I’ve written before about loudness specs for television and content for the web, but not so much for radio as such. From what I can tell there are no real hard specifications for loudness of radio ads here in Australia, and some sample ads I have checked out have quite a range of loudness values. It seems that because there are faders and/or programs to control loudness in radio studios, along with the on-air compression that specific loudness requirements seems less important than with some other forms of media.
Adstream includes a recommended peak value but not a specific loudness as such in their recommend audio format which is:
File type: PCM WAV linear audio, 2 channel stereo; Sample Rate: 48 kHz, 16 bit; Audio peak level: -2dB (ppm)
Those specs come from:
https://www.adstream.com/wp-content/uploads/2018/09/Adstream_Radio_Specs.pdf
The Norwegian radio industry agreed on a -15 LKFS standard loudness over years ago, and a recent radio ad I checked out from Australia had been mixed to that level, so I would suggest that -15 LKFS seems like a reasonable target loudness currently, but -16 to -20 LKFS which is more the standard for online and podcasting would also be appropriate. I don’t feel like ads need to be mixed and mastered as loud as they perhaps might have in the past, but I welcome any feedback on this.