Air quality modeling in the city of Marrakech, Morocco using a local anthropogenic emission inventory

Abstract

Exposure to high levels of suspended particles, and particularly dust, has been associated with increased risk of morbidity and premature mortality. The city of Marrakech is situated at a distance of only 560 km from the Sahara desert, the major dust source in the world, leading to an atmosphere rich in particulate matter all year round. In this study, we use for the first-time local scale information on anthropogenic emissions in the city of Marrakech and conduct urban-scale chemistry-transport model simulations with the CHIMERE-WRF coupled system. We compare simulated airborne particles of diameter lower to 10 μm (PM10), NO2 and O3 concentrations against surface in-situ measurements and quantify the added value of the local inventory compared to the state-of-the-art global anthropogenic emission dataset (CAMS-GLOB_ANT). We show that correlation with measurements increases and the bias decreases in all cases (pollutants, seasons and monitor sites). The major component of summertime PM10 composition is dust particles, whereas in winter PM10 consists mainly of primary organic aerosol. Comparison between simulated and observed aerosol optical depth suggests that the model reproduces accurately most of the observed summertime dust plumes. PM10 simulated concentrations are closer to in-situ surface measurements during summer than during winter with an overestimation of 13% in summer versus an underestimation of 37% in winter. Finally, we show how the Atlas range blocks at some extent the import of dust plumes in the region and the export of local air-pollution to the surrounding area leading to ozone formation at high altitude.Modeling is an important activity to predict air-quality in Africa, where monitor networks are scarce. Our results highlight the necessity of using fine scale, local information on anthropogenic emissions to assess air-quality and in particular to (i) quantify the chemical composition of atmospheric aerosol; ii) compare the relative role of dust transport compared to locally emitted or formed suspended particles and iii) provide evidence of ozone formation at high altitude.