Atmospheric Concentrations, Dry Deposition Fluxes and Compositional Analysis of Suspended Particulate Matter in Nairobi, Kenya

Abstract

Nairobi City is a rapidly growing regional center with divergent land uses, pollutants and pollution sources. Different sizes of particulate matter with varying compositions are released into and dispersed in the air through activities on the earth’s surface that generate such materials. Settleable particulate matter of size greater than 10µm are readily deposited on the ground and on vegetation. The settleable solids/dustfall and their pollution severity is dependent on their composition. Presence of harmful trace elements like Pb, Cu, and organic compounds in the dustfall pose risks to the environment. This study assessed the composition of dustfall in Nairobi. Purple air PM monitors were used to assess the concentrations of particulate matter (PM2.5 and PM10). The British Standard (BS 1747-1:1969) dustfall jar was used for the collection of dustfall. Atomic Absorption Spectrometer (AAS) was used to determine the concentrations of trace elements (Pb, Cu, Zn, Al, B, Co, Cr) in the dustfall samples. The samples were also screened for presence of organic molecules using a Gas Chromatograph with Mass Spectrometer (GC-MS) detector. The mean PM2.5 and PM10 recorded in this study were 20.16 and 24.33 μg/m3, respectively. Four out of the six sites studied recorded PM2.5 levels higher than the WHO daily averages of 15 μg/m3 while only one site recorded PM10 higher than WHO of 45 μg/m3. The sites showed wide variations which can be attributed to land use as well as vehicular traffic volumes at the sites. The study found the overall mean deposition rates of Total Solids to be 127±40 mg m-2day-1 with a range of 69 to 158. The average dry deposition fluxes (Fd) of cobalt, boron, aluminum, zinc, copper and chromium were; 77, 49, 44, 2, 1.4, 0.01 mg m-2 yr-1, respectively. Industrial and residential/industrial zones recorded higher amounts of dustfall than commercial zones. The study revealed that heavy metal contamination in the dustfall is dependent on human activities and therefore mitigation measures should be designed to control causative activities. Organic molecules such as; tridecane, benzenediamine, salicylic acid, hexadecane, dibutyl phthalate, methy salicylate, aniline, undecane, dibutyl phthalate, trichloromethane, ethanedinitrile, propane, exadecenoic acid and phenol were detected in all samples analysed. Their sources can be attributed to industry related sources and motor-vehicle fuel constituents. The findings of this study indicate that air in Nairobi has pollutants at levels that are harmful to its residents. It is therefore recommended that legislation of, adherence to and enforcement of environmental laws should be done to control air pollution.