Submicron Aerosol Pollution in Greater Cairo (Egypt): A New Type of Urban Haze?

Abstract

Greater Cairo, the largest megacity of the Middle East North Africa (MENA) region, currently suffers from major aerosol pollution, a significant concern due to its adverse health impacts. However, atmospheric observations are limited and the main pollution sources remain insufficiently characterized. To bridge this knowledge gap, we conducted a continuous 2-month field study during the winter of 2019-2020 at an urban background site, documenting for the first time the chemical and physical properties of submicron (PM1) aerosols. Crustal material from both desert dust and road traffic dust resuspension contributed as much as 24% of the total PM1 mass (rising to 66% during desert dust events), a figure not commonly observed in urban environments. Our observations showed decreased black carbon concentrations and a fivefold decrease in ammonium sulfate compared to data from 15 years ago, indicating an important reduction in both local and regional emissions as a result of effective mitigation measures. The diurnal variability of carbonaceous aerosols evidenced a strong dependence on emissions from local traffic at rush hours and nighttime open biomass burning. Surprisingly, semi-volatile ammonium chloride (NH4Cl) originating from local open biomass and waste burning was found to be the main chemical species in PM1 over Cairo. Its nighttime formation contributed to aerosol water uptake during morning hours, thereby playing a major role in the build-up of urban haze. While our results confirm the persistence of a significant dust reservoir over Cairo, they also unveil an additional source of highly hygroscopic (semi-volatile) inorganic salts, leading to a unique type of urban haze. This haze, with dominant contributors present in both submicron (primarily as NH4Cl) and super micron (largely as dust) modes, underscores the potential implications of heterogeneous chemical transformation of air pollutants in urban environments.

Keywords: Megacity Submicron aerosolsUrban hazeGreater Cairo