4. Import issues

Import is the proportion of a pollutant's concentration that does not originate from local sources close to the measurement point. The pollution was produced upstream of the zone and then transported by the movement of air masses.

Import concerns all pollutants with a sufficiently long lifespan to be transported. The longer the lifetime, the larger the area affected by the pollution (regional, national or even continental). Typically, ozone and particulate matter, especially the lightest fractions of the latter known as "ultrafine" (size less than a tenth of a micron), are affected by this phenomenon.