Environmental Research Letters (Jan 2021)
Spatial-temporal patterns of ambient fine particulate matter (PM2.5) and black carbon (BC) pollution in Accra
Abstract
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 _2.5 ) 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 _2.5 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 (10 ^−5 m ^−1 ) 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 _2.5 concentrations. The mean annual PM _2.5 across the fixed sites ranged from 26 μ g m ^−3 at a peri-urban site to 43 μ g m ^−3 at a commercial, business, and industrial (CBI) site. CBI areas had the highest PM _2.5 levels (mean: 37 μ g m ^−3 ), followed by high-density residential neighborhoods (mean: 36 μ g m ^−3 ), while peri-urban areas recorded the lowest (mean: 26 μ g m ^−3 ). Both PM _2.5 and BC levels were highest during the dry dusty Harmattan period (mean PM _2.5 : 89 μ g m ^−3 ) compared to non-Harmattan season (mean PM _2.5 : 23 μ g m ^−3 ). PM _2.5 at all sites peaked at dawn and dusk, coinciding with morning and evening heavy traffic. We found about a 50% reduction (71 vs 37 μ g m ^−3 ) in mean annual PM _2.5 concentrations when compared to measurements in 2006–2007 in Accra. Ambient PM _2.5 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.
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