The characterization and source identification of airborne particles in Hong Kong

Abstract

Low visual range indicates an air pollution problem. In Hong Kong, poor visibility has been of increasing concern since 2000. This thesis explores the relationships between local air pollution and visibility degradation by characterizing particle optical and chemical properties, estimating chemical contributions to light extinction, and identifying particulate matter sources. Particle light scattering and absorption coefficients (Bsp, Bap) in Hong Kong differ among urban, sub-urban and rural areas. Highest values were observed in urban areas due to particles from traffic and cooking sources in addition to regionally distributed particles. Regional-scale Bsp and Bap measured outside the urbanized area was dominantly affected by secondary sulfate, nitrate and organics. The highest PM₁₀ concentrations were observed during the fall and the lowest were found during the spring at three sampling sites. Urban particulate matters (PM) contained large increments of organic and elemental carbon (EC), typical of engine exhaust. Dominant species in the urban area were particle organic matters (POM) and EC, which due to high traffic flow. Regional transportation and stagnant meteorological conditions elevated aged and secondary pollutants in the Hong Kong sub-urban area. Most of the SO₄²⁻, NH₄⁺, K⁺, OC and EC, were in the size range of 0.56 m to 1.8 m, NO₃⁻, and Cl⁻, were dominated in the 1.8 m to 10 m size range. Because of complex chemical compositions in droplet mode, eight emission sources profiles have been determined in the mode. The largest contributio of droplet mode source was from anthropogenic emission sources, i.e. secondary SO₄²⁻ (24%), industrial emission (19%), vehicle emission, secondary NO₃⁻ and sea salt (13-15% in each) with 10% of other sources. Relatively simple sources were identified in the coarse mode (3 sources) and the condensation mode (4 sources). Over 80% of PMcoarse mass comprised of sea salt and soil dust. Most of the primary emissions were identified in the condensation mode, including 27% vehicle emissions and 18-19% each from residual oil combustion and coal/biomass burning. Secondary formation i.e. secondary SO₄²⁻ contributed approximately 37% of the particles in the condensation mode The special chemical composition and particles size distribution in Hong Kong was observed, mass scattering efficiency of Hong Kong need to be determined. The Hong Kong mass scattering efficiency (MSE) value was investigated under the clean and polluted air quality situation. It is presented as follows: 2.6 (clean) and 3.2 m²/g (polluted) for ammonium sulfate, 2.2 (clean) and 1.5 m²/g (polluted) for ammonium nitrate, 2.1 (clean) and 4.3 m²/g (polluted) for particle organic matters, 1.5 m²/g for sea salt and 1.5 m²/g for soil dust. In general, the largest contributors to particle light extinction in Hong Kong were traffic/engine emission and secondary formation. In other words, to improve visibility degradation situation in HK, the government should control the local traffic emission and build the collaboration with regional institution or government to improve regional air quality. In addition, the air pollution in Hong Kong was mainly affected by south and south east China during autumn and winter months and by north and central China during spring and summer months.