I'll give you the fact that links are provided are North American based. I find it easier to give credence to organizations I'm familiar with than those I'm not. I'll also give you the fact that European diesel development is further ahead than North American, but most of the people researching diesel here use Euro-spec'd cars and look at European standards.
All that said, I just posted because I find the diesel vs. gasoline (petrol) debate about as solvable as a religious war: both sides believe they are right, and neither is able to prove their point beyond a shadow of a doubt.
As you said, there are too many variables and factors to allow proper determination -- geography and weather patterns determine how pollution settles, as do patterns of human settlement. Fuel production (both diesel and gasoline) varies in quality and standards and oil isn't a perfectly homogeneous substance.
Even those who say they are experts at understanding pathology and physiology and human anatomy really don't have a clue about most of the deep, underlying reasons for how and why things harm us. Sure they can isolate variables in a lab, but the complex chemical soup that makes up our air, food, and water supplies, not to mention the rest of the chemicals and elements we expose ourself to on a daily basis mingle in that vast petri dish that is the human body and interact in ways that are pretty unpredictable.
When you add vehicle pollution to the above mix, we're just basically playing Russian roulette. Does the formaldehyde present in diesel harm us more than say the coronene in gasoline? And what happens when either is combined with an assortment of other chemicals? I really don't think anybody can say with 100 per cent certainty.
As motorists we all are responsible for poisoning the atmosphere. The only choice we get is to pick our poisons. In some places, like Australia and North America, the choice is pretty much made for us -- gasoline -- but the same can be said (in the reverse) for other regions of the world where diesel is king.
As I stated before both gasoline and diesel have their strengths and weaknesses. My point about diesel is that as the diesel engine was refined, and as scientists and engineers thought they were making things better (by making the particulate matter smaller) they were actually making a by-product that is worse for human health. Humans can more easily deal with larger particulate than smaller particulate. It's just one of those little ironies in life. It would have been a healthier engine (in some ways, not all) if it many of the recent developments to make it cleaner hadn't happened.
If you're still looking for more "evidence" in the argument, I'll play devil's advocate and throw some more on the side of petrol. Please note, I don't necessarily believe it. I'm pretty much a fence sitter on this issue, especially because I know how easily "facts and figures" can be manipulated to prove almost any point. For your satisfaction
most of these have been pulled from sources outside North America, or refer to studies performed outside North America. All are quotations taken directly from the sources indicated.
A November 1995 report from the British Department of Health's Committee on the Medical Effects of Air Pollutants (COMEAP), has called for much stricter standards for PM10 (particluate matter less than 10 microns in diameter). It says that the death rate rises by 1% per day when there is an increase in PM10 concentrations of 10 micrograms per cubic metre. The report further suggests that there seems to be no safe level for PM10. People with existing respiratory and cardiac disorders are most vulnerable.
Concentrating on measuring and reducing PM10 levels would be an improvement over the existing situation but the United States Environmental Protection Agency is urging that standards also be set for levels of particles of less than 2.5 microns in diameter (PM2.5). Pollution by this range of particles is rarely even measured at the moment.
The issue is further casting a spotlight on diesel vehicles. Diesel engines have long been identified as a major source of particulate pollution and more than 90% of the particles emitted are of less than 2.5 microns. Cities in the developing world, including most in the Asia-Pacific region, typically have much higher rates of diesel use for transport than high-income cities. Diesel fuel accounts for about half of the transport fuel consumed in cities like Bangkok, Jakarta and Manila. Furthermore, high sulfur content fuels, common in the region, produce the most particulate pollution. Emissions from poorly maintained diesel engines, which are the norm in low income countries, can be 10 to 15 times those of well-maintained ones.
Gasoline vehicles are not off the hook by any means. Non-tailpipe emissions (such as road dust and brake pad wear) are a significant source of particles and gaseous emissions such as nitrogen oxides and hydrocarbons are precursors for tiny acidic aerosol particles ("white smoke"), which may be among the most dangerous.
The petrol and diesel vapours consist almost exclusively of aliphatic alkanes, alkenes and alkynes (and dienes) and aromatic hydrocarbons. Some of these compounds appear in the exhaust gases together with several aldehydes, viz. formaldehyde, acetaldehyde, pentanal, pentenal (acrolein), butenal, and also methanol and ethanol. Acetone, nitric oxide and ammonia are also present, acetone and nitric oxide being much more abundant in the diesel exhaust gas than in the petrol exhaust gas.
Diesel engine exhaust emissions (commonly known as 'diesel fumes') are a mixture of gases,vapours, liquid aerosols and substances made up of particles. They contain the products of combustion. The carbon particle or soot content varies from 60% to 80% depending on the fuel used and the type of engine. Most of the contaminants are adsorbed onto the soot. Petrol engines produce more carbon monoxide but much less soot than diesel engines.
This means that diesel engines have a much higher compression ratio than petrol engines. Therefore they operate more efficiently producing a smaller proportion of unburnt gases such as hydrocarbons and carbon monoxide. Diesel engines can however produce large amounts of smoke and typically emit up to 50% to 80% more particulate matter than petrol engines.
Emissions from petrol cars have been dramatically reduced by the introduction of catalytic converters, which oxidise pollutants such as CO to less harmful gases such as CO2. When compared to petrol cars without catalysts, catalyst cars have much lower CO, HC and NOx emissions, at the expense of CO2 emissions, which increase due to the oxidation of carbon monoxide to CO2. As a consequence of this, a catalyst car will also use slightly more fuel and become less efficient. However, despite these improvements, petrol cars with catalysts still produce more CO and HC than diesel cars, although exhaust emissions of NOx and particulates are much lower than diesel cars. In fact particulate emissions from petrol cars are so low that they are not routinely measured.
Emissions from Diesel Vehicles
Diesel fuel contains more energy per litre than petrol and coupled with the fact that diesel engines are more efficient than petrol engines, diesel cars are more efficient to run. Diesel fuel contains no lead and emissions of the regulated pollutants (carbon monoxide, hydrocarbons and nitrogen oxides) are lower than those from petrol cars without a catalyst. However, when compared to petrol cars with a catalyst, diesels have higher emissions of NOx and much higher emissions of particulate matter.
http://ehp.niehs.nih.gov/realfiles/m...holm-full.html This site has some nice charts with breakdowns of the pollutants produced by gasoline and diesel engines. Admittedly the comparison isn't entirely fair, because the gasoline vehicles are light duty (cars) and the diesel ones are heavy duty (trucks and buses). I'd reproduce the charts, but I'm not retyping them all and since they can't be copied and pasted, you can look them up for yourselves (if you're really interested).
P.S. I still don't think any of this answers Steve's original question about average mileage in a 2.0 TS.