“Most mass in trees was previously part of the surrounding air”
“dry wood is mostly CO2”
From a US Forest Service website:
Wood is best defined as a three-dimensional biopolymer composite composed of an interconnected network of cellulose, hemicelluloses and lignin with minor amounts of extractives, and inorganics. The major chemical component of a living tree is water, but on a dry weight basis, all wood cell walls consist mainly of sugar-based polymers (carbohydrates, 65-75%) that are combined with lignin (18-35%).
FWIW, you appear to be confusing two claims:
“Most mass in trees was previously part of the surrounding air”
“dry wood is mostly CO2”
From a US Forest Service website:
https://research.fs.usda.gov/treesearch/42245
“Trees are mostly made of air” (direct quote from post title) is more clickbait headline than scientific fact.