Impact of Urbanisation on Local Climate and Air Quality in Lagos, Nigeria

Abstract

The impacts of urbanisation on local climate and air quality in Lagos has been investigated in this study. The aim of the study is to measure, and analyze how urbanisation of the Lagos metropolitan area has impacted the city's climate and air quality in terms of air temperature, humidity and CO concentration by determining the trend, location and magnitude of landuse/cover changes in Lagos, evaluating the temporal variation in land surface temperature, assessment of the spatial and temporal dynamics of the urban heat island (UHI) in Lagos, evaluation of the spatial and temporal characteristics of carbon monoxide concentration in Lagos metropolis, investigate the impact of the UHI on thermal comfort and energy demand; and to develop heat vulnerability index map for Lagos. The study was conducted between May 2014 and June 2015 using both remotely sensed data and in situ observations. Remotely sensed data derived from 1984 Landsat 5 TM, 2000 Landsat 7 ETM+ and 2013 Landsat 8 OLI imageries and in situ observation data on temperature, relative humidity, wind speed and direction, solar radiation and carbon monoxide measured with Hygrocron/thermocron/CO sensors/loggers and automatic weather station which were installed in 14 locations in the city has been used. 200 copies of open ended questionnaire was administered in the area to household heads for heat vulnerability assessment. The results of the study shows that between 1984 and 2013 in the Lagos area, that water bodies, forested wetlands, and marshland decreased while the urban built-up areas and the less forested wetlands increased. NDVI was positive for years 1984 and 2013 but negative for year 2000. Land surface temperature (LST) shows an increasing trend with r2 of 0.5 between years 1984-2000 and 2000-2013. UHI exists throughout the day in all months of the year except March, October, November and December where Urban Cool Island (UCI) was observed few hours in the afternoon. The maximum intensity of the UHI was observed at night, about three hours after sunset and it persists till the early hours of the morning, and can exceed 7 °C during the dry season with lower intensity (3.5 °C) during the wet season. However, calm wind (0-0.3 m/s) intensifies the UHI while gentle to moderate breezes (3.4-5.4 m/s) tend to reduce it. Relative humidity was generally higher in the rural area than in the urban area. The Thermohygrometric index (THI) shows that the core urban area was far less thermally comfortable than other locations including the rural area but some times during daytime period in the dry season, even the rural area becomes less thermally comfortable. This gives rise to higher energy demand for cooling in the urban area than rural environ as higher cooling degree day (CDDs) occurs in the urban area especially during the dry season. The carbon monoxide analysis reveals that the 15 minutes, 30-minutes and 1 hour averages were within WHO limits but the 8-hour average exceeded the recommended threshold by over 2 ppm. Finally, the result of the heat vulnerability index mapping shows that the areas with high vulnerability to heat stress in Lagos are Yaba and Mushin. The areas with medium vulnerability to heat stress are Isolo, Oshodi, Ejigbo and Ajangbadi while areas of low vulnerability to heat stress are Ilupeju, Shomolu, Alagbado, Ikotun, and Amuwo-Odofin. The areas that are not significantly vulnerable to heat stress are Abule-Egba, Oko-afo, City hall and Cash and Carry because of high adaptation means. The study concludes that changes in land use/cover is driven by urbanisation and has led to modification in the microclimate of the city thereby leading higher nocturnal UHI intensity levels as a result of high temperature difference between urban and rural area, poor air quality, thermal discomfort, heat stress and high energy demand for cooling in Lagos. Therefore, the study recommends that temperature increase in urban areas should be consistently monitored and smart ways to mitigate the urban heat island effect mainstreamed into climate policy and should include low carbon development strategies such as efficient transportation systems, use of reflective materials for urban building designs and roads and greening of the city.