The requirement for elements that are relatively rare in the atmosphere greatly constrains the potential nanomass and growth rate of airborne replicators. However, note that at least one of the classical designs exceeds 91% CHON by weight. Although it would be very difficult, it is at least theoretically possible that replicators could be constructed almost solely of CHON, in which case such devices could replicate relatively rapidly using only atmospheric resources, powered by sunlight.
It’s from the paper “Some Limits to Global Ecophagy” (which he’s cited in this context before): https://lifeboat.com/ex/global.ecophagy
I see, thanks! Section 8.2, “Gray Dust”: