Re: Oxygen quality and content in the air during winter

The answer is in the abundance of the different atmospheric gases. 
Basically, the amount of oxygen in the atmosphere is much, much larger 
than carbon dioxide. In photosynthesis and respiration processes, about 
11 molecules of O2 are exchanged for 10 molecules of CO2; the imbalance 
is covered by other chemical processes. The plant photosynthesis 
happening in North America results in a change in CO2 concentration of 
about 10 ppmv; that is, in units of molecules of CO2 per total molecules 
of air, the wintertime CO2 over North America is about 0.000010 higher 
than summertime. Since the concentration of CO2 in the atmosphere is 
about 0.000392, this is a relatively big change, about 2.5%, between 
summer and winter. However, the concentration of O2 in the atmosphere is 
roughly 0.2, so even though the seasonal change in O2 concentration is 
actually larger than CO2 (because of processes happening over the ocean), 
a seasonal change of +/-20 parts per million (0.000020) will change O2 
concentrations by only 0.01%, which you could never detect.
Running in the winter is probably easier because of the lower moisture in 
the air. This obviously only works in places where winter is relatively 
mild!

There is lots of really good information about the carbon cycle and 
carbon dioxide available on the WWW (see for instance,  http://en.wikipedia.or
g/wiki/Keeling_Curve), but I had to go to a 
scholarly source to get the numbers for oxygen (Martin Heimann, "The 
Cycle of Atmospheric Molecular Oxygen and its Isotopes", in _Global 
Biogeochemical Cycles in the Climate System_, ed. Ernst-Detlef Schulze et 
al., Academic Press 2001).