Submicron aerosol pollution in Greater Cairo (Egypt): A new type of urban haze?

Greater Cairo, the most populous megacity in the Middle East North Africa (MENA) region, faces severe aerosol pollution, posing a significant threat to public health. Despite its impact, the main sources of pollution remain under-characterized due to sparse atmospheric observations. To address this gap, we conducted a continuous two-month field study at an urban background site, documenting the first chemical and physical properties of submicron aerosols (PM₁). We found that crustal material from both desert dust and traffic dust resuspension accounted for as much as 24% of the total PM₁ mass, increasing to 66% during desert dust events—a level unusually high for urban settings. Simultaneously, our data indicated reductions in black carbon and ammonium sulfate levels, suggesting successful emission reductions through local and regional mitigation efforts. The diurnal patterns of carbonaceous aerosols were linked to peak emissions from local traffic during rush hours and from open biomass burning at night. Contrarily, our analysis identified unexpectedly high levels of semi-volatile ammonium chloride (NH₄Cl) from local open biomass and waste burning, emerging as the predominant PM₁ chemical species in Cairo. Its formation at night significantly influenced morning aerosol water uptake, thereby playing a crucial role in the formation of persistent urban haze. These findings not only confirm the ongoing presence of a significant dust reservoir over Cairo but also reveal a new source of highly hygroscopic semi-volatile inorganic salts, leading to a unique type of urban haze. This haze, characterized by major contributions from both submicron and supermicron particle modes, highlights the complex implications of heterogeneous chemical transformations of air pollutants in urban settings, emphasizing the need for interdisciplinary research to understand and mitigate these impacts in the Mediterranean and similar regions. Full details are available in our publication in Christodoulou et al., 2024.

Christodoulou, A., Bezantakos, S., Bourtsoukidis, E., Stavroulas, I., Pikridas, M., Oikonomou, K., Iakovides, M., Hassan, S. K., Boraiy, M., El-Nazer, M., Wheida, A., Abdelwahab, M., Sarda-Estève, R., Rigler, M., Biskos, G., Afif, C., Borbon, A., Vrekoussis, M., Mihalopoulos, N., Sauvage, S., and Sciare, J.: Submicron aerosol pollution in Greater Cairo (Egypt): A new type of urban haze?, Environ. Int., 186, https://doi.org/10.1016/j.envint.2024.108610, 2024.