It has not been added yet. Adding it to "tbb/atomic.h" should be a matter of cloning the partial specialization for atomic. The hard work (really just tedious) is updating the unit test/test_atomic.cpp to make sure it works.
I added atomic to the development version, so it will show up in the next development release. It was almost a clone of atomic.To shut up gratuitous warnings from a compiler, I added a few !=0 tests to explicitly convert integral types to boolean. The warning was about a performance issue. I consider it gratuitous because modern architectures typically have instructions for converting integers to bools without branching. Attached is the definition of atomic that I added to include/tbb/atomic.h, inside namespace tbb.
tbb::atomic?
Are atomic boolean values supported in tbb::atomic?
I have seen an old blog post that mentions this would be added (see here).