I tried before, but no luck....
Arduino says:
error: invalid conversion from 'char' to 'const char*'
error: initializing argument 1 of 'int atoi(const char*)'
That's because "atoi" takes a C string as its argument, not a single char.
Seriously, this comes up at least once a week - have a search through the forum.
I think Nick Gammon recently posted a tutorial on the subject.