I've seen a study or two that claim that the bulk of some PM, like brake dust for example, is much smaller than 2.5um. Having exposure to the industry, do you

I've seen a study or two that claim that the bulk of some PM, like brake dust for example, is much smaller than 2.5um. Having exposure to the industry, do you have hopes for sensors that can identify smaller particles than .3um (that's the smallest Purple Air goes).