- Original Paper
- Open Access
Investigation and sensitivity analysis of air pollution caused by road transportation at signalized intersections using IVE model in Iran
European Transport Research Review volume 10, Article number: 7 (2017)
The development of urbanization has had many negative outcomes in different societies. Population growth and the increase in the transportation are the consequences of urban growth which has resulted into problems. Vehicles are responsible for 90% of air pollution in Iran and it is essential to use authentic models of traffic emissions in accordance with the current conditions to predict this and future emissions. Iran has a lot of different air pollution dispersion parameters compared with the developed countries.
In this paper air pollution emission parameters in signalized intersections have been modelled and results have been compared to measured concentrations of air pollution in intersections. For this purpose the use of IVE (International Vehicle Emission) model that is common air pollution modelling and besides, Sensitivity Analysis has been performed to show modeling accuracy in comparison with current emissions.
By modeling and measurement results, it’s easily understood in warm seasons emission concentration is more than cold seasons. Minimum and maximum rate of Carbon-monoxide (CO) and Nitrogen-oxides (NOX) has been evaluated.
IVE model has shown a bit difference amount of pollutions by comparison with field measurement emission. It could be said it is appropriate for model vehicle emission in Iran.
The development of urbanization in different societies has had many negative outcomes. Population growth result increases in the transportation which has resulted into problems. Transportation causes lots of problems contrary to its benefits. Two major problems are noise and air pollution. Noise pollution in airports is more important. Lots of studies on noise pollution reduction of aircraft around airport have been done recently by introducing optimal model to decrease noise pollution and fuel use of commercial aircraft [1,2,3]. Another problems is the air pollution that significantly visible in metropolitan areas. It has been estimated that air pollution is responsible for the death of 3.1 million people in the world every year . Among the sources of air pollution motor vehicles movement in transport network is known as the main cause of air pollution to such an extent that it has obtained a share of 60 to 90% of total emissions . Environmental stress, fuel consumption and transport industry pollutions augment when the traffic flow is stopped and delayed and stop-motion phenomenon occurs frequently. These phenomena are often seen at intersections and road junctions, especially when the traffic signals are used. Thus, the highest concentration of pollutants produced by transportation occurs near the signalized intersections and squares and urban air quality near these areas is of lower condition compared to other areas which is due to the changes in vehicle speed when approaching and moving away from the intersection [6, 7]. In other words, there are high traffic dynamics near signalized intersections.
High dynamics for congestion, i.e. stop-and-go traffic and low dynamics for free-flowing traffic. Traffic with high dynamics has significantly higher emissions than traffic with low dynamics [8, 9]. Also Mustafa et al. (1993) revealed that the traffic signals at intersections generate about 50% more emissions than roundabouts and during heavy traffic, signals lead to larger emissions of HC, almost double of that at roundabouts . Coelho et al. by modelling and laboratory studies displayed that existence of signalized intersections in urban areas increases pollution 15 to 40% .
Road vehicle emissions depend on many parameters like transportation and weather factors. Emission models are used to perform the measurments of road transport emissions . Vafa-Arani et al. (2014) have studied on air pollution in Tehran metropolitan city and have shown two source of air pollution affect the weather of this city. They have examined transportation and industrial pollution effect . Sivacoumar et al. (2001) investigated Jamshedpur region in India by using a mathematical programming method to anticipate the air pollution of this region. He illustrated the portion of NOX concentration from vehicles, domestic and industrial . Hong and Shen (2013) conveyed the residential density on CO2 in comparison with vehicle using emission factors based on vehicle and trip characteristics . Wang et al. (2008) used a system dynamic model to predict air pollution in Dalian. They utilized system dynamic model based on the cause-and-effect analysis and feedback loop structures. Their model comprises 7 sub-models like population, economic development, number of vehicles, environmental influence, travel demand, transport supply, and traffic congestion. They suggest Dalian should have been restricted in quantity of vehicle in aim of reducing air pollution  and Anh (2003) by using a dynamic model showed air pollution is the main result of traffic congestion. He thinks development in public transportation system and road network expanding can help to reduce pollutant .
Defining the problem
The field study of the transportation related projects in the field of air pollution is not completely possible and would not provide satisfactory results because this can be performed by a specific devices or tool such as a portable spectrometer device connected to the exhaust which also create some drawbacks and limitations, such as:
These devices are associated with errors.
Using this device a few parameters such as instantaneous velocity can be measured with limits and it is not possible to consider factors, as well as the entire fleet of transportation.
Preparation and maintenance of these tools is very expensive and not applicable in this country .
Hence it is necessary to use authentic models of traffic emissions in accordance with the current conditions. On the other hand, despite the traffic emissions models used to evalute pollution from mobile sources, no steps have been taken to examine their suitability in the countries’ conditions. In this study IVE model have been chosen because of its ability in covering lots kind of vehicle (about 1372) and estimating emission factors based on spot speed instead of average speed. In this research IVE model is evaluated and validated using field studies and measurements to introduce a reliable and convenient model for its eligibility. In this study the IVE model is evaluated based on the sensitivity to changing parameters. To examine this model all conditions of the intersection of the municipality of Najafabad - Esfahan have been applied. Accordingly, to enter the input parameters needed by the model the data collected on some winter and spring days of 2015 at a certain hour (11 to 12 AM) were used. One of the factors that cause changes in the calculated concentrations of pollutants causing an error in the calculations is wind. That’s why among the days of study the selected days did not vary in terms of wind speed and direction.
Hui et al. (2007) implemented IVE model in Guangzhou China city streets in 2005 and observed correlation coefficients of 0.90 and 0.81 for CO and NOX .
The area under study is a signalized intersection with a fixed schedule both sides of which end to a one-way path. Phasing of this intersection traffic signal is presented in the Fig. 1.
The volume of vehicles
In this study, video recording was used as a tool for counting the vehicles on the road at 11–12 within the mentioned days that the results of which are shown in Appendix Table 5. The counts are multiplied by 10 to be rounded. Also the left turn movements of east-west were 19–22% and right turn movements of north-south were 10–12%.
The average speed of the fleet
According to the software user manual of IVE, the speed of a fleet of vehicles that as an input parameter in this model, is movement speed not travel speed . For this reason, the fleet velocity of this intersection, the stop time of the vehicles caused by the red lights was ignored. These numbers show the dominant speed of the cars 50 m before and after the intersections at two intersecting streets obtained by the distance and time (minus the red lights interval). The results of the average approximate speed of vehicles on research days are presented in Appendix Table 6.
The driving style means the acceleration and vehicle speed during driving. The type of driving is the result of the driving culture and traffic conditions of the area. IVE model, in order to define the style of driving, uses a driving factor related to the type of driving associated with vehicle specific power (VSP) and engine stress. VSP is a function of instantaneous velocity, gradient, vehicle weight, air density and …, IVE uses the Eq. (1) which is valid for light vehicles with good accuracy.
Where, V is the velocity (m/s) a stands for acceleration (m/s2) and grade as the slope of the road. In IVE 60 modes (indices) are considered for specific power and engine stress. Specific power and engine stress parameters related to each of these 60 modes in the model has been shown in Appendix Table 7. By setting these parameters in a period, the percentage of time that the fleet exists in each index is determinded. Engine stress is also visible in minimum and maximum periods in Appendix Table 7 too. According to the instantaneous and average velocity as well as VSP, the engine stress of most vehicles is within (−1.6–3.1) and a very small number of them (some imported cars) has the engine stress of 7 or more.
As previously mentioned among the countries that use this model, China has highest similarity with Iran’s cities driving style. In this study, first the urban cycle of China’s Beijing has been used and it has been applied to the study area by repeatedly driving situations under the intended traffic and recording time, velocity and acceleration within continious and short intervals. Also the condition of those vehicles with right or left turns and not-stop before traffic signal have been included in this cycle. Figure 2 presents the cycle has been defined in the IVE related to this study.
Among lots of meteorological factors, most important ones are air temperature, humidity, wind speed and wind direction . Humidity and temperature are the most important meteorological variable that are effective in engine performance and emissions and have been performed in IVE model for this study. Appendix Table 8 shows the humidity and temperature in the area of this study.
In IVE model, 1372 predefined technologies exist for vehicles and it is possible to define additional capabilities by the user. The classifications are based on engine size, fuel type, fuel system, pollution standards, pollution control of exhaust system, the engine operation and etc. In this study for specifying the fleet technology, by analyzing of field video recorded during this period, all vehicle types have been determined and the technological features have been found by the manufacturer information and other authoritative sources and have been entered into the model. Also to determine the age and kilometers usage of vehicle the information on transport and energy performance of vehicles have been used . Appendix Table 9 shows the fleet technology of this study.
Air conditioning systems
This defined parameter shows a percentage of time in which the fleet uses air conditioner at 27 °C or higher temperature. In designing this model, it is assumed that regardless of the user defined amount of parameters at 15° and below none of the fleet uses the parameter and at a temperature of 32 °C or above the whole fleet having an air conditioning system uses this system. Given the field studies and the fact that this study is done in winter and early spring, this parameter is considered to be 0.
Indicating to reliable sources and studies about the quality of gasoline and diesel in the country and the amount of material in them, corresponding parameters to fuel have been determined and entered into the model. Some of these parameters are as follows:
The overall quality of fuel: Medium
The amount of lead in fuel: zero (negligible)
The amount of sulfur in the fuel: between 50 and 300 ppm
The benzene in the fuel: between 5.0 and 5.1% (average)
The oxygen content of gasoline (fuel additive that increases the oxygen content of fuel and improves pollution): zero
The results of modelling
After entering all the parameters mentioned in the previous section, IVE has been run and the results are presented in Table 1.
The results of field measurement
In this study at the intersection, fixed air pollution monitoring device from “Signal group” is installed by Esfahan Organization of Environmental Protection at the height of 2.5 m ground above that the sensors of which are able to measure and record air pollution at the intersection at any moment. Therefore, in order to perform this study, the data provided by this device have been applied and among the urban emissions, NOX and CO are more than any other emissions which are presented in Table 2.
Data comparison and results evaluation
Environmental pollution measuring devices are able to measure the concentration in ppm, volume percentage or the weight of the pollutants based on a determinded volume of the air (such as g/m3), while IVE model shows the emissions in grams and finally by using the kilometers usage of vehicle and travel time by fleet shows emissions on grams per traveled distance or grams per time unit. However, for comparing the pollutants in both methods of modelling and field data collection, having a similar method of measuring is necessary. To solve this problem data have been converted to unit of the pollutants into g/l using the Eqs. (2) and (3) [4, 17].
To convert the concentration units achieved by the environmental pollution gauge device into g/l the following equations are used:
Where M represents the molar mass (28.1 g/mol for CO, 30 g/mol for NO and 46.01 g/mol for NO2), the fuel density (740 g per liter for gasoline), is the share of fuel carbon (Usually 0.85) and, CCO, CCO2 CHC, CNOX are the concentration of pollutants in terms of percent. These equations are mostly used for spectrometer systems that have a function similar to environmental devices and assuming that the ratio of the pollutants in a mass is preserved that these equations can be used in environmental assessments.
For concentrations derived from modelling the Eq. (4) is used as well:
Where, presents the emission factors in terms of g/km and g/lit and F is the average km per consumed liter of fuel .
By converting and standardization of the concentration of pollutants resulting from the two methods of field data collection and modelling, the difference between this data is compared based on the Tables 3 and 4 and the correlation between the concentrations of the models have been calculated and measured (Figs. 3, 4, 5, and 6) . It can be seen that:
The difference between the concentrations of the average CO pollutants has been 1.2 and 1.6 times for winter and spring repectively.
According to Table 4 this difference for NOX emissions has been 1.7 and 1.9 times for winter and spring respectively.
The differences are for the fact that the measuring device of this study is the environmental gauge device and evaluate the pollutant after withdrawal of supply and decreased with ambient air and it cannot measure it as it comes out of the exhaust. The device used in this study was installed on the edge of the intersection and the pollutants are spreaded and decreased before reaching the sensor. It is observed that the difference in both pollutants is higher in spring, the reason of which might be the wind and increased dispersion of pollutants.
Since the objective of using this model in this research is descriptive analysis and in order to evaluate it, the model sensitivity should be analyzed and the amount of following changes by the model must be estimated. When the results of field measurements differ from different parameters, the model must follow these alterations as well. One of the best ways for this sensitivity analysis is measuring the correlation of the model results and real perceptions. The correlation coefficient is applied to specify the sensitivity of two data series versus each other in face of changes as it presents the severity of relation, correlation and proportionality of the data. As the value approaches 1 it showes higher sensitivity of the data in a direct manner. In this study the correlation between CO concentrations in both methods was 0.86 in spring and 0.88 in winter. Also correlation between NOX concentrations in both methods was 0.84 in spring and 0.85 in winter. The results of the model were connected with the results of measurement and change by converting different parameters with a harmonized, linear trend with high regression coefficient.
The high correlation values for the existing data refers to high levels of proportionality of IVE model calculations with field data collections and indicates that the function of this model is appropriate for the conditions stated in the changes, and present a proper calculation response against changes. As a result, the model is suitable for the assessment and interpretation of emissions behavior in different parameters and it is possible to analyze different transportation policies in these areas before and after implementation in terms of environment and make the best decisions.
In this research, a comprehensive field study has been performed at an intersection of Najafabad city in Isfahan province in winter and spring over a period of 59 days. The terms of the intersection entered IVE model and sensitivity analysis was performed and the results were compared to field measurement data for the NO and NO2 and CO pollutants. The outcomes of the correlation between the concentrations of CO pollutant in both methods were 0.86 in the spring and 0.88 in winter.
Also in intersection suburbs and in low traffic hours with the determined fleet combination, 7.58 g/l CO and 0.63 g/km NOX have been produced. This value equals 7.91 g/l for CO and 0.64 g/km for NOX.
Because of the volume of resulting traffic during study period and considering environmental pollution produced by vehicles, only within 50 m from the intersection, the emission share of 1 h under light traffic conditions at this intersection for CO is minimum 1225 g and maximum 2730 g in winter and minimum 1474 g and maximum 3785 g in spring, for NOX is minimum 97 g and maximum 215 g in winter and minimum 119 g and maximum 247 g in spring. It shows the more pollutant production and propagation in spring in comparison with winter. It suggests to study in summer and autumn also do this research in intersections located in other city by different weather and traffic characteristic. By the result of this study and future study in this field can have a unique emission forecasting in intersection.
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1. Emission factors have been measured in both cold and warm seasons.
2. The fleet technology has been considered in modelling.
3. The results of modelling have been compared with field data.
4. Both transportation and weather related parameters have been considered in model.