Abstract
<title>Abstract</title> <p>This study is the second in a two-part investigation of aerosol variability over Cairo, Egypt. The companion paper (Abd AlMalk, submitted, Part I) established a 20-year AERONET/MODIS climatology (2005–2025) reporting a mean aerosol optical depth at 500 nm (AOD₅₀₀) of 0.357 ± 0.27, a statistically significant decreasing trend of −0.049/decade (Mann-Kendall p = 0.004), and 227 extreme dust days (AOD₅₀₀ ≥ 0.5, ÅE ≤ 0.6) over the high-density Cairo_EMA_2 record (2010–2025). Part I, however, characterised aerosol climatology without resolving the meteorological mechanisms driving individual severe episodes. Here we address that gap by combining the same 2010–2025 daily AERONET record with ERA5 reanalysis fields (zonal and meridional wind at 850/700/500/300 hPa, surface pressure, boundary layer height, geopotential height, relative humidity, and air temperature) and NOAA HYSPLIT back-trajectory clustering of the fifteen most severe episodes in the record. Dust days show significantly stronger low- and mid-level winds (850 hPa: +1.15 m/s, p=0.013; 700 hPa: +1.99 m/s, p<0.001), a weakened upper-tropospheric jet at 300 hPa (−6.43 m/s, p<0.001), and a pronounced surface pressure deficit (−5.6 hPa, p<0.001) relative to clean-background days (N=4325 vs. N=227). Back-trajectory clustering identifies six transport pathways, dominated by a Libya/North Africa route (47% of the fifteen most severe events, mean AOD=1.380) and including, for the first time in the Cairo literature, an Arabian Peninsula pathway (2 events). Composite 500 hPa geopotential height fields reveal a ridging anomaly of up to +25 m over the southern and eastern portion of the regional domain during dust episodes, consistent with jet displacement. Boundary layer height shows no significant difference between dust and clean days (855 vs. 867 m, p=0.50), indicating that Cairo's most severe aerosol episodes are governed primarily by synoptic-scale transport rather than local boundary-layer trapping. Together, Parts I and II provide a unified climatological and mechanistic account of aerosol variability over Cairo, with direct relevance to regional air-quality early-warning systems.</p>