Carbon sequestration rates are up to 90 times greater for healthy large trees than healthy small trees. In Chicago's urban forest, a typical 3-in diameter tree sequesters approximately 2.3 lbs annually, which is equivalent to carbon emissions of a passenger vehicle driven 10 miles. Annual sequestration by a 30-in tree is around 204 lbs per year, the carbon emissions of a passenger vehicle driven approximately 900 miles.
McPherson, Gregory E., David J. Nowak, Rowan A. Rowntree. (1994). Chicago’s Urban Forest Ecosystem: Results of the Chicago Urban Forest Climate Project. USDA Forest Service Northeastern Forest Experiment Station General Technical Report NE-186, 63-81.