Monitoring the Diurnal and Seasonal Variation of Ambient of Ambient Particulate Matter (PM2.5) using Low-Cost Sensors in Juja, Kenya
Authors: Kanyeria Ndiang'ui Josephine , Njogu Paul , Westervelt Daniel 
Published: February 2023

Air pollution is a major environmental concern that affects human health worldwide. Despite recent studies indicating ambient air pollution is a growing global concern strongly linked to rapid global urbanization, little has been done to monitor the air quality levels in Africa. Traditionally, air quality monitoring has relied on environmental monitoring stations, that are expensive to build and maintain. In Kenya, for example there is no publicly available national air quality monitoring data. Thus, low-cost sensors offer a practical and cost-effective means of monitoring air quality. We carried out a study in Juja town, located in central Kenya within the outskirts of Nairobi. Juja is one of the largest growing towns and is located along the busy Thika Superhighway. The purpose of this study was to assess the diurnal and seasonal variations of Ambient Particulate Matter (PM2.5) in Juja, Kenya. The data was collected as from November 2019 to April 2021 at JKUAT Institute of Energy and Environmental Technology (IEET) department, a residential area within Toll and Kibariti and an additional site along the busy Thika Superhighway. The PM level was measured using the Purple Air Monitoring Sensor &#8211; PA-II-SD in &#956;g/m3 on a 24hour cycle. The PM2.5 level from the low-cost Purple Air Sensors were later calibrated against a reference BAM-1022 to yield corrected PM values. The results revealed that the overall PM2.5 concentration was higher during the dry season (June - August 2020) compared to March - May 2020 (wet season) where it dropped by 5-10&#956;g/m3 on average. The average daily PM2.5 levels were recorded at 44&#956;g/m3 (Pine Breeze), 20&#956;g/m3 (Toll) and 16&#956;g/m3 (Kibariti) all exceeding the WHO guideline of 15&#956;g/m3. JKUAT had an annual mean concentration of 15&#956;g/m3, also exceeding the WHO guidelines of 5&#956;g/m3. The study also found that PM2.5 levels were highly correlated with vehicle emissions, as the site closest to the highway had the highest PM2.5 levels. The levels also peaked twice a day at 5am and 5pm, possibly due to morning and evening traffic. It is thus evident that traffic related emissions are a great concern within the town and effective mitigation measures are needed to protect the residents. In addition, comparing the month of April 2021 to the previous year, the daily mean dropped by 5-10&#956;g/m3 &#8211; the period of the new Covid -19 lockdown. These results can then be used to model and predict urban air quality within the town. Overall, low-cost sensors have provided an increased availability of data that can be used to identify patterns and trends in air quality over time. Their use can also facilitate greater community engagement, as individuals and organizations can participate in data collection, monitoring and analysis. To conclude, this research provides an important tool for informing urban planning and environmental policies. By understanding the sources, patterns and impacts of air pollution, decision makers can develop strategies to address these issues and improve the health and well-being of urban residents.