A solid particle number measurement system (SPNMS) was developed
using a catalytic stripper (CS) technology instead of an
evaporation tube (ET). The ET is used in commercially available
systems, compliant with the Particle Measurement Program (PMP)
protocol developed for European Union (EU) solid particle number
regulations. The catalytic stripper consists of a small core of a
diesel exhaust oxidation catalyst. The SPNMS/CS met all performance
requirements under the PMP protocol. It showed a much better
performance in removing large volatile tetracontane particles down
to a size well below the PMP lower cut-size of 23 nm, compared to a
SPNMS equipped with an ET instead of a CS. The SPNMS/CS also showed
a similar performance to a commercially available system when used
on a gasoline direct injection (GDI) engine exhaust.
While both the SPNMS/CS and the SPNMS/ET will likely perform
similarly when used within the confinement of Euro 6/VI
regulations, the SPNMS/CS has some advantages over an SPNMS/ET in
its ability to evaporate and oxidize the volatile hydrocarbon
materials instead of simply transferring them to the gas phase as
done with the ET. Oxidizing the volatile hydrocarbon species
promotes faster droplet evaporation and reduces any potential
artifact particle formation downstream of the CS, as compared with
the ET, during dilution and/or cooling. This performance
characteristic may give the SPNMS/CS a wider flexibility for
measuring solid particle number from various combustion sources
with high volatile particulate matter, well beyond the current
scope of regulatory requirements. Relative to the potential
sulfuric acid formation downstream of the CS, we were able to
demonstrate in previous work that sulfuric acid formation does not
occur at the CS operating temperature of 300°C. In addition, using
synthetic gas containing high concentration of SO₂ in air (~80
ppm), this CS showed high adsorption of SO₂ with no sign of SO₃
formation at its operating temperature of 300°C.