Sources of excess urban carbonaceous aerosol in the Pearl River Delta Region, China
Introduction
The Pearl River Delta (PRD) in China refers to the regions or cities alongside the Pearl River Estuary where the Pearl River flows into the South China Sea. Nine cities in Guangdong Province, including Guangzhou and Shenzhen, are also often referred to as the PRD, which has become one of the leading economic areas in China. With a total area covering less than 0.5% of China and a population of about 4% of the Chinese total, the PRD accounts for 19% of the total GDP in China (Zhang et al., 2008a). This is a highly urbanized region including three large population cities: Guangzhou (∼10 million), Hong Kong (∼7 million) and Shenzhen (∼4 million) (Wang et al., 2003). With such rapid increase in both economy and population, it is not surprising that the air quality in the PRD has deteriorated rapidly.
The PRD along with two other major city clusters in China (Beijing-Tianjin and Yangtze River Delta) have suffered from high concentrations of PM2.5 and ozone (Zhang et al., 2008a). Numerous studies have shown that these pollutants can cause adverse impacts on human health, visibility, climate change, and even crop production (Chameides and Bergin, 2002, Deng et al., 2008, Pope et al., 2002). In Guangzhou, the largest city in the PRD, only a few days per year exhibited low visibility (<10 km) between 1954 and 1972. After 1980 visibility degraded quickly (up to 150 days per year) and fine particulate matter (PM), especially those <1 μm in radius, contributed to 70% of the visibility reduction (Deng et al., 2008) and up to 93% in episodic days (Cheng et al., 2008). In Guangzhou, a clear and positive correlation was found between aerosol extinction coefficient and mortality associated with lung cancer (Tie et al., 2009). For PM2.5 pollution, there were few studies in the PRD before 2000, while it became a major concern in recent years (Wang et al., 2005, Zhang et al., 2008b).
The PRD region is under the influence of the Asian Monsoon with the prevailing wind as southwesterly in summer and northeasterly in winter, resulting in inevitable cross-boundary pollution. For example, Lau et al. (2007) reported that regional air masses from upwind Guangdong impacted Hong Kong 36% of the time in a year (132 days) while 53% of the time local air masses dominated. The assessment of the number of days or the extent of regional impact, to some extent, depends on the approaches applied. Compared to Hong Kong, the Pearl River Delta Economic Zone contributed significantly to the emissions in this region based on an emission inventory approach (95% of PM10, 88% of VOC, 87% of SO2, and 80% of NOx) (CH2M HILL, 2002). Organic matter is one of the most important constituents of PM2.5 in this region, accounting for 24–35% of PM2.5 mass (Hagler et al., 2006). This is not surprising considering the rapid economic growth in this region and the significant consumption of fossil fuel and biofuel. In order to develop effective control strategies for PM2.5, it is critical to understand what sources contribute to the excess part of carbonaceous aerosol over Hong Kong since the PRD contributed more than 90% of PM emissions in this region. It has been well recognized that without speciation analysis, it is difficult to conclude what sources cause high OC concentrations in ambient aerosol. A recent special issue published various findings of an air quality study in the PRD (Zhang et al., 2008a), although the contributions of specific sources to particulate OC concentrations are not fully addressed.
Therefore, in this study, the major goal is to better understand source contributions to excess OC in the PRD, defined by the difference between the levels in the PRD and the average in Hong Kong. The method using molecular markers in a chemical mass balance model (CMB-MM) allows quantifying source impacts from individual sources to OC. Among the compounds that can be identified by gas chromatography/mass spectrometry (GC/MS) are important markers to pinpoint the major source types of OC including cholesterol in meat charbroiling smoke, hopanes and steranes from lubricating oil in motor vehicles, levoglucosan from cellulose breakdown in biomass smoke, picene in coal combustion smoke etc. (Cass, 1998).
Most previous PM2.5-related studies have focused on major ions, EC and OC, whereas speciated data for organic carbon are very limited. A few previous studies aimed to assess the composition of organic matter in PM2.5 in Hong Kong (Sin et al., 2005) or in size-segregated aerosols in Guangzhou (Bi et al., 2005) and Hong Kong (Zheng et al., 2000, Zheng et al., 2008), but none was a regional-wide study, covering both Hong Kong and the PRD. This is the first effort to carry out simultaneous regional sampling for detailed molecular marker measurements in PM2.5 in the PRD region and to conduct source apportionment of carbonaceous aerosol from regional sites with the same technique (CMB-MM), thus data from various sites can be directly compared. In addition, we measured a great number of organic compounds, providing rich information on the organic aerosol composition and the related sources in the PRD region.
Simultaneous PM2.5 sampling was conducted at 7 sites in this region (4 in the PRD and 3 in Hong Kong) during four months of one year (October, December 2002; March, June 2003). Hagler et al., 2006, Hagler et al., 2007 reported values for bulk carbonaceous fractions, ionic, and elemental data from these PM2.5 samples, while this study focuses on PM2.5 organic speciation (up to 100 compounds) in order to better understand the sources of excess OC found at the sites in the PRD over Hong Kong. The overall objective of this work is to supply critical source apportionment data to be used for developing effective control strategies for carbonaceous aerosol and mitigating the adverse effects of air pollution in the PRD.
Section snippets
Sampling
The details of the field sampling have been discussed elsewhere (Hagler et al., 2006). For every sixth day in each of the four months, PM2.5 samples were collected over a 24 h period. PM2.5 particles were collected on filter media installed in four parallel channels through a cyclone-based size selection with quartz fiber filter in one of the channels for organic speciation analysis. The flow rate was 16.7 L min−1. Samples were collected simultaneously from three sites of Tap Mun (TM), Tung
Excess OC and EC in the PRD
Summer season samples for organic speciation were collected in June when southerly winds (from the South China Sea) prevailed. However, during the other three seasons, samples were primarily influenced by northerly, northeasterly and mixed flows (Hagler et al., 2006). Averaged OC and EC concentrations and EC/OC ratios at sites in Hong Kong and the PRD are shown in Table 1.
The OC/EC ratio in both areas exhibited a distinctly higher value in rural areas (6.1 in TM and 6.5 in CH) with the highest
Conclusions
Carbonaceous aerosol in PM2.5 was found to be more homogenously distributed in Hong Kong compared to its adjacent PRD, which is known to have much higher pollutant levels and has thus been a major pollution source in this region. Therefore, our study investigated what sources constituted the excess OC at four sites in the PRD as well as the spatial and seasonal variations of these excess OC. The highest excess OC was constantly found at Guangzhou. The results from receptor modeling revealed
Acknowledgments
This research was sponsored by Hong Kong Jockey Club Charities Trust, Castle Peak Power Co. Ltd., the Environmental Protection Department of the Hong Kong Special Administrative Region, the Shell Hong Kong Ltd., through Civic Exchange. We thank Christine Loh, the founder of Civic Exchange, and C.S. Kiang for coordinating the project, Dr. Tao Wang of Hong Kong Polytechnic University for the project assistance throughout this study, and Tao Liu of the Guangzhou Environmental Monitoring Center,
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