Why does Watney need an air day every five days or so on his trip to Schiaparelli?

Question

When Watney must get to the Schiaparelli crater in The Martian, he stops every five days for an "Air Day." Why does he do this?

Answer 1

Watney produces carbon dioxide

Like most humans, Watney inhales oxygenated air and exhales carbon dioxide.

The Atmospheric Regulator then stores this. From the novel:

I thought about the Atmospheric Regulator. It pays attention to what’s in the air and balances it. That’s how the excess O2 I've been importing ends up in the tanks. Problem is, it’s just not built to pull hydrogen out of the air.

Watney can then use the Oxygenator to convert the CO2 to oxygen.

I followed that process for four sols, and then it was time for an “Air Day”. An Air Day turns out to be pretty much the same as any other day, but without the 4 hour drive. Once I set up the solar panels, I fired up the Oxygenator and let it work through the backlog of CO2 the Regulator had stored up. Once it was done, my test run was complete. It converted all the CO2 to oxygen, and used up the day's power generation to do it.

However, that consumes all the power he can generate via the solar panels, so he can't drive that day. Thus the term "Air Day."

It just so happens that after five days, the carbon dioxide concentration has reached unacceptable levels.