How large can local surface CO2 concentration vary due to natural processes?
There are many studies quantifying temporal CO2 variability for a static location. Conducting such studies isn't particularly difficult, the cost of equipment is low, and it's an easy way to publish meaningful results. You may have a hard time wading through all of the publications to select the few that mean anything to you, specifically.

A quick 2 minute search found this paper (among a slew of others), entitled, Diurnal and Seasonal Variation of CO2 levels of surface air in Garwhal Himalaya
in which this graph highlights daily CO2 variations over the course of a year, in mmol/mol. You'll have to look up the conversion to ppm CO2:



You may to search using these key words: CO2 variability, diurnal, yearly
Seasonal and diurnal variations in moisture, heat and CO2 fluxes over a typical steppe prairie in Inner Mongolia, China

Temporal variations of atmospheric CO2 concentration in a temperate deciduous forest in central Japan

Seasonal and Diurnal CO2 Patterns at Diekirch, LU

Interannual variability in the atmospheric CO2 rectification over aboreal forest region