For the benefit of other readers, I think you mean “..correlates CO2 with PM2.5..” I’m basically a layman here and I agree that CO2 and PM2.5 particles are com

For the benefit of other readers, I think you mean “..correlates CO2 with PM2.5..” I’m basically a layman here and I agree that CO2 and PM2.5 particles are completely unrelated. PPM is Parts Per Million, i.e. a general measure of density of one substance in another. So, CO2 at 500 ppm makes sense.