Population-scale COVID-19 curfew effects on urban black carbon concentrations and sources in Kigali, Rwanda

COVID-19 lockdowns enabled researchers to examine air quality in response to economic shifts. However, sub-Saharan African studies are limited, and lockdowns in tropical countries coincided with the wet season. We provide the first record of variations (including diurnal change, daytime and nighttime curfew) in concentrations of black carbon related to fossil fuel burning (BCff) and biomass burning (BCbb) for 2020, focused on three periods: before lockdown (P1: February 8–March 21), lockdown (P2: March 22–May 3), and after lockdown (P3: May 04–June 14) in 2020, with the same periods in 2018 and 2019 used as a control. The annual average BC concentration in Kigali City was 7.8 μg/m3 in 2020. During curfew hours, BCff was reduced by 59% between P2 and P3 (not observed in previous years), which indicates that the lockdown improved transport-related air pollution in Kigali only during curfew hours. No significant change was observed in BCbb concentrations. When COVID-19 restrictions were lifted, black carbon exceeded the same period in previous years. There is evidence for a link between the decline in Rwanda's vehicle use and a reduction in BCff, suggesting that air pollution in African cities could be reduced by promoting sustainable transportation.