Superposition means I can take all of the concentration fields produced when the wind is blowing in different directions and lay them on top of each other in a common grid system. Additivity means I then can add together the contributions from the separate wind directions within each grid block. Superposition is always appropriate. Transfer is the movement of a pollutant between compartments. Transport is the movement to different points in space within a compartment.
Transformation is a change in form of a pollutant within a compartment.
You would want to retain information on temporal variability if this variability was related to the appearance of the effect. An example is an acute effect that occurs only if the concentration exceeds some threshold. You would not realize this if you all you had was the time-weighted average. This would not be important for many chronic effects, particularly those in which only cumulative exposure, rather than exposure intensity, is important. Either because you wanted to characterize variability of exposure in a population, or because an individual was moving in the exposure field and you wanted to characterize the temporal variability of his or her exposure during this movement.