Understanding day–night differences in dust aerosols over the dust belt of North Africa, the Middle East, and Asia
Authors: Tindan Jacob Z. , Jin Qinjian , Pu Bing 
Published: May 2023

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.