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Merra2

Posted: Thu Dec 07, 2023 3:09 pm America/New_York
by dinesh_5497
Hi
I am not a chemistry expert, i need your support on unit conversion.
Sea Salt Surface Mass Concentration - PM 2.5 is in Kg/m3 (Volumetric method), i need to convert into mg/m2 (Deposition method).
For Example if i get 9.95 * 10^-9 kg/m3 as Sea Salt Surface Mass Concentration, what is the value in mg/m2?

Re: Merra2

Posted: Thu Dec 07, 2023 4:12 pm America/New_York
by GES DISC - mgreene
Hello,

Our MERRA-2 data and subject matter experts will examine your question and formulate a reply shortly.

Re: Merra2

Posted: Fri Dec 08, 2023 8:42 am America/New_York
by mbosilov
Well, I'm not a chemistry expert either, but I learned unit conversions quite some time ago. Kg to mg, assuming that the constituent is the same in the conversion, then should just be a multiple of 10. I don't know how many powers of ten to tell you because its not clear if you want, milligram or microgram.

to change volume to area, then you need a length dimension. Since this is representative of the surface, you need the height of the lowest model level. The variable is called HLML and you can find that in the single level variable collection: tavg1_2d_flx_Nx (M2T1NXFLX): Surface Flux Diagnostics. Be aware that HLML varies in space and time and has units of m. so that, Kg/m2 = Kg/m3*HLML

I hope this helps

Re: Merra2

Posted: Sat Dec 09, 2023 1:40 pm America/New_York
by dinesh_5497
Thanks for answering
I didn't understand it.
I searched in google and i got this
(https://developers.google.com/earth-engine/datasets/catalog/NASA_GSFC_MERRA_flx_2)
Here they have mentioned 60 m as HLML.
From the Merra2 File specification(https://gmao.gsfc.nasa.gov/pubs/docs/Bosilovich785.pdf)
I am going to use the below
tavg1_2d_aer_Nx (M2T1NXAER): Aerosol Diagnostics -> SSSMASS and SSSMASS25 .
can i use 60 as HLML for it or is it a complicated calculation

Re: Merra2

Posted: Mon Dec 11, 2023 10:04 am America/New_York
by mbosilov
If you read that carefully, you'll see that 60m is an approximate value. This is because the model uses a terrain following coordinate so that the thickness of the layers is a function of pressure. The variable HLML varies in latitude, longitude and time (space and time). This is why we provide the variable in the output. So, to get the proper conversion, you need to multiply by HLML. If you multiply by 60m you will get a more uncertain approximation.