Low-Cost Hourly Ambient Black Carbon Measurements at Multiple Cities in Africa

Abstract

There is a notable lack of continuous monitoring of air pollutants in the Global South, especially for measuring chemical composition, due to high cost of regulatory monitors. Using our previously developed low-cost method to quantify black carbon (BC) in fine particulate matter (PM2.5) by analyzing reflected red light from ambient particle deposits on glass fiber filters, we estimated hourly ambient BC concentrations with filter tapes from Beta Attenuation Monitors (BAMs). BC measurements obtained through this method were validated against a reference aethalometer in Addis Ababa, Ethiopia, demonstrating a very strong agreement (R2 = 0.95 and slope = 0.97). We present hourly BC for three cities in sub-Saharan Africa (SSA) and one in North America: Abidjan (Côte d’Ivoire), Accra (Ghana), Addis Ababa (Ethiopia), and Pittsburgh (USA). The average BC for the measurement period at the Abidjan, Accra, Addis Ababa Central summer, Addis Ababa Central winter, Addis Ababa Jacros winter and Pittsburgh sites were 3.85 µg.m-3, 5.33 µg.m-3, 5.63 µg.m-3, 3.89 µg.m-3, 9.14 µg.m-3 and 0.52 µg.m-3, respectively. BC made up 14 – 20% of PM2.5 mass in the African cities, compared to only 5.6% in Pittsburgh. The hourly BC data at these sites show a pronounced diurnal pattern with prominent peaks during the morning and evening rush hours on workdays. Comparison between our measurements and the Goddard Earth Observing System composition forecast (GEOS-CF) estimates shows that the model performs well in predicting PM2.5 for most sites but struggles to predict BC at an hourly resolution. Adding more ground measurements could help calibrate and improve performance of chemical transport models. This method can potentially use existing BAM networks, especially BAMs at US embassies around the globe, to measure hourly BC with our method. The PM2.5 composition data, thus acquired, can be crucial in identifying emission sources and help in effective policymaking in the Global South.