An assessment of ambient air quality in a densely populated urban settlement of Harare, Zimbabwe

Abstract

Poor air quality in urban settings poses a significant global challenge with adverse environmental-health effects. In Zimbabwe, a critical knowledge gap exists regarding specific characteristics and seasonal variability of ambient air pollution in low-income, densely populated settlements, with existing studies often relying on indirect assessments. This stems from the lack of a national monitoring network, largely due to the prohibitive cost of research-grade instruments. The purpose of the was to demonstrate a viable approach to generate localized data needed to fill the national data gap through the deployment of a low-cost sensor in a representative densely populated urban settlement of Cold Comfort, Harare over a 12-month period. The deployed sensor measured particulate matter concentrations (PM_2.5 and PM₁₀) at 5-minute temporal resolution, which were then aggregated to hourly averages and analysed using open-air in R statistical packages. Results show that ambient particulate matter concentrations were elevated during winter, with annual means of 34.1 µg/m³ for PM2.5 and 58.1 µg/m³ for PM10. These levels significantly exceeded the WHO annual mean guidelines of 5 µg/m³ and 25 µg/m³ for both PM₂₅ and PM_10, highlighting local air quality concerns. A HYSPLIT trajectory analysis of a peak winter pollution event suggested that the high concentrations were a combination of long-range pollutant transport and enrichment from local emission sources. The low-cost sensor performance was evaluated against gravimetric measurements using Mean Absolute Error, Root Mean Square Error, and coefficient of determination. The low-cost sensor consistently under-estimated PM₁₀ concentrations, showing a Mean Absolute Error of 14.2 µg/m³, Root Mean Square Error of 19.7 µg/m³, and an R² of 0.47. Despite accuracy limitations, the low-cost sensor provided a useful overview of pollution levels. As the first long-term, campaign-based study of its kind in Zimbabwe, these findings are vital for informing air quality management policy and developing targeted interventions.