Well, I’ve officially been called “geeky” on Twitter. I wear the label with pride. Not wanting to let my fans down, I decided to prove my earlier conjecture that the US Embassy linearly extrapolates the AQI-PM2.5 relationship from the EPA’s 400-500 range for concentration values above 500. As I noted before, this is NOT supported by regulatory language in the US, since the PM2.5 concentration almost never goes above 500.
Here’s a plot of the US EPA’s regulatory AQI-PM2.5 relationship definition (blue, stops at 500) as well as a smattering of data from the Embassy monitor (red). Looks pretty clear that the Embassy is just using the EPA’s same AQI-PM2.5 relationship from 400-500 for all values above that.