Investigation of Seasonal dust in northeastern Iran and numerical simulation of extreme dust events using WRF-CHEM model

In recent years, dust storm has become a serious environmental concern and has attracted a lot of attention among atmospheric scientists. Northeast of Iran is a large and strategic population area. Due to its proximity to large arid regions in Central Asia, this region has a high risk of experiensing dust events. In recent years, it has faced many problems regarding dust phenomena. This study is conducted to investigate seasonal dust events in northeastern Iran. To achieve this goal, a combination of station data, reanalysis, satellite and output of the WRF-Chem numerical model have been used simultaneously to improve our understanding of the dust seasonal cycle in northeast of Iran. Accordingly, this research was organized in two parts: monitoring and modeling of dust phenomenon. The results of this study may be useful for forecasting dust storms as well as spatial planning. To investigate the dust events seasonal variabilities, the dust surface mass concentration of MERRA-2 dataset, aerosol optical depth (AOD) of the combined Dark Target (DT) and Deep Blue (DB) algorithms of MODIS sensor of Terra and Aqua satellites were examined during the long-term period (2004-2018). Since the emission of dust is highly dependent on biophysical components, it is necessary to use numerical models. The WRF-Chem numerical model was used for this purpose. The study area includes northeastern Iran and parts of Central Asia. The horizontal resolution of the child domin of 30 km model was performed with 32 vertical levels. The NCEP / FNL is used as boundary conditions with 3-hourly time step and 1-degree horizontal resolution for the model configuration. Four extreme dust events were selected to investigate the transport of dust to northeastern Iran. The selected dust events occurred on November 13, 2007, May 29, 2008, June 8, 2015, and October 17, 2017 in northeastern Iran. Therefore, case events were simulated with a time step of 180 seconds and output every three hours using GOCART, AFWA, UoC_S01 and UoC_S11 schemes. The results showed that the maximum dust activity occurred in spring with AOD value equal to 0.59 and dust surface mass concentration is 645.2 µg m -3. The summer is in the next place. Seasonal analysis of AOD and dust using satellite and reanalysis data, showed that Aralkum, Kyzylkum, Karakum and Kara-Bogaz-Gol are the main dust sources in Central Asia that are active in all seasons. Comparison of dust simulation results for PM2.5 and PM10 variable with observational data of air quality control stations in Mashhad showed that GOCART scheme can well depict dust events and provide a low bias towards station data. Also, the study of correlation coefficient between simulation and observation showed that the GOCART scheme explains nearly 90% of the variance of data. The root mean square error (RMSE) for the GOCART scheme is less than 20 micrograms per cubic meter for PM2.5. Accordingly, the GOCART scheme is a suitable scheme for dust study in Northeast of Iran and the WRF-Chem model can be used to operationally forecast dust storms. The dust detection algorithm (DDA) of the AIRS sensor and the aerosol optical depth (AOD) of the MODIS sensor confirm the contribution of the mentioned sources of dust in transferring dust to the northeast of Iran. The results showed that three of the case studies occurred as a result of the passage of an extratropical Rossby wave and the deepening of the trough on the territory of Turkmenistan. In contrast, the summer case study is the result of the establishment of a summer circulation pattern that has occurred with the simultaneous establishment of an anticyclonic circulation in the southern part of Turkmenistan and the northeastern parts of Iran and a cyclonic circulation in the Sistan plain and southeastern parts of the country.