Abstract
We report the characterisation of anthropogenic magnetic particulate
matter (MPM) collected on leaves from roadside Callistemon trees
from Lahore, Pakistan, and on known sources of traffic-related
particulates to assess the potential of first-order reversal curve
(FORC) diagrams to discriminate between different sources of
anthropogenic magnetic particles. Magnetic measurements on leaves
indicate the presence of surface-oxidised magnetite spanning the
superparamagnetic (< 30 nm) to single-domain
(~30-70 nm) to vortex size range
(~70-700 nm). Fe-bearing particles are present both as
discrete particles on the surface of larger mineral dust or carbonaceous
particles and embedded within them, such that their aerodynamic sizes
may be decoupled from their magnetic grain sizes. FORC diagrams of
brake-pad residue specimens show a distinct combination of narrow
central ridge, extending from 0-200 mT, and a low-coercivity, vertically
spread signal, attributed to vortex and multi-vortex behaviour of
metallic Fe. This is in agreement with scanning electron microscopy
results that show the presence of metallic as well as oxidised Fe.
Exhaust-pipe residue samples display a more conventional
‘magnetite-like’ signal comprising a lower coercivity central ridge
(0-80 mT) and a tri-lobate signal attributed to vortex state and/or
magnetostatic interactions. The FORC signatures of leaf samples combine
aspects of both exhaust residue and brake-pad endmembers, suggesting
that FORC fingerprints have the potential to identify and quantify the
relative contributions from exhaust and non-exhaust (brake-wear)
emissions. Such measurements may provide a cost-effective way to monitor
the changing balance of future particulate emissions as the vehicle
fleet is electrified over the coming years.