While not empirical by any means, every semester, while teaching general chemistry, I always demonstrate the solubility of CO2 in water by taking a fresh sample of DI water, cooled to room temperature and measuring the pH after several hours of exposure to the atmosphere, and show then how a fresh sample has a pH close to 7, while a sample that is a couple of days old has a pH below seven.
I have noticed the pH of the days old sample dropping slightly every year for the past ten years. I know it is now due to the DI machine, since the fresh sample has always been close to seven.
Like I said, not empirical, but it certainly makes me wonder, where the extra acidity is coming from.
Valdo