Comparison of PM_2.5 pollution between an African city and an Asian metropolis

Fine particulate matter (PM_2.5) samples were collected simultaneously at urban sites in Lagos (ULG site) and in Hong Kong (TC site) for four consecutive weeks in July and August 2017, in order to investigate the potential to apply successful pollution control measures from Hong Kong to Lagos. To compare chemical characteristics and sources of PM_2.5 in these two regions, organic carbon (OC), elemental carbon (EC), water soluble ions (WSIs), and elements were analyzed for the first time. It was found that total carbon and elements were much more abundant (p < 0.05) at ULG, indicating severer PM_2.5 pollution in Lagos, while levels of WSIs were comparable (p ≥ 0.05) at both sites. Higher correlation coefficient (0.79) between OC and EC but lower OC/EC ratio (1.81 ± 0.18) at ULG (TC: 0.48; 3.51 ± 0.60) revealed the dominant role of primary sources in Lagos. Furthermore, examination of secondary organic carbon (SOC)/OC ratio implied that only 12 ± 8% of OC were attributable to secondary organic formation at ULG whereas 47 ± 9% at TC. Positive matrix factorization (PMF) model resolved six PM_2.5 sources at each site, among which vehicular emissions contributed the most (32.2 ± 3.18%) at ULG, while secondary inorganic aerosols (including secondary SO₄ ²⁻ and NO₃ ⁻) together with regional biomass burning (36.5 ± 5.21%) dominated at TC. Sea salt source was significant at both harbor cities. For inter-comparison, the sum of vehicular emission and fugitive dust accounted for ~40.9% at ULG, triple that at TC (p < 0.01) in concentration. Severer primary PM_2.5 pollution especially the street-level pollution in Lagos called for effective control measures, such as periodical upgrade of fuel and retrofits on vehicles, which have been successfully promoted in Hong Kong.