UK Air Masses MET
Air masses are categorised twice: once by the temperature of the source region and once by the type of source region (land or sea.)
The latter of these is the easier: if the air mass comes from over the land, it is continental, if it comes from the sea it is maritime.
For the former, look at where the arrow has come from. It could be directly from the arctic, from high (colder) latitudes in which case it is polar or from lower (warmer** latitudes in which case it is tropical.*
Put the temperature descriptor before the land/sea descriptor and you will have your air mass.