Car-free days lead to cleaner air

Every other Sunday in Kigali, Rwanda, instead of driving to shops or public parks, people walk or ride bicycles. Instead of sitting in traffic, they gather together in green spaces to participate in organized sports. Established in 2016 by the Kigali city council, these car-free days are designed to promote the health of Kigali citizens by encouraging physical activity and a sense of community. But in addition to physical activity, these car-free days are having a positive effect on public health.

Using real-time affordable multi- pollutant (RAMP) monitors, a team led by R. Subramanian of the Center for Atmospheric Particle Studies (CAPS) and Paulina Jaramillo of Engineering and Public Policy (EPP) has shown that these car-free days appear to reduce ambient air pollution, particularly fine particulate mass (PM2.5) and black carbon (BC), both of which are associated with premature human mortality and other negative health effects. The team also included Department of Engineering and Public Policy Department Head Peter Adams, Department of Mechanical Engineering Department Head Allen Robinson, EPP doctoral student Nathan Williams, and CAPS postdoctoral researcher Carl Malings, as well as collaborators at the University of Rwanda. Subramanian led the development of RAMP monitors in partnership with a local start-up, SenSevere.

“The World Health Organization estimates that in 2016, ambient air pollution caused about three thousand deaths in Rwanda,” the team writes in the paper. “However, such estimates can be uncertain because exposure is inferred from satellite estimates. There has been no long-term ground-based monitoring in major cities like Kigali to validate estimated exposures. This lack of monitoring due to resource limitations also hampers scientific understanding of the sources contributing to air pollution in these countries, which is essential to formulating effective environmental management policies.”

To fill in this gap in air quality data, the team deployed their RAMP monitors around Kigali. Through their measurements, the team found that from March 2017 to July 2018, the average ambient PM2.5 in Kigali was significantly higher than the World Health Organization’s recommended targets, and the average BC was comparable to mid- sized urban areas in India and China, and significantly higher than similar-sized cities in developed countries.

Certain observed fluctuations in measurements suggest that morning peaks are associated with rush-hour traffic related air pollution, while late evening peaks can be attributed to both traffic and domestic biofuel use, suggesting that traffic restrictions could go a long way toward improving the air quality in Kigali. And indeed, these car-free days do appear to reduce both PM2.5 and BC by 10 – 12 and 1 micrograms per cubic meter respectively, though according to the team, this requires further investigation.

“While these car-free days do appear to moderately reduce certain air pollutants,” says Jaramillo, “there is still a long way to go. These lower-cost monitors can play an important role in the continued monitoring essential to track the effectiveness of pollution-control policies recently implemented in Rwanda and other countries in Africa.”

Subramanian adds, “Due to the high cost of traditional air-quality monitoring, there is a significant infrastructure gap in many parts of the world, including many African countries. Low-cost sensors can fill in that gap as part of a cost-saving hybrid network that includes traditional (but expensive) reference-grade monitoring. Following our study, the Government of Rwanda set up their own hybrid air-quality monitoring network including the RAMPs. Our success in Rwanda was facilitated by Carnegie Mellon Africa, CMU faculty discretionary funding, and College of Engineering support. It has led to the creation of the new AfriqAir network, expanding our efforts to several other cities in Africa. We look forward to working with local partners and governments to improve air quality and reduce premature mortality across Africa, and welcome philanthropic support for these efforts.”