Temporal and spatial changes of dust in Golestan province using AOD (Aerosol Optical Depth) and the affectability of this province from the deserts of Turkmenistan

One of the effects of drought is the loss of vegetation, drying of wetlands, and desertification, which in turn can create an appropriate substrate for the dust and increase the concentration of aerosol. In this research, with the aim of understanding the spatial and temporal changes of dust storms in Golestan province over a period of 20 years (2000 to 2020), AOD (Aerosol Optical Depth) data from MODIS were used. So, dust detection when the AOD value was greater than 0.5, was considered a dust event. Also, the monthly horizontal visibility data of 10 synoptic stations with appropriate distribution in the province were used to validate the AOD product. Monthly horizontal visibility data is obtained by averaging three-hour horizontal visibility at 10 synoptic stations in Golestan province. With the aim of investigating the annual and seasonal distribution of dust in the Golestan province, a diagram of the number of days with dust in the study time was obtained. Then, the map of the spatial changes of dust was also presented. To investigate the intensity of influence of Golestan province from the deserts of Turkmenistan, in addition to tracking the dust transfer path using the output of the HYSPLIT model for a selected event from each season, synoptic analysis was also performed using ERA5 data. Also, the trend of dust changes in the Golestan province and deserts of Turkmenistan was investigated. The validation results with a correlation of 0.66 show the ability of AOD data to present the dust distribution in Golestan province. According to the obtained results, in fact, there are two time periods with low occurrences of a dust storm (2000 to 2007 (except 2003) and 2016 to 2020) and one-time period with high occurrences of a dust storm (2008 to 2015). The highest number of days with dust is in the summer season and the lowest is in the winter season. In terms of spatial distribution, the northern regions of the province, i.e., Gonbad-e kavus, Aqqala, and Gomishan, are more involved in dust phenomena than other parts of the province. During the statistical period, the highest AOD value occurred on 7th September, 2020 with a value of 4.1. Tracing the dust transfer path using the output of the HYSPLIT model in the region showed that the main source of dust in the province was from Turkmenistan and especially the deserts of the Balkan province. Tracking the dust transfer route using the output of the HYSPLIT model in the region showed that the main source of dust in the province is mainly from the desert areas of Turkmenistan. The synoptic analysis of the selected day from each season generally indicates a strong pressure gradient in the region of Turkmenistan and north of Iran in such a way that the northerly winds in Golestan and the east of the Caspian Sea are significant, which causes the soil to rise, and it is transported to Golestan province. Also, on the selected days of spring, summer, and autumn, there is a ridge pattern and stable atmospheric conditions in the middle level of the troposphere, leading to dust persistence. In the winter, due to the presence trough of low in the middle of the troposphere and the occurrence of precipitation, the intensity of the dust is weaker. Also Comparing the annual and seasonal distribution of dust storms in the Golestan province and Turkmenistan shows the simultaneity of events.