Does this work for you?
[{"id":"bfe082243e057b8b","type":"function","z":"bdd7be38.d3b55","name":"forEach()","func":"Object.keys(msg.payload).forEach((element) => {\n if (element.startsWith(\"ATC\")) {\n msg.payload = msg.payload[element]\n node.send(msg)\n }\n})\n","outputs":1,"timeout":0,"noerr":0,"initialize":"","finalize":"","libs":[],"x":860,"y":160,"wires":[["68be1dc221018baa"]]},{"id":"68be1dc221018baa","type":"debug","z":"bdd7be38.d3b55","name":"debug 99","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":1020,"y":160,"wires":[]},{"id":"3586ec0a6edd8630","type":"inject","z":"bdd7be38.d3b55","name":"No ATC","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"mac\":\"a4c1380d5c0c\",\"Temperature\":21.6,\"Humidity\":64.7,\"DewPoint\":14.7,\"Btn\":0,\"Battery\":37,\"RSSI\":-64}","payloadType":"json","x":690,"y":180,"wires":[["bfe082243e057b8b"]]},{"id":"3f53197d243846c3","type":"inject","z":"bdd7be38.d3b55","name":"With ATC","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"mac\":\"a4c1380d5c0c\",\"Temperature\":21.6,\"Humidity\":64.7,\"DewPoint\":14.7,\"Btn\":0,\"Battery\":37,\"ATC1\":\"this is the value of ATC1\",\"RSSI\":-64}","payloadType":"json","x":700,"y":140,"wires":[["bfe082243e057b8b"]]}]