It seems many are making a connection between sustainability and the emissions of particulate matter and other short-lived atmospheric pollutants, but these issues are actually rather separate. Since decisions about what vehicle to use are likely to depend on how much weight you give to each, and I am not sure if one cares more about sustainability or about the air quality problem, I would like to offer some background on the issue of particulate emissions, as this is a rather complex topic.
First, the exhaust from a gasoline engine is different from the exhaust from a diesel engine. Diesel engines produce primarily nitrogen oxides, with a small amount of soot and a small amount of organic compounds with relatively low volatility; these organic compounds can be, however, fairly harmful. Gasoline engines produce primarily carbon monoxide, nitrogen oxides, a wide mix of organic compounds, and some small particles, all in the amounts that are much higher than in diesel exhaust.
The exhaust from gasoline cars has been cleaned up using very high efficiency catalytic converters, and by very precise regulation of the air-fuel ratio using computer controlled fuel injection. The emissions from a new gasoline car has decreased by 96-99% over the last thirty years or so.
Diesel engines are undergoing the same treatment, there has already been approx. 90% decrease in particulate matter emissions, with another approx. 90% decrease (total reduction of 99%) coming up starting with the next model year.
One thing to consider here that if anything deteriorates, gets out of adjustment, breaks down, etc., the emissions from a gasoline car skyrocket, to 100 times and even up to 1000 times the original levels. Older diesel engines do not have catalytic converters or computer controls. Therefore, while a new gasoline car is cleaner than a new diesel car, at some age or mileage, this trend reverses. For example, I had a 1981 Datsun pickup truck, operated on biodiesel. The truck had lower emissions of regulated gaseous pollutants (volatile organic compounds, carbon monoxide, nitrogen oxides) than an "average" gasoline car.
So when you are making your comparison, you should be specific – which diesel engine, and which gasoline engine – and you must consider the mechanical shape of each.
To answer a common question, a passenger car with an old, mechanically controlled diesel engine will produce more emissions of particulate matter than a new gasoline car; this is regardless of what the diesel engine is being run on. (Tests show, so far, that when you properly care for and properly operate your engine, you will get some reduction in particulate matter emissions running on vegetable oil.)
The emissions of regulated air pollutants have, however, a limited bearing on sustainability. Sustainability is a concept which considers the impact of today's actions on future generations. Most atmospheric pollutants originating from internal combustion engines are short-lived, on the order of hours to weeks, after which they are destroyed during the natural cleaning of the atmosphere.
The emissions that do have a significant bearing on sustainability are not particulate matter emissions, but greenhouse gas emissions, primarily of carbon dioxide. The life of carbon dioxide in the atmosphere is estimated to be decades to hundreds of years, which will affect future generations. Burning any fuel releases carbon dioxide. Diesel powered passenger cars typically release less carbon dioxide, per mile, than comparable gasoline cars. The source – where does your fuel come from – makes a huge difference. If your fuel is derived from fossil fuels, all carbon contained in the fuel eventually ends up as "excess" carbon dioxide. If your fuel is derived from a renewable source, like vegetable oil, your engine still emits carbon dioxide, but, the plants from which the oil was made have already absorbed the same amount of carbon dioxide. Thus, with straight vegetable oil, you are emitting co2 that has already been removed from the atmosphere. The net impact of SVO on levels of greenhouse gas in the atmosphere is essentially zero.
Also, on a "sustainability" scale, particulate emissions are not a problem. First, particles emitted into the air counteract, to some extent, the greenhouse effect, slowing down the progress of "global warming", and therefore, this is more of a "benefit". Second, vehicles are getting cleaner, and the air pollution problem remains only because the total vehicle miles traveled double every twenty years or so (even then, the air is getting cleaner). Third, upcoming global shortages of petroleum will likely put a stop to rampant "automobilism"; in the long run, we will only be able to burn fuels at a rate we are able to grow them (or otherwise produce them from renewable sources). By then, you probably won't have to worry about producing "unsustainable" amounts of particulate matter.
This is only a grossly simplified overview; there are many exceptions and issues. As you are a "sustainability consultant", I believe this will serve you as an inspiration to pursue these topics further, as a part of the lifetime of further studies, exploration and thinking that lies ahead of those of us who have chosen this path.
If I may ask, what are your current transportation needs? If you do drive a passenger vehicle on a regular basis, how long are your trips, and how many miles do you travel each year? Also, if I may ask, what kind of energy do you use at home and at your office?
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