Urban air quality in the city of Nairobi, Kenya: application of energy dispersive x-ray fluorescence and principal component analysis

Abstract

Air pollution is a widespread problem affecting millions of people exposed to high levels of air pollution that exceed one or more ambient limit values. One of these air pollutants is the airborne particulate matter (PM), which is composed of a broad class of chemically and physically diverse substances. PM varies in size, chemical composition, formation mechanism and origin. In order to formulate mitigation policies and control measures, there is need to identify the sources. In the recent past Nairobi city has gone through rapid and haphazard urbanization under the pressure of rural to urban migration. On the other hand there has been a huge unprecedented increase in the number of on-road vehicles, rapid establishment and growth of the informal sector, industries and uncontrolled burning of biomass and waste among other anthropogenic activities. These activities contribute heavily to urban air pollution and understanding of the current situation is very important in view of implementing Kenya Vision 2030 and the Millennium Development Goals. In this study airborne PM samples were collected for 8 hours on filter media using BGI 400 personal samplers at two selected locations; at Mines and Geology premises (Industrial site) and University of Nairobi main campus in Nairobi city (CBD site) for a period of 2 weeks in the month of January/February in 2012. PM2.5 was sampled simultaneously at two sites in each location; at 1.5 m above ground (abg) at both locations and at 14m abg at industrial site and 17 m at CBD. Sampling was done at alternate times; morning (0700h-1500h) and afternoon (1200h-2000h) on alternate days. Analysis of trace elements in the samples was done using EDXRF spectrometer and Quantitative X-ray Analysis Software (QXAS) was used for spectra analysis and elemental quantification. Principal Component Analysis (PCA) was used to identify the sources of the sampled particles using the evaluated elemental concentrations in all the samples. Mean daytime concentration of PM2.5 at both sites ranged from 1 to 260 µgm-3. The PCA components apportioned mineral dust, industrial emissions, traffic related emissions and biomass burning as the main sources of particulate matter in Nairobi area. Mn, Fe, Cu and Zn registered high concentration and implied mineral dust was a major source of PM2.5 in Nairobi. In vertical dispersion, PM concentration near ground was higher than those collected at the rooftop by a factor of 1 to 8 in most of the days and less than 1 in a few days. These heavily influences urban air quality thus formal legislation need to be set.