Oh, and I almost forgot.
The emissions in question are particulates and nitrogen oxides.
Diesels tend to emit physically large particulates that are visible, when inhaled don't go far into the lungs, and fall out of the air quickly.
Gasoline engines tend to emit small particulates, and a lot of them, that are not visible, when inhaled go far deeper into the lungs, are more likely to cause cancer, and stay in the air a lot longer. But, their particulate mass is low, so until recently, nobody's cared. (New emissions standards will restrict the particulate number, as well.)
Any engine that runs lean tends to emit high amounts of nitrogen oxides, and diesels have to run lean or they start smoking. Of course, lean burn also reduces fuel consumption, improving energy independence and all... Nitrogen oxides are a smog precursor in certain situations... but those situations are all in areas where volatile organic compounds are low. In a high VOC environment, which is essentially any area with a lot of plant or modern human life, nitrogen oxides actually destroy smog... and all of the areas that have smog problems are high VOC. Yeah, I'm gonna say that that one's completely misguided.
So, modern diesels tend to run high amounts of cooled EGR, and particulate traps and NOx neutralization technology (either traps, or spraying urea into the exhaust stream). The traps require that fuel be sent into them to burn things off, too. Yes, this is ridiculous, and seriously hurts fuel economy.