The Significance of Urban Rail Transit Systems in Mitigating Air Pollution Effects: The Case of China

Abstract

Air pollution is a global problem and the transportation industry is one of its major causes, yet the impact of transportation infrastructure on air quality is little understood. It is vital to know about evaluating the effects of transportation infrastructure on air quality. The urban road has a role in defining air pollution and automobiles significantly contribute to the air quality in urban areas. The issues are a result of the automobile’s impact on air quality. China put substantial investments in its metropolitan subway networks. The information on air quality on an hourly basis, daily metrological data, and demographic profiles of various cities with significant subways from the year 2013 to the year 2014 used. This study examines the effect of substantial investment on urban air quality. The Discontinuity Based Ordinary Least Squares (DB-OLS) on data for further analysis with results are incorporated in the study. The researchers witnessed a 14% reduction in air pollution during the daytime in those not rushed cities. Researchers discovered that the impacts are less severe in urban areas with a lower population density and a higher income. The results will be less dangerous if more subways affect the air quality.

1. Introduction

Academics and practitioners are increasingly concerned about the long-term effects of pollution. Local city economic development, innovative solid skills, and citizens’ life spans directly impact their health [1,2,3,4]. Every year, air pollution kills almost 3 million people, with 94% of those fatalities caused by respiratory infections, lung illnesses (lung cancer), and particulate matter inhalation, as per the report published by the World Health Organization in 2016. Cities with high pollution levels may lose their original talent and experience a significant drop in regional vitality. One of the forms of air pollution is the presence of particle matter, called haze pollution. Haze pollution is more visible than other pollutants such as SO₂, NO₂, or CO; haze pollution, one type of air pollution, is mainly brought on by particulate matter. Finding ways to lessen haze pollution has therefore become a top priority.

In contrast to sulfur dioxide, nitrogen dioxide, and carbon monoxide, haze pollution has high visibility. Therefore, it is simple to spot when it is present. Developing strategies to reduce pollution ought to be a high priority. In the recent past, a primary focus has been placed on technological advancement, increased levels of regulation, and infrastructure enhancement [5,6]. Governments implemented tax exemptions, financial subsidies, penalties, policies, various incentives, and punishments to protect the environment and encourage subordinate departments. Other novel technological techniques, such as extracting hazardous chemicals from the atmosphere, are ways to reduce environmental pollution. It is unknown how many necessary effects the rail transit infrastructure has on air quality, although these consequences are numerous. The author identifies various mechanisms concerning the reduction in air pollution. Due to heavy traffic congestion, the issue of air pollution is still not answered. The annual investment of China in the transportation system is more than two trillion CNY. Many cities in China joined the “rail transit club” to counter urban air pollution. By the end of 2019, 40 towns opened rail transit, reaching a total of 6736.2 km, of which 5180.6 were in the form of subway lines. From 2013 to 2019, the Chinese annual investment in transportation infrastructure stood at more than two trillion CNY. Although urban planners and policymakers have realized the importance of rail transit in improving air quality and traffic conditions, scholars and urbanites still doubt its actual effect. First, the positive connection between the expanded subway and decreasing local pollution is still unclear in China. Bicycle, BRT, and automobile usage are good options to consider for people of a particular age group that is only plausible in specific cases, i.e., within cities. However, coming from other areas toward cities and going back, the bicycle does not serve the purpose to that extent based on the issue of air pollution highlighted in the study by the authors.

Mohring [7] suggested that rail transit is susceptible to increasing returns to scale. Ridership on the transit rail can minimize the average time spent waiting at stations and improve and generate a greater service frequency; therefore, the government should encourage ridership. According to the Mohring Effect, investments in rail transport infrastructure may be able to sway the travel patterns of somewhat more car-dependent passengers. This diversion can reduce air pollution. Vickrey [8] stated that travel demand is introduced by investment in transportation infrastructure, resulting in the traffic creation effect. The net effect of automotive travel on air pollution is uncertain because of the diversion of auto travelers and the subsequent increase in rail transit infrastructure use, as per Figure 1 depicted below.

Since 2005, Beijing’s rail transit infrastructure has undergone significant expansion, as demonstrated in Figure 1. To be more specific, the mileage of the rail transit system increased at an average annual growth rate of 38.6%, going from 114 km in 2005 to 554 km in 2015. In the meantime, the number of people using rail transit has skyrocketed in recent years, reaching an all-time high of 3.39 billion in the year 2014.

Essential strategies, such as road intensity and transportation infrastructure improvements, are recommended to mitigate haze pollution. Few studies have examined the connection between long-distance transportation networks in cities and provinces and pollution [9]. Low carbon monoxide emissions and other pollutants in urban transportation zones are directly linked to the introduction of high-speed transit, bus rapid transit (BRT), and electric cars [10,11]. However, the connection between rail transportation and pollution is still unclear. China has serious air pollution problems; in 2016, the World Health Organization issued a global map of China’s air quality. However, according to a report on China’s air quality, 385 cities fulfilled the state standard (annual average PM2.5 concentrations of 35 g/m³), but none reached the World Health Organization’s requirement of 10 g/m³. China urgently needs legislation to enhance air quality, particularly in the nation’s capital, where the problem is particularly severe because of the prevalence of conventional vehicle traffic as a major contributor to air pollution [12,13,14].

China’s process of “sprawl,” which is the primary source of pollution in cities, is driving up the number of people who drive in cities. Transportation on urban roads releases several pollutants into the air, including PM₁₀ and PM2.5 particles of different sizes, carbon dioxide (CO₂), nitrogen oxides (NOX), carbon monoxide (CO), and sulfur dioxide (SO₂) as Figure 2 depicted.

These pollutants seriously affect people’s health [15,16,17,18,19]. Let us use Beijing, China, as an example. The contribution of vehicles to PM2.5 in the city accounts for 31.1% of the total, ranking them as the primary source of both concentration and emissions [20]. Compared to private engine vehicles, public transportation is considered a more environmentally beneficial mode. Public transport is the primary mode of transportation in most urban areas [21,22,23]. China’s large number of urban automobiles must study the urban rail transit system, an essential aspect of urban public transport. Urban rail transit is a more accurate and effective means of transportation. It is unclear if this will limit building ownership and auto energy usage. The main challenge in current urban rail transit development has to be understood and clarified [15,16,17]. Various methods are present to reduce air pollution such as tax systems, bicycle usage, ownership, and automobile usage. The majority of investments are in those areas by the government that provides low emission mechanisms and traffic congestion. Extending subway systems makes using the subway more convenient for commuters, cutting down on their time spent on the road and the pollution and congestion that comes from people driving to work [9]. Benefits from these expenditures are conditional on how quickly drivers adjust their habits in response to the subway’s cheaper and faster service and on how much more polluting private automobiles are compared to public transit (given the electricity needs of the subways).

Most of the studies have discussed transportation policies and how these policies affect the reduction in pollution. Still, very few studies have addressed the rail transit system in urban areas, especially case studies not exploring the area of the urban rail transit system even though the urban rail transit system boomed over the last decades [24]. The rail transit consists of the longest course in China’s capital, Beijing. Rail travel will improve Beijing’s air quality and offer information on air contaminants. The inception date of rail travel (28 December 2014) was seen as a sharp intermittence and air quality and first-day seasons were associated with noticing the implications.

Most sources of air pollution (PM2.5, PM₁₀, SO₂, NO₂, and CO) were significantly reduced due to the rail transportation system’s operation, although ozone pollution was barely affected. In this paper, we focus on 11 tram track starts that happened in 2013 and 2014, using hourly air quality data from a few records to estimate the air quality every hour. Compared to the high-frequency monthly data analyzed by Gendron-Carrier and Gonzalez-Navarro [25], this paper lets us control for nonlinear connections between air pollution and different variables that change over time, such as the climate. It also allows us to assess the effects over time in the daylight, as evidenced by the fact that pollution is more dangerous during the day than at night because more people are outside during the day. We investigate five vehicle-related air toxins (CO, PM₁₀, PM2.5, SO₂, and NOx) currently underutilized as a quality measure (AQI). However, Ref. [26] investigated NOx and CO, and Gendron-Carrier and Gonzalez-Navarro [25] used a middle ground based on satellite insights and observation. Gendron-Carrier and Gonzalez-Navarro [25] implemented a Discontinuity Based OLS strategy (DB-OLS) and tested for an essential disruption in air pollutant levels later in the introduction or leeway. Introducing trams reduced air pollution, particularly during the morning non-rush hours of heavy traffic. These effects are more modest in cities with higher pay and sublines and the results are more significant in urban regions with higher population densities.

Furthermore, the subway line’s initial opening is tougher and sturdier, and it corresponds to the growth of the present subway framework. The rest of the paper is structured as follows. The material and methods are presented in Section 2. The empirical findings are presented in Section 3. Section 4 finishes by discussing limitations and future research.

2. Materials and Methods

2.1. Data

The study is quantitative. Using four critical sets of data, it explored the significance of the urban rail system, i.e., metro timings since its beginning in 2015, air quality and pollution, climate data at the metrological stations, and city profiles, and compiled them for further analysis. The information sources and the specifics of the determining factors are listed below.

2.1.1. The Schedule of Subway

Researchers have observed and gathered statistics regarding the opening timing of the subway for every one of the urban neighborhoods that had innovative lines of metros open in 2015–2016. Researchers have displayed them in

Table 1. New Subway Lines Opened in 2015 and 2016 in China.

Name of City	Starting Date	Current Stations	New Stations	Revenue (RMB)
Wuhan	December 2013	50	20	39,000
Nanjing	January 2014	60	23	45,000
Changsha	February 2014	2	25	38,000
Shanghai	December 2013	340	27	45,000
Kunming	April 2014	16	18	32,000
Guangzhou	December 2013	135	24	44,000
Xi’an	September 2013	20	23	34,000
Wuxi	January 2014	0	26	42,000
Dalian	January 2014	15	10	34,000
Ningbo	May 2014	2	24	42,000
Zhengzhou	December 2013	3	23	27,000

. According to the information provided in Table 1, 11 different metropolitan areas over 11 other subway lines opened new tram routes in 2015 and 2016. The opening of more new stations began with the number 243. The urban regions with a new subway line across China’s eastern and central portions are massive. The second line we saw of Kunming’s, Changsha’s, and Dalian’s initial date is around May, the Labor vacation lagging behind an extended break. Compared to regular days, the perspective of commuting on lengthy vacation adventures is different. As a result, we restrict the subway lines, which are three related analyses. The one concern is that the decision about a line’s commencement date may be related to the elements that have no impact on pollution in the air. For example, if the opening is on a given workday or at the end of the week and the vehicle’s design on other days is distinct, the assessment may be neglected. From Table 1, we can see that practically all dates for every month’s commencement and ending, every one of the initial days was not a good workday. So, in this context, the difficulties mentioned above have been resolved.

2.1.2. Air Quality Data

We acquired the air quality per hour from the Ministry of Environmental Protection’s source site (MEP). The data were gathered in January 2013 and covered hundreds of air monitor quality in China’s major cities. The PM2.5, SO₂, NO₂, PM₁₀, and CO alliances are recorded, checked from the screen, and pronounced by an MEP with the continual detailed arrangement. The Air Quality Index (AQI) was examined and established on the data/figures depicting pollution.

Table 2. Summary Statistics.

		Before Opening		After Opening		Difference
	Variable	Mean	S.D	Mean	S.D
Pollutants Caused by Traffic	PM2.5	80.50	32.43	54.86	43.87	25.64 ***
	SO2	30.3257	23.5328	27.6521	32.6215	2.6536 ***
	NO2	50.1252	28.6732	44.6211	18.5636	5.5041 ***
	PM10	113.2617	83.7187	92.2517	80.6217	21.01 ***
	CO	3.3264	17.8232	4.6217	2.7218	−1.2953 ***
Weather Condition	Temperature	190.7218	90.5621	190.5212	79.6521	1.8002 ***
	Precipitation	100.6217	105.4157	38.7271	93.6217	7 ***
	Pressure	623.2718	287.2678	575.2681	232.8123	48.0037 ***
	Wind speed	25.2716	10.6217	28.6217	7.5127	17.7589 ***

Note: *** p < 0.01.

reveals the instantaneous statistics about air pollution factors and other parameters. As shown in Table 2, there is a significant difference in the quality of air and groupings of all observed airborne pollutants in the later subway inaugurations than the earlier.

2.1.3. Weather Data

The weather’s unpick effect on air quality, the data on daily wind data, pressure data, rainfall, and temperature are from the Administration of China Meteorological.

2.1.4. City Characteristics

To know about how pollution in the air was affected by the heterogeneity of the subway, we collected data from the annual chrematistics book, such as the personal income and automobile ownership rate.

2.2. Model

Our research is based on the prior work done by Chen and Whalley [9]. The discontinuity approach is used in the air to measure the high-reaching frequency. The subway opening is the best way to improve air quality according to consideration. The regulation of air pollution learning is not related to subway opening. Adjusting a daily air polynomial pollution permits a movement at the time of introduction for a distinct betterment. The subway opening hub is not in one city but in multiple cities. The discontinuity technique was used [9,27,28,29]. After considering the common effect, we examine the heterogeneity in the reflex and investigate the aspect that runs the heterogeneity. The specified equation used to analyze the impact of the subway opening is below:

𝑦𝑖𝑡 = 𝛽0 + 𝛽𝑜𝑝𝑒𝑛_𝑖𝑡 + (𝒕) ′𝜹 + (𝒕) ′𝜸 𝑜𝑝𝑒𝑛_𝑖𝑡 + 𝑿_𝒊𝒕 ′ 𝜽 + 𝛼_𝑖 + 𝜖_𝑖𝑡

(1)

(1). The indicators for measuring the air quality log in one city is time (t) and yit.

(2). If t is adjacent to a subway slot in city I, the dummy variable is opened and set to one. The time progress p(t) is polynomial.

(3). To accommodate for the weather (precipitation, temperature, pressure, and wind speed), X_it is a set of factors being researched for delivering a response to the level of airborne contaminants.

(4). Once again, it is dependent on the day of the week, i.e., a weekend or a winter day in northern cities where region-wise heating is delivered, and bandwidth is h for each of the pollutant variables in the use of Equation (1) separately.

(5). It depends on the day of the week, i.e., a weekend or a winter day in northern cities where the region-wise heating supplied 𝜖𝑖𝑡 is the error term, as is the effect of town fixed, and bandwidth is h for each of the pollutant variables in the use of Equation (1) independently.

The literature suggests that the most significant pollutants from automobiles are carbon monoxide, nitrogen oxides, fine particulate matter (PM2.5), and sulfur dioxide (SO₂). As a result, we anticipate adverse impacts on O₃ and the AQI due to these contaminants. We look at five different pollutants and the Air Quality Index (AQI) to see their influence on ozone.

3. Results

3.1. Major Results

We have estimated Equation (1) and the results are summarized in

Table 3. Does Urban Rail Transit Eliminate Air Pollution.

	Pollution Caused by Transportation
Variables	PM 2.5	SO2	NO2	PM10	CO	AQ1
Control Variables Included	−0.365 ** (0.021)	−0.087 ** (0.043)	−0.086 ** (0.050)	−0.257 ** (0.037)	−0.024 ** (0.027)	−0.345 *** (0.034)
Control Variables Excluded	−0.276 ** (0.029)	−0.145 ** (0.038)	−0.089 ** (0.012)	−0.145 ** (0.039)	−0.049 ** (0.019)	−0.140 *** (0.025)
Interaction of City and Weather	−18.56 ** (0.021)	−24.65 *** (0.005)	−0.1476 ** (0.032)	−0.187 ** (0.047)	−0.056 ** (0.029)	−0.159 ** (0.032)
Observations	6500	6500	6500	6500	6500	6500

Note: *** p < 0.01, ** p < 0.05.

. First of all, researchers have conducted a regression analysis with control variables and have added dummy and weather variables, the day of the holiday, and the city of the week. Researchers have added the equation’s interaction of the dummy city and the weather. Only the coefficient of the subway on the air quality shows the effect reported. There are many results stable for most of the pollutant’s specifications. As the primary regression, we use the specification with the most control. It shows the beginning of the subway reduced PM2.5 by 18%, PM₁₀ by 18%, NO₂ by 14%, CO by 5%, and SO₂ by 24% on average. The AQI decreased by 15% in the opening of the subway. We reported pollutants a day after the opening dates and then the weather effects as dummy variables that are aligned with the results-driven out by [9].

It has been reported that the estimators of the trend of subway openings follows

Table 4. Urban Rail Transit Eliminates Air Pollution Trend.

	Pollution Caused by Transportation
Before, After Opening	PM 2.5	SO2	NO2	PM10	CO	AQ1
Before opening	0.0052 *** (0.004)	0.0043 *** (0.002)	0.0052 *** (0.005)	0.0088 ** (0.003)	0.0029 *** (0.007)	0.0031 ** (0.034)
After opening	−0.0021 *** (0.009)	−0.0065 *** (0.008)	−0.0087 *** (0.002)	−0.0032 *** (0.006)	−0.0032 *** (0.003)	−0.0077 *** (0.007)
Difference	−0.00165	−0.00161	−0.00732	−0.00175	−0.00187	−0.00132

Note: *** p < 0.01, ** p < 0.05.

. It is also indicating that a subway opening decreases the pollution level. On average, the reported reduction in the AQI per day was 0.16%, which is aligned with the scientists’ research [9].

3.2. Heterogeneous Effects

In this part, we interrogate the possible heterogeneity of a metro primary on the air condition. For that reason, the auto use experience of the public is different during the morning, noon, and nighttime. Researchers have expected that the subway’s influence on air quality varies throughout the day; while the tunnel is not operating, there is no effect on air quality, and morning travel time is when people learn about transportation and rush hour congestion. Then, the metro is not considered an excellent alternative to autos. Replacing automobiles with the metro is almost considered more substantial during non-overcrowded hours. To investigate the heterogeneity effect at different times, the researcher reduced the average night pollutant level (0 a.m.–5 a.m.), morning (7 a.m.–11 a.m.), and noon (11 a.m.–3 p.m.) in the way the independent variable is displayed by

Table 5. Does Urban Rail Transit Eliminate Air Pollution During Various Schedules.

	Pollution Caused by Transportation
Schedules	PM 2.5	SO2	NO2	PM10	CO	AQ1
Morning Time	0.027 (0.163)	0.081 (0.231)	0.091 (0.183)	0.062 (0.198)	0.042 (0.187)	0.047 (0.135)
Noon Time	−0.182 (0.187)	−0.187 (0.134)	−0.123 (0.176)	−0.198 (0.142)	−0.190 (0.129)	-0.153 (0.187)
Night Time	−0.297 (0.097)	−0.287 (0.133)	−0.298 (0.198)	−0.276 (0.145)	−0.187 (0.298)	−0.265 (0.245)
Observations	300	300	300	300	300	300

. The exact consequences are to look forward to the most significant effects of the subway on non-crowded hours and the air quality at night is not affected.

3.3. Discussion of Results

The study aims to check the significance of the urban rail system in mitigating air pollution effects. The recorded data were analyzed by extracting results in the form of summary statistics of variables such as pollutants caused by the traffic, i.e., PM2.5, SO₂, NO₂, PM₁₀, and CO. With this, another set of variables under the umbrella of weather conditions, i.e., temperature, precipitations, pressure, and windspeed summary statistics were driven out. Six thousand five hundred observations were analyzed for pollution caused by transportation with control variables included and excluded. For urban rail transit pollution before and after opening, differences were found with the Air Quality Index. All values are highly significant. Again, the heterogenous effect was yielded by digging deep into the variables depicting pollution caused by transportation on various time schedules such as in the morning, noon, and night times. The observations were 300 and the results significant too.

4. Conclusions, Implications, Limitations, and Future Research

Considering the above commentary, it is concluded that numerous cities in China have witnessed a quick construction of their rail transport systems with the purpose of eliminating the problems of urban traffic jams and air pollution. Many cities have one of the most developed urban rail transit systems in China with rail transit and annual growth rates. Identifying the effects of the Beijing Metro on the air quality provides a relevant experience for other cities with rail transportation systems either under construction or preparing for construction. This paper identifies the implications of opening a new rail transit system. The results obtained were highly resilient across a variety of estimation approaches, alternative specifications, and smoothness tests. Recent studies showed that the automobile elements of pollution harm health. As we know, transportation is the cause of air pollution and few studies examined the direct impact of transportation on air quality. This paper will fill the gap in transportation infrastructure in urban rail transit. Later, researchers will apply a matching difference in the present results and method. Researchers have kept each city with a beginning tunnel to match on outcome per head, population, solidity, and GDP. Researchers have chosen the most matching sites for each province as a representative sample. For Kunming, the cities of Wuhan and Xian have no data about the openings compared to different cities in their holding, therefore researchers have chosen the top close city in their bordering jurisdiction.

Moreover, Shanghai is considered the second-largest city with diverse attributes. In Shanghai, the characteristics under control group experiments were examined. No significant variation concerning GDP and income was found. Shanghai has a denser population in the experiment settings. Then, we use differences to point out the beneficial effect of metro primary regulations on these cities’ attributes and the expected outcome is analogous to those from the foremost devolution. Academicians should also strengthen their academic knowledge by adding more variables concerning time, such as summers and winters.

The practical implications are learning for policymakers to devise such frameworks that are helpful for the commuters traveling on the urban rail system and give it more strength, as this system helps minimize the pollutants and their impact on the air quality of those areas.

The potential limitation of this research is three-fold. First, it only absorbed the effect of the operative phase over speed rail on a population out rush, not in all the other levels of the railway field such as the reusing and recovery of a subway train on the impact of environmental protection [30]. Second, because of the availability of transportation foundations, we do not consider repeatability tasks for other approaches of transportation, for example, aircraft and highways. Further investigation will focus on the relationship between haze pollution and transportation foundations. A final limitation is the latest data is needed to proceed further in the future to highlight the significance of the urban rail transit system and its effect on air pollution. There are a few helpful directions for future research. First, claiming a similar discontinuing approach to estimate the air grade effect of other types of transport foundations would be fruitful. The air condition effect of over-speed rail or an airport foundation is a delimitation in the study. Additionally, urban rail transit systems are probably not the best option for all regions; hence, the implementation of other competing public transportation systems, such as BRT systems, would be more suited than the implementation of rail transit systems. Further research can be conducted on the same model in various weather conditions, such as in summers and winters.