Penn State has a useful chart that outlines what time of year each type of control method for removing Tree of Heaven works best. In general, now (late summer) is the time to have the trunk treated with a "hack-and-squirt" type of herbicide application; wait about a month for it to fully be absorbed and kill roots, then the tree can be cut down. An alternative is to make a basal bark spray instead, since the trunk diameter of your tree falls into both categories. Without a delay to make sure the herbicide has time to work, cutting down a Tree of Heaven too soon (or without any herbicide pre-treatment) can make it sucker profusely, which is when new sprouts emerge from the root system. For young saplings only an inch or less in trunk diameter, a foliar spray instead with systemic herbicide should suffice, which will also be absorbed and moved into roots.

If you don't want to apply the herbicide yourself, you can hire a certified pesticide applicator; many landscapers and arborists have this license.

Miri