Definition of a Mass Balance

Steps to Writing a Mass Balance:

1. Identify the specific pollutant for which the mass balance is to be written.
2. Define the system boundaries (the entire building in our case) so that it is clear whether a given molecule of the pollutant is inside or outside the control volume.
3. Draw a sketch or schematic of the system boundaries.
4. Establish all the ways the pollutant can enter or exit the volume, and establish all the ways the pollutant is created or removed from within the control volume. Usually, these mechanisms are labeled within the drawn schematic to aid this process.
5. Write and solve the appropriate mass balance equation.

A mass balance, in our context, treats the entire building as one well-mixed reactor, which means that we assume a uniform concentration of the examined pollutant in all parcels of air in the space. A mathematical statement of the conservation of mass, the mass balance is basically a bookkeeping tool that is used to keep track of how much of a pollutant is in the space at a given time. The mass balance is written to account for all mechanisms that cause mass of the pollutant to enter or exit, react within, and accumulate or lessen within the system.

In a steady-state scenario, there is no accumulation within the system; i.e., the rate at which mass is added to the system equals the rate at which it is removed. For a transient scenario, accumulation or lessening of the pollutant can occur within the control volume boundaries.

These practical examples let you change key parameters within building systems in order to observe how those parameters influence the resulting indoor concentrations of various pollutants. The Scenarios and corresponding pollutants investigated include: smoking in a bar (PM2.5); cooking in a house (NO2 and water vapor); students in a classroom (CO2); and smoking in a bedroom with an air cleaner operating (ETS).

This calculator allows you to calculate the resulting steady-state concentration of indoor pollutants with different parameters of the building as inputs. You can choose common input parameters for a list of common pollutants or enter in your own information.

This calculator allows you to calculate the resulting transient concentrations of indoor pollutants with different parameters of the building as inputs. You set a total time duration of up to 24 hours to investigate, and you can vary the input parameters at four points during the selected total time duration. You can get common input parameters for a list of common pollutants or enter in your own information. A plot of the resulting pollutant concentration as a function of time is provided.