Ground operations refer to the airplane taxing from the gate and back to the gate, or a final parking place at the airfield. During this operation, the airplane rolls at relatively low speed, stops, restarts, turns, etc. 

Gas turbine engines are known to be inefficient in these conditions, and their emissions, particularly NOx and particulates are responsible to poor air quality. 

Some airplanes, notably turboprops, can taxi back to the gate on a single propeller. OEI operations are not normally encouraged, certainly not on departure, when a delay or failure to start up of the inoperative engine could jam the departure schedule. 

Calculation of engine emissions during taxi-out and –in, requires setting up a sequence of events, which include various combinations of acceleration, roll, braking, stopping/idle and turning, such as in this graphical example.