a volume of air, the component makeup and concentrations of which remain relatively static, that tends to travel around the earth changing geographic location and position above earth as an intact unit. Air parcels can be tracked [Graedel, T.D. and Paul Crutzen. Atmospheric Change: an Earth system perspective; 1993; Freeman Press.] [Journal of Climate; v19; 6267–6277; 2006.] [Journal of the Atmospheric Sciences; v63; 952–967; 2006.]