Sure PM2.5 aren’t the smallest particles - people are even trying to focus on the issues of nanoparticles nowadays. But there is a lot better understanding of the increased risk of PM2.5, and the measurement of PM2.5 from human causes, as opposed to the ultra-ultrafine PM0.1 and under particles.
Well, no, you can’t do that, but how does one break down the exact makeup of the entire atmospheric content of PM2.5 over the United States? It’s difficult to do so even for a single point source emission. Within the scope of Cecil’s column, his generalization was valid. Cecil’s column is not a peer-reviewed paper to be sent to pulmonary researchers, nor a white paper on external air pollution. In short, you are making some valid points here, and we appreciate them, but I believe that within the scope of the column what Cecil said was a decent summary. If you look at the United States or EU regulations on PM10 and proposed regulations on PM2.5 from the EPA, they do not break down limits based on an ultimate analysis of the particles, they base their limits on the macroscopic lump sum of “we know this stuff is bad; we don’t need to dig into it that much” standpoint. And yes there are some specific items sometimes called out separately, but particulate emissions standards generally are a bulk lbm/MBtu, grains/ft3, ppm, or mg/Nm3 (or something like ng/J in Japan IIRC).