There is a direct relationship between the pollutant's steady-state concentration and the source rate---as the source rate increases, the concentration increases. There is also a direct relationship between residence time and steady-state concentration---as the residence time increases, the concentration increases. There is an inverse relationship between volume and steady-state concentration---as the volume increases, the concentration decreases.

These are physical processes that act as pollution sources. In the first lecture, we looked at specific activities or things that produced air pollution, and classified them as natural or anthropogenic. The processes here are the actual physical processes used by any the sources determined earlier.

The sink rate is not actually part of the steady-state box model concentration calculation, except that we require it to be equal to the source rate when we have equilibrium conditions. Sink processes do participate in the determination of the steady-state concentration through the residence time factor. That is, as the sink process becomes more efficient, so that pollutants are removed more readily, the residence time of the pollutants decreases.

The true residence time is found by determining the time it takes for a major fraction of a pollutant is removed from the air; it is sufficient for our use to say that it is the time that a pollutant molecule stays in the box before some sink process removes it.

There is an inverse relationship between the efficiency of the sink processes and the residence time: the more efficient the sink processes, the shorter the residence time (it is not correct to say that sink rate is inversely proportional to residence time, because in the steady-state box model, the sink rate is pre-determined by the source rate, and the absolute value of the sink rate is directly connected to the pollutant concentration at a given moment). It then stands to reason that the shorter the residence time, the smaller the steady-state concentration because the pollutants are being removed relatively quickly and not allowed to hang around inside the box.