Assessment of polycyclic aromatic hydrocarbons (PAHs) in air and soil from selected sites in Nairobi
This study was undertaken to investigate the occurrence, spatial distribution and temporal and seasonal variations of sixteen Polycyclic Aromatic Hydrocarbons (PAHs) in Industrial Area, Dandora Municipal dump site and Kabete sites on a monthly basis from February to July 2008. The sixteen PAHs have been identified by various Regional and International Agencies as priority compounds due to their toxicity, potential for human exposure and adverse effects on human health and the environment. Identification and quantification of the P AHs in air and soils was done using a Varian CP 3800 gas chromatograph equipped with a flame ionisation detector. Confirmatory analysis was conducted using Gas Chromatography equipped with a mass spectrometer (HP 6890-HP 5972). Compound detection limits ranged between 0.0165 ng/ filter for acenaphthene and 0.117 ng/filter for anthracene. Levels of P AHs in air were lower at the Kabete site than in Dandora and Industrial Area. The concentrations of individual P AHs in air ranged from below detection level to 3939.66 ±10.28 ng/filter. The sum of the concentrations of the compounds (2.:16 PAHs) ranged from 1704.05 ±191.16 ng/filter to 1881.81 ±202.86 ng/filter in Kabete; 6470.27 ±724.42 ng/filter to 8726.93 ±716.53 ng/filter in Dandora; and between 5853.69 ±631.28 ng/filter and 7822.87 ±851.89 ng/filter in Industrial Area. Individual PAHs in soils ranged between below detection level to 526.66 ±18.62 ng/g. The sum of the concentration of the 8 PAHs (2.:8 PAHs) ranges (ng/g) were 17.30 ±1.19 to 62.21 ±0.97 in Kabete, 141.44 ±1.03 to 726.60 ±4.51 in Dandora; and 63.37 ±1.29 to 138.68 ±1.93 in Industrial Area. Medium weight PAHs, especially phenanthrene were more abundant than the low molecular weight compounds. There was a general increase in the monthly levels of P AHs in air at the Dandora and Kabete sites. On the contrary, there was a general decrease in the mean levels of P AHs in air from industrial area. Levels of P AHs in soils increased slightly in Dandora but declined slightly in Kabete. There was stronger seasonal dependence in levels of P AHs in air than in soils. Higher levels were observed during the cool season than the dry and wet seasons in Kabete and Dandora. A reverse trend was observed at the Industrial Area site. The Pearson's correlation coefficients as determined for the P AHs present between air and soil were positive at all the sites. The strongest and most significant intermatrix correlation was observed in Kabete (r=0.821) followed by Dandora (FO.S03) while that for Industrial Area was weak (r=0.166). Correlation coefficients for temporal and seasonal variations of P AHs were positive except in relation to Kabete soil (r=0.039) and Industrial Area air (r=0.792).