I can't give you a link, but what you need to look for are comparisons between diesel and gas engines on unburned hydrocarbons, NOX (nitric oxides), particulates, and carcinogens.

Diesel smoke contains considerable amounts of benzene and derivatives, all known to cause cancer, and carried in breathable particles. Part of this is due to poor combustion, part to the nature of diesel fuel (the benzenes are already there in relatively high concentrations.

However, when the soot is gone, so are the carcinogens, and automotive/truck diesels are MUCH better in this reguard than they were just 20 years ago. I cannot see ANY smoke behind my 300D, even at night, and I've never even seen smoke marks on a VW TDI!

Off-road vehicles tend to have ancient technology (1950s and 1960s), use huge amounts of fuel, and make smoke like there is no end to it because the manufactures (american, mostly) are bone idle when it comes to research and design. They pollute a LOT due to low compression engines, wear and tear, and dirty cheap fuel with gobs of sulfur in it. None of this in an inditment of diesel equipment, just the result of the typical laziness and cheapness when it comes to machinery in America. The operators will be nearly stunned when they find out how much better the new equipment is (and how little fuel it uses) when someone finally beats the owners into buying something of newer design than 1965!

As far as pollution from engines goes, the typcial automobile is now very clean, and getting cleaner. To the point that LA improved air quality dramatically by outlawing gasoline lawn mowers -- the typical gas lawn mower in S. Cal. made more unburned hydrocarbons and smog mowing a quarter acre lot once than all the cars in the family produced during a month of driving......

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!