Getting PM2.5 from CMAQ

PM2.5 is a big deal. But it is quite a beast to model, and CMAQ does so in a speciated way. This means that CMAQ keeps track of all the different types of PM (from nitrate, sulfate, whatever) and keeps them separate.

That sounds cool, because I mean that would let you look at a specific one if all you actually care about is sulfate for example. Makes sense.

But! It is way less cool for you if you really only care about PM2.5 as a total count, or if you want to use the total count as a frame of reference to a specific kind. The EPA NAAQS regulate PM2.5 as a total, so you really should care about PM2.5 as a whole too.

The reason it is less cool for you is because there is no "total PM2.5" output from CMAQ at all. You have to make it yourself. This sounds easy but can be VERY misleading, so I want to try and help you do this the right way. I did it the wrong way and it caused me more work so let's avoid that. If this confuses you even more I apologize...

There are conflicting options for calculating PM2.5. The way I did it was according to a presentation given by CMAS in 2010. That powerpoint is here: http://arset.gsfc.nasa.gov/umbc/files/Session1Day3/CMAQ-Introduction-for-ARSET.ppt

On slide 10, it says that PM2.5 is equal to the sum of a bunch of different species. So that's what I did. Is that right?

I once thought it was. Then things changed.

Instead, you can follow the approach in this document here:
www.epa.gov/CAIR/pdfs/CMAQ_Evaluation.pdf

This says on page 3 that you did the right thing when you picked your species to count in the mix, but you really should be scaling two of them by 1.167. Is that right?

Possibly? Maybe?

Next we check "Evaluation of the community multiscale air quality (CMAQ) model version 4.5: Sensitivities impacting model performance; Part II—particulate matter" by K. Wyat Appel. Note that this is for version 4.5 and you shouldn't be using that version, but in any case it says not only should you be doing that scaling bit, but also you should be looking at sodium and chlorine species too.

This is also in "A multi–pollutant,risk–based approach to air quality management:Case study for Detroit" by Karen Wesson. Is that right?

Could be?

Sometimes in life and in modeling, you have to make your own answers.