[LaputanMachine]

Multiple physical effects are responsible for filtering. Each of them works best at certain particle diameters.

The combined efficiency of all effects happens to have a minimum at 0.3 microns. [1] This is called the "Most Penetrating Particle Size" (MPPS).

For very small particle sizes, filtering mostly works by diffusion (quoting from [2]):

>[Diffusion] is a result of the collision with gas molecules by the smallest particles, especially those below 0.1 μm in diameter. The small particles are effectively blown or bounced around and collide with the filter media fibers. This behavior is similar to Brownian motion and raises the probability that a particle will be stopped [...]

[1]: https://en.wikipedia.org/wiki/HEPA#/media/File:Filteration_C...

[2]: https://en.wikipedia.org/wiki/HEPA#Mechanism

[schiffern]

In the paper[0] they define "ultrafine emissions" as <0.1 microns.

"Diesel Oxidation Catalyst (DOC) and Diesel Particle Filter (DPF) has been very effective in controlling the emissions of solid particles and hence BC, but that there has been little change in the liquid mode (<30 nm) particles, and that concentrations of ultrafine particles (<100 nm) still well exceed the threshold for “high” concentrations (>10e4 cm^−3 /24-hour mean) defined by WHO. BC declined by 81% between 2014 and 2021, but the ultrafine particle (<100 nm) count declined by only 26%.

Looks like DPF/DOC filters use mainly interception, but they're not terribly effective at filtering via the diffusion mechanism.

[0] https://www.sciencedirect.com/science/article/pii/S016041202...