The MERRA-2 surface data, such as SPEED (Wind Speed) in M2T1NXFLX_5.12.4 and DUSMASS25 (PM2.5 dust surface mass concentration) in M2T1NXAER_5.12.4, are the surface (i.e. lowest) model layer data extracted from the 3-dimensional data and saved as a 2-dimensional variable. The surface model layer height (midpoint of the layer) varies in time and space, generally around 60 meters above the surface, as revealed by the variable "HLML"(Surface Layer height) in the collection M2TMNXFLX_5.12.4 (See the figure below).
The monthly mean MERRA-2 surface layer heights can be explored with Giovanni.
Related documentation: Related Data Collections:
M2T1NXFLX_5.12.4
M2TMNXFLX_5.12.4
M2T1NXAER_5.12.4
M2TMNXAER_5.12.4