عنوان مقاله [English]
Atmospheric visibility is a Common meteorological parameter and almost all meteorological stations have recorded it for a long time. Visibility is defined as the maximum distance at which a selected target can be seen by human eyesight. Horizontal visibility is a kind of atmospheric visibility and it is a visual perception of weather conditions. Its values depend on different weather conditions like the clear sky (sunshine), fog, rain, snow, hail, thunder, and tornado. Furthermore, it depends on air quality and urban-industrial activities that can change it. Low visibility causes psychological problems and effects on flight safety and transportation.
Horizontal visibility variation has both natural and anthropogenic origins and so its values depend on both weather conditions and air pollutants. Almost all large cities of Iran have meteorological stations and for decades, parameters like horizontal visibility and weather condition types have been measured and recorded. On the other hand, atmospheric aerosols are the main air pollutants and retrieval of their parameters needs specific aerosol monitoring stations. Unfortunately, only Zanjan city has an aerosol monitoring station and therefore, we focus on this city in this work.
The Zanjan weather station has recorded data from 1973 until now. In this work, the effects of air temperature and weather type on horizontal visibility are first discussed. Also, data acquisition in Zanjan’s aerosol monitoring station (IASBS sun-photometer site) began in 2010. The aerosol optical depth and Angstrom exponent as two important parameters of aerosol are discussed here. Extinction of direct sunlight throughout the atmosphere is related to both aerosol and molecular concentrations and the pure aerosol portion is named aerosol optical depth or AOD and its higher (lower) values are related to aerosol more (less) number densities. Angstrom exponent or alpha is a qualitative indicator of aerosol mean size and higher (lower) values are related to fine (coarse) aerosols. Therefore, both AOD and alpha determine the dominant type of aerosols and their concentrations.
In this work, all 10,359 days of recorded data from Zanjan’s weather station during 1973-2017 are selected to investigate the correlation between horizontal visibility, temperature, and atmospheric conditions. The results show that the horizontal visibility is reduced in the colder months of the year due to rainy weather conditions, fog, and snowfall. During 70 days in 1990-2017, horizontal visibility dropped below 5 km and 86% of these days have foggy, rainy, and snowy conditions. So, one concludes that air pollution is not the main reason for visibility reduction in Zanjan city. Furthermore, the optical depth and Angstrom exponent parameters of atmospheric aerosols are extracted from the sun-photometer station of Zanjan during 2010-2017. No significant correlation is observed between the horizontal visibility reduction and the increase in the concentration of dust particles as the dominant aerosol of the Zanjan’s atmosphere. In the studied period, 1269 (76) days (dusty days) were recorded, with mean horizontal visibility of 11 (9) km. Therefore, one concludes that the parameter of horizontal visibility is not sharply decreased during dust events and it is mainly related to dust transportation from external sources to the atmosphere of Zanjan, especially at higher altitudes.