Flows, Stocks, and Residence Times

Let us imagine that we have some environmental compartment (e.g., a lake) with some water flow into it and some flow out of it (these could be two rivers in our lake example). Symbolically, we have 

The average time an item of the stock spends in the compartment is called the residence time, and we will use the symbol r (tau). It has units of time for example, days. The reciprocal of r is a rate constant with units of 1/time or time-1 rate constants usually are represented by the symbol k. 

Consider a Spherical Cow, University Science Books, Sausalito, CA, 1988, 283 pp. 

Remember this latter equation Let us do some examples. 

Imagine a one-car garage with a volume of 90 m3 and imagine that air in this garage has a residence time of 3.3 h. At what rate does the air leak into and out of this garage  

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