The physico-chemical characteristics of particulate matter (PM) in African cities remain poorly known due to scarcity of observation networks. Magnetic parameters of PM are robust proxies for the emissions of Fe-bearing particles. This study reports the first magnetic investigation of PM2.5 (PM with aerodynamic size below 2.5 m) in Africa performed on weekly PM2.5 filters collected in Abidjan (Ivory Coast) and Cotonou (Benin) between 2015 and 2017. The magnetic mineralogy is dominated by magnetite-like low coercivity minerals. Mass normalized SIRM are 1.65e−2A□m/kg and 2.28e−2A□m/kg for Abidjan and Cotonou respectively. Hard coercivity material (S-ratio = 0.96 and MDF = 33 mT) is observed during the dry dusty season. Wood burning emits less iron oxides by PM2.5 mass when compared to traffic sources. PM2.5 magnetic granulometry has a narrow range regardless of the site or season. The excellent correlation between the site-averaged element carbon concentrations and SIRM suggests that PM2.5 magnetic parameters are linked to primary particulate emission from combustion sources.

Ultrafine grains of magnetic minerals provide reliable recordings of both naturally occurring and anthropogenically generated particulate matter in polluted air; magnetic data can be used to understand biogenic iron-cycling in anaerobic environments, as well as pedogenesis and palaeoclimate studies of loess soils. The ultrafine fraction is produced under specific conditions and can be easily recognized by its superparamagnetic (SP) behaviour. Many proxies have been proposed to account for the SP contribution by measuring its susceptibility dependency with frequency (frequency effect) or the magnetization loss after removing an external inducing field. Here we introduce the Superparamagnetic Concentration and Dipole Moment (SPCDM) procedure for quantitative interpretation of SP magnetization. This procedure is well suited to SP carriers with a fast magnetization decay (<1 s), as would be expected for magnetic minerals with a grain size distribution lying below the blocking volume for stable, single-domain (SD) magnetization. SPCDM requires a dedicated experimental procedure to isolate the SP response from the paramagnetic and remnant effects, as observed in samples with mixed contributions. The proposed technique was tested using synthetic, nanoparticles of magnetite and then to characterize the magnetic properties of air particulate matter (PM) sampled at Jânio Quadros tunnel in São Paulo, Brazil. For nano-sized magnetite, SPCDM estimates for dipole moment are invariable with mass concentration and consistent with the published results; estimates for particle concentration are strongly correlated with true mass concentration (R 2 = 0.96). For air PM, SPCDM estimates a particle size with a diameter of 7.7 ± 0.1 nm, a kind of ultrafine magnetic material not previously recognized in air pollutants.

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