Nitrogen oxides, ozone and heavy metals analysis of suspended particulate matter (SPM) of air in Nairobi, Kenya.
G.O. Odhiambo1, A.M. Kinyua1, and C.K. Gatebe1, 2.
1 Institute of Nuclear Science, College of Architecture and Engineering, University of Nairobi, Nairobi, Kenya.
2
National Aeronautics and Space Administration (NASA), Goddard Space Flight Center, Climate and Radiation Branch, Code 913, Greenbelt, Maryland 20771, USA.Abstract
Emissions from motor vehicle are a major source of air pollution in most urban centers. In Kenya, Nairobi city has the highest traffic density and is of particular cause for concern due to the poor maintenance standards of most vehicles plus their use of leaded gasoline. This study was carried out to determine the levels of nitrogen oxides (NOx), suspended particulate matter (PM10), ozone (O3) and heavy metals in the PM10 collected from the ambient air of Nairobi city. Sampling was done once every week for a period of three months (February to April 2000). Hourly average concentrations of NO2, NO and O3 were measured simultaneously from 9.00 am to 5.00 p.m., at a site connecting two main highways (University and Uhuru). PM10 was collected using "Gent" Stacked Filter Unit (SFU) air sampler on nucleopore filters (0.4 and 8.0 m m pore size for fine and coarse filters respectively) which were weighed and analysed for trace elements by Energy Dispersive X-ray Fluorescent (EDXRF) technique. Nitrogen oxides were analysed with ThermoElectron's Chemiluminescent NOx Model 14B analyser while ozone was by using DASIBI ozone monitor, Model 1003 AH. An automatic vehicle counter was used for determining the vehicle density at the sampling point. The findings of this study show that the values obtained for Pb, Mn, Fe, Br, Zn, Cu and Ca are within the WHO guidelines. Pb concentrations ranged from 0.051 to 1.106m g/m3; Fe, 0.149 to 3.154m g/m3; Mn, 0.002 to 0.526m g/m3; Cu, lower limit of detection (LLD) to 0.15m g/m3; Br, LLD to 0.43m g/m3; Zn, LLD to 0.14 m g/m3 and Ca, 2.18 to 5.389m g/m3. Concentrations of NO2, NO and O3 were also within the 8-hour WHO limits with levels ranging from 0.011-0.976 ppm for NO, 0.001-0.2628 ppm for NO2 and LLD-0.1258 ppm for O3. The O3 levels were slightly higher in the afternoons when solar intensity was high especially when cloud cover was less than 3 Oktas. However, PM10 levels were above the WHO guidelines for most of the days. The mean (PM10) was 239 m g/m3 while the range was 66.66 - 444.45m g/m3. In February and March, coarse particulate accounted for more than 70 % of PM10 levels. Analysis of the data shows strong correlation (r2=0.966) between fine (0.4m m) particulates and NO, indicating the importance of traffic as a common source for both fine particulates and NO. Coarse particulates, NO2 and O3, however shows poor correlation with motor vehicle density. There is also positive correlation (r2=0.930) between Pb and Br, while Fe, Ca and Mn are positively correlated with coarse particulate matter. The high correlation between Pb and Br and motor vehicles suggests their main source as being motor vehicle emission. Ca, Fe and Mn are most probably from resuspended soil dust since they correlate well with coarse particles. The health implications of these results are also discussed.
Key terms: motor vehicles, PM10, urban air pollution, trace elements and gaseous pollutants.