Diurnal Aerosol Variability and Meteorological Associations over Inland and Coastal Pakistan: Insights from EPIC and MODIS

Abstract

Aerosol properties and effects vary by region because of emission sources, meteorological conditions, topography, elevation, and land–sea interaction. These factors can produce substantial aerosol changes within a day, rendering it important to study the aerosol diurnal behavior. Using available 2015–2025 observations from Quetta and Karachi, this study demonstrates an integrated approach for characterizing diurnal aerosol variability where ground monitoring is limited. The approach combines DSCOVR EPIC aerosol optical depth (AOD) and ultraviolet aerosol index (UVAI), MODIS Ångström exponent (AE), ERA5 meteorology, and Karachi AERONET measurements. Paired representative-scene analysis showed a larger afternoon enhancement over Quetta (ΔAOD = 0.159) than over Karachi (ΔAOD = 0.066). In the GeoDetector analysis, wind speed showed the strongest morning spatial association over Quetta, and multiyear mean annual precipitation the strongest afternoon association. Surface pressure ranked highest in both Karachi periods, closely followed by relative humidity in the Karachi morning. City-specific AE–UVAI models identified four aerosol regimes per city. The models showed a small redistribution toward strongly absorbing regimes in Quetta and a larger coarse-to-fine regime shift in Karachi. Across the nine clustering years, fine–absorbing aerosols increased significantly. These case studies demonstrate how complementary observations and explicit temporal sampling controls can jointly characterize daytime aerosol loading, aerosol regimes, and meteorological spatial associations in sparsely monitored regions.