Spatial Prediction of Pb, Zn and Fe Air Pollution in Urban Environments: A Case of Douala, Cameroon

Abstract Plant biomonitoring is usually used as a tool for characterizing or monitoring air quality in complex urban environments. Trees are reliable bio-accumulators of air pollution and can provide information on the chemical composition associated with vehicular particles through their bark. In this work, geostatistical techniques are employed to examine the content of Pb, Zn, and Fe within 234 tree bark samples analyzed using a portable X-ray fluorescence instrument to map their spatial distribution throughout the city of Douala in Cameroon. We used the universal kriging to predict Pb levels in sampled sites where Pb values are below the instrument's detection limit. Heavy metal levels were predicted at unsampled sites using ordinary kriging. We found that the Zn and Fe levels exceeded the threshold set by the World Health Organization (WHO), at intersections with high traffic intensity and nearby sites. Pb levels also exceeded the threshold value throughout the city and increased with traffic intensity. Our results also suggest that the city of Douala is on high pollution alert, and mitigation measures must be rapidly integrated into the city's master plan, including the establishment of green infrastructures based on the choice of appropriate species to trap chemical elements from the air.