First Ambient PM2.5 Monitoring in the City of Mombasa, Kenya using a hybrid network of reference monitors and air sensors

The paucity of surface measurements of fine particulate matter (PM_2.5) limits estimates of air pollution mortality in Sub-Saharan Africa. If well calibrated, low-cost sensors have the potential to provide reliable data especially where reference monitors are unavailable. We evaluate the performance of Clarity Node-S PM sensors against a Tapered element oscillating microbalance (TEOM) and develop a calibration model for sensor correction in Mombasa. Raw Clarity Node-S data from January 2023 through April 2023 was moderately correlated with the TEOM-1400a measurements (R²=0.609) but also exhibited a mean absolute error (MAE) of approximately 7.026 µg m^–3. Employing three calibration models, namely, multiple linear regression, gaussian mixture regression (GMR) and random forests (RF) decreased the MAE to 4.3, 3.9, and 4.4 µg m^–3 respectively. The R² score improved to 0.63 for the MLR model but all other models registered a decrease in precision (R²=0.44 and 0.6 respectively). The correction factor was then retrospectively applied to a 6-sensor Clarity network in Mombasa that operated between July 2021 and July 2022 to provide an overall survey of the air quality data. The locations represented different diverse environments in Mombasa: University of Nairobi-Mombasa Campus (urban site), JKUAT (urban site), Changamwe (industrial site), Bamburi (industrial site), Nyali (residential site), and Vescon (residential site). Daily mean PM2.5 at the two industrial sites were substantially elevated (up to 20 µg m^-3) compared to the residential sites (~12 µg m^-3) and urban sites (~15 µg m^-3). Concentrations in all sites followed a 6am and 6pm peak, suggesting influences from anthropogenic activities such as traffic emissions. Our results represent the first ever ambient reference and low-cost monitoring in Mombasa and highlight the need for air quality management planning in the rapidly growing Kenyan city.