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Volume 38, Issue 1 (2023)
GeoRes 2023, 38(1): 27-34 |
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Received: 2022/11/12 | Accepted: 2023/03/1 | Published: 2023/03/6
Received: 2022/11/12 | Accepted: 2023/03/1 | Published: 2023/03/6
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Karimi M, Babaei Semiromi F, Bahmanpour H, Tabesh M, Mohammadi A. Modeling the Emission and Spread of Nitrogen Dioxide in Beyhaqi Transportation Terminal. GeoRes 2023; 38 (1) :27-34
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1- Department of Environmental Management, Faculty of Natural Resources and The Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran
2- Department of Environment, Faculty of Engineering, Shahrood Branch, Islamic Azad University, Shahrood, Iran
2- Department of Environment, Faculty of Engineering, Shahrood Branch, Islamic Azad University, Shahrood, Iran
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Introduction
Air pollution is recognized as the fourth leading risk factor for attributable mortality worldwide and the seventh in Iran. According to investigations conducted by the World Health Organization, more than four million people die prematurely each year due to air pollution [Kermani et al., 2020]. For instance, in Austria, Switzerland, and France, 6% of all deaths among adults over 30 years old have been attributed to air pollution [O’Reilly et al., 2015]. Air pollution is one of the most significant factors affecting human quality of life and exerts detrimental effects on health. These effects cause biochemical and physiological changes in the body, which may eventually lead to severe illness or even death [Arnesano et al., 2016]. According to the World Bank report, the health risks associated with air pollution are highest in developing countries [World Bank, 2015].
Urban development projects can be considered as the foundation for economic, social, political, cultural, and legal restructuring of cities. Their primary goals include improving the processes of urbanization and city development, restoring the urban environment, organizing the urban economy, and strengthening the political, social, and cultural dimensions of urban life. The Organization for Economic Cooperation and Development (OECD) has introduced air quality as the most important environmental indicator in discussions of sustainable development [Rohani et al., 2017].
The metropolis of Tehran, due to its topographical and climatic conditions, as well as the movement of nearly five million vehicles and the presence of numerous large and small industrial units, is one of the eight major cities in Iran where air pollution has become a serious challenge for both citizens and officials [Bahmanpour et al., 2021]. In 2016, Tehran experienced 80 days of unhealthy air for sensitive groups and 9 days of unhealthy air for the general population [Motesaddi et al., 2017]. Statistics indicate that on days of severe air pollution in Tehran, the number of patients with respiratory diseases increases by up to 60% [Tavassoli et al., 2019]. The most significant factors contributing to the aggravation of cardiovascular and respiratory diseases are increases in sulfur dioxide, particulate matter, and carbon monoxide concentrations [Bahmanpour et al., 2021]. On average, air pollution in Tehran has reduced the life expectancy of its residents by approximately five years [Haji Seyed Mirzahoseini et al., 2021].
Studies show that more than 1,192 tons of pollutants are emitted daily into Tehran’s atmosphere. The majority of these emissions consist of sulfur oxides (695 tons per day), followed by nitrogen oxides, carbon monoxide, and unburned hydrocarbons, which constitute the main air pollutants in the city [Mohaghegh & Hajian, 2013; IQ Air, 2018].
Nitrogen dioxide (NO₂) irritates the eyes, throat, and chest and can cause shortness of breath. At higher concentrations, it reduces the body’s resistance to respiratory infections [Tabrizian et al., 2019]. In Tehran, trucks are responsible for approximately 7.5%, 4%, and 3% of total nitrogen oxide (NOx) emissions, respectively [Buga et al., 2019]. These pollutants are mainly produced by the combustion of fossil fuels. Due to their role in acid rain formation, they can contribute to environmental pollution even far from their source areas. In the presence of organic compounds and other air pollutants, nitrogen oxides form highly toxic secondary compounds that adversely affect the respiratory system [Mattiuzzi & Lippi, 2019; Can et al., 2019; Bahmanpour et al., 2013].
One of the main sources of nitrogen oxide emissions is motor vehicles. Some studies have shown that passenger terminals can be significant sources of air and noise pollution [Mansoori & Ghasemabadi, 2011]. Research conducted on the emission levels of air pollutants in Tehran’s terminals indicates that the highest concentration of carbon monoxide occurs in autumn, while the lowest levels are observed in spring [Asl et al., 2018].
To date, no comprehensive study has been conducted on the role and significance of passenger terminals in urban air quality. However, a number of scattered studies have investigated specific air pollutants or certain public transport stations. Among the most recent of these are the following: Haji Seyed Mirzahoseini et al. have evaluated gaseous and particulate pollutants in six urban bus terminals operated by the Tehran Bus Company. Their results showed that the highest concentrations of suspended particles occurred in winter, with PM₁ averaging 20.1 µg/m³, PM₂.₅ averaging 72.6 µg/m³, PM₁₀ averaging 100.23 µg/m³, CO averaging 6.95 ppm, NO₂ averaging 0.05 ppm, and SO₂ averaging 0.05 ppm. Temperature inversion, unsuitable terminal locations, and cold weather were identified as the main causes of pollutant accumulation [Haji Seyed Mirzahoseini et al., 2021].
Ramezani and Shabankhoo have developed a management model for mitigating the environmental impacts of passenger terminals. Their findings indicate that factors such as bus type and quality, appropriate terminal location, efficient and regular service, and avoiding unnecessary engine idling can improve environmental quality [Ramezani & Shabankhoo, 2013]. Rahimi et al. have used a Markov chain model to examine the persistence of air pollutant days. Their results show that the highest persistence probabilities occurred at the Fatemi, Bazaar, and Aghdasiyeh monitoring stations. Throughout most of the year, the Fatemi station exhibits the highest probability of two-day pollutant persistence, whereas the Shahr-e Rey station shows the lowest [Rahimi et al., 2013]. Bahmanpour et al. have evaluated the environmental risk of air pollutants in Tehran’s outdoor recreational and sports spaces. They have found that the cleanest air was recorded at the Aghdasiyeh station (77 days), while the lowest is observed at the Pirouzi station (7 days). Approximately 30% of the year’s days fell under the categories of “unhealthy” or “unhealthy for sensitive groups.” The hourly and daily mean concentrations of carbon monoxide at all stations were below the recommended limits, indicating that none of the studied sports complexes were at risk from hourly CO exposure [Bahmanpour et al., 2021].
Given the information obtained from relevant organizations and experts showing that Tehran’s air quality is generally poor and occasionally reaches hazardous levels, it is necessary to examine the contribution of different urban land uses to these conditions. One of the key land uses in this regard is passenger terminals, which accommodate various types of public transportation vehicles. Unfortunately, no extensive studies have yet been conducted on the environmental quality (especially air quality) of the Beyhaghi Passenger Terminal in Tehran. Therefore, the objective of the present study was to model the emission and dispersion of nitrogen dioxide (NO₂) in the Beyhaghi Passenger Terminal in Tehran.
Methodology
This cross-sectional field study was conducted at the Beyhaghi Passenger Terminal in Tehran over a period extending from spring to winter 2021. The terminal, constructed in 1991, covers an area of approximately 13.5 hectares. Three locations within the terminal (the taxi, van, and urban bus station; the intercity bus platforms; and the parking lot) were selected as internal sampling sites, while one location outside the terminal was designated as a control point.
Sampling was performed monthly throughout the year, for three consecutive days each month, and three times a day (morning, noon, and evening). The required instruments included a sampling pump with a flow rate of up to 5 L/min (Quick Take 30-2010; Taiwan), an impinger, a rotameter (Rotameter-Jorg Fischer; Switzerland), and gas detector tubes. The accuracy and performance of all instruments were verified and calibrated in the laboratory prior to field deployment.
Nitrogen dioxide sampling was conducted according to NIOSH Method 6004 and the Saltzman method, using an impinger and Saltzman absorbing solution at a flow rate of 0.2 L/min. The concentration of NO₂ was subsequently determined using a spectrophotometer [The National Institute for Occupational Safety and Health, 2014]. The obtained data were compared with the standards of the World Health Organization [WHO, 2019] and the United States Environmental Protection Agency [Shen et al., 2017].
To estimate the emissions resulting from bus operations at the terminal, it was necessary to compile an emission inventory for each vehicle type and determine their average parking durations within a 24-hour period. For this purpose, data were obtained from the Bus Fleet Management System of the Tehran Municipality, which has been operational since January 2014. The estimated daily and annual emissions of nitrogen dioxide were calculated for different vehicle categories, including diesel and CNG-fueled buses, vans, minibuses, and taxis.
According to official data, approximately 300 intercity buses, 100 urban buses, 40 vans and minibuses, and 100 passenger cars operate daily within the Beyhaghi Terminal. Based on the emission rates of each vehicle type, the total pollutant emissions were calculated accordingly. The average parking duration of buses in the terminal was 160 hours, with the peak period occurring between 9:00 a.m. and 2:00 p.m. [IQ Air, 2018]. Considering that previous studies have estimated up to 70% emission reduction through the implementation of management strategies [TTPO, 2014], an alternative scenario with reduced emissions was also analyzed.
For the simulation and modeling of NO₂ dispersion within the Beyhaghi Passenger Terminal, Austal View 7 software was used. This program serves as the graphical interface of Austal 2000, the air dispersion modeling system developed by the German Environment Agency. It applies a Lagrangian particle-tracking approach for atmospheric dispersion analysis and incorporates an integrated wind-field diagnostic module [Rovira et al., 2020]. Pollutant concentrations were simulated at a height of 5 meters above ground level—slightly above the average human breathing zone. The modeling domain covered a circular area with a one-kilometer radius centered on the terminal site
Findings
The concentration of nitrogen dioxide (NO₂) during autumn and winter was notably higher than in the first six months of the year. According to the comparative analysis of seasonal concentrations, the measured values in these colder months frequently exceeded the air quality standards set by the World Health Organization (40 ppb) and the U.S. National Ambient Air Quality Standards (53 ppb).
Based on the baseline data and statistical analysis of the emission inventory, two modeling scenarios were developed. The one-hour dispersion of NO₂ was initially high but gradually decreased due to meteorological and climatic factors, as well as the chemical transformation of the pollutant—particularly toward the western direction. Within approximately 300 meters from the emission sources in the southern, northern, and western directions, the NO₂ concentration declined significantly.
Under the first scenario, the total NO₂ emissions were estimated at about 108,575 kilograms, while under the second (optimized) scenario, emissions decreased to approximately 32,573 kilograms. In the existing-condition scenario, the pollutant concentration declined much more rapidly toward the west, with the main plume extending predominantly eastward.
In the second scenario, the overall polluted area was noticeably smaller than in the first. Except for the eastern direction, NO₂ concentrations dropped sharply in all directions, indicating the effectiveness of emission-reduction measures
Discussion
The main objective of this study was to examine the emission and dispersion of nitrogen dioxide (NO₂) within the Beyhaghi Passenger Terminal in Tehran. To this end, two scenarios were considered: the first represented the current operational conditions, while the second reflected an optimized state incorporating managerial and engineering strategies aimed at reducing pollutant emissions.
The findings revealed that NO₂ concentrations were higher during autumn and winter compared to the first half of the year. These results are consistent with those reported by Bahmanpour et al. (2021) and Haji Seyed Mirzahosseini et al. (2021). Several factors may explain this pattern, including increased fossil fuel consumption during the colder months, reduced urban vegetation cover in Tehran (due to deciduous trees), and the greater persistence of pollutants in the atmosphere under cold and stable meteorological conditions. Similar explanations have also been reported by Eskani and Siah Pirani (2011), Mirzadeh Tabatabaei et al. (2022), and Asadzadeh et al. (2022).
The emission inventory analysis indicated that vans, taxis, diesel buses, and CNG buses, respectively, contributed the most to NO₂ emissions over a one-year period. These results are in line with those of Ramezi and Shabankhoo (2013) and Ahmadzadeh et al. (2023). According to Ferdowsi et al. (2018), the NO₂ emission factor is not strongly dependent on the quality of fuel used in buses; rather, engine technology and the use of emission-control systems such as diesel particulate filters and catalytic converters play a more significant role, a conclusion also supported by Wang et al. (2014) and Can et al. (2019).
One key reason for the relatively lower NO₂ emissions from CNG-powered buses is the cleaner nature of natural gas compared to gasoline and diesel fuels. CNG combustion produces substantially fewer pollutants and eliminates evaporative emissions from the fuel system. Hence, substituting conventional fossil fuels such as gasoline and diesel with natural gas can significantly reduce air pollution. As Esmailian et al. (2022) have reported, natural gas has a lower carbon content per unit of generated energy, resulting in reduced CO₂ and greenhouse gas emissions. Furthermore, studies show that CNG contains no lead, sulfur oxides, or particulate matter, and generates considerably lower levels of toxic compounds such as benzene, 1,3-butadiene, aldehydes, and polycyclic aromatic hydrocarbons (PAHs) compared to gasoline and diesel (Azizi, 2007). Owing to its lighter molecular structure, natural gas produces fewer unburned hydrocarbons, and because it does not rely on fuel vaporization during operation, it avoids evaporative losses from the tank or carburetor. Additionally, since it does not require a rich fuel mixture for cold starts, CNG emits very few pollutants during engine ignition. Its higher ignition temperature (649 °C compared with 315 °C for gasoline) also provides better safety performance. Finally, the high octane rating of CNG (130 versus 78 for gasoline) contributes to cleaner combustion and reduced emissions, as also confirmed by Tahmasabi and Razavi Nasab (2019).
Modeling results further demonstrated that in both scenarios, NO₂ concentrations were initially high but declined as a result of meteorological dispersion and chemical transformation, particularly toward the western direction. Within a distance of 300 meters to the south, north, and west, the pollutant levels dropped markedly, most sharply in the west, while the pollution plume extended mainly eastward. Because Iran’s national clean air standards do not specify a one-hour permissible limit for NO₂, two international reference standards were used for comparison. According to the U.S. National Ambient Air Quality Standards (NAAQS), the one-hour limit is 100 ppb, and according to the World Health Organization (WHO, 2019), it is 200 ppb. In both modeling scenarios, the estimated NO₂ concentrations were below these thresholds. The pollution zone under the second scenario was smaller than that in the first, and except for the eastern direction, NO₂ concentrations decreased rapidly in all directions.
Comparative analysis of the modeled concentrations confirmed that NO₂ levels in Scenario 1 were consistently higher than in Scenario 2, although both remained below the permissible one-hour limits set by NAAQS and WHO.
Although this study did not explicitly account for certain meteorological and synergistic factors such as temperature inversions, the interaction of multiple pollutants, or stagnant wind conditions. It is recommended that effective management and engineering measures be implemented. These include preventing prolonged idling of engines, using exhaust filters, and reducing parking durations to minimize emissions, potentially by up to 70%. Such actions would significantly improve air quality within and around the terminal area. Moreover, adopting complementary strategies such as vehicle engine optimization and the use of higher-grade fuels could further enhance pollutant reduction and environmental sustainability.
Conclusion
Implementation of the pollutant-reduction scenario would reduce nitrogen dioxide (NO₂) concentrations within a one-kilometer radius of the Beyhaghi Passenger Terminal to levels below the permissible limits.
Acknowledgment: The authors express their sincere gratitude to all experts who contributed to the completion of this research and the preparation of the manuscript.
Ethical Permission: No ethical issues were reported by the authors.
Conflict of Interest: The authors declare no conflicts of interest.
Authors’ Contributions: Karimi M (First Author), Principal Researcher (30%); Babaei Semiromi F (Second Author), Discussion Writer (20%); Bahmanpour H (Third Author), Statistical Analyst (30%); Tabesh MR (Fourth Author), Introduction Writer (10%); Mohammadi A (Fifth Author), Data Analyst (10%).
Funding: This article is extracted from a doctoral dissertation entitled “Developing an Environmental Management Model for Urban Passenger Terminals with a Smartization Approach”, conducted at the Islamic Azad University, Science and Research Branch, Tehran. All related expenses were fully funded by the doctoral student
Air pollution is recognized as the fourth leading risk factor for attributable mortality worldwide and the seventh in Iran. According to investigations conducted by the World Health Organization, more than four million people die prematurely each year due to air pollution [Kermani et al., 2020]. For instance, in Austria, Switzerland, and France, 6% of all deaths among adults over 30 years old have been attributed to air pollution [O’Reilly et al., 2015]. Air pollution is one of the most significant factors affecting human quality of life and exerts detrimental effects on health. These effects cause biochemical and physiological changes in the body, which may eventually lead to severe illness or even death [Arnesano et al., 2016]. According to the World Bank report, the health risks associated with air pollution are highest in developing countries [World Bank, 2015].
Urban development projects can be considered as the foundation for economic, social, political, cultural, and legal restructuring of cities. Their primary goals include improving the processes of urbanization and city development, restoring the urban environment, organizing the urban economy, and strengthening the political, social, and cultural dimensions of urban life. The Organization for Economic Cooperation and Development (OECD) has introduced air quality as the most important environmental indicator in discussions of sustainable development [Rohani et al., 2017].
The metropolis of Tehran, due to its topographical and climatic conditions, as well as the movement of nearly five million vehicles and the presence of numerous large and small industrial units, is one of the eight major cities in Iran where air pollution has become a serious challenge for both citizens and officials [Bahmanpour et al., 2021]. In 2016, Tehran experienced 80 days of unhealthy air for sensitive groups and 9 days of unhealthy air for the general population [Motesaddi et al., 2017]. Statistics indicate that on days of severe air pollution in Tehran, the number of patients with respiratory diseases increases by up to 60% [Tavassoli et al., 2019]. The most significant factors contributing to the aggravation of cardiovascular and respiratory diseases are increases in sulfur dioxide, particulate matter, and carbon monoxide concentrations [Bahmanpour et al., 2021]. On average, air pollution in Tehran has reduced the life expectancy of its residents by approximately five years [Haji Seyed Mirzahoseini et al., 2021].
Studies show that more than 1,192 tons of pollutants are emitted daily into Tehran’s atmosphere. The majority of these emissions consist of sulfur oxides (695 tons per day), followed by nitrogen oxides, carbon monoxide, and unburned hydrocarbons, which constitute the main air pollutants in the city [Mohaghegh & Hajian, 2013; IQ Air, 2018].
Nitrogen dioxide (NO₂) irritates the eyes, throat, and chest and can cause shortness of breath. At higher concentrations, it reduces the body’s resistance to respiratory infections [Tabrizian et al., 2019]. In Tehran, trucks are responsible for approximately 7.5%, 4%, and 3% of total nitrogen oxide (NOx) emissions, respectively [Buga et al., 2019]. These pollutants are mainly produced by the combustion of fossil fuels. Due to their role in acid rain formation, they can contribute to environmental pollution even far from their source areas. In the presence of organic compounds and other air pollutants, nitrogen oxides form highly toxic secondary compounds that adversely affect the respiratory system [Mattiuzzi & Lippi, 2019; Can et al., 2019; Bahmanpour et al., 2013].
One of the main sources of nitrogen oxide emissions is motor vehicles. Some studies have shown that passenger terminals can be significant sources of air and noise pollution [Mansoori & Ghasemabadi, 2011]. Research conducted on the emission levels of air pollutants in Tehran’s terminals indicates that the highest concentration of carbon monoxide occurs in autumn, while the lowest levels are observed in spring [Asl et al., 2018].
To date, no comprehensive study has been conducted on the role and significance of passenger terminals in urban air quality. However, a number of scattered studies have investigated specific air pollutants or certain public transport stations. Among the most recent of these are the following: Haji Seyed Mirzahoseini et al. have evaluated gaseous and particulate pollutants in six urban bus terminals operated by the Tehran Bus Company. Their results showed that the highest concentrations of suspended particles occurred in winter, with PM₁ averaging 20.1 µg/m³, PM₂.₅ averaging 72.6 µg/m³, PM₁₀ averaging 100.23 µg/m³, CO averaging 6.95 ppm, NO₂ averaging 0.05 ppm, and SO₂ averaging 0.05 ppm. Temperature inversion, unsuitable terminal locations, and cold weather were identified as the main causes of pollutant accumulation [Haji Seyed Mirzahoseini et al., 2021].
Ramezani and Shabankhoo have developed a management model for mitigating the environmental impacts of passenger terminals. Their findings indicate that factors such as bus type and quality, appropriate terminal location, efficient and regular service, and avoiding unnecessary engine idling can improve environmental quality [Ramezani & Shabankhoo, 2013]. Rahimi et al. have used a Markov chain model to examine the persistence of air pollutant days. Their results show that the highest persistence probabilities occurred at the Fatemi, Bazaar, and Aghdasiyeh monitoring stations. Throughout most of the year, the Fatemi station exhibits the highest probability of two-day pollutant persistence, whereas the Shahr-e Rey station shows the lowest [Rahimi et al., 2013]. Bahmanpour et al. have evaluated the environmental risk of air pollutants in Tehran’s outdoor recreational and sports spaces. They have found that the cleanest air was recorded at the Aghdasiyeh station (77 days), while the lowest is observed at the Pirouzi station (7 days). Approximately 30% of the year’s days fell under the categories of “unhealthy” or “unhealthy for sensitive groups.” The hourly and daily mean concentrations of carbon monoxide at all stations were below the recommended limits, indicating that none of the studied sports complexes were at risk from hourly CO exposure [Bahmanpour et al., 2021].
Given the information obtained from relevant organizations and experts showing that Tehran’s air quality is generally poor and occasionally reaches hazardous levels, it is necessary to examine the contribution of different urban land uses to these conditions. One of the key land uses in this regard is passenger terminals, which accommodate various types of public transportation vehicles. Unfortunately, no extensive studies have yet been conducted on the environmental quality (especially air quality) of the Beyhaghi Passenger Terminal in Tehran. Therefore, the objective of the present study was to model the emission and dispersion of nitrogen dioxide (NO₂) in the Beyhaghi Passenger Terminal in Tehran.
Methodology
This cross-sectional field study was conducted at the Beyhaghi Passenger Terminal in Tehran over a period extending from spring to winter 2021. The terminal, constructed in 1991, covers an area of approximately 13.5 hectares. Three locations within the terminal (the taxi, van, and urban bus station; the intercity bus platforms; and the parking lot) were selected as internal sampling sites, while one location outside the terminal was designated as a control point.
Sampling was performed monthly throughout the year, for three consecutive days each month, and three times a day (morning, noon, and evening). The required instruments included a sampling pump with a flow rate of up to 5 L/min (Quick Take 30-2010; Taiwan), an impinger, a rotameter (Rotameter-Jorg Fischer; Switzerland), and gas detector tubes. The accuracy and performance of all instruments were verified and calibrated in the laboratory prior to field deployment.
Nitrogen dioxide sampling was conducted according to NIOSH Method 6004 and the Saltzman method, using an impinger and Saltzman absorbing solution at a flow rate of 0.2 L/min. The concentration of NO₂ was subsequently determined using a spectrophotometer [The National Institute for Occupational Safety and Health, 2014]. The obtained data were compared with the standards of the World Health Organization [WHO, 2019] and the United States Environmental Protection Agency [Shen et al., 2017].
To estimate the emissions resulting from bus operations at the terminal, it was necessary to compile an emission inventory for each vehicle type and determine their average parking durations within a 24-hour period. For this purpose, data were obtained from the Bus Fleet Management System of the Tehran Municipality, which has been operational since January 2014. The estimated daily and annual emissions of nitrogen dioxide were calculated for different vehicle categories, including diesel and CNG-fueled buses, vans, minibuses, and taxis.
According to official data, approximately 300 intercity buses, 100 urban buses, 40 vans and minibuses, and 100 passenger cars operate daily within the Beyhaghi Terminal. Based on the emission rates of each vehicle type, the total pollutant emissions were calculated accordingly. The average parking duration of buses in the terminal was 160 hours, with the peak period occurring between 9:00 a.m. and 2:00 p.m. [IQ Air, 2018]. Considering that previous studies have estimated up to 70% emission reduction through the implementation of management strategies [TTPO, 2014], an alternative scenario with reduced emissions was also analyzed.
For the simulation and modeling of NO₂ dispersion within the Beyhaghi Passenger Terminal, Austal View 7 software was used. This program serves as the graphical interface of Austal 2000, the air dispersion modeling system developed by the German Environment Agency. It applies a Lagrangian particle-tracking approach for atmospheric dispersion analysis and incorporates an integrated wind-field diagnostic module [Rovira et al., 2020]. Pollutant concentrations were simulated at a height of 5 meters above ground level—slightly above the average human breathing zone. The modeling domain covered a circular area with a one-kilometer radius centered on the terminal site
Findings
The concentration of nitrogen dioxide (NO₂) during autumn and winter was notably higher than in the first six months of the year. According to the comparative analysis of seasonal concentrations, the measured values in these colder months frequently exceeded the air quality standards set by the World Health Organization (40 ppb) and the U.S. National Ambient Air Quality Standards (53 ppb).
Based on the baseline data and statistical analysis of the emission inventory, two modeling scenarios were developed. The one-hour dispersion of NO₂ was initially high but gradually decreased due to meteorological and climatic factors, as well as the chemical transformation of the pollutant—particularly toward the western direction. Within approximately 300 meters from the emission sources in the southern, northern, and western directions, the NO₂ concentration declined significantly.
Under the first scenario, the total NO₂ emissions were estimated at about 108,575 kilograms, while under the second (optimized) scenario, emissions decreased to approximately 32,573 kilograms. In the existing-condition scenario, the pollutant concentration declined much more rapidly toward the west, with the main plume extending predominantly eastward.
In the second scenario, the overall polluted area was noticeably smaller than in the first. Except for the eastern direction, NO₂ concentrations dropped sharply in all directions, indicating the effectiveness of emission-reduction measures
Discussion
The main objective of this study was to examine the emission and dispersion of nitrogen dioxide (NO₂) within the Beyhaghi Passenger Terminal in Tehran. To this end, two scenarios were considered: the first represented the current operational conditions, while the second reflected an optimized state incorporating managerial and engineering strategies aimed at reducing pollutant emissions.
The findings revealed that NO₂ concentrations were higher during autumn and winter compared to the first half of the year. These results are consistent with those reported by Bahmanpour et al. (2021) and Haji Seyed Mirzahosseini et al. (2021). Several factors may explain this pattern, including increased fossil fuel consumption during the colder months, reduced urban vegetation cover in Tehran (due to deciduous trees), and the greater persistence of pollutants in the atmosphere under cold and stable meteorological conditions. Similar explanations have also been reported by Eskani and Siah Pirani (2011), Mirzadeh Tabatabaei et al. (2022), and Asadzadeh et al. (2022).
The emission inventory analysis indicated that vans, taxis, diesel buses, and CNG buses, respectively, contributed the most to NO₂ emissions over a one-year period. These results are in line with those of Ramezi and Shabankhoo (2013) and Ahmadzadeh et al. (2023). According to Ferdowsi et al. (2018), the NO₂ emission factor is not strongly dependent on the quality of fuel used in buses; rather, engine technology and the use of emission-control systems such as diesel particulate filters and catalytic converters play a more significant role, a conclusion also supported by Wang et al. (2014) and Can et al. (2019).
One key reason for the relatively lower NO₂ emissions from CNG-powered buses is the cleaner nature of natural gas compared to gasoline and diesel fuels. CNG combustion produces substantially fewer pollutants and eliminates evaporative emissions from the fuel system. Hence, substituting conventional fossil fuels such as gasoline and diesel with natural gas can significantly reduce air pollution. As Esmailian et al. (2022) have reported, natural gas has a lower carbon content per unit of generated energy, resulting in reduced CO₂ and greenhouse gas emissions. Furthermore, studies show that CNG contains no lead, sulfur oxides, or particulate matter, and generates considerably lower levels of toxic compounds such as benzene, 1,3-butadiene, aldehydes, and polycyclic aromatic hydrocarbons (PAHs) compared to gasoline and diesel (Azizi, 2007). Owing to its lighter molecular structure, natural gas produces fewer unburned hydrocarbons, and because it does not rely on fuel vaporization during operation, it avoids evaporative losses from the tank or carburetor. Additionally, since it does not require a rich fuel mixture for cold starts, CNG emits very few pollutants during engine ignition. Its higher ignition temperature (649 °C compared with 315 °C for gasoline) also provides better safety performance. Finally, the high octane rating of CNG (130 versus 78 for gasoline) contributes to cleaner combustion and reduced emissions, as also confirmed by Tahmasabi and Razavi Nasab (2019).
Modeling results further demonstrated that in both scenarios, NO₂ concentrations were initially high but declined as a result of meteorological dispersion and chemical transformation, particularly toward the western direction. Within a distance of 300 meters to the south, north, and west, the pollutant levels dropped markedly, most sharply in the west, while the pollution plume extended mainly eastward. Because Iran’s national clean air standards do not specify a one-hour permissible limit for NO₂, two international reference standards were used for comparison. According to the U.S. National Ambient Air Quality Standards (NAAQS), the one-hour limit is 100 ppb, and according to the World Health Organization (WHO, 2019), it is 200 ppb. In both modeling scenarios, the estimated NO₂ concentrations were below these thresholds. The pollution zone under the second scenario was smaller than that in the first, and except for the eastern direction, NO₂ concentrations decreased rapidly in all directions.
Comparative analysis of the modeled concentrations confirmed that NO₂ levels in Scenario 1 were consistently higher than in Scenario 2, although both remained below the permissible one-hour limits set by NAAQS and WHO.
Although this study did not explicitly account for certain meteorological and synergistic factors such as temperature inversions, the interaction of multiple pollutants, or stagnant wind conditions. It is recommended that effective management and engineering measures be implemented. These include preventing prolonged idling of engines, using exhaust filters, and reducing parking durations to minimize emissions, potentially by up to 70%. Such actions would significantly improve air quality within and around the terminal area. Moreover, adopting complementary strategies such as vehicle engine optimization and the use of higher-grade fuels could further enhance pollutant reduction and environmental sustainability.
Conclusion
Implementation of the pollutant-reduction scenario would reduce nitrogen dioxide (NO₂) concentrations within a one-kilometer radius of the Beyhaghi Passenger Terminal to levels below the permissible limits.
Acknowledgment: The authors express their sincere gratitude to all experts who contributed to the completion of this research and the preparation of the manuscript.
Ethical Permission: No ethical issues were reported by the authors.
Conflict of Interest: The authors declare no conflicts of interest.
Authors’ Contributions: Karimi M (First Author), Principal Researcher (30%); Babaei Semiromi F (Second Author), Discussion Writer (20%); Bahmanpour H (Third Author), Statistical Analyst (30%); Tabesh MR (Fourth Author), Introduction Writer (10%); Mohammadi A (Fifth Author), Data Analyst (10%).
Funding: This article is extracted from a doctoral dissertation entitled “Developing an Environmental Management Model for Urban Passenger Terminals with a Smartization Approach”, conducted at the Islamic Azad University, Science and Research Branch, Tehran. All related expenses were fully funded by the doctoral student
Keywords:
References
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