Units in HEMCO

Overview

Attention

We recommend that you provide explicit scale factors for unit conversions in the HEMCO configuration file. This will avoid some known issues with unit conversions that were recently discovered.

HEMCO classifies all data fields as fluxes, concentrations, or unitless data. Data are internally stored in HEMCO standard units of [kg emitted species/m2/s] for fluxes, and [kg emitted species/m3] for concentrations. No unit conversion is performed on unitless data.

The classification of a data field depends on the units attribute in the netCDF file, the SrcUnit attribute in the HEMCO configuration file, and the unit tolerance setting in the HEMCO configuration file (see below). In general, the original units of the input data is determined based on the units attribute on the netCDF file, and data is converted to HEMCO units accordingly. The mass conversion factor is determined based on the species assigned to the given field throuh attribute Species in the HEMCO configuration file. It depends on the species molecular weight (MW), the MW of the emitted species, and the molecular ratio (molecules of emitted species per molecules of species). If the input data is found to be in non-standard units (e.g. L instead of m3, g instead of kg, etc.), HEMCO will attempt to convert to standard units. This feature is not fully tested yet, and it is recommended to provide input data in standard units wherever possible.

SrcUnit attribute

The SrcUnit attribute in the HEMCO configuration file gives the user some control on unit conversion.

If SrcUnit is set to 1, data are treated as unitless irrespective of the units attribute on the file. This option works on all fields only if unit tolerance is relaxed to 2 (for unit tolerance of 1, the input data must be in one of the units recognized by HEMCO as unitless).

If SrcUnit is set to count, the input data is assumed to represent index-based scalar fields (e.g. land types). No unit conversion is performed on these data and regridding will preserve the absolute values.

Special attention needs to be paid to species that are emitted in quantities other than mass of species, e.g. kg C. For these species, the species MW differs from the emitted species MW, and the molecular ratio determines how many molecules are being emitted per molecule species. By default, HEMCO will attempt to convert all input data to kg emitted species. If a species is emitted as kgC/m2/s and the input data is in kg/m2/s, the mass will be adjusted based on the emitted MW, species MW, and the ratio emitted MW / species MW. Only input data that is already in kgC/m2/s will not be converted. This behavior can be avoided by explicitly set the SrcUnit to the same unit as on the input file. In this case, HEMCO will not convert between species MW and emitted MW. This is useful for cases where the input data does not contain data of the actual species, e.g. if VOC emissions are calculated by scaling CO emissions (see examples below).

Unit tolerance setting

Unit tolerance determines how HEMCO will handle discrepancies between the units variable attribute (as read from the netCDF input data file) and the SrcUnit setting specified in the HEMCO configuration file.

Value

What it does

0

No tolerance. A units mismatch will halt a HEMCO simulation.

1

Medium tolerance. A units mismatch will print a warning message, but will not halt a HEMCO simulation. (Default setting)

2

High tolerance. A units mismatch will be ignored.

Unitless data

The following units are currently recognized as ‘unitless’ by HEMCO

  • 1

  • count

  • unitless

  • fraction

  • factor

  • scale

  • hours

  • v/v

  • v/v/s

  • s-1

  • m2/m2

  • kg/kg

  • K

  • W/m2

  • pptv

  • ppt

  • ppbv

  • ppb

  • ppmv

  • ppm

  • ms-1

  • m

  • cm2cm-2

  • dobsons

  • dobsons/day

  • hPa

  • Pa

Examples with units

Attention

We recommend that you provide explicit scale factors for unit conversions in the HEMCO configuration file. This will avoid some known issues with unit conversions that were recently discovered.

File file1.nc contains field DATA in units of kg/m2/s. It shall be applied to species acetone (ACET), which is emitted as kg C. The species molecular weight of ACET is 58, the emitted molecular weight is 12 (i.e. that of carbon), and the molecular ratio is 3 (3 molecules of carbon per molecule of acetone).

The following entry in the HEMCO configuration file will interpret the input data as kg acetone/m2/s, and convert it to kg C/m2/s using a scale factor of 0.62 (= 12/58*3):

#--> data is converted from kg acetone/m2/s to kgC/m2/s
0 ACET  /path/to/file1.nc  DATA 2000/1/1/0 C xy kgC/m2/s ACET - 1 1

The following entry will avoid the unit conversion from kg to kgC:

#--> data is kept in kg species/m2/s
0 ACET  /path/to/file1.nc  DATA 2000/1/1/0 C xy kg/m2/s ACET - 1 1

Note that the opposite does not work: If file2.nc contains data in units of kgC/m2/s, it is not possible to convert to kg species/m2/s and the following two entries have the same effect:

 #--> data is converted from kgC/m2/s to kg emitted species/m2/s,
 #    which is also kgC/m2/s``
0 ACET  /path/to/file2.nc  DATA 2000/1/1/0 C xy kg/m2/s  ACET - 1 1

#--> data is kept in kgC/m2/s
0 ACET  /path/to/file2.nc  DATA 2000/1/1/0 C xy kgC/m2/s ACET - 1 1

However, if one wants to use file2 for a species not emitted as kg carbon, say CO, the source unit attribute matters!

 #--> data is converted from kgC/m2/s to kg CO/m2/s
0 ACETasCO  /path/to/file2.nc  DATA 2000/1/1/0 C xy kg/m2/s  CO - 1 1

#--> data is kept in kgC/m2/s
0 ACETasCO  /path/to/file2.nc  DATA 2000/1/1/0 C xy kgC/m2/s CO - 1 1

Tips for testing

The unit factor applied by HEMCO is written into the HEMCO log file if Verbose is set to true.