North Sea / Irish Sea (NS/IS). Arriving from northern sectors after traversing the North Sea and / or Irish Sea with additional input from coastal regions of Ireland and the UK. • Northern European (NE). Polluted air masses originating over northern Europe, Scandinavia, Russia or Baltic regions, usually traversing the UK. • Southern European (SE). Polluted air masses originating from southern European and North African sectors. • Mixed (M). Difficult to assign category; mixed maritime / continental type. • South Atlantic (SA). Clean maritime air masses with no continental contact for at least 5 days before reaching the Irish coast. • Mid-Atlantic (MA). Normally fast moving air masses, could originate from eastern USA. • North Atlantic (NA). Clean air masses from North Atlantic or polar regions, with no continental contact for at least 5 days.