Semi-arid mountain-basin regions, such as Tehran Province in Iran, often experience significant dust pollution in summer, yet the factors controlling its diurnal variability remain poorly understood. This study examines the daily patterns of summer dust events in Tehran, combining hourly MERRA-2 (1980–2023) and ERA5 (2023) reanalysis data with surface PM2.5 and PM10 measurements from six air-quality monitoring stations collected during summer 2023. Reanalysis results show that dust concentrations near the surface are strongly associated with local wind patterns, atmospheric mixing, and transport from surrounding arid plains. Dust emissions peak in the morning due to strong southeasterly winds, which carry dust from desert sources toward populated areas by upslope flows. Surface observations confirm these patterns: southern stations exhibit pronounced nocturnal PM2.5 and PM10 peaks, while central and northern stations display daytime maxima, consistent with northward advection of dust-laden air. MERRA-2 dust surface concentrations exhibit distinct spatial behavior. In southern Tehran, concentrations peak at night and remain elevated into the early morning, followed by a marked afternoon decline as boundary layer height increases. In contrast, central stations display dominant late-morning maxima before midday. In northern areas, the maximum occurs later and the afternoon decrease is less pronounced due to sustained northward dust transport that partially offsets vertical dilution. At night, cooler mountain breezes reduce local dust concentrations, although levels remain elevated over the desert plains. The observed diurnal cycle results from the interplay of thermal mountain breezes, shifting wind directions, and boundary layer dynamics, which together govern dust emission, transport, and dilution. These findings highlight how mountainous terrain and daily temperature changes shape dust pollution patterns, providing insights for air quality management in semi-arid mountain-basin regions worldwide.
Ahmadi et al. (Thu,) studied this question.