Excellent Resource for Tailpipe Emissions

And to add balance for this side of the Atlantic, there's a similar search tool for European cars;

Select a search : Directgov - Car fuel data, CO2 and vehicle tax tools (select the 'new' option)

Some of our lastest 'clean' diesel cars on the Euro 5 emission limits still emit 25 times more NOx than the Prius (and other petrol cars).

 

But again, what about the higher HC emissions of the petrol cars? Why are you fixated on NOx emissions?

 

Also, EPA's "Green Vehicle Guide" only addresses exhaust emissions. Gasoline inherently has much higher upstream VOC emissions which IMHO need to be taken into account. See Table 4-12 on page 4-42 [366 of 479] of http://www.epa.gov/otaq/climate/documents/420d11901.pdf for a comparison of upstream emissions of gasoline and diesel fuel (ULSD).

 

I compared the emissions of the 2012 VW Passat certified as PZEV by CARB with the 2012 VW Passat TDI certified as ULEV, adding the upstream emissions to each vehicle (i.e., "well-to-wheels" analysis). The results are at Gas vs. Diesel Well-to-Wheels Emissions if anyone is interested.

 

I do not know, I always heard they were bad. I'm not a scientist. Can a scientist or engineer tell me which is the worst pollutant from a car - diesel or petrol.

Anyhow, I live in Europe where diesel is popular and our cities and towns have got worse for pollution in the last ten years or so. Twenty years ago pollution improved when petrol catalysts were required, then when diesel models become more popular because of better fuel economy people switched to those and emissions got worse. Before you say the later emission levels are better I'll let you know we had ultra low sulphur diesel for well over 10 years (unlike US where it's only about 5 years) and diesel particle filters came out about that time too.

 

The fixation is because VOC + NOx = SMOG. If you have diesel vehicles creating NOx then your big cities will have smog unrelated to oil and gas production VOC you are trying to highlight. Apparently EPA is coming out imminently by about mid-April with new VOC regs. So lets see how this goes.

Thank you however for the post.

 

The question is really what are the appropriate levels. Both of these 2012s will produce much lower emissions than the cars they are replacing. The only way to get to 0 tailpipe is EV and I don't think we need to get all the way down there.

But.... IMHO it seems hybrid gasoline is the best way to reduce tailpipe emissions, but the cost of the cars are more. VW is putting out a jetta hybrid, lets see how those emissions compare.

 

WXMAN that is a very interesting calculation and I thank you very much for sharing. Basically you are saying the GREET model uses an upstream VOC emission factor of approx +40 gram VOC (gasoline vapors) per million BTU is lost to the atmosphere in the process of transporting gasoline to gas station. Diesel is heavier so little VOC lost in that step.

You propose that the upstream gasoline VOC losses need to be assigned to gasoline vehicles, and you have made that calculation. Since I am not an air modelling expert, I do not know if your basic assumption is valid. Below however is my psuedo-layman's critique:

I would assume the air quality issue we talking about is smog in the cities. Background VOC is always going to be there, even some from trees, so what cars/trucks really contribute to the smog equation is NOx, seen as reddish-brownish tint in the sky over cities. The upstream gasoline vapors (VOC) from the GREET factors only matters if it is happening right in the local city itself. I question also, is it fair to combine NOx+VOC in your plots? Seems like you are trying to hide higher NOx from diesel, which I think may be primary cause of Grumpy's smoggy London. VOC maybe secondary cause.

In other words, I am not sure adding upstream VOC losses to vehicle exhaust data gets us closer to the truth of which car is cleaner.

 

I believe he is using the test results from CARB. For example, 0.07 for NOx is the maximum allowed value, not what the car actually emits.

 

I apologize for suddenly dropping out of this discussion, but I was called out of town because of a family emergency and haven't had access to the internet for a week.

I have been involved in various aspects of air quality since 1985. I have been an air quality forecaster for NOAA since 2004 (see NWS Air Quality Program), and was a modeller for at least 10 years prior to that.

To generically address a few point that have been raised, NOx DOES contribute to ground-level ozone, but the ozone that is produce immediately is destroyed in what is called the "NOx photolytic cycle", and ozone accumulation cannot be explained by ambient NOx regardless of how high the concentrations are (there are other issues like NO2 concentrations that need to be considered, so NOx emissions DO need to be contained to some degree). Ground-level ozone accumulates because of ambient hydrocarbons whose oxidation products interfere with the NOx photolytic cycle.

As wjtracy mentioned, VOCs do occur naturally (although NOx also occurs naturally - a study by NASA showed that an average CG lightning strike produces about 715 moles of NOx). However, extensive research in the early 2000s showed that large cities in the U.S. were all "VOC-limited" with respect to ozone production, meaning that VOC was the limiting factor in ozone formation. I invite interested folks to read http://www.altfuels.us/pdf/lawson%5B1%5D.pdf for a good discussion of this urban phenomenon.

There have also been studies which have shown that gasoline exhaust and gasoline vapor contributes 50% to as much as 85% of the ambient VOC levels in urban locations (e.g., John G. Watson, Judith C. Chow and Eric M. Fujita; “Review of volatile organic compound source apportionment by chemical mass balance.” Atmospheric Environment, Volume 35, Issue 9, March 2001, Pages 1567-1584).

I combined HC+NOx in my analysis because EPA and CARB combine HC and NOx in the supplemental FTP emissions (US06 and SC03 test cycles) and also because CARB will be combining HC+NOx in LEV III even in the actual FTP emission cert levels.

I also took the actual reported emissions based on the CARB cert sheets (listed under "CERT" on the pdf cert sheets) as seftonm suggested.

I can go into more detail on the atmospheric chemistry of ground-level ozone formation if anyone is interested.

 

So is Nox bad or is it HC?

In Europe city pollution has got worse and it seems to have increased with the popularity of diesel cars. These diesel cars emit 25 to 100 times more Nox than a Prius say. Are they the problem? Or would we be better reducing HC more aggressively?

I have often said that one Prius emits the same NOx as 25 similar sized diesels. Is that correct? Is it as simplistic as encouraging petrol rather than diesel in towns and cities? (or better still, encourage EV's).

 

GC - since you live in the UK, I'm not familiar enough with your local atmospheric chemistry to offer an informed opinion. I am aware that the "weekend ozone effect" has been documented in Europe (Finland, IIRC). Both are "bad" as far as those two emissions are concerned, and are considered pollutants, but the local chemistry determines which is "worse".

It's my understanding that many "pre-clean-diesel" vehicles were sold in Europe, so most of the emissions from diesel cars are likely from those legacy diesels.

A study just released yesterday confirms that the emissions from clean diesel technology (diesel trucks in this case) have virtually no direct health effects - http://pubs.healtheffects.org/getfile.php?u=708.