Song: I could not miss Barca opportunity
He told El Mundo Deportivo: "It was in July. Barca spoke with Arsenal and Arsene Wenger came to tell me.
"It was hard to convince him and also say goodbye because he really helped me and my family when I arrived at Arsenal at 17 years old.
"When a club like Barcelona want you you can't say no.
"A chance like this only happens once in life and you must take it."
Failing to win any trophies in England has served to increase his appetite for success.
He said: "I'm hungry. I've never won anything and I have come here to play many games and win it all."
In little over a week, and if selected, Song will face Cristiano Ronaldo when Barca meet Real Madrid on October 7.
Tito Vilanova's side will be without Carles Puyol, out with a knee injury, while Gerard Pique has a sprained ankle.
Such a prospect does not in any way concern Song.
"Clearly, with Puyol and Pique we are stronger but I am ready and I've played against Cristiano in the Premier League.
"I have not just started playing today. I've played over 200 games at Arsenal and I have nothing to be worried about.
"I am not just thinking about stopping Cristiano.
"What is important is to play and do the job. That is why I have come."