Understanding day-night differences in dust activities over the dust belt of North Africa, the Middle East, and Asia

Abstract. Utilizing the well-calibrated, high spectral resolution, and equal-quality-performance for day and night observations (9:30 a.m. and 9:30 p.m. equator passing time) of the Infrared Atmospheric Sounder Interferometer (IASI) products, this study investigates the day-night differences in dust activities over the dust belt of North Africa, the Middle East, and Asia. Both daytime and nighttime dust optical depth (DOD) at 10 microns shows high consistency with solar and lunar observations from AErosol RObotic NETwork (AERONET) sites across the dust belt, with correlation coefficients of 0.8–0.9 for most sites. IASI reveals significant (95 % confidence level) day-night differences in dust activities over the major dust sources within the dust belt. Annual mean daytime DOD at 9:30 a.m. is significantly higher than that of nighttime at 9:30 p.m. in the central to northern Sahara Desert, the central to eastern Arabian Peninsula and dust source regions in South and East Asia including the Taklamakan Desert, but lower over the southern Sahel to the Guinea Coast, and the central to southern Indian subcontinent. The magnitude of the day-night difference in DOD is larger and more significant in boreal winter and spring than other seasons. An analysis of 10 m wind fields and dust uplift potential using the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) suggests that the positive day-night differences in DOD over the central Sahara, the Middle East, and Asia are associated with enhanced dust emissions driven by stronger wind speed. Dust layer heights demonstrate negative day-night differences (i.e., lower daytime versus higher nighttime values) over dust source regions in the central Sahara, central Arabian Peninsula, and Asia, and positive height differences in the southern Sahel to the Guinea Coast, southern parts of the Arabian Peninsula, and large parts of the Indian subcontinent. The higher dust layer height over the Guinea Coast and the Indian subcontinent during daytime is associated with a deeper planetary boundary layer height and greater convective instability around 9:30 a.m. than that during 9:30 p.m., which promotes vertical transport and mixing of dust. The corresponding lower daytime DOD over these downwind regions indicates a possible dilution of dust aerosols when they are transported to higher altitude by convection and are more susceptible to horizontal transport.

Ground observations from the Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA) and AERONET show surface PM₁₀ concentration and dust aerosols exhibit a spatially varying diurnal cycle across the dust belt with peak coarse-mode aerosol optical depth (CAOD; around 7–9 a.m.) and PM₁₀ concentrations (around 9–11 a.m.) in the morning hours and late afternoon to midnight in the Sahel, peak CAOD from morning to early afternoon (around 9 a.m.–1 p.m.) and around midnight in the Middle East and Asia, generally consistent with day-night differences in dust activities revealed by IASI. An examination of DOD from Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) and ECMWF Atmospheric Composition Reanalysis 4 (EAC4) products reveals that reanalysis products largely capture the temporal and spatial variability of DOD on the seasonal scale but failed to capture the day-night differences in DOD in large parts of the dust belt except in a few dust source hotspots over North Africa, such as the northeastern Bodélé Depression and the northeastern North Africa. Overall, this study provides a detailed and comprehensive analysis of the day-night differences in dust activities over the dust belt, which could improve our current understanding of physical mechanisms of dust cycle at the diurnal timescale in various dust source and downwind regions.