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### MERRA-2 PM 2.5 vs. PM 10

Posted: Tue Jul 19, 2022 9:48 pm America/New_York
Hello,

I am reconstructing MERRA-2 sourced PM 10 and PM 2.5. My primary question is it correct to assume that PM 10 will ALWAYS be greater than PM 2.5?

My follow up question is can you offer any comments on equations for PM 2.5 and PM 10 shown here: https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/FAQ/

Previously there had not been an equation for PM 2.5, and the one that I had used differs slightly. Are these the official or best practice equations for calculating PM 2.5 and PM 10? Are they subject to change?

For PM 2.5 when an equation was not provided I turned to literature and found several options. I chose one based of this paper https://link.springer.com/article/10.1007/s11869-015-0338-3

It differs slightly, with an OC coffeiecient of 1.8, which I suspect is why I am seeing PM 2.5 exceed PM 10 in some cases.

This is the current equation I am using.
PM2.5 (kg/m^3) = DUSMASS25 + SSSMASS25 + BCSMASS + (1.8 * OCSMASS) + (1.375 * SO4SMASS)

Thanks!

### Re: MERRA-2 PM 2.5 vs. PM 10

Posted: Wed Jul 20, 2022 1:40 pm America/New_York
We have received your question and are providing it to our MERRA-2 and aerosol experts for response.

### Re: MERRA-2 PM 2.5 vs. PM 10

Posted: Wed Jul 20, 2022 3:03 pm America/New_York
Here is the reply from our GES DISC staff:

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Yes, it is correct to assume PM 10 will ALWAYS be greater than PM 2.5. Inhalable coarse particles PM10 have a diameter of 10 micrometers (μm) or less. Finer particles, designated PM2.5, have a diameter of 2.5 μm or less.

In the MERRA-2 FAQ page https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/FAQ/#Q4, the number 4 FAQ shows how to calculate PM2.5 and the associated collections. and the number 5 FAQ shows how to calculate PM10 and the associated collections (different from those for PM2.5). These equations are official and will be valid for the MERRA Version 2 variables only.

Note that for the PM2.5 formula in the number 4 FAQ, i.e.,
PM2.5 = DUSMASS25 + OCSMASS+ BCSMASS + SSSMASS25 + SO4SMASS* (132.14/96.06)

the component OCSMASS in MERRA-2 is actually organic matter surface concentration, thus there are no need to multiply with any factor like 1.8.

Feel free to let us know if you have further questions.

GES DISC MERRA-2 support group