While technological advancements have cleaned up diesel engines, modern clean diesel vehicles may not be as "green" as previously thought. The reason, according to some analysts, is that soot emissions from these new powerplants are underestimated.
In Britain, for example, vehicles are exempt from road tax if their carbon dioxide emissions are below 100 grams per kilometer. However, this exemption scheme does not account for soot, which some analysts think contribute to global warming through a substance called black carbon. Stanford University's Mark Jacobson says that:
The UK's Department for Transport claims that its scheme excludes soot because "black carbon emitted by modern diesel cars is practically undetectable." But even reduced levels of soot could still equal an additional gram of CO2 emissions per kilometer, according to some experts. It would be an interesting dilemma for automakers if this black carbon was counted, since most of the diesel vehicles that qualify for zero road tax do so because they are just under the law's 100-gram threshold. If regulators start adding in that single gram of emissions, some of these vehicles could no longer be exempt from the UK's road tax.Each effect will vary from vehicle to vehicle but the point is they are not accounting for this [black carbon]. The claim that barely detectable black carbon causes no warming is misinformation.
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[Source: Ward's Auto – sub.req.]