An estimation of daily cumulative UVA radiation and some related influential factors in the central regions of Iran

Ultraviolet radiation (UVA) is a dangerous part of solar radiation that makes a small percentage of total solar radiation energy (5 to 7 percent). Despite the beneficial role of solar energy in the production of vitamin D3, it can cause irreparable damage to human cells. The majority of previous studies on ultraviolet radiation in Iran have focused on in-vitro impacts of UV radiation on human health and plant physiology in a limited study area. The present study estimates daily cumulative UVA radiation in central regions of Iran and compare it with total column ozone (TCO), cloud optical depth (COD) and aerosol optical depth (AOD) in different seasons.

The present study estimates daily cumulative UVA radiation (320-400 nm) over a 13-year reference period (2005-2017) in a large area in Central Plateau of Iran with arid and semi-arid climate using TUV5 multilayer radiative transfer model (Madronich, 1993). 22 synoptic stations in 9 provinces were investigated in this study. Daily cumulative UVA radiation under three different sky conditions (clear-sky, overcast and real-sky) was also compared with geographical distribution of total column ozone (TCO), cloud optical depth (COD), aerosol optical depth (AOD) and surface albedo (SALB). Required data were extracted from satellite images (downloaded from http://disc.gsfc.nasa.gov) and Iran Meteorological Organization data center.

In general, maximum daily UVA radiation was recorded in the southern half of the study area. During warm seasons of the year, the eastern part of the study area (Kerman and Khorasan-e-Jonubi Provinces) and during the cold seasons of the year, central and southwestern part of the study area (Yazd and Fars Provinces) experience maximum daily UVA radiation. Maximum cloudiness in spring has occurred in northeastern and western parts of the study area and a lower level of cloudiness has always been recorded in its southern parts. Thus, the highest level of UVA radiation has been recorded in southeastern parts of the study area and especially in Birjand station (1071.12 kj/m2 per day). As expected, maximum UVA radiations in all sky conditions and all stations were recorded in summer. The lowest level of cloudiness was also recorded in this season. During autumn and in overcast condition, the highest concentration of UVA was recorded in southeastern parts of the study area and Birjand station (725.85 kj/m2 per day). This is consistent with cloud optical depth and total column ozone, and so, the lowest amount of ozone in this season was recorded in Birjand station (276.57 Dobson). The highest values of atmospheric aerosol with an average of 0.59 optical depth were recorded in winter in the eastern parts of the study area. Thus unlike other seasons, maximum UVA radiation in overcast condition moves toward central stations in winter. Comparison of daily cumulative radiation maps in overcast condition shows that there is a good agreement between daily cumulative radiation and cloud optical depth (COD) and aerosol optical depth (AOD). This indicates that in overcast condition, total column ozone (TCO) have a weaker impact on UVA radiation as compared to other sky conditions. However, UVA radiation is consistent with total column ozone in clear-sky conditions.

Geographical distribution of UVA radiation indicates that maximum daily radiation in warm seasons has often occurred in the eastern parts of the study area. However, maximum concentration of UVA radiation moves towards southwestern parts of the region in cold seasons. Therefore, residents of the eastern and southwestern regions face a higher risk due to daily cumulative UVA radiation. Findings indicate high biological risk of solar UVA wavelengths in clear-sky condition within the study area. Overcast conditions can reduce daily UVA radiation up to 52% in winter and 21% in summer as compared to clear sky conditions. In real-sky conditions, daily UVA radiation decreases up to 19% in summers and up to 32% in winters as compared to clear-sky conditions. As a result of lower solar zenith angle, the impact of cloudiness on surface UVA radiation in summer is relatively less than cold seasons.