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### Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Sun May 28, 2023 9:07 am America/New_York
I am constructing PM2.5 using the MERRA-2 dataset following the equation (PM2.5 = DUSMASS25 + OCSMASS+ BCSMASS + SSSMASS25 + SO4SMASS* (132.14/96.06)) provided in FAQ.

Dataset: https://disc.gsfc.nasa.gov/datasets/M2T1NXAER_5.12.4/summary
FAQ: https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/FAQ/#Q4

I knew that the unit of each variable DUSMASS25 to SO4SMASS is kg m^{-3}. I calculated the average daily PM2.5 for Indian states using the equation and I got very small numbers of PM2.5, such as 4.579283e-08. The values I expected are 10-50, with the unit of μg/m^3 (← also how do you call/pronounce this?).
Example is https://www.nasa.gov/topics/earth/features/health-sapping.html

I guess I need to adjust the unit, but since I am not a climate or any natural science major (I study social science) and as it is my first time using such climate data, I have no idea how to adjust the calculated/obtained PM2.5 from using the equation.

Any thoughts, comments, or advice is appreciated!

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Tue May 30, 2023 1:15 pm America/New_York
Hello,

We will have our MERRA-2 staff examine your question; however, you appear to be doing the calculation correctly. The conversion for kg to μg (micrograms) is 10^9. So, multiplying your value (rounded) of 4.6e-8 kg / m^3 by 10^9 gives a value of 45, which is in the range you were expecting. The full name of the unit is micrograms per cubic meter.

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Wed May 31, 2023 4:22 am America/New_York
Thank you for responding quickly. As I do not have any scientists/experts in this field around, I just would like to confirm the following two things:

(1) Is the conversion for kg m^{-3} to μg m^{-3}, multiplying kg m^{-3} by 10^9, generally applicable and frequently used method in practice?

(2) Is there anything that I need to (scientifically) pay attention to this conversion? I plan to use PM 2.5 data for my research and plan to submit it to journals in social science fields.

Thank you for your support, and I am also looking forward to hearing from your MERRA-2 staff.

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Wed May 31, 2023 11:24 am America/New_York
Hello. In answer to your first question, this is a simple units conversion, so the multiplication factor would be generally applicable. I am not an atmospheric scientist, but I expect that the use of μg / m^3 is much more common, both because the numbers are easy to comprehend and because of the actual concentrations in the atmosphere.

As for the second, the only concern would be significant digits, as the decimal places (as a random example, for the number 45.386 μg / m^3, the ".386" represents a very small number that is likely beyond the capability of measurements. In other words, 45 μg / m^3 is a high precision number.

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Wed May 31, 2023 12:41 pm America/New_York
From our MERRA-2 staff: you may have seen this, but this also confirms the conversion factor.

Here is the related information about converting the surface PM2.5 units from kg/m3 to micro-grams/m3

PM2.5 = (DUSMASS25 + OCSMASS+ BCSMASS + SSSMASS25 + SO4SMASS*1.375) *1.e9
# To convert MERRA-2 aerosol output data to fine particular matter PM2.5,
# the formulation described in Buchard et al. (2017) is used. The factor
# of 1.375 (=132.14/96.06) is applied to convert sulfate ion (molar mass
# of 96.06 g mol-1) concentration output by MERRA-2 to ammonium sulfate
# (132.14 g mol-1) assuming that sulfate is primarily present as neutralized
# ammonium sulfate.
#
# Buchard, V., C. A. Randles, A. da Silva,A. S. Darmenov, P. R. Colarco,R. C.
# Govindaraju, R. A. Ferrare, J. W. Hair, A. Beyersdorf, L. D. Ziemba, and
# H. Yu, 2017. The MERRA-2 Aerosol Reanalysis, 1980-onward, Part II: Evaluation
# and Case Studies. J. Clim, 30, 6851-6872. DOI: 10.1175/JCLI-D-16-0613.1

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Mon Aug 28, 2023 10:37 am America/New_York
I want to do pm2.5 validation and now I found two datasets for each:
(1): MERRA-2 tavgU_2d_aer_Nx: 2d,diurnal,Time-averaged,Single-Level,Assimilation,Aerosol Diagnostics V5.12.4 (M2TUNXAER)
(2) MERRA-2 tavgM_2d_aer_Nx: 2d,Monthly mean,Time-averaged,Single-Level,Assimilation,Aerosol Diagnostics V5.12.4 (M2TMNXAER)
If I want to do validation of pm2.5 concentration for monthly mean, can I use a second dataset validation code?

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Mon Aug 28, 2023 6:45 pm America/New_York
From our data specialist: "Regarding the question posted at viewtopic.php?t=4594&p=16264#p16264, your method for calculating the monthly mean BCSMASS (M2TMNXAER) from the hourly mean (M2T1NXAER) is incorrect. The two datasets, i.e., M2T1NXAER for hourly mean aerosol surface concentration, and M2TMNXAER for monthly mean, are correctly selected. Note that the units of surface mass concentration are kg/m^3 for both hourly and monthly mean. To obtain the monthly mean, you should average all hourly mean data in this month instead of adding up each hour for a day and then each day in a month. You can calculate the MERRA-2 hourly PM2.5 and monthly PM2.5 with individual components following the formulas posted at https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/FAQ/#Q4."

We are not sure what this question means: “Can I use this month's data?”. If your focus is on monthly mean instead of hourly variation, you can directly use PM2.5 calculated from the monthly dataset or collection M2TMNXAER.

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Tue Aug 29, 2023 12:43 am America/New_York
Thank you for your serious answer, it is very helpful for my research. In the last sentence of your answer you mentioned that you don't understand the meaning of the phrase: "Can I use this month's data?". This sentence means can I utilize the following dataset:
MERRA-2 tavgM_2d_aer_Nx: 2d,Monthly mean,Time-averaged,Single-Level,Assimilation,Aerosol Diagnostics V5.12.4 (M2TMNXAER)
What I mean is to use this dataset above for the validation of pm2.5 concentration to validate the monthly mean for a particular area.
Another question is if I want to validate the mean for a particular quarter of the year or the mean for a particular year, do I just need to download the monthly data (3 months of data) for that quarter in the dataset mentioned above (M2TMNXAER) and do a simple arithmetic mean to find the quarterly mean for that particular quarter of the year?

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Tue Aug 29, 2023 3:29 pm America/New_York
From our data specialist:

"I am glad that my previous response was helpful. The MERRA-2 reanalysis, including aerosol reanalysis, is not pure observation. It results from assimilating various observations into a model, i.e., the NASA GEOS model. Thus, you can use PM2.5 from the dataset M2TMNXAER as a reference, especially when/where there are no observations, but you can’t use it to validate any other products, such as model outputs. For additional information on MERRA-2, please read the following three primary references:

• Gelaro, Ronald, McCarty, Will, Suarez, Max J., Todling, Ricardo, Molod, Andrea, Takacs, Lawrence, Randles, Cynthia A., Darmenov, Anton, Bosilovich, Michael G., Reichle, Rolf, Wargan, Krzysztof, Coy, Lawrence, Cullather, Richard, Draper, Clara, Akella, Santha, Buchard, Virginie, Conaty, Austin, da Silva, Arlindo M., Gu, Wei, Kim, Gi-Kong, Koster, Randal, Lucchesi, Robert, Merkova, Dagmar, Nielsen, Jon Eric, Partyka, Gary, Pawson, Steven, Putman, William, Rienecker, Michele, Schubert, Siegfried D., Sienkiewicz, Meta, Zhao, Bin. 2017. The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2). Journal of Climate. Vol. 30, No. 14, pp. 5419-5454. DOI: 10.1175/JCLI-D-16-0758.1

• Randles, C. A., da Silva, A. M., Buchard, V., Colarco, P. R., Darmenov, A., Govindaraju, R., Smirnov, A., Holben, B., Ferrare, R., Hair, J., Shinozuka, Y., Flynn, C. J.. 2017. The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part I: System Description and Data Assimilation Evaluation. Journal of Climate. Vol. 30, No. 17, pp. 6823-6850. DOI: 10.1175/JCLI-D-16-0609.1

• Buchard, V., Randles, C. A., da Silva, A. M., Darmenov, A., Colarco, P. R., Govindaraju, R., Ferrare, R., Hair, J., Beyersdorf, A. J., Ziemba, L. D., Yu, H.. 2017. The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part II: Evaluation and Case Studies. Journal of Climate. Vol. 30, No. 17, pp. 6851-6872. DOI: 10.1175/JCLI-D-16-0613.1 ISSN: 0894-8755, 1520-0442

As for your other question, if you want to obtain a particular quarter of the year or the mean for a particular year, you can download the monthly data (3 months of data) for that quarter in the dataset mentioned above (M2TMNXAER) and do a simple arithmetic mean to find the quarterly mean for that particular quarter of the year. Here is a FAQ that guides users to subset data with various cases, in case you find it useful: “How can I subset and download a large amount of the MERRA-2 data?" https://disc.gsfc.nasa.gov/information/faqs?keywords=subset&title=How%20can%20I%20subset%20and%20download%20a%20large%20amount%20of%20the%20MERRA-2%20data%3F

### Re: Magnitude and Unit of PM2.5, MERRA-2: Conversion Necessary?

Posted: Sun Sep 10, 2023 4:37 am America/New_York
Thank you very much for the help you have provided, I have recently encountered a new problem, the problem is this, I have downloaded the nc data from the following dataset:
MERRA-2 tavgM_2d_aer_Nx: 2d,Monthly mean,Time-averaged,Single-Level,Assimilation,Aerosol Diagnostics V5.12.4 (M2TMNXAER).
And with matlab extracted from it, dusmass25/ocsmass/bcsmass/sssmass25/so4smass, these five parameters of the tif format image, please ask me is it possible to according to the formula, with the arcgis in the raster calculator will be a month of these five parameters are added up to get the month of the pm2.5 concentration of the average value.

PM2.5 = DUSMASS25 + OCSMASS+ BCSMASS + SSSMASS25 + SO4SMASS* (132.14/96.06)
Now I calculated based on this formula above and got the monthly mean value of pm2.5 concentration, but I found in arcgis that the pixel size of the original five parameters (dusmass25/ocsmass/bcsmass/sssmass25/so4smass) is not the same as the pixel size of the monthly mean value of the pm2.5 concentration, which is confusing to me and I am not sure if this method is correct. If you have a way to synthesize the image of the monthly mean of pm2.5 concentration based on these five parameters, please let me know about it, I am very much looking forward to your help.