Undoubtedly this study can result in findings that show increased hazard to human health - if the funders find that to be expedient to their business.

Apparently someone is pissed that biodiesel fuel “is favorably viewed.”
I read the Intro of the report. They ,Swanson, Madden and Ghio have got nothing. They're just soliciting for work.

Rafael Seidl posted: Biodiesel does not contain PCAs, nor is it likely that any are formed during the combustion process. However, detailed knowledge of the chemical species produced during the combustion of all automotive fuels remains patchy.

Upon what basis do they claim that PCAs (normally called Polynuclear Aromatic Hydrocarbons or PAHs) are unlikely to form during biodiesel combustion? It is my impression that PAH formation can occur in the combustion of nearly any carbonaceous fuel. The formation is disfavored by a very lean mixture. They admit that (their own?) detailed knowledge of speciation in combustion is "patchy", although they may not have talked to any combustion experts.

Biodiesel may be cleaner than petrodiesel, but it's still diesel, and as such is not as clean as gasoline.

A note to the science-bashers here: When scientists say "more research is needed", they are saying that they don't have all the answers. I find the claims that scientists are only in it for the money or are otherwise unethical to be offensive.

George -

petrodiesel combustion produces lower engine-out PCA/PAH emissions than any gasoline engine, see e.g.

http://www.wrapair.org/APACE/SPECIATION/summary_topic3.htm

And that's considering that US petrodiesel contains a larger fraction of isomers, polyalkanes and aromatics than European petrodiesel does, because US refineries have to hydrocrack even valuable middle distillates to satisfy the demand for gasoline. The portion of the hydrocracking residue known as light cycle oil is mixed in to #2 diesel. This is why US diesel features lower cetane numbers than the European grade, i.e. it's harder to combust and tends to produce higher particulate emissions.

Biodiesel consists of fatty acid methyl esters. Can we be certain that the PM produced when combusting this fuel does not contain any PAHs or other carcinogenic compounds? No, the highly complex speciation of real-world diesel combustion processes is still too poorly understood. So by all means, let the scientists research this.

Note that European diesel vehicles have featured oxidation catalysts for over a decade and most new ones now feature a wall-flow DPF either standard or as an option. They will become standard on all cars when Euro 5 kicks in in 2010. What matters for population health is not engine-out but tailpipe emissions.

Cervus points out that in the US, the LDV diesels about to be released in or after 2008 will meet Tier 2 Bin 5, making their as clean as gasoline cars in the standardized drive cycle. This is possible only because the US has switched to ULSD, comparable to the 10ppm standard that will come into force in Europe in 2010 (though some countries' refineries already meet it today).

It would be unwise to assume a priori that these clean diesels will still pose a greater risk to population health than the vast majority of gasoline-powered vehicles do. On the contrary, establishing a modern clean diesel LDV fleet in the US will prompt refineries there to raise cetane levels, yielding a modest PM reduction even from legacy diesel vehicles.

Biodiesel may be cleaner than petrodiesel, but it's still diesel, and as such is not as clean as gasoline.

Good lord, how is gasoline clean?!?

Have you tried walking outside in Houston, LA, Atlanta on a summer day? Emissions from burning anything is not going to be good, gasoline included. Gasoline emissions are currently treated more comprehensively than diesel emissions (from an engine-out perspective gasoline is dirtier by every measure except PM by mass), but only for regulated pollutants. Regs say nothing about aromatics, aldehydes, PM by number, secondary particulate formation, etc. And regs still say nothing about CO2.

Anyway ... Upon what basis do they claim that PCAs (normally called Polynuclear Aromatic Hydrocarbons or PAHs) are unlikely to form during biodiesel combustion? It is my impression that PAH formation can occur in the combustion of nearly any carbonaceous fuel. The formation is disfavored by a very lean mixture.

A diesel typically runs at air-fuel ratios of 20-60:1, depending on load. By gasoline standards, that's extremely lean. In general, gasoline exhaust has higher PAH content than diesel exhaust. In terms of leakage emissions (such as at the filling station) biodiesel doesn't have aromatics to begin with; diesel ideally has low aromatic content (aromatics are great octane boosters but terrible for cetane); gasoline has high aromatic content. Gasoline also has a vapor pressure nearly 1000 times that of diesel, and bio even less.

Levels of "engine out" pollutants are irrelevant. What matters is what comes out the (catalyst) tailpipe, and diesel presently loses that competition without question.

According to the EPA, definitely. But if you think the EPA is incomplete wrt toxic emissions, that statement is difficult to defend. The engine vs tailpipe out is only relevant in that you do have to work more to clean up gasoline, but it is true that today we do in fact do more to clean up gasoline emissions.

Evaporative losses are not likely to be a significant source of PAHs, as their vapor pressure is not high relative to the major components of gasoline, given that they are C14 or so.

Except that lighter components react in ambient air to form a huge number of heavier compounds with much lower vapor pressures (ie, form PM)

At some point in the future, probably around the time that I can buy a nice plug-in hybrid, I suspect that we will see "clean diesel" in America.

I'm hoping for primarily BEV, with a small biodiesel genset for longer runs.

... University of North Carolina School of Public Health and the US EPA’s National Health and Environmental Effects Research Laboratory call for more research into the possible health effects of biodiesel exhaust ... Who is pushing this from UNC and EPA? Is it Exxon and Mobil funding and contributing to the debate? You always need to look into who is pushing for the "research" ... Follow the money ...