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Study finds gasoline exhaust contributes more to formation of secondary organic aerosols than diesel

Contrary to expectations, exhaust from gasoline vehicles contributes more to the production of secondary organic aerosols (SOA) than exhaust from diesel vehicles, according to a new study by scientists from the Cooperative Institute for Research in Environmental Sciences (CIRES), NOAA’s Earth System Research Laboratory (ESRL) and other colleagues.

Based on their study, the team estimated that within a day of processing, SOA from gasoline exhaust emissions may reach 4 Tg/yr, which is ~16% of recent global estimates of biogenic SOA. As a result, the researchers suggest that a decrease in the emission of organic species from gasoline engines may significantly reduce SOA concentrations on local and global scales. A paper on their findings is in press in the journal Geophysical Research Letters.

SOAs are tiny particles that are formed in air and make up typically 40% to 60% of the aerosol mass in urban environments. This is important because fine-particle pollution can cause human health effects, such as heart or respiratory problems.

The team used airborne and ground-based measurements of organic aerosol (OA) in the Los Angeles (LA) Basin, California during May and June 2010 to assess the amount of SOA formed from diesel emissions. Diesel emissions in the LA Basin vary between weekdays and weekends, with 54% lower diesel emissions on weekends. Despite this difference in source contributions, in air masses with similar degrees of photochemical processing, formation of OA is the same on weekends and weekdays, within the measurement uncertainties.

The surprising result we found was that it wasn’t diesel engines that were contributing the most to the organic aerosols in LA. This was contrary to what the scientific community expected.

—CIRES research scientist Roya Bahreini

Due to the harmful nature of SOA and the fact that they can also impact the climate and can reduce visibility, scientists want to understand how they form, Bahreini said. Researchers had already established that SOAs could be formed from gases released by gasoline engines, diesel engines, and natural sources—biogenic agents from plants and trees—but they had not determined which of these sources were the most important, Bahreini said.

Los Angeles proved to be an ideal location. Flanked by an ocean on one side and by mountains to the north and the east, it is, in terms of air circulation, relatively isolated. At this location, the scientists made three weekday and three weekend flights with the NOAA P3 research aircraft, which hosted an arsenal of instruments designed to measure different aspects of air pollution.

From their measurements, the scientists were able to confirm, as expected, that diesel trucks were used less during weekends, while the use of gasoline vehicles remained nearly constant throughout the week. The team then expected that the weekend levels of SOAs would take a dive from their weekday levels, Bahreini said. But that was not what they found.

Instead the levels of the SOA particles remained relatively unchanged from their weekday levels. Because the scientists knew that the only two sources for SOA production in this location were gasoline and diesel exhaust, the study’s result pointed directly to gasoline as the key source.

According to the findings, diesel exhaust’s contribution to SOA is at max 20%; that leaves gasoline contributing the other 80% or more.

While diesel engines emit other pollutants such as soot and nitrogen oxides, for organic aerosol pollution they are not the primary culprit.

—Roya Bahrein

If the scientists were to apply their findings from the LA study to the rest of the world, a decrease in the emission of organic species from gasoline engines may significantly reduce SOA concentrations on a global scale as well. This suggests future research aimed at understanding ways to reduce gasoline emissions would be valuable.


  • Bahreini, R., et al. (2012), Gasoline emissions dominate over diesel in formation of secondary organic aerosol mass, Geophys. Res. Lett., doi: 10.1029/2011GL050718, in press



Yeahhhhhh, even MORE reasons to love our gasoline monsters. What is that saying again? Oh yes: "Drill baby drill!" LOL


Even though half the electricity for EVs may come from coal fired power plants, it is easier to put controls on a few smoke stacks than on millions of tailpipes.


Add another 17% or so from gasoline from Tar Sands and you have a prime product.

Stan Peterson

Gee Whiz,

Having cleaned up most toxic emissions and the CAGW farce is now grinding down to a complete comical collapse, urgent action is required. We must find a new and ominous culprit to justify the EPA bureaucrats jobs.

Now SOAs are discovered. Of what they are culpible, we should soon discover as soon as the proper condemnitory studies can be funded and created.

WEll done, and just in the nick of time!


Stan, don't misrepresent the findings of this study. SOAs are not a newly discovered "culprit" and nobody in this study is saying they are. We've known about SOAs for years: In fact 10 years ago I was even hearing climate change deniers trying to use the existence of SOAs as proof that global warming wasn't man-made because SOAs could also be formed from gases released by natural sources (biogenic agents from plants and trees). The only thing new this study found is that "gasoline exhaust contributes more to formation of secondary organic aerosols than diesel."


BTW, those climate change deniers I just mentioned? They were of course just rehashing an older Reagan quote;


If Stas didn't misrepresent things he'd have nothing to say to support his side.


The same deniers claimed that tobacco, asbestos, BCPs, Tar Sands and emissions from 4000+ other industrial products are good for humans, plants etc as long as friends can make $$$$ from them.

There are no reasons why USA cannot produce enough clean e-power for 200+ electrified vehicles. Two to three decades may be required but that's not eternity.

Bob Wallace

SJC - we don't need to put scrubbers on coal plants.

We need to install a lot more renewable generation and shut coal plants down.

Then we need to install even more renewable generation and shut gas plants down.

Driving with wind-electricity costs "$1/gallon" or less. And we avoid the costs of health care for all those being harmed by petroleum and coal use.

We also eliminate the cost of military intervention in oil-producing countries.

That make sense to you folks who have saving tax dollars as their number one issue?


I look at what we need to do versus what is likely to happen, I believe that to be more reality based.

I would like to see us convert coal fired power plants to IGCC and make fuels as well, but it is not likely to happen.

I think that if we go with what is likely, we are better off actually have something happen rather than making sweeping statements about what "should" happen.

It has been said that politics is the art of the possible, so if true that is the tide we should ride. We need to quit tilting at windmills and get more practical if we want to get something actually done.


YES B.W.....why so many prefer to drive on $4/gal to $9/gal dirty fossil liquid fuel while they could drive on $1/gal-e clean wind electricity.

Resistance to change and massive ICEVs Ads programs are deeply embedded into our mind. For most of us, it is not easy to discard.


My point is that we will not discard it in any significant way in the next 10 years. Something as large as 200 million gasoline powered cars does not change to all electric in 10 years.

Let's quit with the "should" and get with the "can". We need to go with what can be done and what probably WILL be done, otherwise we are all just kidding ourselves that some day real soon now we will all be driving electric cars on wireless roads.


I think we will make a very substantial dent in petroleum demand in 10 years.  There are several factors behind this:

  1. The typical LDV travels half its lifetime mileage in its first 6 years.  The cars and trucks rolling off the line today will already have consumed 60-70% of their lifetime fuel demand by 2022.  What really matters is the models selling around 2019 and beyond.
  2. Basic fuel demand for the same style of vehicle will be down at least 20% due to engine improvements, such as Ecoboost.
  3. Conventional hybrids will expand their market share.  20% penetration means 8-10% fuel savings.
  4. Easily 20% of US motor fuel demand could be shifted to CNG/LNG in 10 years (heavy trucks alone are almost 25% of motor fuel demand).  See the two GCC articles on CNG pickups in the last couple of days.
  5. EREVs such as Via Motors' drivetrain will make inroads.
  6. Vehicle-sharing schemes such as the Relay Rides P2P system will make a smaller number of CNG and EREV vehicles serve a larger number of users, pushing out conventional drivetrains and accelerating the payback from the more expensive systems.  If 20% of pickups and vans are CNG/EREVs but they account for 50% of the mileage, that is a lot of fuel saved/substituted.
  7. Pure EVs such as the E-CELL Crewbus will replace diesels in applications like hotel airport shuttles, both for fuel savings and for air-quality reasons.
There is no one panacea in the short term, but there are a host of ways to reduce and substitute.

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