Spatial-temporal variation of particulate matter less than 2.5 microns (PM2.5) concentration in Kigali

Abstract

In our study, PM2.5 concentration data of two monitoring sites in Kigali were collected continuously from 06th of July 2017 to 31st of May 2018. These data were used to investigate the diurnal, seasonal and day of week characteristics of PM2.5 in Kigali. Special days and meteorological parameters were also studied for their contribution to the variation of PM2.5 levels in Kigali. Observations of hourly PM2.5 showed a distinct diurnal, seasonal, and day of week patterns. The diurnal variations of PM2.5 were observed with higher concentration at night than daytime, with the peak in the morning between 6:00 am and 8:00 am exactly during the rush hour. The lowest values appeared in the late afternoon between 4pm and 5pm. The probable reason of the higher concentrations at night is the built up of particles under inversion conditions and/or atmospheric stability. The reduction in traffic volume in the afternoon, dilution of aerosols associated with the deeper Planetary Boundary Layer depth and higher wind speed are responsible of the lowest values in afternoon hours. Regardless of the month or weather conditions, PM2.5 concentrations across the city are significantly lower on Saturdays and Sundays and high on the other days of the week. This comes from the decrease in human activities, and implementation of the Umuganda and Car free days on weekends. The observed facts indicate that the variations of PM2.5 concentrations are also influenced by meteorological factors such as high wind speed and abundant rainfall that played a big role in reduction of PM2.5 levels in wet seasons compared to the dry seasons.