Black carbon aerosols at an urban site in North Africa (Kenitra, Morocco)

Abstract

Although black carbon is one of the most important atmospheric particulate components driving climate change and air quality, there is a lack of comprehensive air quality data from North African urban areas. Seven-wavelength aethalometer-collected data were used to provide first insights into the levels and patterns of equivalent black carbon (eBC) mass concentrations, particularly into the seasonal and diurnal cycles, that exist over an urban area in the North of Africa, Kenitra city, Morocco. The average BC mass concentration was observed as 0.90 ± 0.80 μg/m3. This BC value is overall largely lower than reported African and European urban location levels, although the role of COVID-19 restrictions could have influenced this result. The diurnal pattern revealed two BC peaks corresponding to morning and evening traffic rush hours. This variation is found to be seasonally dependent, with the maximum daily amplitude occurring in winter. The BC bimodal behaviour is observed in all the seasons but with varying degrees of the magnitude of the peaks (winter > autumn > summer). This observed seasonal heterogeneity was explained by varying meteorological conditions (wind speed, wind direction, relative humidity, air pressure, air temperature, rainfall, and boundary layer height), and air mass trajectory height and pathways. It was pointed out that the daily and seasonal variations are significantly influenced by local traffic sources, wind speed, and boundary layer dynamics. Hysplit backward trajectories revealed that the long-range aerosols transport through both south-western (Atlantic region), and western (over populated areas) regions can as well contribute to BC concentrations, although to a different extent. Finally, a common thread that runs through seasonal, and diurnal variations were that typical low hourly BC concentrations (distinguished in the afternoon) do not exceed the value of 0.5 μg/m3 representing a kind of urban background level.