Which car is more polluting..

[rufusgti]

Chatting to a friend earlier, I was ribbing him that he'd be getting rid of his 05 Volvo xc90 under the forthcoming diesel scrappage scheme. He said I couldn't really get on my high horse about pollution whilst driving my '90 Porsche 944 s2. Just mindless banter as neither of us knew which is the more polluting vehicle.
I get the impression we're going to becoming much more focused on emissions and pollution in the coming years which is a good thing of course. I've been thinking since, just how polluting are the cars I drive. Is there anyway of comparing diesel and petrol when it comes to actual air pollution? I have to admit I just pay the road fund licence by direct on both our cars. The other is a petrol 03 golf estate 1.8turbo. Neither of my cars are new or frugal on fuel. But my thoughts are they're cleaner than an old xc90 Volvo. Am I right or wrong?