English to Portuguese Medical Chemistry; Chem Sci/Eng biodegradation

"cumulative gas volume (background corrected)"


this is the heading of a table. The paper is about biodegradation of plastics in landfill conditions.
I am assuming they measure these gasses, but correct the results by excluding other sources in the background. the text that follows the table is:
" The evolution of gases, methane and carbon dioxide was monitored as the indicators of slow but continuous biodegradation of the plastics containing the prodegradant additive was monitored. This slow gas evolution was monitored for 750 days until at least a half life of biodegradation for the exposed Oxo-biodegradable plastic was achieved, before termination of the experiments. The percent of biodegradation, calculated based on the total carbon evolved as gas relative to the carbon content of the samples is shown in Table 4. The results of these gas evolutions are also shown in graphical form for cumulative gas evolution and percent biodegradation."

I am trying to find a good translation for "background corrected" rather then a lenghy explanation in PT - BR.
thank you all!