Analysis of horizontal visibility trends in southwestern Iran between 1998-2020

Vision usually refers to the horizontal distance at which the contrast between a target and its sky background is equal to the threshold of the human eye. In an atmosphere that is standard and without pollution, visibility often varies in the range of 145-225 kms. However, in polluted places, atmospheric visibility is reduced by air pollutants through the dispersion and absorption of fine particles and gases in the atmosphere. In some highly polluted areas, the horizontal visibility can be as low as one kilometer. Due to the adverse effects of air pollution on human life, horizontal visibility has been one of the main concerns in air pollution and climatology studies. The trend of changes in horizontal vision as well as the effects of air pollution on the vision process has attracted the attention of all researchers. The results of many studies have also shown that horizontal vision has a negative correlation with public health, especially in developing countries. One study also found that horizontal vision was associated with an increase in deaths and cardiovascular disease in Shanghai, China. One of the effective climatic phenomena in horizontal view is dust. This meteorological-environmental phenomenon is called a mass of fine dust particles and sometimes smoke emitted into the atmosphere, which increases aerosols and reduces horizontal visibility. The location of southwestern Iran at the top of the basin of dusty winds known as the north wind, which spread dust from the deserts of Syria and Iraq to southwestern Iran, frequently reduces the horizontal visibility in these areas. The aim of this study is to use horizontal vision data, mean values of aerosol optical depth and atmospheric extinction coefficient to study the trend of horizontal vision changes in southwestern Iran between 2000-2020; Therefore, by reviewing the available resources in this field, it is clear that relatively limited methods have been used to study and analyze the process of horizontal vision, among which Ridit analysis is the most widely used and efficient method in this field.Given the importance of the issue, the need for such a study in this area becomes apparent.

The study area is southwestern Iran, which includes the political boundaries of the provinces of Khuzestan, Ilam, Lorestan, Chaharmahal and Bakhtiari, Kohkiluyeh and Boyer-Ahmad. The highest point of this region with an altitude of 4283 meters is located along the northwest-southeast, which is part of the Zagros folds, while the lowest point with a height of -105 is located in the southwest of this region, which (leads to) stretches the Persian Gulf. Also, the area of the study area is 28199 / 42 square kilometers. The data consists of two different types of climatic data, including observational data from meteorological stations and satellite data. Observational data are daily horizontal data of synoptic stations in Ilam, Khuzestan, Chaharmahal Bakhtiari, Kohgiluyeh, Boyer-Ahmad and Lorestan provinces in the period 1998 to 2020. This information is often reported at three-hour intervals. Ridit statistical technique was used to study the horizontal vision trend. Ridit analysis is a useful statistical technique that is widely used in trend study. Ridit indicates the possibility that observation of vision over a period of time is better than distribution of reference vision. Aerosol optical depth index (AOD) and extinction coefficient were also used. Because the aerosol optical depth (AOD) is one of the important parameters in the study of dust and affects the horizontal viewing process. Also, the amount of optical depth can vary with the density of the number of aerosols and the properties of those particles.

First, the frequency value of each horizon floor was determined and then the annual Ridit value was calculated for each of the stations studied and the relevant graphs were drawn. By studying the Ridit diagrams of the studied stations, it is clear that in Masjed-e-Soliman, Aligudarz, Dehloran, Khorramabad and Yasuj horizons, the horizon is increasing. Also, in these stations, the trend line is below the reference line until 2009, but from 2009 to 2020, the trend line is above the reference line. But in Shahrekord, Koohrang, Ilam, Do Gonbadan, Boroujerd, Borujen, Ramhormoz, Omidieh, Dezful, Bandar Mahshahr, Bostan, Ahvaz and Abadan stations, the horizontal visibility is decreasing. While in Ilam stations and two domes, there is no change in horizontal vision in the studied years. AOD diagram of the studied stations shows that in Ahvaz, Abadan, Borujen, Boroujerd, Bostan, Dezful, Do Gonbadan, Ilam, Bandar Mahshahr, Koohrang, Omidieh, Ramhormoz and Shahrekord stations, the amount of aerosols is increasing. Unlike 13 previous stations in Aligudarz, Dehloran, Khorramabad and Masjed Soleiman stations, the amount of aerosol has a decreasing trend. By examining and comparing the blackout coefficient diagrams in the studied stations, it was found that the blackout coefficient values decrease in Masjed Soleiman, Aligudarz, Dehloran, Yasuj and Khorramabad stations. But in the next 13 stations of the study area, the rate of blackout increases. After reviewing and studying the Ridit values, the aerosol optical depth index and calculating the extinction coefficient to determine the condition of the horizon in southwestern Iran Horizon was classified into five quality groups: very good, good, medium, bad and very bad.

In this study, the trend of horizon vision changes based on daily horizontal data of 18 synoptic stations in southwestern Iran, the amount of AOD changes based on Modis satellite data and also by calculating the extinction coefficient in the mentioned stations were studied. The results showed that in Masjed-e-Soleiman, Aligudarz, Dehloran, Khorramabad and Yasuj stations, in the studied years, instead of decreasing the horizontal vision, we are facing a situation of improving the horizontal vision. While in Shahrekord, Koohrang, Ilam, Do Gonbadan, Boroujerd, Borujen, Ramhormoz, Omidieh, Dezful, Mahshahr, Bostan, Ahvaz and Abadan stations, the horizontal visibility is decreasing. And in these stations, we are faced with a decrease in horizontal visibility, with the difference that this decrease in Bostan, Bandar Mahshahr, Koohrang, Omidieh and Boroujerd stations has a sharp trend and compared to Ahvaz, Abadan, Borujen, Shahrekord, Ramhormoz and Dezful stations have worse conditions. Also, in Ilam and Do Gonbad stations, no change in the horizontal visibility status was observed in the studied years. AOD study in the studied stations showed that in Ahvaz, Abadan, Borujen, Boroujerd, Bostan, Dezful, Do Gonbadan, Ilam, Bandar Mahshahr, Koohrang, Omidieh, Ramhormoz and Shahrekord stations, the amount of aerosol increases. While in other stations, AOD decreases, which indicates an increase in horizontal visibility in these areas. Also, in Masjed-e-Soliman, Aligudarz, Dehloran, Yasuj and Khorramabad stations, the values of the extinction coefficient are reduced. This indicates that in these areas, the horizontal visibility conditions are better than other stations. But in other stations, the values of the blackout coefficient increase and show that in these areas the horizontal visibility is not in the desired condition; Therefore, it can be concluded that in general, in the southwest of Iran, the horizontal visibility trend is decreasing. It is also suggested that this statistical technique be used in other parts of the country to better determine its efficiency.