1. Introduction
In recent years, with the rapid development of social economy and urbanization, as well as the continuous improvement of people’s living standard and travel demand, the number of vehicles in China has shown an explosive growth [
1,
2]. The huge number of vehicles and their high activity level have led to the increasing contribution rate of vehicle pollutant emissions to haze pollution represented by a high fine particle (PM
2.5) concentration, and photochemical pollution represented by high ozone (O
3) concentration, especially in urban areas [
3,
4]. In addition, because most of the vehicles are driven in densely populated areas, the impact of the emissions of CO, HC, NO
x (a general term for all types of nitrogen oxides such as NO and NO
2), PM and other pollutants on people’s health is more direct and serious [
5].
Facing the increasingly severe situation of vehicular pollution, China has taken a series of measures and achieved great success, including conformity inspection of new vehicle production, environmental protection inspection of in-use vehicles, elimination of old vehicles, accelerated implementation of higher emission standards, promotion of clean-energy vehicles, etc. [
6]. However, vehicle emission control is a relatively complex decision-making process, which needs a series of technical methods as support. Among these supporting means, vehicle source emission characterization is one of the most core basic contents [
7].
At present, for the estimation of vehicle exhaust emissions, Chinese researchers mainly draw lessons from and refer to mature models developed by the US and Europe, such as the MOBILE model [
8] and the MOVES model [
9], developed by the US Environmental Protection Agency, and the COPERT model [
10], developed by the Joint Research Centre of the European Commission. Most of these models are developed on the basis of vehicle emission in the US and Europe [
11]. However, there are great differences between China’s domestic vehicle types, real road driving characteristics, fuel quality, etc. and those of foreign countries [
12]. Therefore, when the above models are applied in China, a large amount of localization-correction work needs to be carried out, and their applicability needs to be evaluated [
13].
In order to guide the domestic vehicle pollutant accounting and emission inventory preparation, China’s Ministry of Ecological Environment issued the Technical Guide for the Compilation of Air Pollutant Emission Inventory of Road Vehicles (Trial) in 2015, which filled in the gap of China’s vehicle emission accounting basis from the official perspective [
14]. However, because the data in the Guide was based on the national statistical average, it was more suitable for supporting the preparation of road vehicle emission inventory at the macro scales of cities, urban agglomerations and regions. However, it was very limited at the meso- and micro- scale and for refined vehicle emission simulation, and it was also difficult for the Guide to reflect the impact of actual road driving conditions on vehicle emissions.
In the latest research on the vehicle emission model, the driving conditions closely related to vehicle emission level are basically taken into account [
15]. Compared with the macro emission model, the emission model based on driving conditions is able to analyze the impact of the real road driving state (such as uniform speed, acceleration, deceleration, idle speed, etc.) of vehicles on their emissions more comprehensively [
16]. The driving-conditions-based mode emission model can more comprehensively consider the impact of vehicle driving characteristics on emissions, and its “substitute parameters” are more representative [
17]. The modeling idea of the emission model based on driving conditions is to take the mathematical law embodied between the measured emission data and the “substitute parameters” as the core, and then use a mathematical means such as statistical regression to fit the mathematical function closest to the law [
18]. In order to more accurately express the relationship, the emission model based on driving conditions will also rely on the physical relationship between vehicle emission and “substitute parameters” when establishing the mathematical function [
19]. Furthermore, in recent years, with the continuous maturity of real road vehicle test methods and the rapid development of portable exhaust measurement technology, especially the improvement of measurement accuracy, so that it becomes more and more feasible to monitor the micro and transient emission characteristics of vehicles under real road driving conditions and to develop local emission models accordingly [
19].
This study focuses on Tianjin, China, which has great pressure on vehicle emission reduction, as the research object [
6]. Based on the demand of vehicle emission research and control, this paper selected the typical vehicles to carry out on-board emission tests on the local representative roads, developed the vehicle emission model based on the real-road driving conditions by taking vehicle specific power (VSP) as the “surrogate variables”, and completed the calculation and validation of emission factors. The research results were of great significance for the establishment of regional high spatial-temporal resolution and the refinement management of vehicle emission.
4. Conclusions
Based on the demand of vehicle emission research and control, this paper selected the typical vehicles to carry out on-board emission testing on the local representative roads in Tianjin, and then developed the vehicle emission model based on real road driving conditions by taking VSP as the “surrogate variables”, and completed the calculation and validation of emission factors. The research results were of great significance for the establishment of regional high-resolution inventory and the refinement management of vehicle emission.
In addition, through this study, it could be found that due to the great differences in traffic development modes and vehicle driving conditions in different cities in China, the emission model based on driving conditions was a better choice to carry out the research on vehicle emission in Chinese cities. On the other hand, it should be noted that when using the existing international driving-conditions-based models (such as IVE, MOVES, etc.) to directly simulate China’s vehicle emissions, the technical differences and maintenance status differences between Chinese and foreign vehicles would introduce certain errors. This part of the error could be reduced by carrying out field test based on PEMS and correcting the international model emission factor database. In this sense, the establishment of China’s local working condition emission model can more accurately study the vehicle emission characteristics of Chinese cities. From this perspective, the establishment of an emission model based on local driving conditions could more accurately study the vehicle emission characteristics of Chinese cities.
However, it should also be clear that, like all emission models, the accuracy of emission models based on driving conditions would also be affected by the systematic error of test instruments and the representativeness of test samples. For the mature model based on a large number of test samples, these effects were far less than the emission model with a small number of samples. Therefore, the international mature models had obvious advantages in test samples and should be fully utilized. In addition, due to the rapid development of China’s urban traffic and the rapid change of driving conditions, it was of great significance to regularly update China’s urban conditions to improve the accuracy of the model, no matter which model was chosen.