Assessment of levels of Urban Air Pollutants and Trace Elements in Roadside Soils: A Case Study in Eastleigh Area, Nairobi.

In this study, the assessment of five most common air pollutants from vehicular emissions were done along Juja road in Eastleigh area, Nairobi.  Sampling was done for soil samples, particular matter (PM10), sulfur dioxide (SO2), carbon monoxide (CO), nitric oxide (NO) and nitrogen dioxid (NO2).



Sampling for PM10 and soil samples was done during dry and rainy season, while sampling for SO2, CO NO and NO2 was done during the dry season.  Rainy season sampling was done during the month of April 2005, while sampling for dry season was done from November 2005 to January 2006.


Gases and PM10 sampling was done in 3 places along Juja road (described as Area 1, 2 and 3).  Area 1 was at St. Theresa Catholic Church, which is near Eastleigh Avenue 1 and Juja road junction.  Sampling Area 2 was at Radiant Hospital near Pangani roundabout, while Area 3 was near Huruma Chief’s office, near Mathare North road and Juja road junction.


Sampling for SO2, NO2, No and Co pollutants was done using Z-1300XP, Z-1400XP, Z-700XP and Z-500XP monitors respectively, and “Gent” stack filter unit (SFU) was used for PM10 sampling.  IN area 1, the sampling was done for 5 days while in area 2 and 3 it was done for 4 days.  The measurements were taken for 8 hours (from 9.30 am to 5.30 pm).  This was repeated for PM10 during rainy season and a total of 55 samples were collected and analyzed using EDXRF for elemental content.  Vehicle tallying was done in area 1 and 3, for 8 hours on Fridays while sampling for the gases.


Soil samples were collected at an interval of approximately 250 meters along Juja road.  And at point 4 other samples were collected in consecutive places (about 20 meters apart) away from the road.  Top surface soil sample weighing 0.5 to 1 kg (0-2 cm), was scooped out within 1 meter from the road kerb.  A total of 50 soil samples were collected in both dry and rainy season.


The soil samples were analyzed in triplicate, and the mean obtained.  Association between air pollutants levels and the vehicle density was assessed by performing a simple correlation analysis.  ANOVA was used to indicate any statistical differences in concentration levels for different pollutants at P≤0.05.


The results showed that the PM10 was high during dry season sampling with an average of 441± 116 mg/m3 corresponding to an average of 393 ± 102 mgm3 for coarse particles and 48 ± 45 m g/m3 for the fine particles.  While the result for rainy season sampling showed an average PM10 of 427 ± 105 mg/m3 corresponding to 404 ± 102 mg/m3 and 24 ± 14 mg/m3 average levels for coarse and fine particles respectively.  The daily range for PM10 variation was between 233 and 621 mg/m3; a value that exceeds the WHO permitted level of a daily average of 150-230 mg/m3.


In coarse particles, the elements average concentration in aerosols during dry season sampling (in mg/m3) was observed to be 1.48 ± 0.99 for Mn, 13.29 ± 5.67 for Fe; 0.02±0.02 for Cu; 0.15±0.05 for Zn; 0.07±0.03 for Br; 0.14±0.06 for Zr; 0.03±0.01 for Nb; and 0.2±0.08 for Pb.  While during rainy season sampling, it was observed to be 1.87±0.92 for Mn; 17.0±7.96 for Fe; 0.01 ±0.01 for Cu; 0.17±0.08 for Zn; 0.04±0.01 for Br; 0.18±0.09 for Zr; 00.04±0/02 for Nb; and 0.13±0.05 for Pb.  Bromine and lead in aerosols showed a correlation of R2=0.843 and r2=0.898 during rainy and dry season sampling respectively.  The lead level in ambient air was observed to be within WHO permitted guideline of 0.5-1 mg/m3.


The average concentration for CO in area 1, 2 and 3 was observed to be 5.7, 3.9 and 2.5 mg/m3 respectively and for SO2 was 4.9, 37.1 and 119.4 mg/m3 in area 1, 2 and respectively.  The average level for NO in area 1, 2 and 3 was observed to be 5.2, 1.8 and 1.1 mg/m3 respectively.  The daily range for gases during the sampling period was; CO, 2.1 – 6.6 mg/m3; SO2, 0.2 – 476.5 mg/m3; No, 0.4 – 7.3 mg/m3; and NO2, 0.1 – 245.5 mg/m3.


The daily average levels of gases was found to be within WHO daily limit of 10 mg/m3 for CO and NO, and 150 mg/m3 for NO2 and SO2.  Fridays were the days that low pollutants levels were observed followed by Saturdays (except for the case of SO2), while from Tuesday to Thursday, there were high concentration levels.


The traffic density was observed to be low between noon 3:00 pm, and then increased gradually thereafter.  The public vehicles, consisting of Matatu’s (36-sitter) and Nissan’s (14-sitter), were the majority on the road and contributed to 56% and 63% in area 1 and 3 respectively.  Nitrogen dioxide showed a correlation of r2=0.59 with the traffic density.


In this study, the range of elements in soil during the dry season sampling was found to be; Pb, 87.4-242.4 mg/g; Br, 2.1-20.9 mg/g; Zn, 70.0-648.0 mg/g; Fe, 72.4-133.0 mg/g; and Mn, 4.2-14.9 mg/g.  During the rainy season, the range was; Pb, 41.1-177.9 mg/g; Br, 0.5-13.6 mg/g; Zn, 70.1-1187.0 mg/g; Fe, 16.4-168.2 mg/g; and Mn, 1.4-17.1 mg/g.  The level of lead and bromine was seen to decrease gradually with increasing distance from the road.


Most of pollutants levels especially gases and trace element in soil and aerosols were found to be within WHO permitted guideline.  But the results of PM10 levels were shown to exceed the WHO guideline, which could pose possible adverse health effects to the Eastleigh residents.






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