A procedure for quantitative characterization of superparamagnetic minerals in environmental magnetism

Abstract

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.

Magnetic signatures of a creosote oil contaminated site: case study in São Paulo, Brazil

“…We carried out the SP Concentration and Dipole Moment (SPCDM) procedure developed by Leite et al 26 which essentially isolates the SP response from paramagnetic and remnant effects in the magnetic moment decay after the applied field is shut off and by the Langevin function for a magnetization curve, estimates the number of SP particles in the samples. The experimental protocol consists of the application of external fields of amplitudes ranging from 5 to 340 mT.…”

Ultrafine Magnetic Particles: A DIET-Proxy in Organic Rich Sediments?

“…The SPCDM procedure developed by Leite et al (2018) is based on Neél's model for superparamagnetism for which sample magnetization M(B i , T) is dependent of the external applied…”

Barking up the Right Tree: Using Tree Bark to Track Airborne Particles in School Environment and Link Science to Society

“…They have been used to monitor anthropogenic PM trapped on vegetation such as leaves, lichens and bark (Chaparro et al., 2020 ; Dawai et al., 2021 ; Limo et al., 2018 ; Moreno et al., 2003 ; Muñoz et al., 2017 ; Sagnotti et al., 2009 ; Urbat et al., 2004 ; Vezzola et al., 2017 ; Zhang et al., 2008 ). Such methods rely on tracking magnetic minerals, for example, iron oxides like magnetite, hematite and maghemite (Leite et al., 2018 ; Marié et al., 2018 ; Winkler et al., 2021 ). For traffic‐related sources, these magnetic minerals can originate from internal fuel combustion of vehicles, disk brakes abrasion and erosion of the pavement.…”