Polycyclic aromatic hydrocarbons in PM_2.5 and PM₁₀ in Lagos, Nigeria: Sources, characteristics, and health risks

Particulate matter (PM) bound polycyclic aromatic hydrocarbons (PAHs) were assessed across Lagos, Nigeria for their spatial-temporal variations, sources, and health risks. PM samples were collected with low-volume samplers at six sites during 12 months and analyzed by GC-MS for 16 PAHs. Sources were identified using Positive Matrix Factorization (PMF), and health risks were assessed using the United States Environmental Protection Agency's Incremental Lifetime Cancer Risk (ILCR) methodology with site-appropriate exposure times following ATSDR guidelines. Annual mean concentrations of the sum of 16 PAHs (Σ16PAH) were 44.9 ± 38.5 ng/m³ in PM_2.5 and 50.3 ± 42.6 ng/m³ in PM₁₀ across Lagos. Dry season values (PM_2.5 PAHs: 76.2 ng/m³, PM₁₀ PAHs: 85.7 ng/m³) were >3 times the wet season values (21.9 and 24.1 ng/m³, respectively). These concentrations are higher than in many cities but comparable to and generally lower than the most polluted megacities. Industrial and port-adjacent sites consistently recorded the highest levels, while suburban/coastal sites were the lowest. PMF identified factors for PM_2.5 as diesel (18%), gasoline (15%), petroleum handling (14%), industrial (14%), marine (14%), biomass burning (13%), and background (12%). For PM₁₀, factors include biomass burning (21%), background (20%), petroleum (15%), diesel (14%), industrial (14%), gasoline (8%), and marine (7%). Most of the Σ16PAH (89%) occurred in PM_2.5. Meteorological factors, particularly temperature influenced PAH concentrations by affecting the gas-particle partitioning of semi-volatile PAHs, dispersion, and removal processes, with particle-bound concentrations typically higher in cooler periods. Health risk assessments estimated ILCRs of 2.0 to 6.1 × 10^-6, exceeding the USEPA benchmark (1 × 10^-6) by 2-6 times at all sites. The corresponding loss of life expectancy ranged from 12 to 38 min over a 30-year exposure period with residential and industrial zones most affected. These results highlight the need for targeted emission controls and health-focused policies in Lagos.