Introduction

In recent years, air pollution has become a serious problem in Iran, which can have various consequences in the social, cultural, economic, and health dimensions. In a recent report by the Research Center of the Islamic Consultative Assembly, the damage caused by air pollution has been estimated at nearly 12 billion dollars, which, when including medical costs, reaches 23 billion dollars (5 percent of GDP). Part of the air pollution problem can be attributed to the activities of factories and industries, which, due to inappropriate location, proximity to residential areas, and lack of air pollution monitoring and control technology, exacerbate this environmental issue.
The asphalt production industry is one of the important industries in Iran. Due to the high concentration of pollutants emitted from stacks and its proximity to residential areas, it requires evaluation and study in the field of air pollution. The construction of urban roads, as well as their repair and maintenance, often requires the use of hot asphalt concrete technology. However, this technology involves drying non-metallic materials such as sand, gravel, and mineral powder, as well as heating bitumen. Surface emission sources in asphalt production include trenches, conveyors, conveyor belts, and roads within the plant. Point emission sources from the process also include stacks, boilers, combustion from the incineration system, and the asphalt plant stack. Approximately 300 MJ energy are consumed per ton of asphalt produced, and 28.8 kg of carbon dioxide is released into the atmosphere. Particles and gases produced in plant activities are mainly associated with screening, rotary kiln storage tank operations, mixers, plant site pollutants, and product loading.
With this introduction, the present study attempts to evaluate the impact of the activities of an asphalt production industry in Khorramdareh City, Zanjan Province, on air pollution. The concentration of exhaust from the stack, measurement of the level of PMs in the industrial environment, and modelling of emissions are among the most important goals of this study.
Materials and methods
The Omran Khaseh asphalt plant is situated 8 km from Khorramdareh city and 4 km from Hidaj city. It is 1,100 meters from the village of Palas, 700 meters from the surrounding gardens, and 2 km from the village of Alvand.
To measure the pollutants emitted from the stack, two instruments were utilized: the OPTHMA7 MRU and the Vastech ISO 9096. The OPTHMA7 MRU, equipped with an internal gas analyzer, employs an electrochemical method to assess exhaust gases. This device measured the concentrations of SO2, NO2, CO, and NO gases emitted from the stack.
The Vastech was employed to quantify the amount of total suspended particles (TSP) released from the stack. The concentration of these particles was determined using the gravimetric method in accordance with the ISO 9096 standard.
Additionally, an Aeroqual Series 500 device was used to measure particulate matter concentrations of sizes 2.5 and 10 micrometres (PM2.5 & PM10) at the industrial site. Measurements were taken at various locations, considering the presence of personnel and workers. Sampling height was set at 1.5 meters above ground (breathing height) to evaluate individuals' exposure to particulate matter in the workplace. Alongside pollutant concentrations, stack characteristics were also documented for modelling purposes. A modelling process was implemented to analyze the dispersion patterns of air pollutants. For this analysis, the SCREEN3 model, approved by the US Environmental Protection Agency (EPA), was utilized.

Results and discussion
Based on the model outputs, there is no apparent difference in appearance between the emission patterns of different pollutants, different stability classes, or the point of occurrence of maximum concentration. For all pollutants studied, the point of occurrence of maximum concentration for stability class A occurs at the closest distance from the stack (about 281 meters), and for subsequent stability classes (B, C, D, E, and F, respectively), it occurs at further distances from the stack.
The model of the distribution and dispersion of ambient concentrations of TSP and their spatial variations under different stability conditions shows that in stability classes A, B, and C, the maximum concentration occurs at a distance of about 281 to 619 meters from the stack, while for stability classes D, E, and F, it occurs at further distances (1130 - 3538 meters). Based on the annual wind pattern of the region, 91% of the prevailing winds in the study area are generally westerly, northern, and northeastern. Therefore, the transfer of pollution to the west, north, and northeast is possible, and considering the location of the factory (northeast of the village), the possibility of pollutants spreading towards the village and residents' dissatisfaction is not far-fetched.
In another part of the study, the emission rate from the industrial stack was analyzed and examined in comparison to the standard values. In all cases, the compounds and pollutants emitted from the stack of the factory are lower than the standard values for the stack output of this group of industries.
In the next step, the exhaust values from the stack were compared with the clean air standards of Iran. As a result of this comparison, it was determined that the concentration of gases emitted in all cases is far lower than the permitted and recommended limits. Regarding particles, the standards are based on particle size (PM2.5 and PM10), and measurements from the factory on the suspended particle scale (TSP) do not allow for direct comparison.
In the last part of this study, the measured values of the concentration of particles (PM2.5 and PM10) at the workplace of workers in this industry were examined. The concentrations emitted are far higher than environmental standards, but since the emissions occurred instantaneously and, based on natural mechanisms, the dispersion of particles occurs in the short term, it is not possible to compare the numbers with environmental standards. Based on the possibility of breathing suspended particles or particles entering the eyes of people, it is necessary to reduce the level of emissions as much as possible and, on the other hand, to use and require the use of appropriate safety equipment such as respirators and safety glasses for employees and personnel involved.

Conclusion
Measurement of the exhaust values from the stack confirmed that the emission levels are standard based on the activity group of this industry. The concentration of particles during activity is very high and can have serious health effects on personnel. Although these values are emitted instantaneously and natural atmospheric mechanisms can significantly reduce and dilute their concentration, the use of protective equipment and initial measures such as water spraying is still recommended to control emissions.
Considering the distance of 1.1 km from the village under study to the asphalt plant, the occurrence of the maximum concentration at a distance of 281 meters from the plant, and comparing the emission values with the Iranian clean air standard, it can be claimed that the gaseous pollutants are far below the standard level and there is no concern about this. However, we cannot make a definitive statement about the suspended particles. Part of this ambiguity is due to the difference in the pollutant under study, which is analyzed at the industrial site based on TSP and in the clean air standards based on particle size (PM2.5 and PM10). In addition, in the EPA's proposed model, SCREEN3, there is no distinction between particles and gaseous pollutants, and considering the parameters affecting the emission, such as density and boundary layer effects, this issue can create errors in the emission pattern of this model.
In conclusion, it can be stated that there is a possibility of the emission of suspended particles in the village area, and based on the emission from the stack and the direction and speed of the prevailing wind, this possibility is not far-fetched. It is suggested that, to address this problem, appropriate spacing should be considered between industries and residential areas, and that the prevailing wind direction of the region be taken into account in the location of the industry in a precise and long-term manner. In addition, the requirement to use pollutant control equipment along with continuous monitoring can play an effective role in improving the air quality in the environment.