Abstract
Urban air pollution is a major global challenge. Industrial activities, vehicular
emissions, frequent use of private power-generating sets, and indiscriminate and
open burning of solid waste have contributed to air pollution. In Nigeria, there is a
paucity of research and data on air pollution studies that incorporate either singly or
jointly the effects of land use, road density, road intersection density, and elevation.
Furthermore, available data on ambient air quality are concentrated around major
roads and industrial establishments. In contrast, traffic-related air pollution monitoring
and modelling in developed countries employ different sampling strategies and
statistical analysis based on the availability of air monitoring networks; however, the
influence of land-use diversity, road density, road intersection density, and elevation
in moderating not only the spatiotemporal dynamics of air pollution emanating from
vehicular movement but also in siting such monitoring stations has not been
adequately addressed. This study, therefore, was designed to investigate the
influence of land-use diversity, road density, road intersection density, and elevation
on selected air pollutants in the Lagos metropolis, to understand the spatiotemporal
dynamics of the air pollution emanating from the vehicular movement in the Lagos
metropolis.
An existing land-use map of the area of study was obtained from the Ministry of
Physical Planning, Lagos State, while a contemporary land-use map was developed
from an extremely high-resolution satellite image of the study area. Road network
data was obtained from OpenStreetMap.org, while elevation data was obtained from
Shuttle Radar Topographical Mapping (SRTM). Data from OpenStreetMap.org
provided information on road length, road density, and road intersections. A grid of
100 x 100 metres was overlaid on the road network to obtain the road density and
intersection (node) density, while the SRTM was processed to obtain elevations as
percentages. A weighted linear combination method was used to combine all these
data layers and subsequently classify the resulting map into three categories. This
classified map was subsequently overlaid on the land-use map. Eighty-five percent of
each grid within the different land-use types were selected. The centre (centroid) of
each grid was a location where air quality data were obtained. Ambient air quality
parameters collected at these locations were CO, NO2, and PM2.5. Sample readings
were taken using air quality monitoring equipment from 08:00 to 20:00 hrs over a
period for 30 days, in both wet and dry seasons, in two selected local government
areas (LGAs), namely Ikeja LGA and Lagos Mainland LGA. Vehicular counts were
conducted simultaneously.
A Pearson correlation analysis was used assess the correlation of the daily average
pollutant concentrations compare to traffic density. CO and NO2 concentrations
showed positive correlations (p<0.05) with traffic density in the sampling zones. A
Wilcoxon test was used to analyse regional variations in the observed measurements
across different land uses and seasons in the two selected LGAs. On the whole,
significant variations in the air quality parameters were recorded at different times of
the day and in different land-use areas within the two LGAs of the Lagos metropolis.
In some cases, pollutant concentrations, i.e. PM2.5, were higher for the wet than for
the dry season, which is attributed to excessive delays of vehicles due primarily to
traffic congestion associated with poor physical planning, lock-up shops along the
highways and arterial roads, and a poor road maintenance culture. Combined
vehicular traffic density analysis revealed its contribution to the seasonal variations
(p<0.05). Lastly, a land-use regression model was used to estimate the levels of
ambient air pollutants and determine the possible predictors of the air pollutants,
especially land use, within the two LGAs of the Lagos metropolis. The result confirms
that land use is an important independent variable in the determination of PM2.5
concentrations, especially during the wet season.
The results underscore the need to consider land use and time of day when
assessing the impact of traffic on ambient air quality. Furthermore, there is a need to
regularly monitor the micro-variations and dynamics of ambient air quality parameters
in the Lagos metropolis.