A procedure for quantitative characterization of superparamagnetic minerals in environmental magnetism
Résumé
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 (R2 = 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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