Study of Visibility Degradation over the Pearl River Delta Region: Source Apportionment and Impact of Chemical Characteristics.

摘要

Fine particulate, also called PM2.5 (particulate matter less than 2.5 μm in diameter), is the known major culprit for visibility degradation. It is generally observed that the degree of visibility degradation increases with concentration level of PM2.5. This thesis work investigates potential reasons beyond PM mass, such as PM size and chemical composition, in impacting the visibility degradation. In addition, apportionment of visibility degradation to major aerosol source categories has been carried out and empirical formulas linking light extinction with aerosol constituents are established for the PRD region.;Source apportionment of light extinction at Tsuen Wan, an urban site in Hong Kong, to seven aerosol source categories was carried out by applying Positive Matrix Factorization (PMF) to PM2.5 chemical composition data and visibility data in the periods of November 2000 - October 2001 and November 2004 - October 2005. The seven sources and their respective contributions to light extinction are: secondary sulfate (31±9%), mixture of biomass burning / industrial combustion emissions (BBIC) (23±6%), automobile exhaust (22±8%), secondary nitrate (10±5%), sea salt (10±7%), and residual oil combustion (3±4%). Between the two campaign years (2000/01 vs. 2004/05), the contributions to light extinction by the top three sources have changed. The contribution of BBIC dropped from 27±7% (79.1 Mm-1) in 2000/2001 to 19±5% (65.7 Mm-1) in 2004/2005; the contribution of automobile exhaust decreased from 26±9% (74.9 Mm-1) in 2000/2001 to 15±7% (51.9 Mm-1 ) in 2004/2005; however, the contribution of secondary sulfate increased from 25±8% (73.1 Mm-1) in 2000/2001 to 36±10% (121.4 Mm-1) in 2004/2005. The source apportionment results indicate that the control strategies on vehicular emissions and industrial combustion emissions have taken effect and that the formulation and implement of control strategies on SO2 emissions need to be strengthened in order to lower the ambient PM2.5 levels and improve visibility in Hong Kong.;Examination of high PM hours under conditions of similar PM2.5 mass and RH reveals significant differences in light scattering coefficient (Bsc) between winter and summer. Similar observations were found in the daily samples, for which detailed chemical composition are available. A case study using two daily samples 20041103 and 20050812 was conducted. The ISORROPIA-Mie model was used to examine how aerosol size distributions and the mixing state of elemental carbon (EC) could significantly impact light extinction by aerosols. The modeling results show that size variation in the droplet-mode size of aerosols (in the range of 0.5-1.2 μm, near visible wavelength) may cause a change in Bsc larger than 50% for aerosols having the same bulk PM2.5 mass and chemical composition. The different mixing state of EC (external versus internal mixture) could also contribute to the relative difference of Bsc as large as 34%. Weather chart, wind field data and back trajectory analysis as well as chemical tracers show that the summer and winter samples examined here were influenced by air masses of different origins.;The long-term record of PM and visibility at the urban site of Tsuen Wan and the suburban site of Tung Chung from 2000 to 2007 were analyzed using the ISORROPIA-Mie model. The aim is to explain the non-linear relationship between annual PM concentration and the number of visibility degradation days. The increase of Bext (272 to 333 Mm-1) from 2000 to 2004 at Tung Chung was mainly caused by the increase in the PM mass concentration (44.9 to 62.1 μg/m3). From 2004 to 2007, although the mass concentration decreased (62.1 to 54.1 μg/m3), the annual Bext stayed at a high level (∼ 325 Mm-1) due to the increase contributions from sulfate (16% to 23%) and organic matter (25% to 31%). At the Tsuen Wan site, the increase of mass concentration (49.8 to 62.7 μg/m3) was largely responsible for the increase of annual Bext from 2000 to 2004 (280 to 334 Mm-1). The slightly decrease of annual Bext from 2004 to 2007 (334 to 304 Mm-1) was more related to the variation of aerosol size, which likely occurred due to reduction in local vehicular emissions and increase in source types at the urban site. (Abstract shortened by UMI.).