MERRA-2 references
First pick the correct variable here:
https://disc.gsfc.nasa.gov/datasets?page=1&project=MERRA-2&keywords=%22MERRA-2%22
When you click on the correct variable, it will take you to a second webpage with tabs that you can click that include: (1) documentation papers you need to cite, and (2) the correct variable citation information. (And you apparently need both types of citation). For example, below is a link to the page for 3d assimilated, 3-hourly, model level data (i.e. the equivalent of what everyone uses for 6-hourly ERA-Interim variables like temperature).
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https://disc.gsfc.nasa.gov/datasets/M2I3NVASM_V5.12.4/summary?keywords=%22MERRA-2%22