Fine-scale variations in black carbon concentrations and the chemical composition of total suspended particulate matter at a heavy traffic road intersection in Addis Ababa, Ethiopia.

Abstract

Atmospheric aerosols in Ethiopian urban environments remain poorly characterized. This study investigated the spatiotemporal variability of equivalent black carbon (eBC) concentrations and the molecular composition of organics in total suspended particulate matter (TSPM) collected in urban Addis Ababa. The level of eBC was monitored at major traffic intersections from January 2024 to December 2025. eBC concentrations were measured using a portable MicroAeth AE51. TSPM samples were collected on Teflon filters using a Leland Legacy sampler and analyzed by reversed-phase liquid chromatography coupled to electrospray ionization interfaced to quadrupole time-of-flight mass spectrometry (RPLC-ESI-QTOFMS) operated in the negative ion mode. The study revealed substantial spatiotemporal variability in eBC concentration, with a mean of 26.24 ± 42.81 µg m^-3. Episodic peaks exceeding 1000 µg m^-3 were recorded at heavily trafficked locations such as Kality. Diurnal patterns exhibited distinct morning and evening rush-hour maxima. Non-targeted analysis identified 74 compounds (with molecular weights spanning from 102 to 370 g mol^-1), 41 of which were structurally assigned with high confidence. The organic fraction of TSPM was dominated by oxygenated (CHO) and nitrogen-containing (CHON) species, including carboxylic acids, oxocarboxylic acids, phenolics, aromatic acids, and nitro-aromatic compounds. Spatial heterogeneity was evident, particularly at Awotobistera, where it showed enrichment in biomass-burning tracers and lignin-derived compounds. Twenty-three identified compounds functioned as brown carbon chromophores, including nitro-aromatic and oxygenated aromatic species. Their occurrence suggests contributions from both primary combustion sources and atmospheric processing, although source apportionment was beyond the scope of this study. These findings provide a baseline for future investigations of aerosol sources, atmospheric processing, and urban air quality dynamics in Addis Ababa.