Pm2.5 is a specific particle size, yes, we do agree on that. Importantly, it is also a common measurement used in assessing air quality. In that context it is typically composed of particles from a set of common sources, like wildfires, industrial sources, automobile pollution, etc…. It is, in the context of air quality monitoring, never composed of amino acids. It seems valid to make claims about pm2.5 and health, even though the claim does not distinguish wildfire pm2.5 from pm2.5 from automobile pollution or other sources. Maybe I misunderstood you to be saying otherwise.