The AIRTRACER file gives the residence time (in seconds) of the air
masses over the geographical grid in a time span of 0.25, 1, 7, 10 or 30
days of transport. This quantity is proportional to the relative
contribution of each grid bin to the air composition at the ship
position.

When this quantity, also called emissions sensitivity, is multiplied
with emission fluxes (e.g., from an inventory), it gives the
contribution of each grid point (the source contribution) to the total
concentration at the ship location.

We report here the source contribution (in ug/m3) of BC (Black Carbon),
CO (Carbon Monoxide) and SO4-S (Sulphate Sulphur) concentration
resulting from the coupling with anthropogenic emissions
(https://iiasa.ac.at/web/home/research/researchPrograms/air/ECLIPSEv6b.
html)

The 100m or 3000m label refers to the thickness of the layer (above
ground level) considered for the emission coupling. So the 100m, which
represents the lowest model layer, take into account only the
interactions with emissions when the air masses in their transport has
been traveling close to the surface.