Traffic and mineral dust impact on air quality in Nairobi, Kenya

Abstract

Air quality issues remain a challenge in major cities of Sub-Saharan Africa and Nairobi, the capital city of Kenya, is no exception. However, there has been many short study campaigns assessing both ground level, about 1.5 m above ground level (agl) and at regional air mass heights above 10 m agl. None of the studies addressed measurements at both heights simultaneously until the study by Kinney et al. (2010). This study was focused on traffic influence in the Central District (CBD) of Nairobi and it showed concentrations of PM2.5 remained high but attenuated in the vertical profile to heights above 17 m agl. There were no obvious implications of regional air mass influence in the CBD measurement during that regional winter of 2009. The current short study revealed a marked influence by regional air mass at an urban site where mineral dust elements at heights above 17 m agl were higher than those measured at 1.5 m agl simultaneously. The measurements were for 8 hours a day at an industrial and urban site during the regional summer of 2012. The study included manual counting of vehicles, filtered PM2.5 particles and subsequent trace element analysis using EDXRF at the university of Nairobi. At the urban site average concentrations of PM2.5 were 73 ?g m-3 at 17 m agl and 48 at 1.5 m, and the vehicle count was 442 h-1. Mineral dust elements of Ca, Ti and Fe were 11.5, 2.3 and 29.1 ?g m-3 respectively at 17 m agl and 6.7, 2.8 and 20.5 at 1.5 m. At the industrial site PM2.5 values were 47 at 14 m agl and 71 at 1.5 while Ca, Ti and Fe registered 8.1, 2.3 and 19.8 ?g m-3 respectively at 14 m agl and 6.4, 1.8 and 16.1 at 1.5 m. The average number of vehicles was 335 h-1. These results showed clear influence of regional air mass on air quality in Nairobi. They also indicated the ground level pollutants are impacted more by the type of vehicles than numbers. The vehicles transiting the industrial site were mainly heavy-duty trucks and lorries while saloon vehicles transited the urban site. Most trucks and lorries have diesel engines while saloon vehicles have mainly petrol engines.