I have some very heavy doubts about the validity of their data. Look at their primary findings:

"Using a popular family hatchback running on brand new, correctly inflated tyres, we found that the car emitted 5.8 grams per kilometer of particles."

Most tires seem to be around 7kg, which means according to their data a car would have all 4 tires reduced to nothing after 4828km. With most tires going bald after ~50,000km (not fully vaporized mind you), there's clearly something wrong here.

My guess is they tested brand new tires which may shed particulate quickly at the start, similar to how a new carpet sheds exponentially more at the start.

There's more information here:

https://www.ukimediaevents.com/publication/955cc42f/30

They said that all four tires combined lost 1.8kg, which implies they only drove 340km? Also, this is literally based on measuring the weight of the tires apparently, so how much of that became PM2.5 class particles is a mystery.

The article also mentions "track testing", which if used for the test, would easily explain the enormous material lost after a short time. Track racing is very different than highway driving...

It's also the material on the road being heated and ground up, not just the material from the tyres themselves.

I think you're right, new tires have a lot of flash and channel runners that are artifacts of the rubber molding process and are she'd very early in the tire's life.

Is the rubber oxidizing? Some of the mass might be coming from the air.