Spatial-temporal patterns of ambient fine particulate matter (PM) and black carbon (BC) pollution in Accra.

Sub-Saharan Africa (SSA) is rapidly urbanizing, and ambient air pollution has emerged as a major environmental health concern in growing cities. Yet, effective air quality management is hindered by limited data. We deployed robust, low-cost and low-power devices in a large-scale measurement campaign and characterized within-city variations in fine particulate matter (PM) and black carbon (BC) pollution in Accra, Ghana. Between April 2019 and June 2020, we measured weekly gravimetric (filter-based) and minute-by-minute PM concentrations at 146 unique locations, comprising of 10 fixed (∼1 year) and 136 rotating (7 day) sites covering a range of land-use and source influences. Filters were weighed for mass, and light absorbance (10m) of the filters was used as proxy for BC concentration. Year-long data at four fixed sites that were monitored in a previous study (2006-2007) were compared to assess changes in PM concentrations. The mean annual PM across the fixed sites ranged from 26 g m at a peri-urban site to 43 g m at a commercial, business, and industrial (CBI) site. CBI areas had the highest PM levels (mean: 37 g m), followed by high-density residential neighborhoods (mean: 36 g m), while peri-urban areas recorded the lowest (mean: 26 g m). Both PM and BC levels were highest during the dry dusty Harmattan period (mean PM: 89 g m) compared to non-Harmattan season (mean PM: 23 g m). PM at all sites peaked at dawn and dusk, coinciding with morning and evening heavy traffic. We found about a 50% reduction (71 vs 37 g m) in mean annual PM concentrations when compared to measurements in 2006-2007 in Accra. Ambient PM concentrations in Accra may have plateaued at levels lower than those seen in large Asian megacities. However, levels are still 2- to 4-fold higher than the WHO guideline. Effective and equitable policies are needed to reduce pollution levels and protect public health.