Hello. I am rereading Hiroshima and early in the book the author states that the residents of the city had heard rumors that the "Americans were saving something special for the city", did the Japanese know anything at all of Atomic weaponry in August 1945? If they did, were they aware of it's destructive potential and that the Americans had tested bombs and were close to using them?
No. There's no evidence that even Japanese intelligence, much less the average Japanese citizen, had any clue about the Manhattan Project. It is important that one not read either the wrong thing into such rumors (e.g., assuming they were thinking about an atomic bomb — they weren't), or, frankly, too much into such rumors anyway (it is a form of confirmation bias to grant them much status after the fact).
The concept of an atomic bomb was known prior to Hiroshima, and had been discussed in popular culture and scientific journalism for many years. But there was a big jump between the concept and the reality.