After applying the growth factors to the SMOKE intermediate inventory file, the Grwinven program writes a new SMOKE intermediate inventory file that contains the new, grown emissions. Grwinven only writes the I/O API part of the inventory and not the ASCII part, because the ASCII part that contains the state/county codes, SCCs, and other character strings does not change between the base and future or past year. All of the sources in the base and grown inventory files are the same; the difference is in their emissions values.

Grwinven adds a header element to the I/O API part of the inventory file that indicates the file was created using Grwinven and a growth matrix. The header element is /FYEAR/, which is followed by the date of the future year (for example, /FYEAR/ 2018). Other programs (such as Temporal) will recognize the /FYEAR/ header element and alter its messages to indicate that a future year is being processed.