Polycyclic Aromatic Hydrocarbons (Pahs) and Carbonyl Compounds in Urban Atmosphere of Hong Kong S.C

Atmospheric Environment 35 (2001) 5949–5960

Polycyclic aromatic hydrocarbons (PAHs) and carbonyl compounds in urban atmosphere of Hong Kong S.C. Leea,*, K.F. Hoa, L.Y. Chana, Barbara Zielinskab, Judith C. Chowb

a Research Center for Urban Environmental Technology and Management, Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, P.O. Box 60220, Reno, NV 89506, USA

Received 14 February 2001; received in revised form 25 May 2001; accepted 6 July 2001

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and carbonyls compounds are becoming a major component of atmospheric toxic air pollutants (TAPs) in Hong Kong. Many studies in Hong Kong show that traﬃc emission is one of the most signiﬁcant contributors in urban area of Hong Kong. A twelve months monitoring program for PAHs and carbonyl compounds started on 10 April 1999 including a two weeks intensive sampling in winter had been performed at a roadside urban station at Hong Kong Polytechnic University in order to determine the monthly and seasonal variations of PAHs and carbonyl concentrations. The objective of this study is to characterize the roadside concentrations of selected TAPs (PAHs and carbonyl compounds) and to compare with the long-term compliance monitoring data acquired by Hong Kong Environmental Protection Department (EPD). Monthly variations, seasonal variations and winter/summer ratios at the monitoring station are discussed. r 2001 Elsevier Science Ltd. All rights reserved.

Keywords: PAHs; Carbonyls compounds; Seasonal variation; Road-side; Hong Kong

1. Introduction particle and gas phases (Bidleman et al., 1986). Low molecular weight PAHs tend to be more concentrated Toxic air pollutants (TAPs) are defined as substances in the vapor phase while the ones with higher mole- that have potential to cause serious and adverse health cular weight are often associated with particulates. effects and damage to the environment (Boubel et al., The recognized carcinogenic PAHs are mostly asso- 1994). These pollutants differ from other primary air ciated with particulate matter (Lyall et al., 1988). PAHs pollutants such as sulfur dioxide, nitrogen dioxide, total are formed from both natural and anthropogenic suspended particulates (TSP) and respiratory suspended sources, with the latter being now the major contributor particulates (RSP), in that even when they are present (Masclet et al., 1986). These organic compounds are at substantially lower concentrations in the ambient produced by high-temperature reactions such as incom- environment, their health effects are typically carcino- plete combustion and pyrolysis of fossil fuels and other genic in nature. organic materials. Baek et al. (1991) showed that Polycyclic aromatic hydrocarbons (PAHs) are motor vehicle exhaust is more likely to be the major ubiquitous environmental pollutants and many of them PAHs contributor in urban and suburban area. PAHs are known to be carcinogenic (IARC, 1984) and participate in various chemical reactions and decompose mutagenic compounds. PAHs are semi-volatile organic under strong solar radiation. They undergo thermal compounds (SVOCs) which are partitioned between decomposition and react with a number of atmospheric chemicals producing derivatives, which can be *Corresponding author. Fax: +852-2334-6389. more toxic than the original compounds (Nicolaou E-mail address: [email protected] (S.C. Lee). et al., 1984).

Carbonyls are among the major species of organic monitoring of TAPs at Central/Western and Tsuen Wan compounds involved in photochemical air pollution. monitoring stations. Both of these stations are roof top Aldehydes and ketones play an important role as stations with sampling inlet 17–18 m above the ground products of photooxidation of gas-phase hydrocarbons level. Since it is well known that these pollutants are in the form of free radicals (Baez et al., 1995). Motor emitted in large quantity from vehicle exhaust, knowl- vehicle exhaust is the primary emission source of edge of the roadside concentrations of these traffic- carbonyl compounds in urban areas (Granby et al., related TAPs is considered crucial to establish the 1997). Formaldehyde is the most abundant atmospheric baseline of environmental database. The obtained aldehyde, followed by acetaldehyde (Williams et al., database will be useful to fill up the gap in the air 1996; Muller, 1997). Atmospheric photooxidation is an quality data and for the attributions of source emissions important secondary source of aldehydes (Grosjean of TAPs in urban area. et al., 1990; Hoekman, 1992). Cleverland et al. (1977) A 12-month study including two weeks of winter observed seasonal variations of formaldehyde. Aldehyde intensive sampling were carried out at the Hong Kong concentrations dependent on photochemical activity, Polytechnic University (PolyU) roadside monitoring with concentrations being higher on days with more station for selected PAHs and carbonyl compounds. intensive photochemical activity (Williams et al., 1996). The concentrations measured at this roadside study were Carbonyl compounds are toxic, particularly aldehydes compared with two compliance monitoring stations (Carlier et al., 1986), the most observed effects are (Central/Western and Tsuen Wan) operated by Envir- irritants of the skin, eyes and nasopharyngeal mem- onmental Protection Department (EPD). branes. Even more seriously, formaldehyde and acrolein are suspected carcinogens. Past studies (e.g., Baez et al., 1995; Williams et al., 2. Methodology 1996; Panther et al., 1999; Oanh et al., 2000) documen- ted PAHs or carbonyl concentrations at different air 2.1. Study site shed. The objective of this study aims at monitoring PAHs and carbonyl compounds simultaneously at The TAPs samples were collected from the PolyU roadside station, which have not been routinely done roadside station (the location is shown in Fig. 1), which in Hong Kong. Since 1997 Hong Kong EPD initiated was located at podium level of university (about 6 m

Fig. 1. The sampling locations in this study. S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960 5951 above ground level). The monitoring station is 8 m away mode onto a 30 m long 5% phenylmethylsilicone fused from the heavily trafficked Hong Chong Road, which is silica capillary column (J&W Scientific type DB-5). connected to the Cross Harbour Tunnel from Kowloon Deuterated PAHwas used as internal standards. A field to Hong Kong Island. The traffic volume of the road is blank was treated exactly as a sample except that no air extremely high which is more than 170,000 vehicles per was drawn through the filter/adsorbent cartridge assem- day. bly. Samples were stored at 01C in an ice chest until they The two EPD air quality monitoring stations which were received at the analytical laboratory, after which were chosen for data comparison are located at Central/ they were refrigerated at 41C. Based on U.S. EPA Western and Tsuen Wan. Central/Western monitoring Method TO-13, the following 18 PAHs were selected for station represents an urban residential area while Tsuen analysis: napthalene (NAP), acenapthylene (ACY), Wan monitoring station is classified as mixed urban acenapthene (ACE), fluorene (FLU), phenanthrene residential, commercial and industrial area. The samples (PHEN), anthracene (ANTH), fluoranthene (FLT), were collected on the rooftop of 18 and 17 m tall pyrene (PYR), benzo[a]anthracene (BaA), ben- buildings, respectively. zo[b+j+k]fluoranthe (BbF, BjF, BkF), chrysene (CHR), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP), 2.2. Sampling and analysis indeno[123cd]pyrene (IND), benzo[ghi]perylene (BghiP), dibenzo[ah]anthracene (DbahA), coronene (COR) and The sampling period was 1 yr, starting from April benzonaphthothiophene (BNT). 1999 to April 2000 including a two-week intensive sampling program in December 1999 and January 2000. 2.4. Carbonyls compounds The samples were collected for 24-h. The PAHs and carbonyl samples were analysed by the Desert Research Carbonyls samples were collected by drawing air Institute (DRI), Nevada, USA. through a cartridge impregnated with acidified 2,4- dinitrophenylhydrazine (Waters Sep-Pak DNPH-silica), 2.3. PAHs which was very reactive toward carbonyls. A flow rate of about 60–80 ml/min was used for the 24-h duration. The Atmospheric samples were collected using the medium flow rate through the cartridges was measured with a volume total suspended particulate PUF sampler rotameter before and after each sampling period. The (Andersen Instrument, Smyrna, GA, Model GPS-1). rotameter was calibrated in the laboratory against a PAHs in both particulate and vapor phases were soap bubble flow meter. Ozone scrubber was connected trapped by means of Teflon-impregnated glass fibers before DNPH-silica cartridge in order to prevent the (ultrapure quality, Gelman) followed by polyurethane interference of ozone. The resulting products (hydra- foam plug (PUF)/XAD4/PUF. The PAHsampler was zones) in the cartridge were measured using reverse calibrated using a manometer and top loading orifice phase high performance liquid chromatography (HPLC) plate. Sampling was conducted at a flow rate of 180 l/min to determine the levels of the carbonyl compounds for 24-h. A total of 33 samples were collected for PAHs originally present in air. The HPLC instrument used was analysis. Filters and XAD-4 were extracted with 45 ml of manufactured by Waters and was equipped with two dichloromethane using microwave extraction technique Waters 501 high-precision pumps, Waters 484 variable for 15 min at 80 psi. Since PUF could not be extracted wavelength ultra violet (UV) detector, Waters system with dichloromethane, they were extracted separately interface module, Waters Autosampler, and was inter- using a Soxhlet or microwave extractor with 10% faced to a NEC PowerMate SX Plus using the Baseline diethyl ether in hexane over 6 h at a rate of at least 3 810 Chromatography Workstation Version 3.30 data cycles per hour. Extracts were combined and concen- system. trated to 1 ml by rotary evaporation at 201C under Typically, C1–C6 carbonyl compounds, including gentle vacuum, and filtered through a 0.2 mm AnotopTM benzaldehyde, are measured effectively by this techni- 10 (Whatman), rinsing the flask twice with 1 ml of que, with a detection limit of 0.2 ppbv. A total of 41 dichloromethane each time. samples were analyzed for formaldehyde and acetalde- Approximately, 200 ml of acetonitrile was added at hyde. Using DRI’s standard carbonyl sampler with one this time and the solvent was added to 100 ml. The final channel, cartridges could be exposed on a predetermined extract volume was adjusted to 1 ml with acetonitrile. schedule. Since the reagent is extremely reactive, a The samples were analyzed by the electron impact (EI) cartridge left open would continue to absorb carbonyl GC/MS technique, using a Hewlett-Packard Mass compounds from the air until all the reagents were Selective Detector (with Selective Ion Monitoring) or completely consumed. Thus, the cartridges were plugged Varian Star 3400 CX with a model 8200CX Automatic at both ends and placed inside the glass screw-capped Sampler and interfaced to a Saturn 2000 Ion Trap Mass vials. They were further placed into a tin can for Spectrometer. Injections were 1 ml in size in the splitless protection during shipment and storage. The exposed 5952 S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960 cartridges were stored inside a refrigerator and returned The mean concentration of NAP was 993 ng/m3 and to the DRI laboratory in a cooler chilled with blue ice. ranged from 65 to 3508 ng/m3. Other than NAP, the All analysis procedures met the requirements of the mean concentration of PHEN (34.8 ng/m3), FLU USEPA Method TO-11. (17.5 ng/m3), FLT (8.2 ng/m3) and PYR (7.5 ng/m3) were relatively high. Emissions from motor vehicles 2.5. Quality control and assurance contribute to these elevated concentrations. Actually, the concentration levels of the many other selected Quantification of PAHs and carbonyl compounds PAHs were similar to or even lower ( Table 2) when compared to other urban cities (Harrison et al., 1996; were according to the retention times and peak areas of the calibration standards. The instruments were calcu- Panther et al., 1999; Oanh et al., 2000). It is because the lated using at least five standard concentrations covering traffic volumes at those sites are not so high and the distances to the sources are not so close when compared the concentration of interest for ambient air work. Correlation coefficient of the calibration curve was to PolyU station. >0.999 for a linear least square fit of the data. The BaP has been the most extensively measured Collocated sampling was conducted, precision of dupli- PAHin urban areas around the world due to its high cate samples within 15% and 25% for PAHs and carcinogenic property and was used as an indicator for total PAHs concentrations. Average BaP concentration carbonyl compounds, respectively. The detection limits 3 3 were 5 ng/sample for PAHs and 0.015 ppbv for carbonyl was 0.49 ng/m and ranged from 0.05 to 1.34 ng/m . compounds. However, in some cases, BaP was not an ideal choice as a profile index because BaP was a relatively reactive PAH, and subject to reactivity losses during summer when solar radiation and ambient ozone levels are 3. Results and discussion expected to reach the highest (Greenberg, 1989; DeWeist et al., 1981). In order to investigate the effect of 3.1. Polycyclic aromatic hydrocarbons (PAHs) photochemical degradation, a comparison had been made between the ratio of BeP/BaP. BaP has a halflife of A total of 33 samples were collected and 18 PAH 5.3 h under strong sunlight conditions, while BeP has a species were identified. The summary of statistical data half life of 21.1 h which is stable to photolysis (Katz was shown in Table 1. The concentrations of napthalene et al., 1979). Nielsen (1988) demonstrated that the ratio (NAP) were much higher than that of other PAHs. The of BeP to BaP can be used to indicate the decay of BaP source of NAP was mainly from fuel combustion. The in the atmosphere, assuming emissions of the two species majority of napthalene is present in the gaseous phase. were similar (Nielsen et al., 1984). The variations of BeP/BaP ratio were plotted in Fig. 2. Generally, the ratios were low (between 0.8–1.6) in PolyU station over the year, except on 20th July 1999 (BeP/BaP=2.4) and Table 1 29th March 2000 (BeP/BaP=1.9). Poor correlation Statistical summary of PAHs concentration acquired at PolyU between BaP concentration and solar radiation was station between April 1999 and April 2000 (n ¼ 33) (concentra- observed (r=0.32). There were many other factors (e.g., 3 tions in ng/m ) temperature, humidity and pollutants mix time) that PAHs Maximum Minimum Median Mean may affect the degradation of BaP as well.

7 NAP 3508.54 65.05 611.79 993.14 871.08 3.2. Seasonal variation ACY 12.99 0.99 4.68 4.8472.89 ACE 61.96 1.05 6.73 8.39710.45 FLU 57.54 2.88 15.76 17.5379.81 In Hong Kong, winter season includes the 4 months PHEN 129.38 7.18 30.88 34.79721.24 of November through February; while summer season 7 ANTH12.95 1.01 2.73 3.31 2.28 includes the four months of May through August. FLT 22.57 3.27 7.09 8.1473.94 PYR 21.52 2.63 6.67 7.5373.57 Strong monsoon wind and dry weather characterize BaA 1.51 0.08 0.49 0.6070.35 winter seasons, whereas hot and humid climate with BbF, BjF, BkF 2.65 0.01 0.74 0.7870.55 occasional showers and thunderstorms represent sum- CHR 4.58 0.33 1.25 1.3870.83 mer seasons. Fig. 3 illustrates the PAHconcentration 7 BeP 1.21 0.06 0.52 0.56 0.31 variations between the winter and summer months. The BaP 1.34 0.05 0.46 0.4970.25 IND 2.16 0.01 0.54 0.6670.56 concentrations of all selected PAHs were higher in BghiP 3.90 0.01 0.67 0.8870.80 winter than in summer except for COR. This is because DbahA 0.70 0.01 0.13 0.1970.17 most PAHs are attached to particulates and rainy 7 COR 1.63 0.01 0.33 0.38 0.34 summer days cause them to be washed out in the BNT 1.60 0.03 0.33 0.3670.26 atmosphere. Also, there is photochemical degradation of S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960 5953

Table 2 abundant carbonyl compounds, formaldehyde and Comparison between selected PAHs of the present study and acetaldehyde, were identified at the PolyU roadside other countries station. For the 41 samples collected, the automobile Sampling site Concentration (ng/m3) exhaust is the most important source of carbonyl compounds in urban areas, and they are key compounds BaP IND of photochemically generated air pollution (Carlier PolyU station 0.49 0.54 et al., 1986). The measured concentrations of formalde- Tsuen Wan 0.23 0.30 hyde and acetaldehyde are plotted in Fig. 4 along Central Western 0.17 0.22 with the summary statistics in Table 3. Annual and Jakartaa 4.37 3.28 seasonal formaldehyde concentrations are 2–10 times Seoula 1.17 0.79 higher than acetaldehyde. Annual average of formalde- a Bangkok 0.98 1.06 hyde concentration was 4.6 mg/m3, ranged from 0.82 to b Bangkok 11.3 mg/m3, average acetaldehyde was 2.1 mg/m3, ranged EE Building (site 1) 1.2 0.5 from 0.16 to 6.8 mg/m3. Formaldehyde/acetaldehyde Cafeteria (site 2) 1.4 0.5 ratio measured during this study (n ¼ 41) was 2.03 AIT Gate (site) 1.7 1.2 which was quite similar to the study by Baez et al. (1995) a Panther et al. (1999). (ratio=2.33). b Oanh et al. (2000). some PAHs under high solar radiation in summer. 3.4. Monthly and seasonal variation Physical dispersion/transportation maybe the reasons for higher concentrations in winter. The prevailing The meteorological conditions (temperature, wind winds in Hong Kong are north-easterly especially in speed and solar irradiation etc.) had a strong influence winter time, therefore the transportation of pollutants on concentrations of carbonyl compounds. During the from mainland China was one of the possible source. summer with high photochemical activities, resulted in The exception of COR maybe due to sources other than elevated concentrations of carbonyl compounds. The vehicles during summer. monthly variation of carbonyls was shown in Fig. 4. Both formaldehyde and acetaldehyde showed similar 3.3. Carbonyl compounds trends, double peaked in June 1999 and March 2000, and reached lowest levels during October to December Primary emissions of carbonyl compounds from in 1999. Both pollutants might come from the same gasoline and diesel vehicle were identified in many emission source and affected by meteorological condi- studies. In this study, concentrations of the two most tions. Similar correlation patterns were found in this

Fig. 2. The variation of BeP/BaP ratio at PolyU station. 5954 S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960

Fig. 3. Comparison of selected PAHconcentrations between winter (November 1999–February 2000) and summer (May–August 1999) for samples acquired at the PolyU station.

Fig. 4. The monthly variation of formaldehyde and acetaldehyde.

study. There existed a strong correlation between direct vehicular emissions were their principal source in formaldehyde and acetaldehyde (r ¼ 0:83). winter. It is possible to distinguish two factors depend- The seasonal variation of carbonyls was shown in ing on solar ﬂux and weather conditions. Table 3 gives a Fig. 5. The concentrations of formaldehyde and acet- summary of average formaldehyde and acetaldehyde aldehyde in summer (especially in early summer) were concentrations versus general weather conditions (tem- 50–100% higher than in winter. Atmospheric photo- perature and solar radiation). It was found that the oxidation was an important secondary source of levels of carbonyl compounds increased as the ambient aldehydes. Aldehydes were found to produce temperature or solar radiation increased. Also, the well- photochemically at a large extent in summer, while ventilated summer condition let the pollutants easily S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960 5955

Table 3 3.5. Comparison at three toxic air pollutants monitoring Summary of carbonyls concentrations and general weather stations conditions at PolyU station for samples acquired between April 3 1999 to April 2000 (n=41) (concentrations in mg/m ) Due to the diﬀerences in commercial, industrial and Formaldehyde Acetaldehyde residentiallandusesinurbanandsuburbanareasofHong Kong, it is worthwhile to compare the levels of PAHs and Maximum 11.34 6.75 Minimum 0.82 0.16 carbonyls acquired from a vehicle dominated PolyU Medium 4.53 2.05 station with two long-term compliance monitoring sta- Annual average 4.6572.46 2.1171.36 tions (Central/Western and Tsuen Wan) operated by Summer (May–August) 5.6471.42 2.4670.43 Hong Kong EPD. Monthly average concentration of toxic 7 7 Winter (November–February) 2.82 1.35 1.44 0.66 air pollutants (PAHs and carbonyls) collected by EPD at Summer Winter Central Western and Tsuen Wan stations during the same Global solar radiation (MJ/m2) 14.4 10.5 period (April 1999–April 2000) were used for comparison. Monthly average temperature (1C) 27.8 17.9

3.6. PAHs transport to our monitoring stations when compared to In this study (PolyU station), the sources of PAHs stagnant winter condition. were dominated by vehicle exhausts. As shown in Good correlations were found between formaldehyde Figs. 6a and b, all selected vehicle markers for PAHs and some PAHs (e.g., indeno[123cd]pyrene, benzo[ghi]- (PHEN, FLT and PYR are diesel vehicle markers; perylene, dibenzo[ah]anthracene) that originated from IND and BghiP are gasoline vehicle markers) (Harrison vehicle exhaust at PolyU station, especially in the et al., 1996) at PolyU station were higher than other intensive sampling periods between December 1999 two stations. Among the five selected PAHs, the and January 2000. The correlation coefficients (r) were highest concentration of Phenanthrene, PHEN 0.83, 0.86 and 0.89, respectively. High correlations imply (61.4 ng/m3) was found at PolyU station with an average that these organic compounds came from the same concentration of 34.5 ng/m3 which is the major compo- sources (vehicles exhaust) during wintertime. It was also nent of diesel engine emission source. All the other because the washing out effect for particulates during markers of PAHs concentrations at PolyU station are rainy days and photochemical degradation during high much higher than the other two stations. Industrial solar radiation were minimized in wintertime. combustion source and busier traffic in Tsuen Wan is the

Fig. 5. Comparison of formaldehyde and acetaldehyde concentrations during the summer (June–August 1999) and winter (November 1999–February 2000) season. 5956 S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960

Fig. 6. (a) Comparison of selected PAHs representing diesel emissions at three Hong Kong monitoring stations for the period of April 1999 to April 2000. (b) Comparison of selected PAHs representing gasoline vehicle emissions at three monitoring stations for the period of April 1999 to April 2000. reason for concentrations at Tsuen Wan station to be the PolyU roadside station. On-road vehicles were the higher than Central/Western station. primary source for PAHs at PolyU station when Site to site variations are highest for phenanthrene, compared to EPD stations. The monthly variations were with a factor of 3–5 higher concentrations observed at the significantly influenced by weather patterns (rainfall, PolyU station. While the concentrations of fluoranthene, wind speed and wind direction) which would affect the pyrene, indeno[123-cd] pyrene and benzo[ghi]perylene are dispersion of toxic air pollutants. similar for the two EPD urban-neighborhood commer- Winter to summer ratios for selected PAHs were listed cial/residential stations, the levels are 2–3 times higher at in Table 4. All selected PAHs concentrations in the S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960 5957

Table 4 times. Therefore, the photo-degradation of PAHs and Winter/summer ratio of selected PAHs the washout eﬀect were not signiﬁcant because they need PAHs PolyU Tsuen Wan Central/Western longer time to take action.

PHEN 1.46 0.95 1.10 3.7. Carbonyls FLT 1.11 1.07 1.20 PYR 1.34 1.38 4.49 As shown in Fig. 7, the average concentrations of IND 2.25 4.52 5.94 formaldehyde (5.25 mg/m3) and acetaldehyde (2.53 mg/ BghiP 2.62 2.90 5.45 m3) detected in Tsuen Wan station were slightly higher than other two stations. But the difference of the concentrations among the three monitoring stations was relatively small in the range of 10–30%. According winter were higher than those measured in the summer, to other studies, the major source of formaldehyde and except for PHEN at the Tsuen Wan site (0.95). Winter to acetaldehyde from outdoor environment was on-road summer ratios of FLT, PYR, IND and BghiP exceeded vehicles. However, at Tsuen Wan station, industrial 4 at the Central/Western station, ratios of IND also emissions were also one of the main sources for carbonyl exceeded 4 at the Tsuen Wan station. The high compounds in addition to vehicle exhaust. concentrations of PAHs during winter at Central/ As shown in Figs. 8a and b, significant temporal and Western and Tsuen Wan station were due to less spatial variations of formaldehyde and acetaldehyde photochemical degradation of PAHs in winter. Rainy were found at three monitoring stations. This phenom- days in the summer caused the washout effect of enon is consistent with the hypothesis that emissions pollutants. This effect was significant for PAHs because sources other than on-road vehicle exhaust also con- PAHs were attached to the particulate, therefore the tribute to the elongated carbonyl concentrations, the concentrations of PAHs were low after rainy days, secondary product due to photochemical reaction is also especially when the stations were not near the source one of the factors that affect the result. (Central/Western and Tsuen Wan stations). At PolyU Winter to summer carbonyls ratios were listed in roadside station, the winter to summer PAHs ratios were Table 5. Both ratios at PolyU station were o1, which lower than the other two stations. The distance between means the concentrations of carbonyls were higher in the source (on-road vehicles) and the sampling location summer than those in winter. It was because formalde- (PolyU station) was shorter, resulted in lower residence hyde and acetaldehyde were very volatile organic

Fig. 7. Comparison of carbonyls at three monitoring stations. 5958 S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960

Fig. 8. (a) Monthly variation of formaldehyde at three monitoring stations. (b) Monthly variation of acetaldehyde at three monitoring stations.

Table 5 summer. This indicated that chemical photo-oxidation Winter/summer ratio of selected carbonyl compounds was an important secondary source of formaldehyde Carbonyls PolyU Tsuen Wan Central/ and acetaldehyde. compounds Western Since the concentrations of pollutants are affected by atmospheric dispersion and topography, the variations Formaldehyde 0.500 0.865 1.120 were due to the distance between the source and the Acetaldehyde 0.583 1.098 1.498 receptor. The percentage differences between EPD monitoring station (Central/Western) and roadside compounds. The higher temperature in summer in- monitoring station (PolyU) for TAPs were shown in creased the evaporation of carbonyls. Also, formalde- Table 6. Central/Western was chosen for comparison hyde and acetaldehyde might be the by-product of with PolyU station because in Tsuen Wan Station, there photochemical reaction under high solar radiation in are industrial sources that affect the results. Generally, S.C. Lee et al. / Atmospheric Environment 35 (2001) 5949–5960 5959

Table 6 tively. The roadside monitoring data can reflect the The percentage difference between rooftop (CWS) and roadside effect of the primary sources of vehicle emission and station (PolyU) for selected TAPs minimize the effect of chemical reactions due to the Selected TAPs (Roadside conc. rooftop conc.)/ change in meteorological conditions. Traditionally, markers Rooftop cone. *100% organic compounds in particulate phase are used to identify the sources. However, it would be more effective PHEN 276.66 to apportion the sources using the combined gaseous FLT 33.01 and particulate phase of organic compounds. This study PYR 168.20 IND 78.54 also demonstrated the feasibility characterizing the main BghiP 67.21 organic pollutants from vehicle emissions with selected Formaldehyde 46.87 PAHs and carbonyl compounds. Acetaldehyde 41.30

Acknowledgements the concentrations of vehicle exhaust pollutants at PolyU station were much higher than that collected at residential The authors would like to thank Hong Kong Central/Western monitoring station. Large variation was Environmental Protection Department (EPD) for sup- observed for PAHs, range from 31% to 277%. The plying TAPs data of two EPD stations. They also thank concentration of PHEN at PolyU station was even two Mr. Mark McDaniel (DRI group) for chemical analysis. times (277%) higher than that at Central/Western This project is supported by Research Grants Council of station. The percentage diﬀerences were 46% and 41% Hong Kong (BQ-303) and a grant (G-V951) from The for formaldehyde and acetaldehyde, respectively. Hong Kong Polytechnic University.

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