an average single scattering albedo (ω_o) of 0.82±0.07. The theoretical method of light extinction calculation was used to determine the extinction of the size-resolved particulate matters (PM). The reconstructed light scattering coefficient correlated well with the measured scattering value in the Hong Kong urban area. Droplet mode (0.56-1.8μm) particles contributed most to the particle light extinction (~69%). Organic matter, ammonium sulphate and elemental carbon were the key components causing visibility degradation in the droplet (0.56-1.8μm) and condensation (0.1-0.56μm) size ranges. Five sources contributing to particle light extinction have been identified using positive matrix factorisation (PMF). Traffic/engine exhausts and secondary aerosols accounted for ~36% and ~32% of particle light extinction, respectively, followed by sea salt (15%). The remaining sources, soil/fugitive dust and tire dust, contributed by ~10% and 7%, respectively, to particle light extinction.