Air pollution in Kigali, Rwanda: spatial and temporal variability, source contributions, and the impact of car-free Sundays

Abstract

Ambient air pollution, particularly fine particulate mass (PM25) and ozone (03), is associated with premature human mortality and other health effects, but monitoring is scarce to non-existent in large parts of Africa. Lower-cost real-time affordable multi-pollutant (RAMP) monitors and a black carbon (BC) monitor were deployed in Kigali, Rwanda to fill the air quality data gap here. PM25 data were corrected using data from a coincident, short-term campaign that used standard filter-based gravimetry, while gas data were verified by collocation with reference carbon monoxide (CO) and 03 monitors at the Rwanda Climate Observatory at Mt Mugogo, Rwanda. Over March 2017-July 2018, the ambient average PM25 in Kigali was 52 μg/m3, significantly higher than World Health Organization (WHO) Interim Target 1. Study average BC was 4 μg/m3, comparable to mid-sized urban areas in India and China and significantly higher than BC in cities in developed countries. Spatial variability across various urban background sites in Kigali appears to be limited, while PM25 at Mt Mugogo is moderately correlated with PM25 in Kigali. A sharp diurnal profile is observed in both PM25 and BC, with the Absorption Angstrom Exponent (AAE) indicating that the morning peak is associated with rush-hour traffic-related air pollution (TRAP) while the late evening peak can be attributed to both traffic and domestic biofuel use. PM25 in the dry seasons is about two times PM25 during the following wet seasons while BC is 40-60% higher. Local sources contribute at least half the ambient PM25 during wet seasons and one-fourth during dry seasons. Traffic restrictions on some Sundays appear to reduce PM25 and BC by 10–12 μg/m3 and 1 μg/m3 respectively, but this needs further investigation. Dry season ozone in Kigali can exceed WHO guidelines. 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.