Spatial and temporal variation of atmospheric particulate matter in Nyanza town and Kigali City in Rwanda

Abstract

Exposure to atmospheric particulate matter remains the global environmental cause of premature human mortality in developing countries. This is a particular concern in African countries such as Rwanda, with rapid urbanization and population growth. However, little is known about the spatial and temporal variation of these microscopic air particles in Rwanda. This thesis provided the first spatial and temporal variation of fine and coarse particulate matter with an aerodynamic diameter ≤ 2.5 μm and ≤ 10 μm (PM2.5 and PM10) in two sites (urban and rural) located in a large city (Kigali) and small town (Nyanza). PM2.5 and PM10 data were collected using the low-cost air quality sensors for a four months’ period in the wet and dry season in 2021. The results showed that the 24 hour-mean concentrations of PM2.5 and PM10 were higher in the dry season than in the wet season at all sites. The 24-hourmean concentrations of PM2.5 and PM10 were higher in a rural site in Kigali (40.43 µg/m3, 47.83 µg/m3) than in a rural site in Nyanza Town (25.54µg/m3, 29.57µg/m3), respectively. The 24-hour means of PM2.5 and PM10 at all sites were more than two higher than the World Health Organization air quality guidelines for 24-hour mean. The rural site of Kigali City exhibited higher concentrations of PM2.5 and PM10 than the urban site, suggesting an impact of COVID-19 lockdown in Kigali City. In contrast, the higher PM2.5 and PM10 concentrations were observed in urban than rural site in Nyanza Town. These results prove that vehicle emissions contributed to air pollution in urban areas while biomass burning highly polluted the rural areas. Spearman correlation showed that there was no influence of meteorological conditions on air pollution at all sites and the results indicated that measures to control air pollution in large cities should be also applied in small towns. Further long-term studies are required and should cover many sites and include other meteorological conditions such as wind speed and direction.