| dc.description.abstract | Air pollution is a major environmental problem, affecting developed and developing
countries around the world. Increasing amounts of potentially harmful gases and particles
are being emitted into the atmosphere, resulting in damage to human health and the
environment. The respiratory system is equipped to filter out larger particles, but is
vulnerable to respirable particulates (particles less than l Oum in diameter), which poses a
serious health threat to the population of Nairobi. Therefore the knowledge of the
predominant particle size, concentration and distribution in the air over a certain area is
important for the assessment of the potential adverse health effects.
This study investigated the spatial-temporal variability of respirable suspended
particulates in Nairobi. The spatial-temporal variability of respirable suspended
particulates are explained on the basis of varying meteorological parameters and
particulate pollutants emission sources. The temporal characteristics investigated in the
study included the diurnal, monthly\seasonal, and weekday-weekend variations. The
spatial variability was based on four sampling sites located at residential (Nairobi West,
Eastleigh), commercial and industrial zones (Industrial area and Uhuru park).
The sampling of respirable particulate was done using cyclone Sampler. It samples
particles in two sizes, namely respirable and non- respirable. The samples were analyzed
using gravimetric analysis. A 37mm cellulose membrane filter of O.811mpore size with
collection efficiency of 95% was used during the sampling period and filters were
reconditioned 24 hours before sampling.
The data used in the study were obtained from four sampling sites III Nairobi.
Meteorological data such as wind speed and relative humidity were taken at the sampling
sites using portable hand-held anemometer and a whirling Hygrometer respectively. The
average wind directions were estimated within the sampling period. The other
meteorological elements such as rainfall were obtained from the Kenya Meteorological
Department (Dagorreti station). Samples were taken twice a day at each sampling site in
the morning at 7.00 am and afternoon at 2.00 pm. The sampling duration was six months
(March- August).
The measured concentrations exhibited a distinct diurnal pattern with relatively high
concentration levels of respirable particulates in the morning in all the sampling sites and
low concentration levels of samples taken in the afternoon. Morning concentration levels
were ranging from 36.9I-lgm-3 to 90.7I-lgm-3and were associated to low windspeeds and
high frequency of stable atmospheric conditions which inhibits the dilution of the
particulate pollutants. The afternoons were characterized by relatively strong wind speeds,
strong solar insolation and high frequency of unstable atmospheric conditions which
enhanced the dilution mechanism of the particulate pollutants, low concentrations were
recorded ranging from 33.1I-lgm-3to 74.2l-lgm-3.
The results show that there is a distinct seasonal variations in concentration levels, with
low concentrations ranging from 27!lgm-3 to 65!lgm-3 recorded during rainy season
(April, May) due to washout. While the dry months (July, August) recorded relatively
high concentration levels (33!lgm-3 to 93.8!lgm-3) due to lack of scavenging mechanism
and increased rate of re-suspension of dust particles from unpaved surfaces in Nairobi.
The month of June, though dry, recorded low particulate concentrations because the
previous month of May was wet and the atmosphere had been cleaned by washout and resuspension
of dust was also reduced.
Concentration levels during weekends and weekdays were found to vary both at
residential sites and industrial area. At residential sites concentrations were significantly
high during the weekend than during the weekdays, this is attributed to high population in
the estates during weekend than during weekdays. In the industrial area the weekend
pollutant levels were lower than during weekdays, this is due to the closing of most of the
industries during the weekend. The city centre also recorded lower concentrations during
the weekend as compared to the normal working-weekday and this is attributed to the low
traffic volume and low-key activities in the city centre.
The concentrations of respirable particulates in Industrial area were rangmg from
48.9!lgm-3 to 93.8!lgm-3. Uhuru Park had a mean concentration of 51.5!lgm-3 While in
the residential areas the mean concentrations in Eastleigh was 41.9!lgm-3 and Nairobi
west was 65.4!lgm-3. Nairobi West had relatively high concentrations than St.teresa
Xl
(Eastleigh) since it is lying downwind of Industrial area and hence a major recipient of
industrial particulate pollutants.
The study shows that air quality in Nairobi is closely associated with pollutant emission
sources, diurnal and seasonal cycles of meteorological elements such as the wind speed
and direction, atmospheric stability and deposition. Moreover, these meteorological
parameters were found to have influence on respirable particulate concentrations. | en |
