3.1 Localized emission factors
The pollutant emission factors were obtained through on-board test, the Guidebook, and the IVE model. Figure 2 shows that emission factors of each pollutant gradually decrease, with the improvement of the emission standards for HDDTs. Taking CO as an example, the emission factors of China IV and China V standards are 58% and 64% lower than China III, respectively. The comparison of emission factors among different emission standards (as shown in Fig. 2) implies that the emission factors of the China III HDDTs are possibly overestimated by modifying the parameters following the Guidebook. On-road test results of CO, NOx and PM emission factors are 0.8 times, 0.7 times and 0.5 times the guide method, respectively. However, pollutant emission factors are underestimated by using the IVE model. On-road test results of CO, NOx and PM emission factors are 1.3, 1.2 and 1.1 times the IVE model, respectively. In terms of China IV and China V HDDTs, the CO and NOx emission factors obtained by the guidance method and the IVE model are underestimated, while the PM emission factors are overestimated by the guidance method. The PM emission factors obtained by the IVE model are similar to the practical measurement.
These three methods cover temperature, humidity, altitude, speed and actual road conditions, providing a better estimate of the emission inventory. Accordingly, the CO, NO and NO2 emission factors of HDDTs are obtained through the on-board test results and Eq. (1) in the guide method. The PM emission factors are calculated by the localized IVE model. Finally, we established an emission factor database of HDDTs in Northeast China.
3.2 Emission inventory
HDDTs emissions in Northeast China were calculated to establish the emission inventory based on VKT, the number of HDDTs and the localized emission factors. In 2020, total emissions of CO, NO, NO2 and PM from HDDTs in Northeast China were 172.2 kt, 531.5 kt, 11.2 kt and 921.4 t, respectively.
The differences in emissions among 42 cities in Northeast China present to be pronounced. Figure 3 shows the main results of the emission inventory. The province with the highest emissions is Liaoning, contributing to 42.63% of all pollutants, followed by Jilin with 29.38%, Heilongjiang with 23.28% and Eastern Mongolia with 4.69%. Liaoning Province is an important economic hub and industrial center in Northeastern China, with a considerable number of HDDTs contributing to high levels of emissions due to urban traffic congestion. In terms of emissions of CO, NO, NO2 and PM, Shenyang has the highest emissions in Liaoning Province, and Changchun ranks the top among cities in Jilin Province. Harbin is the city with the most active industrial activities in Heilongjiang Province, which the highest emissions of CO, NO, NO2 and PM pollutants. In eastern Inner Mongolia, the city with the highest emissions is Tongliao, followed by Hulun Buir, Hinggan League, Chifeng and Xilingol League. These results are mainly due to the dense road network, heavy traffic flow and large vehicle population in capital cities. This reflects differences in HDDTs populations in each municipality as well as the differences in emissions ascribing to the spatial diversity of environmental factors (Ibarra-Espinosa et al., 2021).
3.2.1 Emission by different emission standards
As shown in Fig. 4, HDDTs with different emission standards have distinct effects on the emission contribution rate of exhaust pollutants. The CO and NO emissions from China III HDDTs reached 43.92% and 42.51% of the total emissions, respectively, and the NO2 and PM contribution rates are over 75%. Therefore, China III HDDTs are the main contributors to pollutant. This is consistent with the conclusions drawn in prior studies (Yao et al., 2015). During the emission standard upgrading process, since more freight companies and drivers choose China V HDDTs as the main means of transportation, the contribution rate of China V HDDTs in CO and NO is higher than that of China IV HDDTs. In summary, it is necessary to accelerate the elimination of China III HDDTs and promote the advanced automobile tailpipe treatment system, which are crucial to reduce tailpipe emissions from HDDTs.
3.2.2 Emission sharing rates of different seasons
The winter of Northeast China is chill and long, summer is warm and rainy. We analyzed the pollutant emissions from HDDTs in Northeast China during the winter and summer seasons. The temperature change has a remarkable effect on HDDTs emissions, as presented in Fig. 5. Overall, the shares of various pollutants in winter are higher than those in summer, mainly due to more precipitation and snowfall in winter. The slippery roads decrease the speed of motor vehicles, which increases emissions from motor vehicles (Hansen et al., 1995). Simultaneously, low temperature leads to insufficient fuel combustion, HDDTs exhaust pollutants are more difficult to disperse under low-temperature circumstances in winter than under high-temperature circumstances in summer (Mathis et al., 2005). Hence, it is indispensable to implement the mandatory use of tailpipe treatment devices to reduce environmental pollution from HDDTs in Northeast China.
3.2.3 Emission composition from different road types
Figure 6 shows that suburban roads, as the main operating sections of HDDTs, contributed most of the pollutants, followed by urban roads and highways. Due to traffic congestion, frequent start-stop and slow driving of HDDTs, urban roads have a high contribution to the emissions of various pollutants, the CO and NO2 contribution rates are over 40%. HDDTs are the key emission sources of pollutants NOx and particulate among all types of roads, and are the top priority of motor vehicle pollution prevention. Therefore, the government should implement stricter regulations for HDDTs to certain urban areas to prevent severe health-related issues caused by tailpipe exhaust. At present, some cities have implemented traffic restriction policies. For instance, Shenyang prohibits HDDTs from driving in third-ring roads (Shenyang Municipal People’s Government, 2021).
3.2.4 Comparison with other inventories
We compared our emission inventory of HDDTs in Northeast China with the annual reports of motor vehicles and the emission inventory of HDDTs in other cities, as shown in Table 2. The overall trend of pollutant emission in Northeast China are consistent with prior studies, with NOx emissions higher than other pollutants (MEE, 2021). Nevertheless, there is also evident difference in pollutant emissions in the various studies. The HDDTs emissions estimated in this study are approximately 6.0 times higher than those reported by Wang et al. (2020), and far higher than the emissions reported by Sun. (2015) and Zhang et al. (2017).
Table 2
Comparison of emission inventories of HDDTs in Northeast China and other cities
Research region | Year | HDDTs (104 vehicles) | Tailpipe emissions (104 tons) | Data sources |
CO | NOx | PM |
Northeastern China | 2020 | 70.22 | 17.23 | 53.13 | 0.10 | This study |
China | 2020 | 840.64 | 214.25 | 254.17 | 27.8 | MEE, 2021 |
Tangshan | 2018 | 18.13 | 3.53 | 9.45 | 0.25 | Wang et al, 2020 |
Chengdu | 2012 | 4.51 | 0.40 | 1.40 | 0.85 | Sun, 2015 |
Nanchang | 2014 | 1.21 | 0.25 | 0.59 | 0.02 | Zhang et al., 2017 |
These differences can be attributed to the following reasons. Firstly, the emissions of HDDTs pollutants in Northeast China have their own distinct characteristics. There are notable discrepancies in the number and types of vehicles between the Northeast region and other areas. Moreover, cities like Chengdu, Nanchang, and Tangshan experience relatively higher winter temperatures, resulting in stronger pollutant dispersion capabilities than the Northeast region, leading to distinct variations in HDDT pollutant emissions across regions. Secondly, the methodologies employed for computing emission factors in these research works vary. Zhang et al. (2017) and Wang et al. (2020) use the Guidebook to calculate emission factors, while Sun. (2015) estimated pollutant emission factors based on the actual measured IVE model. This contrasts sharply with the actual measured results in this study. On-road tested emission factors are included in the Guidebook in this study, and combined with the three methods to calculate the emission factors of CO, NOx and PM. In contrast, we emphasized the importance of localized emission factors in accurately estimating HDDTs emissions.
3.3 High temporal-spatial distribution of the emission inventory
3.3.1 Spatial distribution
Figure 7 displays the high-resolution spatial distribution of HDDTs pollutant emissions in Northeast China. Overall, the geographical distribution characteristics of CO, NO, NO2, and PM pollutants are similar. The high-emission intensity areas are mainly concentrated in city centers, such as Shenyang, Changchun, Dalian, Jilin, and Harbin. In terms of the distribution interval density of each province, Liaoning Province has significantly higher pollutant emissions than Jilin, Heilongjiang, and eastern Inner Mongolia. Within each province, Liaoning Province radiates pollutants from Shenyang to surrounding cities, while other provinces exhibit similar characteristics.
The high emission intensity in city centers is due to the high road level, large number of lanes, and frequent congestion. Within each city, logistics, freight, and steel enterprises are relatively concentrated in the commercial areas of the urban area, with dense road networks and large traffic flows. The pollutant emission intensity of HDDTs shows a decreasing trend from the inner city to the city edge. Our spatial distribution is consistent with previous research results, indicating that core urban are critical to carry out environmental governance and emission reduction strategies (Chen et al., 2017). Reducing the emissions of HDDTs in major cities and commercial circles can effectively reduce the total emissions.
3.3.2 Temporal distribution
We calculate hourly emissions from HDDTs using a bottom-up approach, the pollutant emission trends of different road types are consistent at different time intervals (Cheng et al., 2020). Emissions are highest on suburban roads, followed by urban roads and highways. Since CO, NOx and PM showed similar spatial characteristics, only NOx emission inventory was used in the following studies. The temporal variation of NOx emissions from HDDTs is described in Fig. 8. The 24-hour variation trend of pollutant emissions from HDDTs on roads is obvious in Northeast China. The temporal emission changes on three types of roads presented a "single-peak", which was different from the peak of traffic flow (the morning peak hours are 7:00 ~ 9:00 and the evening peak hours are 17:00 ~ 19:00). The reason for the single peak is that HDDTs are basically used for steel transportation and logistics, where drivers deliberately avoid the morning and evening peaks hours. Additionally, changes in emissions from HDDTs on the road are closely related to the logistics and transportation time and the working hours of residents (Ghaffarpasand et al., 2020). These results show that with a high spatial and temporal resolution emission inventory, it is possible to comprehend in detail the operating patterns of HDDTs at different road types and different time intervals, which can help to devise targeted emission reduction strategic policies.
3.4 Emission reduction scenarios and strategy
Under the baseline scenario, the elasticity coefficient method is used to predict the number of HDDTs in Northeast China will reach 952,300 in 2030 in this study (Cao et al., 2008). Based on the HDDTs activity level data, we can forecast the pollutant emission inventory and the emission reduction of HDDTs under different scenarios in 2030. The reveal that the total emissions of CO, NO, NO2 and PM from HDDTs in Northeast China in 2030 will be 233.6 kt, 720.8 kt, 15.1 kt and 1.2 kt, respectively. Compared to 2020, the emissions of all pollutants have shown an upward trend. This underscores the need to devise scientifically effective emission reduction strategies as the number of HDDTs continues to rise annually.
As showed in Fig. 9, the results from Scenario 1 show that CO, NO, NO2 and PM from HDDTs are reduced by 62.9%, 69.7%, 83.9% and 83.7%, respectively. In scenario 2, where mandatory use of exhaust gas treatment devices is implemented, CO, NO, NO2 and PM emissions from HDDTs are reduced by 79.7%, 80.2%, 65.9% and 67.2%, respectively. The mandatory use of tailpipe treatment device has a greater impact on CO and NO, and the emission reduction of NO has reached 426 kt. When implementing restrictions of HDDTs on certain urban roads, according to Scenario 3, CO, NO, NO2 and PM emissions are reduced by 30.6%, 15.0%, 9.6% and 29.7%, respectively. In contrast with scenario 1 and scenario 2, Scenario 3 appears to be less effective in reducing emissions from HDDTs. In general, all the three emission reduction scenarios can effectively improve the atmospheric environment.
Beijing, Shanghai and Guangzhou cities are pioneers for implementing motor vehicle emission control policies in China. For instance, Beijing has imposed restrictions on the traffic and purchase of motor vehicles (lottery purchase), subsidies for the elimination of old vehicles, and increased parking fees (Cheng et al., 2020; Wu et al., 2011). Especially, time restrictions and regional restrictions are implemented for HDDTs (NMRPEPTTM, 2017). Shanghai implemented restrictions to highly polluting vehicles in certain areas for certain periods of time (Wu et al., 2023). Guangzhou has imposed a 24-hour restriction for high polluting vehicles, and imposed regional and road restrictions on vehicles outside the city (Lee, 2016). These cities mainly formulate emission reduction measures from the perspective of economic development according to the national motor vehicle pollution prevention plan. In light of unique climate conditions of Northeast China, on the basis of localized emission factors and high-resolution emission inventory, this study proposes targeted emission reduction strategies for Northeast China.
(1) The old vehicle elimination policy has to be strictly implemented. The key point is to accelerate the elimination of HDDTs that follow China III standards. (2) Install tailpipe treatment devices in light of product technical standards, such as diesel particulate filter (DPF), selective catalytic reduction device (SCR). (3) A cross-regional HDDTs emission supervision platform is established. The Chinese government should formulate the penalty standards for exceeding the emission standards of motor vehicles across regions. (4) Further restricting the driving of HDDTs in urban areas is very important to ameliorate the living environment of residents. (5) In addition, promoting natural gas HDDTs can also help address pollution from HDDTs. In 2022, the state council released Fourteenth Five Year Comprehensive Work Plan for Energy Conservation and Emission Reduction (State Council of the PRC, 2022), which pointed out that China should deeply implement the clean diesel engine action and encourage the replacement of HDDTs to address pollution from HDDTs. (6) At present, the China VI emission standard has been implemented, which is expected to cut the emission of CO and NOx by 50%. Accordingly, stricter vehicle emission standards can be implemented in advance, which will effectively control pollutant emissions. (7) The priority should be given to the fuel quality. There are still many challenges in commercialization of diesel oil. Particularly, the ethanol content does not meet the standard, and fuel is not purchased through formal channels. Hence, the government should strengthen the supervision of vehicle fuel quality and strictly implement the quality inspection standards of fuel products, in order to prevent the high-polluting vehicle fuel from entering the market.