In-Vehicle Carbon Monoxide Exposure Level While Traversing Different Roadway Types in Hong Kong L.Y. Chan, Y.M. Liu Department O

In-vehicle carbon monoxide exposure level

while traversing different roadway types in Hong Kong

L.Y. Chan, Y.M. Liu Department of Civil & Structural Engineering, The Hong Kong

Polytechnic University, Hong Kong SAR, People's Republic of China.

Abstract

Automobile emission is the major source of air pollution in Hong Kong. Many people are not aware of the importance of air quality inside vehicle compartments. In this study, in-vehicle CO level was measured in a light goods vehicle. The vehicle was driven over different districts and commuting roads. Results indicate that the in-vehicle CO level is the highest while traversing tunnels. Higher in-vehicle CO level was recorded in area related to commercial activities. Lower in-vehicle CO level was measured in remote area and highways. Self-emitted and surrounding vehicle exhaust are the sources of in- vehicle CO level. CO intrudes into vehicle compartment through the leaks. Ambient CO level and AADT influence the in-vehicle CO level directly.

1. Introduction

Automobile emission is the major source of air pollution in many countries.

Vehicle exhaust consists of various air pollutants and some of them are carcinogenic. The Hong Kong Environmental Protection Department has set up 14 air monitoring stations over the territory. 11 monitoring stations are 12-25 metres above ground level. Chan et al. [1] observed that the roadside respirable suspended particulate level is substantially higher than that in nearby fixed monitoring stations. Some studies show that air pollution level inside vehicle compartment is higher than fixed site measurement [2,3,4,5]. Commuters are required to spend a considerable time inside vehicles every day and they are exposed to higher air pollutant concentration levels during commuting. There is an increasing concern on ambient air pollution in Hong Kong but many people

do not aware of the importance of the air quality inside vehicle compartments. Carbon monoxide is a toxic gas and its production is due to incomplete combustion. It is relatively stable and is good for examining traffic pollution studies. In this study, we monitored the in-vehicle CO level while commuting in different roadway types.

2. Field work study

A light goods vehicle was driven over different districts in Hong Kong in the winter of 1998 to measure the in-vehicle CO level. Portable CO monitor was used to measure the concentration level. The vehicle is a 1998 Toyota light goods vehicle with about 4000 km at the beginning of study. Air pollutant concentration levels are different in different transportation modes [6,7]. The use of a single vehicle eliminates the variation of in-vehicle CO level due to different brand and types of vehicles. The different commuting routes included urban roads, sub-urban roads, rural roads, highways and tunnels. Each type of route has its own characteristics. The urban roads are narrow and usually there are 1-2 lanes in each direction. High- rise buildings locate along the roads. Numerous traffic lights and intersections are present. The traffic volume is heavy. For the sub-urban roads, there are 2-3 lanes in each direction and the roads are wider than the urban roads. Large pedestrian pathways are reserved from the road to nearby buildings. Flyovers and subways are present to facilitate pedestrians to cross the roads. The number of traffic lights is lesser and the traffic flow is medium. The rural roads are gradient and narrow but high-rise buildings are absent. The traffic volume is low. Highways are usually 3 lanes in each direction. Some of them are close to the sea or locate within country parks and buildings are absent. There are 11 tunnels in Hong Kong. 3 of them are cross-harbour tunnels and the others traverse mountains. The lengths of highways range from 1 to 4 km. A total of 5 tunnels and 7 highways were traversed in this study. The Annual Average Daily Traffic (AADT) and microenvironment characteristics of these tunnels and highways are shown in table 1.

3. Measurement methodology and quality assurance

The in-vehicle CO level was measured in the middle of the vehicle and at passenger's respiratory level. Air-conditioning system was turned on during sampling. In order to prevent the direct intrusion of pollutants, all the windows and air vents were closed. Weather condition, traffic volume, selected locations of the vehicle and the corresponding time were recorded. The CO concentration was measured by an electrochemical voltametric sensing monitor (InterScan Co, model 4148). All CO levels were recorded in part per million (ppm) and the detection limit is 0.1 ppm. Readings were recorded every fifteen seconds by a Metrosonics dl-714 portable data logger and the output was programmed to give half minute averaged intervals. The monitor was calibrated by a standard CO span gas and zero air before and after each survey trip. The drift of the monitor

Air Pollution VIII 295 was always less than 2% at the end of each sampling. The monitor was turned on and allowed to stabilise for around 5 minutes before the start of each trip.

Table 1 Commuting microenvironmental characteristics of tunnels and highways.

Tunnel/highway AADT Microenvironment characteristics Lion Rock Tunnel 95,200 Dual two-lane, uni-directional, connects urban area and new town, traverse a mountain

Cross Harbour Tunnel 120,290 Dual two-lane, immersed tube, uni- directional, links between two urban commercial areas, cross harbour tunnel Eastern Harbour 70,360 Dual two-lane, immersed tube, uni-

Tunnel direction, links residential area and urban mixed residential/commercial area, cross harbour tunnel Cheung Tsing Tunnel 56,730 Dual three-lane, uni-directional,

traverse a small hill in an island. Tai Lam Tunnel 29,710 Dual three-lane, uni-directional, locates within country park, links 2 new towns Tolo Highway 124,410 Dual three-lane, locates along coastal

line and next to Tolo Harbour Island Eastern Corridor 70,360 Dual three-lane, locates at elevated bridge or reclaimed land, links urban commercial centre and residential area,

mainly petrol cars North Lantau Express 25,960 Dual three-lane, locates at an outlying island, commercial & residential buildings are absent Tuen Mun Highway 95,354 Dual three-lane, gradient road, links

two new town centres, mainly bus & goods vehicle Yuen Long Highway 66,750 Dual three-lane, extension of Tuen Mun Highway, major route to China in the west of Hong Kong , mainly heavy

diesel goods vehicle Fanling Highway 60,800 Dual two-lane, only route to China at the east of Hong Kong, mainly heavy diesel goods vehicle

Route 3 Country Park 29,710 Dual three-lane, locate within country Section park, mainly private car, another route to China

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o 0:00:00 0:14:24 0:28:48 0:43:12 0:57:36 1:12:00 1:26:24 1:40:48

Travelling Time

Figure 1. Temporal CO variation

4. Result

Figure 1 shows a typical temporal in-vehicle CO variation. The in-vehicle CO level is the highest while traversing tunnel. The in-vehicle CO level is different in different land-use categories.

Table 2. CO concentration in different land-use categories.

No. of No. of Cone (PPim) Land-use category districts samples min avg max traversed Urban mixed commercial/residential 2 8 1.7 2.6 4.3

Urban residential 4 9 1.4 2.3 2 9 Urban commercial 3 10 1.4 2.6 40

Remote 1 4 1.3 1.5 1 9 New town mixed 3 12 1.0 2.4 5,.3 commercial/residential New town residential 4 18 1.0 1.8 2 9 Industrial 3 12 1.2 2.0 26

The vehicle traversed 20 districts in Hong Kong. They are divided into 7 categories: urban mixed commercial/residential (C/R), urban residential, urban

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commercial, remote, new town mixed commercial/residential (C/R), new town residential and industrial area. Table 2 shows the in-vehicle CO levels in these 7 land-use categories. The highest average CO level was recorded in urban commercial and urban C/R area with an average level of 2.6 ppm. The concentration levels in new town C/R and urban residential area are close. The average CO levels are 2.3-2.4 ppm. It is followed by new town residential and industrial area. The average level is between 1.8 and 2.0 ppm respectively. The concentration level in remote area is the lowest with an average value of 1.5 ppm. Figure 2 shows the average in-vehicle CO level while traversing different tunnels and highways. The annual average daily traffic (AADT) in each tunnel and highway is shown in the same graph. The in-vehicle CO level has a direct relationship with the AADT. Tunnels and highways with high AADT have higher CO level. This is obvious in tunnels. The higher the AADT, the higher is the CO level. There is some derivation from this phenomenon for the highways. The in-vehicle CO levels are between 1.3-2.0 ppm while traversing highways except for Island Eastern Corridor that the average in-vehicle CO level is 3.2 ppm. The AADT in Tolo Highway is the highest but the average in-vehicle CO level is relatively low while traversing this highway.

5. Discussion

The in-vehicle CO level was measured in a single light goods vehicle while traversing various districts and commuting routes. Results indicate that self- emitted and surrounding vehicle exhaust intrude into the vehicle compartment during idling or moving. This can be illustrated from figure 1. In the field survey,

Figure 2. Average in-vehicle CO level and AADT while travesing highways and tunnels

all the windows and air vents are closed during sampling. If there is no leakage into vehicle, the in-vehicle air would not allowed to exchange with the out- vehicle air. The CO level would keep constant but it still fluctuates.Thi s shows that leakage exists in this vehicle. The vehicle is not completely sealed and CO intrudes into the vehicle compartment through joints and leaks. What we observe is that the in-vehicle CO level has a direct relationship with the out-vehicle level. The variation of in-vehicle CO level is small in highway. This is because highways locate at open space where dispersion is better. The out-vehicle CO level is lower. The variation is different in tunnels. The air pollutants are confined inside the tunnels and the concentration level is high. Such high CO concentration level intrusion into the vehicle compartment leads to an increase in in-vehicle CO level. Time lag exists between the in-vehicle CO level and the location traversed by the vehicle. The in-vehicle CO level attains peak level after exiting the tunnel in a minute. High in-vehicle CO level is usually associated with commercial activities. High-rise buildings locate along the roads in commercial or C/R area. The air pollutants are trapped by the buildings. Bus, minibus and taxi are the major vehicles in the large shopping centre areas.

Bus-stops and car parks are nearby. Traffic congestion occurs frequently and our vehicle had to stop repeatedly. The ambient CO levels are higher in these areas. This results in higher measured in-vehicle CO levels. In contrast, the traffic volume is lower in new town residential and remote area, and the CO levels are lower. There is a close relationship between in-vehicle CO level and AADT. The traffic flow does not only influence the ambient CO level but also the in-vehicle CO level. The exceptional high in-vehicle CO level recorded in Island Eastern Corridor is due to vehicle type. There are about 37% and 54% of vehicles belonging to private cars during morning and evening peak hours. The CO concentration in petrol vehicle exhaust is higher and hence the ambient CO level is higher. Relatively low in-vehicle CO level was measured in Tolo Highway but the AADT is the highest. It is due to the location of this highway which is just next to the Tolo Harbour. The strong wind from the harbour help to disperse the air pollutants away.

6. Conclusion

In-vehicle CO level was measured in a light goods vehicle which was driven over different land-use categories and commuting routes in Hong Kong. Results indicate that self-emitted and surrounding vehicle exhaust are the sources of in- vehicle CO level. CO intrudes into the vehicle compartment through the leaks. The ambient CO level and AADT directly influence the in-vehicle CO level. CO is a good tracer for traffic air pollution. In this study, we see that there is a variation of in-vehicle CO level while traversing different land-use and commuting microenvironments.

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