Chess Online

I think the black pawn has to be at a4 to take the black pawn en passant.

En Passant was to remedy the problem of a pawn skipping death from another pawn by passing through a guarded space with a double advancement. In other words, en passant only happens when a single space forward would mean that an enemy pawn could capture and so even if your pawn advances two in that formation, the enemy can still capture it as if it had only moved one space forward.