GEM does not calculate or advect aerosols. However, the radiation scheme 'cccmarad' uses a predefined background Aerosol Optical Depth/Thickness (AOD/AOT). 

‘cccmarad’ consideres the following 5 types of background aerosols:

• Organic carbon
• Sea salt
• Mineral dust (from deserts)
• Black carbon
• Sulphate

The their AOD can be different for all model levels.

In GEM:

• Organic carbon only exists over land where it’s AOD goes linearly from 0.25 at the equator to 0.05 at the poles – see red line in image below
• Sea salt only exists over the ocean where it’s AOD goes linearly from 0.13 at the equator to 0.03 at the poles – see blue line in image below
• Mineral dust, black carbon and sulphate are set to zero everywhere

The background AOD is distributed evenly (in term of meters) over all model levels of the planetary boundary layer.

In CRCM6-GEM5:

In CRCM6-GEM5 one has the possibility of using prescribed 2-D background AODs for all 5 types of aerosols. The fields can vary from one month to the next, but so far only 2-D fields can get used.