In terms of absolute figures China has the no. 1 spot at least when it comes to airborne pollution. At least according to these sources.
http://www.guardian.co.uk/environmen...9/china.usnews
http://news.bbc.co.uk/2/hi/7347638.stm
Although it's possible that the UK media might have an agenda against China and therefore they might be inclined to put out info that paints China negatively and gives a more positive image of their war ally the US.