IME they only wear out maybe 15-20% faster than you'd think. On the other hand, over the span of 40,000 miles, a tire loses a LOT more rubber by weight/volume than a brake pad loses pad material. No idea what the PM2.5 breakdown is though.

The difference is that only about 1% of the worn rubber ends up in the air whereas most of the brake pad ends up in the air. Most of the worn rubber stays on the road. Where it will get washed away by the rain to end up as microplastics in the water.