Seasonal characteristics and regional transport of PM2.5 in Hong Kong

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Atmospheric Environment 39 (2005) 1695–1710 www.elsevier.com/locate/atmosenv

Seasonal characteristics and regional transport of PM2.5 in Hong Kong Peter K.K. Louiea,, John G. Watsonb, Judith C. Chowb, Antony Chenb, Della W.M. Sinc, Alexis K.H. Laud a

Environmental Protection Department, 33/F, Revenue Tower, 5 Gloucester Rd., Wanchai, Hong Kong Special Administrative Region of the People’s Republic of China b Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA c Government Laboratory, 7/F, Homantin Government Offices, 88 Chung Hau St., Homantin, Hong Kong, Special Administrative Region of the People’s Republic of China d Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 17 May 2004; received in revised form 9 November 2004; accepted 17 November 2004

Abstract The Asiatic monsoon is the dominant meteorological feature of the Pearl River Delta (PRD) region in China, and influences the accumulation of locally emitted pollutants, as well as regional transport. To investigate the effect of meteorological characteristics on PM2.5 mass and chemical composition in Hong Kong, a major city of the PRD, 24-h (0000–2400 LST) PM2.5 samples were collected every sixth day from 6 November 2000 to 26 October 2001. The sampling network included a roadside site at Mong Kok (MK), an urban site at Tsuen Wan (TW), and a rural site at Hok Tsui (HT). Air parcel back trajectory and residence time analyses indicate a predominantly northeasterly transport during winter and fall in contrast to a southwesterly-to-southeasterly transport in summer. The highest seasonal PM2.5 concentrations were found in winter, followed by fall, at each of the sampling sites due to elevated organic material (OM) and ammonium nitrate (NH4NO3), consistent with the increased air parcel residence time over potential source regions. Though local mobile sources appeared to dominate the emission of carbonaceous material, a higher organic carbon/elemental carbon (OC/EC) ratio and water-soluble potassium (K+) concentration in winter imply an additional contribution from vegetative burning. Episodic pollution events were identified and investigated with back trajectories and species enrichment factors. Local and regional sources both contributed to the formation of a pollution episode on 28 February 2001. These findings reflect the degree of PM2.5 pollution in Hong Kong and provide valuable insights for planning future monitoring and modeling studies in the PRD region. r 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon, elemental; Carbon, organic; PM2.5; Pearl River Delta region

1. Introduction

Corresponding author.

E-mail address: [email protected] (P.K.K. Louie).

The Pearl River Delta (PRD) is one of the most highly populated areas of China and contains some of the country’s highest concentrations of heavy industry (Cao et al., 2003, 2004; Chow et al., 2004). This region of

1352-2310/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.11.017

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more than 50 million people includes the 10 major cities of Hong Kong (
1100 km2, 6.8 million), Shenzhen (1949 km2, 7 million), Guangzhou (7434 km2, 9.94 million), Macao (25.8 km2, 440,000), Dongguan (2465 km2, 6.45 million), Foshan (3914 km2, 2000 census population: 5.34 million), Huizhou (11,158 km2, 3.22 million), Jiangmen (9541 km2, 3.96 million), Zhongshan (1800 km2, 2.36 million), and Zhaoqing (14,856 km2, 3.37 million). These cities are interspersed with smaller towns and farms. The Hong Kong Special Administrative Region (HKSAR)—including Hong Kong Island, Kowloon Peninsula, and the New Territories at the southeastern end of the PRD—leads the PRD in economic development and modernization. Owing to a high density of local and regional emitters that are not adequately characterized or regulated, particulate air quality has been one of the critical environmental issues for the HKSAR government. Both long-term and episodic particulate matter (PM) concentrations in Hong Kong have exceeded the ambient air quality standards accepted by developed countries (e.g., the United States), and this has implications for health, visibility, and regional climate change (Vedal, 1997; Watson, 2002; White et al., 2003). Primary sources of PM emitters in Hong Kong include vehicle exhaust, cooking, road dust, electricity generation, construction, fleet shipping, and industrial processes (e.g., sand/gravel production, cement kilns). Leung (1999) reported a strong correlation between fine PM (PM2.5, particles with aerodynamic diameters less than 2.5 mm) mass and carbonaceous aerosol (organic and elemental carbon). Carbonaceous aerosol and secondary ammonium sulfate ((NH4)2SO4) were found to account for more than 90% of PM2.5 mass at Tsuen Wan (TW), a typical urban site in Hong Kong. Wintertime organic carbon (OC) concentrations were twice those during summer. Pathak et al. (2003) estimated that
40% of the (NH4)2SO4 originated from regional transport of non-local emissions. Yu et al. (2004) attributed elemental carbon (EC) in Hong Kong to both year-round marine vessel emissions and to the northerly transport of residual air masses with elevated carbonaceous aerosol, which occurred frequently in winter. The non-local sources likely result from major industries such as steel and petrochemical manufacturing, power generation, and other manufacturing, as well as from agricultural burning, vehicle exhaust, and residential heating/cooking in urban and rural areas of the PRD and southeastern China. To better understand the seasonal nature of PM2.5 and its local or regional components, a year-long study (2000–2001) was conducted from November 2000 to October 2001 that acquired 24-h PM2.5 mass and chemical composition every sixth day at middle-, urban-, and regional-scale (Chow et al., 2002) sampling sites. Large urban vs. regional contrasts were found for

carbonaceous aerosol, which accounted for 52–75% of PM2.5 mass at the middle-scale roadside and urban-scale sites but just 32% at the regional-scale (Louie et al., 2004) site. (NH4)2SO4 and crustal material (CM) showed limited spatial variations. This paper examines the effects of meteorology on seasonal variations in PM2.5. Back trajectories, coupled with air parcel residence time analysis, are used to link meteorological conditions to episodic pollution events. A conceptual model was formulated (Louie et al., 2005) to relate ambient PM2.5 to pollution sources in Hong Kong. Findings from this study may influence future monitoring and modeling studies.

2. Monitoring network and methodology Sampling sites are illustrated in Fig. 1. The Mong Kok (MK) site is located in a mixed commercial and residential plaza where many diesel bus routes converge with numerous restaurants in the vicinity. The TW site is set back from heavily traveled roads in a highly populated residential area with many stores, offices, and light industry. The Hok Tsui (HT) site is located at the southeast coast of Hong Kong Island with no roadways, houses, commercial establishments, or industry. A lightly traveled access road leads to the site. Twenty-four-hour (0000 to 2400 LST) PM2.5 samples were taken every sixth day from 6 November 2000 to 26 October 2001, at each site. Partisol samplers (Rupprecht & Patashnick, Albany, NY, USA) equipped with Andersen SA-246 PM10 (particles with aerodynamic diameters less than 10 m) size-selective inlets followed by PM2.5 Well Impactor Ninety-Six (WINS) inlets sampled at a flow rate of 16.7 L min1 (Watson and Chow, 2001). Two samplers were collocated at each site: one configured with a Teflon-membrane filter (]R2PJ047, Pall Life Sciences, Ann Arbor, MI, USA) and the other with a quartz-fiber filter (]2500 QAT-UP, Pall Life Sciences, Ann Arbor MI, USA). The Teflon-membrane filter collected particles for measurement of mass by gravimetry and for 40 elements (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Yt, Zr, Mo, Pd, Ag, Cd, In, Sn, Sb, Ba, La, Au, Hg, Tl, Pb, and U) by X-ray fluorescence (XRF) spectroscopy (Watson et al., 1999). The quartz-fiber filter was also analyzed for mass by gravimetry; for 2 chloride (Cl), nitrate (NO 3 ), and sulfate (SO4 ) by ion chromatography (IC) (Chow and Watson, 1999); for ammonium (NH+ 4 ) by automated colorimetry (AC); for water-soluble sodium (Na+) and potassium (K+) by atomic absorption spectrophotometry (AAS); and for eight fractions of OC and EC by the IMPROVE thermal/optical reflectance (TOR) protocol (Chow et al., 1993, 2001, 2004).

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Fig. 1. Locations of three sampling sites: middle-scale MK (22.32 N, 114.1 E), urban-scale TW (22.38 N, 114.1 E), and regional-scale HT (22.21 N, 114.26 E). Table 1 Seasonal meteorological characteristics in Hong Kong Seasona

Surface pressureb (hPa)

Daily max Daily Daily min RHb Cloud Total Bright b T (1C) T b (1C) mean (%) coverageb precipitationb sunshineb (%) (mm) (h) T b (1C)

Average Average prevailing wind wind speedc directionc (deg) (km h1)

Winter Spring Summer Fall Annual

1018 1011 1005 1013 1012

20.6 25.5 30.4 28.8 25.8

45 65 124 60 63

18.6 23.3 28.1 26.9 23.7

16.8 21.5 26.2 25.3 21.9

76.7 83.4 82.6 69.5 79.4

65.6 81.1 75.3 54.0 70.6

215.3 225.5 2716.6 10.6 3168.0

5.4 4.4 5.6 7.4 5.5

25.2 19.1 19.9 26.6 22.3

a

Winter: 06/11/2000–13/03/2001; spring: 19/03/2001–15/05/2001; summer: 17/05/2001–20/09/2001; fall: 23/09/2001–26/10/2001. Hong Kong Observatory (
3 km south of the Mong Kok (MK) site). c Waglan Island, an outlying island site monitoring regional wind flows. b

3. Meteorological characteristics Synoptic-scale meteorology in the PRD is influenced by the Asiatic monsoon (Chang and Krishnamurti,

1987). During winter, strong radiative cooling over the continent creates a high-pressure anticyclone that drives cold, dry polar air from the continent into the surrounding oceanic areas, resulting in weak to

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moderate northeasterly winds or strong northerly winds (Murakami, 1979). Spring is characterized by predominant easterly winds with moderate speed and rising temperatures. During summer, a low-pressure trough

draws moist warm air inland from the ocean. Surface winds shift from southwesterly in May to southeasterly in June with an increase in precipitation. Most of the precipitation occurs during summer. The winds become

Fig. 2. Residence time analysis for clustered 72-h air parcel backward trajectories during the (A) winter, (B) spring, (C) summer, and (D) fall. See Table 1 for days included in each season. Backward trajectories were initiated at 1000 m above the regional-scale HT site in Hong Kong every 2 h. The domain of trajectory modeling is 15–351N, 100–1301E, and from the surface to an altitude of 4 km above ground level (AGL). The upper and lower panels show the horizontal and vertical movement of air parcels, respectively. The gridded residence time was normalized to the total air parcel residence time during each season and the logarithm values are plotted on the shaded (z) axis.

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stronger and east/northeasterly during fall. The winter circulation re-establishes itself by mid-November. Wind speeds are generally weak to moderate. The strongest winds are found during typhoons, which occur 2–6 times a year and are of about two days’ duration. Strong northerly winds are recorded during cold winter surges; moderately strong southwesterly winds are reported during early summer when the Meiyu rainband is active. Table 1 summarizes conditions during the study period. Based on these observations and an examination of synoptic weather maps, seasons were defined as winter (6 November 2000 to 13 March 2001), spring (19 March 2001 to 15 May 2001), summer (17 May 2001 to 20 September 2001), and fall (23 September 2001 to 26 October 2001). An examination of the historical climatology records shows that meteorological characteristics during the study period did not deviate from the norm. They represent a typical monsoonal climate, with a prolonged winter and summer, and transitional, short falls and springs.

4. Air trajectory residence time analysis Air mass back trajectories, which trace air parcels backward in time from a receptor, are used to understand synoptic-scale atmospheric circulation and the associated transport. Three-day (i.e., 72-h) backward trajectories were calculated every 2 h for each sampling day, using the hybrid single-particle Lagrangian integrated trajectories (HY-SPLIT) model (Draxler, 1988; Draxler and Hess, 1997). HY-SPLIT was configured with wind fields from the National Center for Environmental Prediction Final Analyses observation database (http://www.arl.noaa.gov/ready/hysplit4.html). When comparing HY-SPLIT calculated trajectories with tracer gas releases, Draxler (1991, 1999) estimated a potential error of 20–30% for total travel distance. Back trajectories were clustered by season and their residence time over each 11  11 cell (15–351N, 100–1301E) was calculated (Chen et al., 2002). The gridded residence time was then normalized to the total air parcel residence time of each season, with results shown in Fig. 2. Winter was dominated by air parcels originating from the East China Sea passing over China’s highly populated coastal area and the western part of Taiwan before turning northeast toward the PRD. Fig. 2A corroborates that most trajectories resided over the southeast coast of China with limited inland and oceanic contributions. Back trajectories in fall resembled those in winter, with even lower inland and oceanic contributions (Fig. 2D). Summertime air parcels (Fig. 2C) originated mostly from the South China Sea and advanced northward, reaching Hong Kong with a

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strong marine influence. The transport pattern in spring was a mix of the summer and fall patterns but generally with more marine than continental influences (Fig. 2B). The persistent northeast monsoon in winter carries pollutants emitted from potential source regions in southeastern China, while the summer monsoon shifts to southwesterly winds that bring in cleaner marine air. This summer–winter contrast in prevailing wind direction is associated with appreciable seasonal variations of ambient PM2.5 concentration in Hong Kong.

5. PM2.5 seasonal variations Table 2 compares the seasonal averages for PM2.5 mass and chemical components. The highest PM2.5 mass occurred during winter (33–69 mg m3) followed by fall (27–56 mg m3). PM2.5 masses during spring and summer were lower, especially at the HT site. The lowest seasonal average of 14.8 mg m3 was measured at the HT site during summer. Seasonal PM2.5 mass and its major components are compared in Fig. 3. The gravimetric PM2.5 mass is in good agreement with the sum of these components (Louie et al., 2005). The highest and lowest (NH4)2SO4 concentrations were observed during fall (14.5– 20.4 mg m3) and summer (9.2–11.1 mg m3), respectively, differing by
50–100%. This contrasts with regional measurements from the eastern US coastal region (Chen et al., 2002, 2003), where summertime SO2 is highest downwind of a major sulfur dioxide 4 (SO2) source region. Frequent back trajectories with cleaner marine origins during summer are associated with the lower SO2 4 in Hong Kong. For this study the fall differed from the summer not in temperature (averaging just 1.2 1C lower) but in prevailing wind direction and precipitation (Table 1). Since the fall was relatively hot and dry, the elevated (NH4)2SO4 concentration may reflect the regional accumulation of secondary SO2 around the southeast coast of China, 4 which is upwind of Hong Kong and the PRD. The regional nature of (NH4)2SO4 is confirmed by its spatial uniformity across the three sampling sites (varying by
5% in fall and o15% in other seasons). (NH4)2SO4 dominated PM2.5 mass at the HT site (460%) and contributed nearly as much as carbonaceous material at the TW site. NH4NO3 levels were relatively low, with seasonal averages of less than 4 mg m3. The seasonal variation in NH4NO3 was inversely related to ambient temperature, being 3–5 times higher in winter than in summer and moderate in spring and fall. NH4NO3 concentration depends on atmospheric temperature and relative humidity, which affects the gas/particle partitioning between precursor gases (NH3 and HNO3) and

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Table 2 Seasonal averages (concentration7standard deviation, mg m3) of PM2.5 mass and chemical composition in Hong Kong between 6 November 2000 and 26 October 2001 Wintera

Springa

MKb

TWb

HTb

MKb

Number of samples

20

20

20

Mass (Teflon) Chloride (Cl) Nitrate (NO 3) Sulfate (SO2 4 ) Ammonium (NH+ 4 ) Soluble sodium (Na+) Soluble potassium (K+)

69.15719.61 0.4270.56 2.6572.33 10.3274.89 3.8472.43 0.3870.27 0.7270.53

44.50723.62 0.2270.59 2.2672.35 9.9074.76 3.5972.51 0.4170.35 0.7770.56

32.76715.05 0.2170.31 1.4071.57 9.6474.35 2.7971.80 0.9670.76 0.6370.41

50.2674.86 0.2070.05 1.8270.13 10.1670.65 3.1570.22 0.3470.37 0.3170.02

29.31712.54 0.0870.05 1.2170.67 9.1074.53 2.7371.52 0.4070.31 0.3370.25

21.84710.73 0.1470.15 0.5070.41 9.6774.77 2.1371.23 0.4970.37 0.2570.15

OC1c OC2c OC3c OC4c Pyrolyzed OCc Total OCc EC1c EC2c EC3 Total ECc Total carbon (TC)c

3.6670.94 3.0670.96 6.4672.41 9.3874.60 0.0170.01 22.5877.31 18.9473.49 0.2370.22 0.0070.01 19.1673.53 41.7476.72

1.0670.69 1.7670.70 4.3771.91 5.4173.11 0.0170.01 12.6276.07 4.8471.66 0.1970.07 0.0170.03 5.0371.64 17.6577.27

0.3770.37 0.9870.55 2.6771.77 3.0171.79 0.0270.03 7.0574.23 1.8370.88 0.1770.07 0.0070.00 1.9970.88 9.0474.94

2.7170.63 1.6970.27 2.7470.43 4.6570.44 0.1270.09 11.8970.97 18.8771.79 0.1470.06 0.0070.01 18.8971.94 30.7871.96

0.6470.29 1.3270.32 1.5070.80 2.9771.36 0.0070.00 6.4172.36 4.9371.28 0.3370.11 0.0070.00 5.2671.26 11.6773.17

0.1970.20 0.5370.14 0.8570.27 1.2370.61 0.0370.07 2.8171.04 1.6370.64 0.2570.09 0.0070.00 1.8470.62 4.6671.58

Sodium (Na) Magnesium (Mg) Aluminum (Al) Silicon (Si) Phosphorus (P) Sulfur (S) Chlorine (Cl) Potassium (K) Calcium (Ca) Titanium (Ti) Vanadium (V) Chromium (Cr) Manganese (Mn) Iron (Fe) Cobalt (Co) Nickel (Ni) Copper (Cu) Zinc (Zn) Gallium (Ga) Arsenic (As) Selenium (Se) Bromine (Br) Rubidium (Rb) Strontium (Sr) Yttrium (Y) Zirconium (Zr) Molybdenum (Mo) Palladium (Pd) Silver (Ag) Cadmium (Cd) Indium (In) Tin (Sn) Antimony (Sb)

0.1570.11 0.0470.04 0.1570.15 0.5970.43 0.0170.01 3.7671.60 0.2470.43 0.8370.54 0.2170.15 0.0170.01 0.0170.02 0.0070.00 0.0270.01 0.3470.17 0.0070.00 0.0070.01 0.0270.01 0.2570.17 0.0070.00 0.0170.01 0.0070.00 0.0270.02 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0270.01 0.0170.01

0.1170.12 0.0370.05 0.1670.17 0.5570.49 0.0170.01 3.6371.45 0.2170.55 0.8970.54 0.1770.18 0.0170.01 0.0170.01 0.0070.00 0.0270.01 0.2570.19 0.0070.00 0.0070.01 0.0170.02 0.2370.18 0.0070.00 0.0170.01 0.0070.00 0.0270.03 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0370.02 0.0170.01

0.3770.19 0.0770.05 0.1470.16 0.4870.45 0.0070.00 3.4471.49 0.2870.31 0.7570.46 0.1570.17 0.0170.01 0.0170.01 0.0070.00 0.0170.01 0.1770.15 0.0070.00 0.0070.00 0.0170.01 0.1770.12 0.0070.00 0.0170.01 0.0070.00 0.0270.01 0.0170.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0270.01 0.0170.01

0.1770.03 0.0570.02 0.1470.01 0.5470.03 0.0170.01 3.4670.19 0.0370.07 0.4170.02 0.1670.01 0.0170.03 0.0270.01 0.0070.00 0.0170.00 0.2770.02 0.0070.00 0.0170.00 0.0170.00 0.1470.01 0.0070.00 0.0070.01 0.0070.00 0.0170.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.01 0.0070.01 0.0070.01 0.0070.01 0.0270.02 0.0070.02

0.1870.09 0.0570.03 0.1470.13 0.4770.40 0.0070.00 3.3571.66 0.0070.00 0.4370.30 0.1370.10 0.0170.01 0.0270.02 0.0070.00 0.0170.01 0.2070.15 0.0070.00 0.0170.00 0.0170.01 0.1270.10 0.0070.00 0.0070.01 0.0070.00 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0270.02 0.0070.00

0.2270.12 0.0670.05 0.1570.18 0.5070.57 0.0070.00 3.1271.47 0.0570.10 0.3470.24 0.1270.12 0.0170.01 0.0270.01 0.0070.00 0.0170.01 0.1770.19 0.0070.00 0.0170.00 0.0070.00 0.0870.06 0.0070.00 0.0070.00 0.0070.00 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0170.01 0.0070.00

9

TWb

HTb

9

9

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Table 2 (continued ) Wintera MKb Barium (Ba) Lanthanum (La) Gold (Au) Mercury (Hg) Thallium (Tl) Lead (Pb) Uranium (U)

0.0470.03 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.1170.10 0.0070.00

Springa TWb 0.0270.02 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.1270.12 0.0070.00

HTb 0.0170.02 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.1070.07 0.0070.00

Summera

MKb 0.0270.08 0.0170.11 0.0070.01 0.0070.00 0.0070.00 0.0570.00 0.0070.00

TWb 0.0270.02 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0570.06 0.0070.00

HTb 0.0170.01 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0470.03 0.0070.00

Falla

MKb

TWb

HTb

Number of samples

21

21

21

Mass (Teflon) Chloride (Cl) Nitrate (NO 3) Sulfate (SO2 4 ) Ammonium (NH+ 4 ) Soluble sodium (Na+) Soluble potassium (K+)

51.40716.93 0.1870.14 0.8570.60 7.7276.10 2.3772.15 0.3570.20 0.2770.35

25.58715.05 0.1170.10 0.5770.30 7.6075.86 2.1572.03 0.3470.20 0.2870.36

14.83712.33 0.1270.12 0.2370.12 6.7075.82 1.4471.59 0.4570.23 0.2370.34

56.0378.36 0.1070.09 1.0570.36 11.4373.81 3.6571.43 0.6770.29 0.5270.40

34.4278.87 0.0470.03 0.6470.22 11.6173.70 3.6071.40 0.6370.29 0.5170.37

27.1679.21 0.0370.04 0.3070.21 11.0374.00 2.7471.39 0.8770.33 0.4870.36

OC1c OC2c OC3c OC4c Pyrolyzed OCc Total OCc EC1c EC2c EC3 Total ECc Total Carbon (TC)c

1.5670.91 2.3370.73 3.8171.53 6.2674.38 0.3871.65 14.3476.52 21.7175.30 0.4670.63 0.0070.00 21.7975.20 35.7577.52

0.5470.47 1.1470.41 1.9871.08 2.6972.20 0.0470.13 6.3373.81 5.5671.07 0.4270.34 0.0070.01 5.9571.00 12.2473.86

0.0770.09 0.4770.39 0.7370.81 0.9671.32 0.0970.13 2.2672.51 1.1271.01 0.2770.09 0.0070.00 1.3071.01 3.5273.33

1.6670.63 2.3770.60 3.5571.40 4.6971.77 0.2270.49 12.3873.05 19.6573.05 0.4170.19 0.0070.01 19.8573.09 32.1672.50

0.3570.32 1.4470.52 2.2670.53 3.4370.60 0.0170.00 7.3871.48 4.4671.38 0.2470.13 0.0070.00 4.6971.47 12.0072.64

0.2170.27 0.7170.35 1.1270.40 1.4870.82 0.3070.32 3.7172.01 1.8871.08 0.2270.12 0.0070.00 1.8170.81 5.4572.77

0.1770.09 0.0470.02 0.0770.06 0.3670.17 0.0170.01 2.8372.08 0.0770.09 0.3370.40 0.1470.05 0.0170.01 0.0270.02 0.0070.00 0.0170.01 0.2170.08 0.0070.00 0.0170.01 0.0170.01 0.1370.10 0.0070.00 0.0070.00 0.0070.00 0.0170.00 0.0070.00

0.1770.08 0.0370.02 0.0770.07 0.2370.17 0.0170.00 2.7672.13 0.0370.06 0.3470.41 0.0970.05 0.0170.01 0.0270.01 0.0070.00 0.0170.01 0.1370.07 0.0070.00 0.0170.00 0.0170.01 0.1570.21 0.0070.00 0.0070.00 0.0070.00 0.0170.00 0.0070.00

0.2070.09 0.0470.02 0.0670.06 0.1670.17 0.0070.01 2.3671.99 0.0870.13 0.2770.39 0.0570.04 0.0070.01 0.0170.01 0.0070.00 0.0070.00 0.0670.07 0.0070.00 0.0170.00 0.0070.01 0.0670.10 0.0070.00 0.0070.00 0.0070.00 0.0170.00 0.0070.00

0.2970.10 0.0570.03 0.1170.03 0.4370.09 0.0070.00 4.6071.56 0.0370.05 0.6670.47 0.1670.04 0.0170.01 0.0170.01 0.0070.00 0.0170.00 0.2470.06 0.0070.00 0.0070.00 0.0170.00 0.1770.05 0.0070.00 0.0070.00 0.0070.00 0.0270.00 0.0070.00

0.2770.10 0.0670.03 0.1170.02 0.3370.08 0.0070.00 4.5171.49 0.0070.00 0.6370.43 0.1270.02 0.0170.00 0.0170.01 0.0070.00 0.0170.00 0.1670.03 0.0070.00 0.0070.01 0.0170.00 0.1570.07 0.0070.00 0.0070.00 0.0070.00 0.0170.00 0.0070.00

0.3970.11 0.0770.02 0.1170.04 0.3270.12 0.0070.01 4.2271.55 0.0670.14 0.5970.42 0.1070.04 0.0170.00 0.0170.01 0.0070.00 0.0170.00 0.1170.05 0.0070.00 0.0070.00 0.0070.00 0.1070.05 0.0070.00 0.0070.00 0.0070.00 0.0270.00 0.0070.00

Sodium (Na) Magnesium (Mg) Aluminum (Al) Silicon (Si) Phosphorus (P) Sulfur (S) Chlorine (Cl) Potassium (K) Calcium (Ca) Titanium (Ti) Vanadium (V) Chromium (Cr) Manganese (Mn) Iron (Fe) Cobalt (Co) Nickel (Ni) Copper (Cu) Zinc (Zn) Gallium (Ga) Arsenic (As) Selenium (Se) Bromine (Br) Rubidium (Rb)

MKb 6

TWb

HTb

6

6

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Table 2 (continued ) Summera MKb Strontium (Sr) Yttrium (Y) Zirconium (Zr) Molybdenum (Mo) Palladium (Pd) Silver (Ag) Cadmium (Cd) Indium (In) Tin (Sn) Antimony (Sb) Barium (Ba) Lanthanum (La) Gold (Au) Mercury (Hg) Thallium (Tl) Lead (Pb) Uranium (U)

0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0170.01 0.0070.00 0.0270.01 0.0270.02 0.0070.00 0.0070.00 0.0070.00 0.0370.04 0.0070.00

Falla TWb 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0170.01 0.0070.00 0.0170.01 0.0170.01 0.0070.00 0.0070.00 0.0070.00 0.0370.04 0.0070.00

HTb 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0170.01 0.0070.00 0.0170.01 0.0270.02 0.0070.00 0.0070.00 0.0070.00 0.0370.04 0.0070.00

MKb 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0270.01 0.0170.00 0.0370.03 0.0170.02 0.0070.00 0.0070.00 0.0070.00 0.0670.04 0.0070.00

TWb 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0270.01 0.0070.00 0.0170.01 0.0270.02 0.0070.00 0.0070.00 0.0070.00 0.0670.04 0.0070.00

HTb 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0070.00 0.0170.00 0.0070.00 0.0070.00 0.0170.02 0.0070.00 0.0070.00 0.0070.00 0.0670.04 0.0070.00

a

Winter: 06/11/2000–17/03/2001; spring: 18/03/2001–15/05/2001; summer: 16/5/2001–20/09/2001; fall: 21/09/2001–26/10/2001. Middle-scale roadside Mong Kok (MK) site, urban-scale Tsuen Wan (TW) site, and regional-scale Hok Tsui (HT) site. c For thermal/optical reflectance (TOR) carbon analyses following the IMPROVE protocol, the temperature levels in a pure helium (He) atmosphere are 120 1C (OC1), 250 1C (OC2), 450 1C (OC3), and 550 1C (OC4). Each stage begins when the flame ionization detector (FID) response returns to baseline or remains at a constant value. The temperature levels in a 98% He/2% O2 atmosphere are 550 1C (EC1), 700 1C (EC2), and 800 1C (EC3). The fraction of pyrolyzed organic carbon (POC) is detected in the He/O2 atmosphere at 550 1C prior to the return of reflectance to its original value. OC is defined as OC1+OC2+OC3+OC4+POC, and EC is defined as EC1+EC2+EC3–POC (Chow et al., 1993, 2001, 2004). b

particulate NH4NO3 (e.g., Watson et al., 1994a). Lower temperatures and less precipitation during winter favor particulate NH4NO3 over HNO3. Although this seasonal pattern was consistent across the three sampling sites, a higher concentration of NH4NO3 appeared at the MK site, followed by the TW site; HT nitrate was less than half the MK nitrate, indicating that NH4NO3 is more local than regional in origin. Urban sources, especially vehicle exhaust and shipping, dominate anthropogenic emission of reactive nitrogen oxides (NOx), which likely leads to the elevated NO 3 level in urban areas. EC is often regarded as a marker for motor vehicle— especially diesel—emissions in many air quality studies (Chen et al., 2001). About 60% of the annual vehicle miles traveled (VMT) in Hong Kong are attributed to diesel vehicles, and this was reflected in the high EC concentration at the MK site (420 mg m3); the EC concentration was much lower at the other two sites. This spatial variation is similar to that of NH4NO3. However, EC at MK, TW, and HT showed a limited seasonal trend, independent of the prevailing wind direction and temperature (Fig. 3). This implies that (1) the emissions were relatively constant throughout the year and (2) seasonal variations in local meteorology,

such as wind speed and boundary layer depth, were not large enough to produce appreciably different dispersion efficiencies. OC originates from a combination of mobile sources, industrial combustion sources, vegetative burning, and secondary organic aerosol (SOA). SOA is formed in the atmosphere from gas-phase oxidation of heavy (C46) volatile organic compounds (OCs). Residential coal and refuse burning is common in Chinese rural areas, especially during winter. These oxidative reactions are more intense in summer due to higher evaporative VOC emissions at higher temperatures and more intense photochemical activity. In Hong Kong, the seasonal variation in organic material (OM) was more pronounced than EC, showing a winter high and a spring or summer low at all three sites (Fig. 3). OC/EC ratios at the MK site were 1.2, 0.63, 0.67, and 0.62 for winter, spring, summer, and fall, respectively. This is consistent with the hypothesis that elevated OC resulted from a combination of locally generated vehicle emission and the regional transport of vegetative burning emissions. SOA does not appear to play a major role in summer. Non-polar solvent extractable organic compounds also showed a higher overall concentration in winter than in summer (Sin et al., 2005).

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50 Mass

80

10 (NH4)2SO4

40 30

6

40

20

4

20

10

2

0

0 Sp

Su

50

0 W

F

Sp

Su

F

W

Organic Material

EC

40 30

3

20

20

2

10

10

1

0

0

Sp

Su

F

5

F

0 W

Sp

Su

F

W

Sp

Su

F

0.02

2

Sea Salt

Su

Crustal Material

4

30

W

Sp

5

50

40

NH4NO3

8

60

W

1703

4

K

+

Ni

3 1

0.01

2 1 0

0 W

Sp

Su

F

W

MK

Sp

Su

0

F

TW

W

Sp

Su

F

HT 3

Fig. 3. Seasonal variations of PM2.5 mass and major chemical components (mg m ) between November 2000 and October 2001 for winter (W), spring (Sp), summer (Su), and fall (F). (NH4)2SO4 ¼ ammonium sulfate (1.29  SO2 4 ); NH4NO3 ¼ ammonium nitrate; OM ¼ 1.4  OC; EC ¼ elemental carbon; Crustal material ¼ (1.89  Al)+(2.14  Si)+(1.4  Ca)+(1.43  Fe); Sea salt2.54  water-soluble sodium (Na+); K+ ¼ water-soluble potassium; and Ni ¼ elemental nickel.

CM was lowest (0.6–1.4 mg m3) during the summer. The higher seasonal averages for the spring and winter (Fig. 3) are associated with large standard deviations, reflecting contributions from intense regional dust events. Elevated CM was found on 3 days: 6 March 2001 (CM ¼ 7.9–8.6 mg m3), 17 April 2001 (CM ¼ 4.9–5.9 mg m3), and 5 May 2001 (CM ¼ 2.9–4.2 mg m3). On these episode days, CM was uniformly distributed across the three sites and was 2–5 times the annual average. Asian dust storm events are most frequent in late winter and early spring (Xuan et al., 2000). Based on observations and model simulations, Gong et al. (2003) identified four major dust storm episodes during spring 2001: 2–6 March, 21–26 March, 4–14 April and 29 April–4 May. Allowing a transport time of 2–3 days, these episodes coincided with elevated CM concentrations observed during the study period. Gong et al. (2003) attributed the dust emissions to deserts in Mongolia and northern China. This longrange transport is a result of prevailing northeast monsoons that carried dust aloft to the PRD. Fig. 3 also indicates generally higher CM concentrations at the MK site for each season, suggesting the contribution of

suspended road dust from the nearby highly trafficked streets. The relative contribution from local road dust warrants further investigation. Marine aerosol, represented by sea salt, was highest at the coastal HT site and peaked in winter and fall (
2 mg m3). The lower sea salt concentration in summer, despite the frequent marine back trajectories, might be explained by wet deposition of highly soluble sodium chloride (NaCl) during more frequent rainstorms. Water-soluble potassium (K+), a marker for vegetative burning, showed two- to threefold seasonal variations from winter to summer (Fig. 3). The higher K+ concentrations occurred in winter (0.51– 0.76 mg m3) and were spatially similar. This is consistent with the elevated carbon concentrations and vegetative burning emissions in winter from rural China. Although open and residential burning is prohibited in Hong Kong, it is common in the nearby agricultural areas of mainland China. K+ concentration remained uniform through spring and summer. PM2.5 nickel (Ni), a marker for oil combustion, did not show clear spatial or seasonal trends. Residual oils (bunker fuels) are commonly used in ships en route from the ports of the

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PRD to the South China Sea. Air trajectories over the ocean may explain the slightly elevated Ni concentration in summer.

6. Air quality standard exceedance days The US Environmental Protection Agency (EPA) 24-h PM2.5 National Ambient Air Quality Standard (NAAQS) of 65 mg m3 (US EPA, 1997) was exceeded on 19 days at the MK site, 3 days at the TW site, and once at the HT site (Table 3). Approximately 65% of the exceedances occurred during winter, and 17% during fall. These episodes often corresponded to subsidence inversions over the PRD in combination with slow movement of air masses laden with pollutants from southeastern China. Fig. 4 presents back trajectories for six of the days (12 trajectories per day) listed in Table 3. These show (A) 6 November 2000, moderate northeasterly transport (relatively high OC/EC ratio and Na+ concentration); (B) 24 December 2000, moderate northwesterly turning-

northeasterly transport (relatively high OC/EC ratio and K+ concentration ); (C) 20 December 2000, weak winds with predominantly northerly transport (elevated trace elements); (D) 28 February 2001, nearly stagnant conditions (elevated trace elements); (E) 6 March 2001, strong northwesterly turning-easterly transport (elevated crustal and marine aerosol); and (F) 14 September 2001, weak winds with northeasterly transport (elevated SO2 4 and Ni). A regional PM2.5 exceedance occurred on 28 February 2001, when all three sites exceeded 65 mg m3, reaching as high as 13177.7 mg m3 at the MK site. Along with the moderation of the northeast monsoon, ambient temperature started to increase on 27 February which was characterized by cloudy skies. Fig. 4D shows that on 28 February morning and afternoon trajectories originated from the Pacific Ocean, traveled through Taiwan, and remained near the sea surface before arriving at Hong Kong. By evening, the air became stagnant and the trajectory residence time over the PRD increased. These trajectories reflect a combination of synoptic-scale monsoon and mesoscale subsidence that gradually became dominant by evening. It was foggy on

Table 3 Summary of 24-h PM2.5 exceedances of 65 mg m3 of US National Ambient Air Quality Standard (NAAQS; US EPA, 1997) (values in bold) and corresponding PM2.5 at other monitoring sites Date

Seasona

MKc (concentration7standard deviationb, mg m3)

TWc (concentration7standard deviationb, mg m3)

HTc (concentration7standard deviationb, mg m3)

11/06/2000 11/30/2000 12/06/2000 12/18/2000 12/24/2000 12/30/2000 01/11/2001 02/04/2001 02/16/2001 02/22/2001 02/28/2001 03/06/2001 03/18/2001 04/17/2001 05/29/2001 07/04/2001 09/08/2001 09/14/2001 09/26/2001 Annual averaged

Winter Winter Winter Winter Winter Winter Winter Winter Winter Winter Winter Winter Spring Spring Summer Summer Fall Fall Fall

76.675.6 67.375.3 77.875.7 70.875.4 65.675.3 92.976.2 71.875.5 75.075.6 69.675.4 83.975.9 131.477.7 68.275.3 72.975.5 67.275.3 71.875.5 75.175.6 70.375.4 97.776.4 69.075.4 56.777.0

50.374.8 32.874.4 54.674.9 47.074.7 37.074.5 71.675.5 57.675.0 58.375.1 45.674.7 57.175.0 122.077.4 37.174.5 49.074.8 47.974.8 39.574.6 49.774.8 n/a 68.575.4 51.074.9 33.576.3

35.4574.5 25.574.3 32.574.4 22.874.3 28.874.4 60.875.1 37.274.5 48.374.8 42.174.6 42.374.6 68.375.4 38.874.6 29.274.4 39.974.6 34.374.5 43.374.7 n/a 38.274.5 42.074.6 24.172.5

a Winter: 06/11/2000–17/03/2001; spring: 18/03/2001–15/05/2001; summer: 16/05/2001–20/09/2001; fall: 21/09/2001–26/10/2001; annual: 06/11/2000–26/10/2001. b Measurement uncertainty (s). c MK ¼ Mong Kok, TW ¼ Tsuan Wan, and HT ¼ Hok Tsui. d Arithmetic average of all samples from 06/11/2000 through 26/10/2001.

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the morning of 28 February. Visibility inside the harbor fell below 1 km during the day (http://www.weather. gov.hk/wxinfo/pastwx/mws.htm). At the surface level, prevailing northeasterly winds gradually diminished in the afternoon and shifted to northerly winds that cleared the fog. In addition to PM2.5 mass, major ions (Cl–,

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2 + NO 3 , SO4 , K ), OC, total carbon (TC), and trace elements (Mn, Cu, As, Se, Br, Rb, Zr, Sb, Pb) at the MK and TW sites all reached their maxima. The MK and TW sites both reported PM2.5 of 465 mg m3 on 30 December 2000, and 14 September 2001, while PM2.5 reached
61 mg m3 at the HT site on

Fig. 4. Seventy-two hour air parcel backward trajectories (every 2 h) of: (A) 6 November 2000, (B) 24 December 2000, (C) 30 December 2000, (D) 28 February 2001, (E) 6 March 2001, and (F) 14 September 2001. The initial point is at 1000 m AGL at the HT site. The domain of trajectory modeling is 15–351N, 100–1301E, and from the surface to an altitude of 4 km AGL. The upper and lower panels show the horizontal and vertical movement of air parcels, respectively. Local standard time is used in the modeling and the date is shown as ‘‘MM/DD/YY’’ in the lower panel.

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Fig. 4. (Continued)

30 December 2000. Figs. 4C and F show similar trajectories with (1) short travel distances, (2) subsidence, and (3) influences from southeastern China. Short trajectories and subsidence imply stagnant air that suppresses pollutant dispersion; these lead to the local mixing of pollutants trapped by the land–sea breeze circulation. The late summer episode (14 September 2001) was dominated by regional (NH4)2SO4, while the winter episode (30 December 2000) was mostly caused by elevated carbonaceous aerosol. The trajectory for 6 March 2001 (Fig. 4E) shows a distinct, fast-moving air mass resulting from a strong anticyclone moving from southeastern China to the Pacific Ocean. Air parcels originating over Inner Mongolia were pushed offshore and entrained by marine air before turning back toward the PRD. This trajectory may be less influenced by local emissions because of the high wind speeds. The trajectories for 6 November 2000 (Fig. 4A) and 24 December 2000 (Fig. 4B) both traveled through China’s southeastern coast 24 h prior to arriving at the PRD. Elevated K+, K, Na+, and OC concentrations on these days are consistent with vegetative burning and marine aerosol contributions.

7. Species enrichment during PM2.5 episodes PM2.5 chemical composition can be characterized by enrichment factors (EF), defined as the ratio of the

episode mass fraction to the average mass fraction of a species (Watson et al., 2002; Watson and Chow, 2004). Fig. 5 compares the species EF at the urban TW site on the six days described above. For the two typical winter episodes (30 December 2000, and 28 February 2001), PM2.5 EFs of NO 3 and 2 NH+ 4 were higher, but the EF of SO4 was lower; this was especially apparent on 28 February the most severe episode. OM was not enriched (EFOM
1), with low (0.3–0.5) EFEC. This leads to a relatively high OC/EC ratio. Elements associated with vegetative burning and industrial emissions (e.g., K+, Se, Cu, Mn, Rb, and Pb) showed EFs greater than unity, while EFs for crustal elements—EFAl, EFSi, and EFCa—were less than unity. These winter PM2.5 episodes were driven in part by stagnant airflow. Stagnation at such a low temperature resulted in the accumulation of NH4NO3. In contrast, NO 3 and most of the combustion- and industry-related species showed mass fractions less than the annual averages (EFso1) for 14 September 2001. The moderately enriched SO2 is consistent with 4 regional transport. Precipitation was
6 mm on 14 September compared with only trace levels on 30 December 2000 and 28 February 2001; precipitation scavenging might have reduced the elemental concentrations. Fig. 5 shows that the EFs of crustal elements (e.g., Al, Si, Ca, and Fe) ranged from 3 to 7 on 6 March 2001, consistent with the passage of Asian dust over the

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Enrichment Factor 0.10

1.00

10.00

mass Sulfate 11/6/2000 Nitrate

12/24/2000 12 /30/2000

Ammonia

2/28/2001

OM

3/6/2001 EC

9/14 /2001

Na+ Al Si K+ K Ca Mn Fe Ni Cu Zn Se Br Rb Pb PM2.5 Species at TW Fig. 5. Enrichment factors of selected PM2.5 species at the urban-scale TW site on six episode days. The enrichment factor for species i designated date (EFi) is the ratio of that species’ PM2.5 mass Pfraction on the P  to the ratio of its annual average concentration to the k k annual average of PM2.5 (i.e., EFik ¼ F ik 1= n¼1 F ik PM2:5;n = n¼1 PM2:5;n where EFik ¼ enrichment factor for species i on sample k, Fik ¼ ratio of species i concentration to PM2.5 concentration on sample k, K ¼ total number of samples, and PM2.5,n ¼ PM2.5 concentration on sample n).

region. Compared with other days, air parcel residence time over the marine boundary layer was substantially longer on March 6 (Fig. 4E), consistent with the

enriched Na+ on that day. In addition, the moderately enriched NO 3 might result from reactions between HNO3 and sea salt aerosols (Watson et al., 1994b).

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K+ and K were most enriched at the TW site on 6 November 2000 and 24 December 2000 (EFs of 3.2 and 2.2, respectively), when EFAl, EFSi, and EFCa were only moderately above unity. These two episodes are related to vegetative burning. At the TW site, the PM2.5 mass on these two days was 50.374.8 and 37.074.5 mg m3, respectively. OM was also significantly enriched (Fig. 5). The trajectory for 6 November (Fig. 4A) suggests a stronger marine influence from the East China Sea than does the trajectory for 24 December (Fig. 4B). The sea salt concentration (3.8 mg m3) reached its maximum at the HT site on 6 November.

8. Conclusions The synoptic meteorology in Hong Kong, influenced by the Asiatic monsoon, results in large winter–summer contrasts in PM2.5 mass and chemical composition. Strong radiative cooling of the Asian continent in winter is accompanied by subsidence and low precipitation that suppress the dispersion and deposition of pollutants emitted from local sources. Air parcel back trajectory and residence time analyses indicate that predominantly northeasterly transport during winter and fall were influenced by potential PM2.5 source regions in southeastern China and western Taiwan. Southwesterly to southeasterly transport dominated in summer and brought in cleaner marine air. During the study period (November 2000 to October 2001), PM2.5 mass concentrations were highest in winter, followed by fall, at the urban-scale and regional-scale sites. This winter high PM2.5 level resulted mostly from the elevated OM and NH4NO3. EC was spatially inhomogeneous, increasing with the nearby traffic density from the regional-scale to the middle-scale roadside sites, but showed limited seasonal variability. Local mobile source emissions highly affected ambient OC and EC concentrations. However, the OC/EC ratio increased from 0.67 in summer to 1.2 in winter at the roadside site. Vegetative burning emissions could be important in winter. (NH4)2SO4 and CM appeared to originate from more distant sources; their seasonal variation can be explained by the variation of source strength and the transition of the monsoon pattern. A regional pollution episode occurred on 28 February 2001, with PM2.5 concentrations reaching 1317 7.7 mg m3 at the roadside MK site, 12277.4 mg m3 at the urban TW site, and 68.375.4 mg m3 at the regional HT site. In addition to PM2.5 mass, major ions (Cl, 2 + NO 3 , SO4 , K ), OC, and trace elements (Mn, Cu, As, Se, Br, Rb, Zr, Sb, Pb) also reached their maxima at the roadside and urban sites. This episode resulted from a combination of synoptic-scale monsoon and mesoscale subsidence with moderate to stagnant easterly transport.

Elevated PM2.5 mass, NO 3 , and trace metals were found during a similar winter episode (30 December 2000) with northeasterly transport. In contrast, long-range transport of SO2 4 with elevated residual oil combustion (Ni) characterized a late summer episode (14 September 2001). An Asian dust storm affected all three sites on 6 March 2001. PM2.5 exceeded 65 mg m3—the US EPA NAAQS for 24-h PM2.5—for 23 samples on 19 sampling days; 65% of these exceedances occurred during winter and 17% during fall. Excluding the roadside site, which represents a microenvironment, there were just three sampling days (a total of four samples) exceeding 65 mg m3 of PM2.5. The annual PM2.5 concentrations, however, were well above the annual NAAQS standard of 15 mg m3 at both urban and rural sites. The consistently high PM2.5, carbonaceous material, and trace metals in the air, especially during winter, raise concerns about adverse health effects.

9. Disclaimer The content of this paper does not necessarily reflect the views and policies of the Government of the HKSAR, nor does any mention of trade names or commercial products constitute an endorsement or recommendation of their use.

Acknowledgments This study was sponsored by the Hong Kong Environmental Protection Department under Contract ]90563. The authors wish to thank Steven Kohl of the Desert Research Institute (DRI) Environmental Analysis Facility (EAF) for conducting chemical analyses and performing data validation of PM2.5 measurements, and Norman Mankim and Eric Dieterle for assembling and editing the manuscript. The authors also wish to thank Hong Kong Polytechnic University and Pacific Century CyberWorks Ltd. for the use of the monitoring site at Hok Tsui.

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