Spatial and Temporal Trends of Ambient PM_2.5 and Source-Apportioned Black Carbon in Addis Ababa, Ethiopia

Ambient fine particulate matter pollution (PM_2.5) is the leading global environmental risk factor for human health. This study leverages three years (April 2022-March 2025) of continuous, in situ ambient PM_2.5 and black carbon (BC) measurements from the Multi-Angle Imager for Aerosols (MAIA) investigation's surface monitoring network to characterize particulate air pollution in Addis Ababa, Ethiopia. Continuous PM_2.5 data were collected from 10 PurpleAir sensors deployed across Addis Ababa and calibrated using collocated Beta Attenuation Monitors at two contrasting U.S. Embassy sites (Central and Jacros). Continuous BC data were collected with AethLabs MA350 microAethalometers operating at these two sites. During the study period, the average PM_2.5 concentration across all 10 sites was 30 μg/m³, and the average BC concentrations were 3.7 μg/m³ and 7.7 μg/m³ at Central and Jacros, respectively. Both PM_2.5 and BC concentrations exhibited strong diurnal and seasonal variation, with peaks during morning traffic and nighttime inversion, especially during the wet season. PM_2.5 levels were highest at sites west of the city center and exceeded the WHO guideline for daily PM_2.5 of 15 μg/m³ on 99% of days. The Aethalometer Model for source apportionment estimated that, on average, 94% of total BC in Addis Ababa was attributable to fossil fuel combustion, while 6% was associated with biomass burning. Directional and trajectory analyses indicate distinct local and regional sources of biomass and fossil fuel BC, including potential long-range transport of shipping emissions from the Gulf of Aden. This study presents, to our knowledge, the first long-term, multisite characterization of continuous PM_2.5 and source-apportioned BC in Ethiopia. Findings from this study establish a pollution baseline for this MAIA target area ahead of the satellite's launch, improving understanding of particulate pollution levels and sources and supporting future assessment of air quality trends in this region.