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Difference between TOT_DUST and DUST_1 to DUST_5


I want to know the difference between TOT_DUST and DUST_1 to DUST_5 in WRF-Chem output. Is TOT_DUST equal to the sum of DUST_1 to DUST_5?
However, I plotted TOT_DUST and the sum of DUST_1 to DUST_5 and they look similar but not identical. Also, TOT_DUST and DUST_1 to DUST_5 have different units;
DUST_1:description = "dust size bin 1: 0.5um effective radius"
DUST_2:description = "dust size bin 2: 1.4um effective radius"
DUST_3:description = "dust size bin 3: 2.4um effective radius"
DUST_4:description = "dust size bin 4: 4.5um effective radius"
DUST_5:description = "dust size bin 5: 8.0um effective radius"
units = "ug/kg-dryair"

I hope my question is clear.
Any feedback would be appreciated.
DUST_1, DUST_2, ... DUST_5 are size-resolved dust concentrations in terms of a mixing ratio (ug/kg-dryair; micrograms of dust per kg of dry air). TOT_DUST is the total dust concentration for all 5-size bins in terms of micrograms of dust per unit volume of air (ug/m3).

There's a parameter in WRF called "alt" that represents the inverse density of dry air (in units of m3/kg-dry air). I don't think "alt" is included in wrfout files by default anymore, but you can add it to the history output by adding an "h" to the WRF registry file. The easiest way to convert dust mixing ratio values to mass per unit volume concentration is to divide the mixing ratio value by the alt parameter. Alternatively, you could sum the 5 size-resolved mixing ratios and diagnose the % mass contribution to the total per bin [e.g., %bin_1 = DUST_1/SUM(DUST_1, DUST_2, ... DUST_5)]. The % distribution per bin should also apply to the mass concentration per volume of air [e.g., bin 1 dust in terms of ug/m3 = %bin_1 * TOT_DUST].