تحلیل روند دید افقی در استان خوزستان بین سال های 2020-1998

Abstract
Horizontal vision is the maximum horizontal distance that a person with the naked eye can clearly detect features on the ground.The location of Khuzestan provinc 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 the present study is to study the trend of horizontal vision changes using horizontal vision data and mean values of aerosol optical depth. The data used in the present study include daily horizontal vision data of synoptic stations in Khuzestan province in the period 1998 to 2020. Ridit statistical method was used to analyze the horizontal vision data. Horizon of view in the studied stations was grouped into five categories and the frequency of each category was determined and the value of Ridit was calculated for each category. Then the relevant diagrams and maps were drawn. Also, the fluctuations of the average monthly AOD in the statistical period under study in the geographical location of meteorological stations were calculated and their values were extracted. Comparison of the graphs of the studied stations shows that in all stations except Masjed Soleiman station, the horizontal visibility has a decreasing trend.

Introduction
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 Khuzestan provinc 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 to study the trend of horizontal vision changes in Khuzestan provinc between 1998-2020; 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.
Data and Method
The study area is southwestern Iran, which includes the political boundaries of the provinc of Khuzestan. The highest point of this region with an altitude of 3741 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 63928/44 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 Khuzestan provinc 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) 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.
Results and Discussion
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, the horizontal visibility is increasing. . Also, in these station, 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 Ramhormoz, Omidieh, Dezful, Bandar Mahshahr, Bostan, Ahvaz and Abadan stations, the horizontal visibility is decreasing. AOD diagram of the studied stations shows that in Ahvaz, Abadan, Bostan, Dezful, Bandar Mahshahr, Omidieh and Ramhormoz stations, the amount of aerosols is increasing. Unlike 7 previous stations in Masjed Soleiman station, the amount of aerosol has a decreasing trend. After reviewing and studying the Ridit values, the aerosol optical depth index to determine the condition of the horizon in Khuzestan provinc the horizontal visibility was classified into five quality groups: very good, good, medium, bad and very bad.
Conclusion
In this study, the trend of horizon vision changes based on daily horizontal data of 8 synoptic stations in Khuzestan provinc, the amount of AOD changes based on Modis satellite data in the mentioned stations were studied. The results showed that in Masjed-e-Soleiman station, in the studied years, instead of decreasing the horizontal vision, we are facing a situation of improving the horizontal vision. While in Ramhormoz, Omidieh, Dezful, Bandar 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, Omidieh and stations has a sharp trend and compared to Ahvaz, Abadan, Ramhormoz and Dezful stations have worse conditions. AOD study in the studied stations showed that in Ahvaz, Abadan, Bostan, Dezful, Bandar Mahshahr, Omidieh and Ramhormoz stations, the amount of aerosol increases. While in other stations, AOD decreases, which indicates an increase in horizontal visibility in these areas This indicates that in these areas, the horizontal visibility conditions are better than other stations. Therefore, it can be concluded that in general, in the Khuzestan provinc, 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.
Keywords: Khuzestan provinc, horizontal visibility, trend, AOD, Ridit analaysis.