Pollutant concentration measurement and emission factor analysis of highway tunnel with mainly HGVs in mountainous area
Introduction
With the rapid development of traffic infrastructure in China, a large number of highway/railway tunnels have been or will be constructed in the coming decade (Chen et al., 2020, Luo et al., 2017a, Luo et al., 2017b, Liu et al., 2018, Li et al., 2020, Wang et al., 2020). By the end of 2018, there were 17,738 highway tunnels in China, covering a total length of 17236.1 km; this corresponds to an increase of 1509 in the number of highway tunnels and an increase of 1951 km in the total length, with growth rates of 8.5% and 11.3% respectively. The newly constructed tunnels included 1058 extra-long tunnels (L > 3000 m) with a total length of 4706.6 km, and 4315 long tunnels (1000 m < L ≤ 3000 m) with a total length of 7421.8 km (Ministry of Transport of the People's Republic of China, 2019).
The challenge posed by tunnel ventilation is a bottleneck that restricts the development of long and extra-long highway tunnels. Recently, China has increased investment in economic development and infrastructure construction in the central and western regions, which are mostly covered by mountains and hills. Accordingly, an increasing number of long highway tunnels have been or are being built to provide more convenient traffic for people and facilitate regional freight transportation in the mountainous areas of central and Western China(Li et al., 2018, Lai et al., 2018, Luo et al., 2017c, Luo et al., 2018, Liu et al., 2020). Furthermore, with the sharp increase of freight transportation demand, both the traffic volume of HGVs and its proportion in the total traffic of highway tunnels are increasing year by year in the mountainous areas of central and Western China (Luo et al., 2019). The traditional ventilation strategy is inadequate to meet the requirements of daily operation of such tunnels with a large HGV traffic, and hence, it has become a challenge to dilute the pollutants emitted by vehicles in tunnels. The emissions contain many harmful substances, with CO, NO, SO2, and NH3 as the main components (Yan and Chen, 2002).
Many researchers studied the concentration and emission factors of air pollutants in highway tunnels considering LCVs as the representative vehicle type. Deng et al. (2000a) statistically calculated the average vehicle pollutant emission factors of vehicles in urban tunnels in China, classified vehicle pollutant concentrations, and compared the results with those of Western developed countries. Chang et al., 2009, Li et al., 2011, and Ma et al. (2011) continuously monitored the air quality of the Hsuehshan Traffic Tunnel in Taiwan (12.9 km), and the measurement results showed that the pollutant emission factor uphill was twice that downhill and the pollutant concentration near the exit was higher than that near the entrance. On-site environmental measurement in urban tunnels, conducted in 2014 in Shenzhen, China, indicated that the in-tunnel CO concentrations stayed at a lower level, and the maximum CO concentration in four case tunnels reached 39 ppm, which was below the 40% of design threshold and within the safety limit (Liu et al., 2020a). Based on the measured data on traffic flow, air velocity, and CO concentration, Liu et al. (2020b) evaluated the ventilation environment in urban tunnels and found that the average CO emission factors of mixed traffic in Henglongshan Tunnel, Cejiexian Tunnel, Jiuweiling Tunnel, and Dameisha Tunnel were 1.075 g/(km·veh), 1.245 g/(km·veh), 4.154 /(km·veh), and 1.739 g/(km·veh), respectively
The concentrations and emission factors of air pollutants in highway tunnels with mixed vehicle traffic too have been widely studied. Colberg et al., 2005, Kristensson et al., 2004 monitored the emission factors of motor vehicles by the tunnel testing method. They found that the longitudinal slope of the road in the tunnel had a great influence on the emission of NOx from HGVs but had little influence on the emission of CO and NOx from LCVs. Wang et al. (2001b) conducted the multi-point synchronous monitoring of gaseous pollutants and atmospheric particulate matter of highway tunnels in North and South China. Further, they analyzed the vehicle flow, visibility, and meteorological factors in tunnels. Brimblecombe et al. (2015) studied the concentration distribution of pollutants and particulates in three tunnels in Hong Kong and analyzed the average emission factors of diesel vehicles. Cheng et al. (2006) determined the actual emission factors of PM2.5, NOx, and CO2 of mixed vehicles in Shing Mun Tunnel in Hong Kong through field measurement. They found that the emission factors of PM2.5 and NOx had a positive correlation with the proportion of diesel-fueled vehicles. Bešlić et al. (2005) measured the particulate matter, CO, NO, and NO2 in Tuhobić Road Tunnel, Croatia and showed that the level of air pollution in the tunnel was closely related to traffic density and pollution time. Indrehus and Vassbotn (2001) measured the CO, NO, NO2 concentration, traffic density, visibility, and air velocity in a Norwegian tunnel (7.5 km) by using a continuous analyzer and suggested that CO and NO2 concentration models should be established.
Jamriska et al. (2004) studied the emission factors of a motorcade of about 300 diesel vehicles in Brisbane, Australia, and evaluated the correlation among submicron particle numbers, PM2.5, and diesel vehicle emissions. However, many harmful NOx emitted by HGVs were not considered and the study had limitations in terms of the control of pollutant concentration in highway tunnels and the improvement of driving ambient air quality.
The previous studies on the emission factors in highway tunnels mostly focused on LCVs or mixed traffic, and there has been limited research on the contribution of HGVs to air pollutant concentration and emission factors in highway tunnels. With the development of the national economy and the rapid advances in the logistics industry, in particular, China is witnessing a gradual transition of the vehicles plying on the mountain roads from mixed traffic to HGVs. The increase in the proportion of HGVs traveling on highway tunnels in mountainous areas gradually increases the NOx emission level. As the NOx emission level increases, the emission factor increases as well, and this factor is distinctly different from that studied previously for tunnels with LCVs or mixed traffic as the representative vehicle type. Hence, previous research on the emission factors based on LCVs or mixed traffic is not suitable for the current economic development scenario and cannot meet the current road transportation requirements.
In this study, a field measurement was conducted in Qinling No. 3 Tunnel, China, with HGVs as the representative vehicle type. The tunnel pollutants were detected on-site, and their emission factors were studied. Besides, the traffic volume, air velocity, and pollutant concentration were examined. The variation law of pollutant concentration and the emission factor of pollutants in the tunnel were obtained. Thus, this work provides a scientific method for predicting tunnel air pollutant concentrations and the supporting data for establishing air pollutant control standards for highway tunnels.
Section snippets
Tunnel description
Qinling No. 3 Tunnel is one of the key projects of Xi'an–Hanzhong Section on Beijing–Kunming National Expressway (G5), which is located in Shaanxi Province and passes through the Qinling Mountain. Qinling No. 3 Tunnel is a two-way, four-lane highway tunnel with a design speed of 80 km/h; the roads in the tunnel are one-way longitudinal grades with a southwest bound line length of 4683 m and a slope of 2.1%, and a northeast-bound line length of 4930 m and a slope of −2.54%. This tunnel is fitted
Traffic volume
The traffic volume of the southwest bound tunnel was studied. The one-way daily traffic volume and peak hour traffic volume in the tunnel were also compared with the original design data. The traffic volume of the southwest bound tunnel is listed in Table 4, and the number of motor vehicles (veh) has been converted into passenger car units (pcu) according to passenger car equivalents.
The data in Table 4 show that the tunnel had high overall traffic volume. The measured one-way daily traffic
Calculation and analysis of in-use fleet emission factors
The emission factor of motor vehicle pollutants can reflect the overall emission level of motor vehicle pollutants and is an important parameter for tunnel ventilation design and pollutant emission calculation. Hence, studying the emission factors of vehicle pollutants is of great significance for evaluating the characteristics of vehicle pollutant emissions and the total amount of vehicle pollutant emissions.
Conclusions
According to the principle of conservation of mass, the emission factors of different types of motor vehicles were calculated by multiple regression equations. The main conclusions are as follows:
- (1)
The measured one-way daily traffic volume in Qinling No. 3 Tunnel ranged from 18,890 to 25,690 pcu/d, and the one-way hourly volume traffic volume ranged from 1571 to 2243 pcu/h. These values exceed the one-way daily traffic volume of 15,784 pcu/d and peak hour traffic volume of 1042 pcu/h,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to acknowledge the financial support provided by the National Key R&D Program of China (Grant No. 2018YFB1600100), the National Natural Science Fund Project of China (Grant Nos. 51978065 and 51678063), the China Postdoctoral Science Foundation (Grant No. 2016M602738) and the Chang Jiang Scholars Program (Grant No. Q2018209).
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