Volkswagen diesel car wins "Green Car of the Year"

It's great having the chance to chat, btw. I realize I have a lot of unanswered questions just waiting to spring on someone like you

Could you summarize the relative importance of the main pollutants as they relate to global warming, ozone depletion, and ground level smog in auto emissions ? If you have to write a book, I'll wait ... E.g., it is my impression that CO is at a low enough level in gassers, that the superior profile seen in diesels does not have practical importance.

 

Thanks. Same here. I love a good discussion on emissions.

The information I have is generic gas vs. diesel, all I know specifically about the Prius emissions are what are in the emission certifications.


Global warming...

CO2 is the main anthropogenic global warming gas, as I'm sure you're aware. There are also other auto emissions that are considered "greenhouse gases" - N2O (nitrous oxide - not the same as NOx (NOx = NO + NO2)) and CH4 (methane). Both are much more potent greenhouse gases than CO2 (the global warming potential of N2O is 310 times more potent than CO2; CH4 is 21 time more potent). According to EPA gassers typically have much higher emissions of both, although they’re swamped by the magnitude of the CO2 emissions. Nevertheless, according to EPA, in 2005, gassers (highway gasoline) were responsible for 108 Gg of N2O emissions, while “highway diesel” was responsible for 1 Gg. CH4 – highway gassers 92 Gg; highway diesel – 1 Gg.


Ground-level ozone production...

Oxides of nitrogen (NOx = NO + NO2), volatile organic compounds (VOC), and carbon monoxide (CO) are all considered “ozone precursors”. Ozone, the main component of “smog”, is not emitted directly from motor vehicles but is formed secondarily in the atmosphere from reactions of these ozone precursors in the presence of sunlight (solar radiation).

Diesel engines generally produce more NOx, while gasoline engines generally produce more VOC (called NMHC or NMOG as direct emissions from vehicles) and CO.

Many reports on ground-level ozone or smog formation typically use the adjective “smog-forming” when describing NOx. However, this is quite misleading since NOx can also be “smog-destroying” in many common conditions (NOx reacts directly with ozone). NOx (actually NO2) is required for the formation of ozone since it is the source of “odd” oxygen in the atmosphere:

NO2 + hv (solar radiation) --> NO + O (“odd” oxygen or atomic oxygen)
O + O2 (molecular oxygen) --> O3 (ozone)

However, the NO that is produced from the photo-dissociation of NO2 is available to immediately react with O3:

O3 + NO --> O2 + NO2

And the cycle starts all over again. Thus, this little pseudo-equilibrium (NOx photolytic cycle) does not produce net ozone and NOx in and of itself cannot account for ozone accumulation in the atmosphere. This is where VOC and CO enter the picture. VOC and CO emissions are oxidized in the atmosphere mainly by the hydroxyl radical (OH):

CO + OH --> H + CO2
H + O2 + M --> HO2 + M

RH + OH --> R + H2O
R + O2 + M --> RO2 + M

(M is a non-participating molecule which carries off excess energy – e.g. N2)

HO2 and RO2 (peroxy radicals) are capable of oxidizing NO to NO2 and thus compete with O3 in this process:

HO2 + NO --> OH + NO2
RO2 + NO --> RO + NO2

Without as much NO to consume the O3, the O3 can accumulate.

When ambient levels of VOC are very high relative to ambient NOx levels (more than about 8:1), the VOC oxidation products consume enough NO that the only limiting factor in O3 formation is the amount of NOx in the atmosphere. Under this condition, ozone formation is “NOx-limited”.

When ambient levels of VOC are low (or at least not very high) relative to ambient NOx levels (less than about 8:1), there is enough ambient NO around to react with (consume) some of the O3 and preclude high ambient levels of ozone. Under this condition, ozone formation is “VOC-limited”.

Studies by the U.S. Department of Energy, National Renewable Energy Lab (NREL), of the “weekend effect”, have empirically shown that all large urban areas studied thus far are “VOC-limited” with respect to ozone formation. The “weekend effect” is a phenomenon that’s been observed for several decades, especially in the Southern California Air Basin (SoCAB, where ambient ozone levels tend to be much higher on weekends in spite of significantly lower ozone precursor emissions (especially NOx).

So as you can see, “smog” formation is quite complex and not clear-cut from an emissions strategy.

By the way, I agree with you that CO emissions are almost uniformly low and one-tenth of an already trivial amount is just splitting hairs.

Does that clarify any of your questions?

 

Thank you ! Very informative. I'll have to digest this, so don't take a break from questions as lack of interest.

For now, two simple questions if I may:
1. Is the 8:1 ratio moles or weights ? Just by weight, CO dwarfs everything but formaldehyde. Maybe less reactive ?
2. Are other NO scavengers or catalysts present in the air in appreciable quantities ? If no VOC or CO was emitted, what would be the eventual fate of NO2 at increasing concentrations ?

In a manner of speaking, it sounds like you are saying that diesel + gasser = smog. Fascinating, and worrisome.
It's interesting that Euro V has focused on PM reduction, and left NOx high. Do you think this is a question of political expediency, or technical difficulty in solving the diesel Nox problem ? Is there a reason why a CAT converter cannot solve diesel NOx, like it is solved in gassers ?

 

In looking over the CARB cert for the 09 TDI that Jhinton provided, I see that NMOG is 0.09 grams/mile, which is about 3X fleet average, and 10x Prius. Am I reading this correctly, and am I correct in thinking that NMOG is the major NO scavenger on a weighted basis leading to smog in an NO2 rich sky ?

I do realize that Prius is not a typical gasser

 

OK, let's agree on those numbers. What you say makes sense now, thanks for elaborating a bit more.

Now what I don't get, and what is (for me) getting surreal by the minute, is that we are drifting away from the emissions that get from the tailpipe at rates of g/km and are therefore massive impact on the environment, and focusing on emissions that get out at (believing your numbers) 0.0004 g/mi.

Is is just me my perception or are you in fact streaching as far as you can to find any gas at all, any emission, no matter how vague or how low, anything at all, as long as you can find a "good diesel" that can compare???

You agreed on another post that the CO emission levels are so low that they are not worth considering. I find that statement outrageous. The CO emissions are very high, considering the ammout that get out the tailpipe for so little distance, and multiplied by millions of cars.

The CO emissions on the Prius for example are 0.180 g/km. The Blumotion Golf, the state-of-the-art clean diesel from VW emmits 0.391 g/km. So, as usual, since the best in class diesel gets a lesson from the hybrid (Bluemotion = over 2x the CO emissions of a Prius), you just say that these are hair spliting numbers... we are talking about 0.391 g/km...

Then you consider vaporative emissions important, and you go and do your best to prove the Prius does have vaporative emissions. Good for you. But then... wait a minute... you are talking about emissions that are in the range of 0.0004 g/mi!!! So now this is important? This is not spliting hairs? A value that is almost a thousand times less than CO is now important???
I find this very amusing, thanks for posting indeed!

What about the other emissions? Why do you keep on discussing PM and evaporative emissions? Why don't you discuss other emissions altogether? What are you avoiding here? Why did you not comment on me saying there are a lot of other emissions coming out as well like NOx, As, Se, Cd, Zn, SOx, several poly cyclic hydrocarbons, etc?

Talk about being biased...

 

Thanks a lot for this link jhinton, I will most definitely have a good reading on it later tonight!

 

This is absurd, can you really state this things and sleep at night?
What about the context for these numbers? What about comparing the average engine size for both types? What is the average engine displacement on your gasoline engines? What is the average engine displacement on the very few diesels you have? C'mon!

You are comparing oranges to apples and you know it. The USA strives on huge SUVs and trucks with huge engine displacements. The inneficiency of those engines is one of the big reasons the Prius was such a success there in the USA.

We in Europe use much more efficient vehicles, very small engine sizes, for example standard here in Portugal is about 1400cc for gasoline and 1900cc for diesel, and diesel are comming down to 1700cc and as low as 1300cc already! For that very same reason the fuel consumption of a Prius is not all that great comparing to other cars, as it is in the USA.

Now you import most of your diesels, and therefore you have an average engine size of your diesels a lot smaller than your gassers. Obviously the gassers emmit more, they have huge engines, compared to the diesels. This is not to say diesel is better than gasoline, it is obviously a very biased statement. Anyway, I don't defend pure gasoline engines anyway, I just like EVs and because I can't own one I have to live with a Hybrid.

Well, I've seen enough of this thread, and where it's headed.

Too bad that we had a chance to talk to someone who actually seems to understand and have knowledge about the emissions standard and it turned out to be a biased person. I guess it happens a bit everywere to any person on any activity area.

I'm out of this thread, thanks for the information you all shared.

 

Hello kettle, this is the pot... wait a minute, are you black or am I????

 

1. I THINK it's by mass, although I'm not certain. Good question.

CO has an average atmospheric lifetime about 50 times longer than HCs (although HC vary greatly by species). Thus, it could be expected that CO is about 1/50th as reactive as HC on average.

2. the hydroxy radial - OH - reacts with NO, but the average lifetime of NO with resepct to OH is 2.5 days; NO lifetime with respect to O3 is about 1 minute. OH also reacts with NO2 (reaction rate about 1 day) which oxidizes it into NO3 (nitrate) which is the removal process for NOx.

As far as Europe concentrating less on NOx, I guess it's just different approaches by the respective regulatory agencies.

 

I'm not sure what you're referring to. The CARB certs I'm referring to have 0.009 g/mi NMOG (@ UL) for the Prius and 0.012 g/mi for the LEV II Jetta TDI (it's 0.014 g/mi for the ULEV II Jetta TDI; not sure why there's a difference).

All of the 2009 CARB certs are available at Subject Top Page: Passenger Car, Light Duty Truck, and Medium Duty Vehicle Executive Orders - 2009

 

I consider evaporative emissions important because up to 50% of ALL ambient HCs (not just anthropogenic HCs) come from GASOLINE evaporation ( Brown et al, “Source apportionment of VOCs in the Los Angeles area using positive matrix factorizationâ€. Atmospheric Environment, Volume 41, Issue 2, January 2007, Pages 227-237).

The SOA exercise was just to point out that the PM emissions from the T2B5 Jetta TDI are so low, that EVEN the tiny amout of evaporative emissions from the Prius itself (based on the 2009 CARB cert) would produce about the same amount of PM (as SOA) as the TDI. Just saying that the TDI's PM emssions are vanishingly small.

 

The values come directly from a USEPA publication, Tables 3-23 and 3-24 ( http://epa.gov/climatechange/emissions/downloads06/07Energy.pdf ). Tables 3-21 and 3-22 give the CO2 equivalence of the N2O and CH4 emissions.

The U.S. uses about 3 times as much gasoline as diesel fuel for on-highway use. So even normalizing the data by multiplying the "diesel highway" values by 3 would still be far less than "gasoline highway" for both emissions.

You seem to have the impression that the only diesel vehicles in the U.S. are little 1.9 liter VWs. Virtually all HD trucks are diesels, and they typically have around 15 liter diesel engines in them.

 

Do you have any data suggesting these emissions are unique to diesels?

I've already acknowledged that diesels tend to be higher in NOx, but I've also explained why I think HCs are more critical than NOx. SOx has been dramatically reduced since we've (the U.S.) switched to ULSD.

All of those heavy metals you mentioned would exist in the solid phase, so would just be part of the PM mass.

As far as PAHs go, a EUROPEAN study (Ecotraffic of Sweden) showed that a light-duty diesel with DPF (Peugeot I believe) had FAR lower levels of PAH than the gas version of the same vehicle - 0.96 micrograms/km (diesel) vs. 9.8 micrograms/km (gas), about an order-of-magnitude difference ( Environmental and Health Impact From Modern Cars, Ecotraffic).

 

I was reading the wrong number. Thanks

 

wxman,

I'm not grasping the kinetics of these NOx reactions as you write them. Chemistry was a long time ago for me, and I'm used to thinking in terms of dissociation constants and half lives. Anything else confuses me.

I presume that saying that NO2 -> NO3 takes one day means that if we had introduced a labeled NO2 molecule into the air today, we would find it as NO3 tomorrow ? Is that the same thing as saying that ALL NO2 is transformed after 24 hours in the sky if we could freeze new NO2 deposition at zero hour ? And while we are talking about NO3 -- is that a bad molecule ?

 

Correct - if you only had NO2 and OH in a confined space, all the NO2 would be oxidized to NO3 in about 1 day (under typical ambient conditions).

I guess assuming sufficient OH, the NO2 would be completely depleted within a day (although NO2 reacts with other species in the atmosphere besides OH, e.g., those peroxy radicals I mentioned in my previous post).

I obtained these reaction rates from a handout we received at an air quality forecasting training course. It referenced "Atkinson 1998" as the source of these reaction rates.

NO3 is nitrate and exists mostly as ammonium nitrate (NH4NO3) in the ambient environment. Ammonium nitrate is used for agricultural fertilizer. It's also a particle so it adds to the ambient PM.

Being an MD, you would probably have a much better idea of its effects on human health than me; I only peripherally am involved with toxicology. However, it would be my guess that since NH4NO3 is highly water soluble, it would completely dissolve in human lungs.

 

Welcome wxman, I haven't had much time to devote to this lately but its nice to see you still kicking around!

Rob

 

Thanks Rob. I enjoyed our discussions at TDIClub.

You've actually raised some very good points which I have found very useful. Thanks for your contributions.

 

This is an exceedingly complex topic, and I'm not really going to be able to do it proper justice due to limited time and the limits of my own understanding (and lack of time to do more research). So I'll just throw a bunch of thoughts out there to try and catch up on the discussion as I can

- Diesel + Gas = Smog. I think to some degree Sage hit the nail on the head. When you take evaporative emissions into account, gas vehicles generally put more HCs. Diesels put out more NOx. Thats a perfect recipe for smog.

- Oil = diesel + gas. Unfortunately there is an unavoidable linkage between these fuels. When you distill oil into its representative parts you get light components that make things like gasoline, and heavy components that make things like diesel. Heavy components can be cracked down into lighter ones through additional processing (requiring more energy and cost), but the inverse is not possible.

- HCs. PZEV vehicles like the Prius do an admirable job of reducing on-board evaporative emissions, although as wxman notes they are still not zero. It should be noted though that the Prius is now 5 years old and no longer state of the art in this regard. The PZEV '09 VW Jetta 2.5L for example has slightly higher 3 day diurnal results (0.23g) but running losses of 0 and NMOG of 0.004g/mi. Using the previous formula, this would work out to 0.004 + 0 + 0.23/50 = 0.0086 g/mi, which is less than its diesel counterpart's 0.012 g/mi. There is still the issue of offboard evaporative emissions, thought this gets even more complicated. Because of its volatility, every time gasoline is transfered or sloshed around or even just sits in an environment that is not completely sealed, there are evaporative losses. The oil refining industry is obviously worried about this, as their product is literally disappearing into the air. They have made some efforts to improve, but they generally seem feel like if they keep things under 1% or so thats good enough. Thats still a huge amount from an emissions perspective, so the EPA has forced them to adopt a number of other measures. One of them is the vapor recovery hood thingies you see on fuel pumps in smoggy cities. While these help, at this point the off-board evaporative emissions are probably far greater on a per-mile basis than tailpipe or on-board evaporation. Trying to apportion and prioritize this emission source is complicated though. To some degree, production of gasoline and other light distillates is unavoidable in the production of diesel, so apportioning these emissions solely to gasoline vehicles seems unfair. There is also a question of location. While most on-board emissions occur in urban areas, where HCs matter most, much of the production and transportation evaporation occur in rural areas where HCs are not the driving factor in air quality. This is an inherent challenge for gasoline, and one that still needs a lot of work. Fuel efficient vehicles like the Prius are an important part of reducing this emissions source, as off-board emissions are on a per-gallon basis. Using fewer gallons per mile means less emissions per mile. A lot more needs to be done on the production, transportation, and fueling component though.

- PM: In my mind its pretty clear that vehicle PM is harmful. There are those that still debate this topic, but IMHO this is the crowd that gives diesels a bad name. IE the lets chip our trucks to save a couple hundred bucks a year on fuel while we drive around blowing black smoke on everyone and then laugh about it with our fellow horseless cowboys online crowd. Fortunately modern filtering is having a very dramatic impact on PM emissions from diesels. While I think its pretty clear that gasoline engines are generally better in this regard, there is an area of concern. The testing done to confirm emissions certification are done at pretty sedate speeds and accelerations that are the same for all vehicles regardless of how powerful they are. Even for the less powerful vehicles, the acceleration rates are generally not so great that you would ever have to floor it. The problem then, is that most vehicles go into a different operating mode at WOT (wide open throttle). My knowledge of this subject is far from perfect, but here's how I understand it. For most of its power range, the engine is run in a closed loop mode. This means there are all sorts of sensors and computers making sure the engine is running with the ideal air/fuel ratio, and that emissions are optimized. At WOT most cars enter open loop mode, where essentially all the sensor data is thrown out the window and the car just dumps fuel and air into the cylinder in best guess amounts to make as much power as possible. In this mode the car is likely to be putting out a lot more emissions, including particulates, but this doesn't show up in the test data as the car is never in this mode under test. So emissions in this realm are essentially unregulated. This can often come into play in performance cars, which tend to be driven more aggressively, but also in under powered cars that tend to be driven at WOT more often in an attempt to make them perform reasonably. From personal experience, I know my WRX had a permanent black soot stain on the chrome tailpipe in my bygone days of driving like a crazy man. I have never seen that on my Prius (I have done the finger test), which I don't think has ever seen WOT. From the infamous Top Gear photo it is obviously possible to flog a Prius (at least a first gen) hard enough to get it to make soot. I think its also very true that vehicles like the Prius, where there is a computer sitting between you and the engine, has a much greater ability to reduce this sort of high emission state. Since it runs in pre-tuned rpm steps and uses the electric motor quick power boosts, I'm not even sure that the Prius has an open-loop mode. Another complication is that finer PM2.5 particles are probably even more dangerous than the currently measured PM10 particles. As of yet I believe there is no real testing or regulation for PM2.5 for either diesel or gasoline vehicles.

- NOx: I can say that there does seem to be good evidence for the VOC/HC limited behavior of smog production in urban areas, but I can't go along with the idea that we just shouldn't worry about NOx at all though. As I understand it smog is only one of the problems related to NOx. According to the EPA its also related to acid rain, oxygen depletion of surface water, lung damage via inhaled acidic particles, and that a number of the products of NOx reactions with organics and ozone are toxic and/or carcinogenic. Based on that, I do feel it is important that we are are taking steps to reduce NOx emissions. Someone mentioned that this is not the case in Europe, but thats not quite right. The Euro V standard may be focused more on PM, but the Euro VI standard seems focused on NOx. It seems likely that is just the reality of the large number of diesel vehicles in Europe and the time required to update them to newer low NOx designs. New diesels like the '09 TDI are much improved in terms of NOx, but it does seem like this comes at a price. While the new TDI reduces NOx by 10x, CO is up by 10x. Another traditional strong suit of diesels, HCs, has remained about constant, while that of many gasoline vehicles has improved to 5x lower than the TDI.

Overall I think many of us would agree that the best answer is reducing and eventually eliminating the use of oil all together. I do think diesels with strict emissions controls can have a part to play in that effort, but to me hybrids, then plug in hybrids, then full electrics powered by renewables coupled with an increased use of mass transport seems the most direct path to that goal. In my mind we don't need a better fuel to burn, we need to stop burning fuel.

Rob

 

Likewise. I wish I had more time to devote to this area of research, as it is one that interests me a great deal. Between work and a 3 y/o and a 6 month old there's not a lot of time for much of anything these days

Rob