CDO

consider a reginoal emission inventory ot 0.002deg,whose girddes is

#

1. gridID 2 #
   gridtype = lonlat
   gridsize = 291760
   xname = Lon
   xlongname = Lon
   xunits = degree_east
   yname = Lat
   ylongname = Lat
   yunits = degree_north
   xsize = 560
   ysize = 521
   xfirst = -86.694
   xinc = 0.002
   yfirst = 40.3788
   yinc = -0.002

and since the unit of emission is Kg, I want to change it into the form of Kg/m2,so I use the following command

cdo div infile -gridarea infile outfile

to get the data value with Kg/m2; It is noted that the dimension of emission is (time,lat,lon),where time is equal to 8760.

For this inventory, I want to remap it to a 87*87 WRF domain with grid-spacing about 1km.The griddes of this domain is also attached here.

But if I directly use remapbil, the result is much smaller than the original file;
if I use

gridboxsum,5,5

first, and then remapbil, this result is close to the original file.
I use

cdo output -fldsum

to compare these results.
These results are also attached finally.

But how to explain it? Is there something wrong?
I would appreciate that anyone can give me some advice.
Thanks.