Natural experiments in urban air quality: lessons from car-free days and COVID-19 lockdowns in Kigali, Rwanda

A lack of long-term air quality monitoring data in African countries such as Rwanda poses a significant challenge as urbanization leads to declining air quality. This study uses four years of data on particulate matter air pollution (PM _2.5) to understand the current drivers of air pollution, the success of current interventions and the potential for further actions. PM _2.5 data were collected using low-cost and reference monitors in two sites in Kigali. Results show that PM _2.5 levels in Kigali exceeded the recommended WHO air quality guidelines. Using the COVID-19 lockdown as a natural experiment, we find that reduced travel activity of over 80% led to PM _2.5 levels declining by 33%, suggesting that transport may account for a smaller share of particulate emissions than is assumed in government literature. We also find that a program to encourage non-motorized transport in Kigali called ‘Car-Free Days’ reduced by PM _2.5 15% when it was held between 2017 and 2020. This reduction is expected to have resulted in more than 200 disability-adjusted life years saved in Kigali annually, about 150 hospital visits, and 600 lost working days being avoided. We conclude by reflecting on the policies for improving air quality in Kigali City.