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Abstract. Utilizing the well-calibrated, high-spectral-resolution equal-quality performance of daytime and nighttime (09:30 and 21:30 local solar Equator-crossing time (local solar ECT)) products of the Infrared Atmospheric Sounder Interferometer (IASI) from the Laboratoire de Météorologie Dynamique (LMD), this study investigates the day–night differences in dust aerosols over the global dust belt of North Africa, the Middle East, and Asia. Both daytime dust optical depth (DOD) and nighttime DOD at 10 µm show high consistency with solar and lunar observations of coarse-mode aerosol optical depth (CAOD) from AErosol RObotic NETwork (AERONET) sites across the dust belt, with correlation coefficients of 0.8–0.9 for most sites. Both IASI DOD and dust layer height show a significant (95 % confidence level) day–night difference over the major dust sources within the dust belt. Daytime DOD over the central to northern Sahara, the central to eastern Arabian Peninsula, and the Taklamakan Desert is significantly higher than that of nighttime but lower than nighttime over the southern Sahel to the Guinea Coast and the western to central Indian subcontinent in the annual mean. The magnitude of the day–night differences in DOD is larger and more evident in boreal winter and spring than in other seasons. The positive day–night differences in DOD (i.e., higher daytime values than nighttime) over the central Sahara, the Middle East, and Asia are likely associated with greater dust emissions driven by higher dust uplift potential (DUP) and stronger wind speeds during daytime. Dust layer heights demonstrate negative day–night differences over dust source regions in the central Sahara, central Arabian Peninsula, and Taklamakan Desert 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 the daytime is associated with a deeper planetary boundary layer height and greater convective instability during daytime than nighttime, which promotes vertical transport and mixing of dust aerosols. The corresponding lower daytime DOD over the Sahel and the Indian subcontinent indicates a possible dilution of dust aerosols when they are transported to higher altitudes by convection where they are more susceptible to horizontal transport. Ground-based observations of dust show surface PM10 concentration and CAOD exhibit a spatially varying diurnal cycle across the dust belt. CAOD and PM10 concentrations generally peak in late morning and from late afternoon to midnight in the Sahel and in early afternoon and around early morning in the Middle East, the timings of which are largely consistent with the day–night differences in IASI DOD. It is also found that DOD from reanalysis products (e.g., Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) and ECMWF Atmospheric Composition Reanalysis 4 (EAC4)) failed to capture the day–night differences in IASI DOD in large parts of the dust belt except in small dust source hotspots over North Africa.

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.

Figure S1: Seasonal cycle of AERONET solar (blue) CAOD and IASI daytime (red) DOD (scaled to 500 nm) over AERONET sites across the dust belt. Time is variable between 2008-2020 depending on the number of records available at individual AERONET sites.

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