Spatial-temporal patterns of ambient fine particulate matter (PM_2.5) 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^-5m^-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 <i>μ</i>g m^-3 at a peri-urban site to 43 <i>μ</i>g m^-3 at a commercial, business, and industrial (CBI) site. CBI areas had the highest PM_2.5 levels (mean: 37 <i>μ</i>g m^-3), followed by high-density residential neighborhoods (mean: 36 <i>μ</i>g m^-3), while peri-urban areas recorded the lowest (mean: 26 <i>μ</i>g m^-3). Both PM_2.5 and BC levels were highest during the dry dusty Harmattan period (mean PM_2.5: 89 <i>μ</i>g m^-3) compared to non-Harmattan season (mean PM_2.5: 23 <i>μ</i>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 <i>μ</i>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.