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Study finds lubricating oil the dominant source of primary organic aerosol from both diesel and gasoline vehicles

Comparison plot showing mass fractions (Fm) of chemically characterized components of lubricating oils and POA. Credit: ACS, Worton et al. Click to enlarge.

Findings from a study by researchers at the University of California, Berkeley and Berkeley National Laboratory suggest that lubricating oil is the dominant source of primary organic aerosol (POA) from both gasoline- and diesel-powered vehicles. Unburned diesel fuel makes an additional smaller contribution, with an additional smaller contribution from unburned gasoline. A paper on the work is published in the ACS journal Environmental Science & Technology.

Motor vehicles are major sources of organic carbon emissions, with implications for human health and air quality, especially in urban areas. The emitted organic carbon is in the form of both primary particulate matter (PM) and gas phase organic compounds of a wide range of volatilities that can be oxidized in the atmosphere to form secondary organic aerosol (SOA). (Earlier post.) The majority of fine PM from vehicles is carbonaceous in the form of either black (BC) or organic carbon, the latter of which is directly emitted as primary organic aerosol (POA).

Heavy duty (HD) diesel trucks are responsible for a disproportionate amount of BC and POA relative to light duty (LD) vehicles due to emission factors that are ∼50 and ∼15 times larger, respectively, the researchers note.

In contrast to BC, POA contains many thousands of organic compounds that are thought to be derived from unburned fuel, unburned lubricating oil, and as byproducts of incomplete combustion. Previous chassis dynamometer studies have estimated the contributions of unburned fuel and unburned lubricating oil to POA emissions for a small number of vehicles (<100 per study). These studies showed a wide range in the contribution of unburned lubricating oil to POA emissions (20−80%). A substantial fraction of POA has been shown to be semivolatile under atmospheric conditions. Following emission these organics can evaporate due to dilution to form low volatility vapors that can be oxidized to form SOA.

Characterizing the chemical composition of POA and apportioning the contribution of these sources to the total POA mass is important as the chemical composition affects the rate and composition of SOA formation following evaporation, as well as the physical properties and toxicity of the aerosol.

The number of possible hydrocarbon isomers increases exponentially with increasing carbon number, which results in a very high number of constitutional isomers at large carbon numbers. Due to the large number of hydrocarbon compounds present in POA, many of which are constitutional isomers, the vast majority of the mass cannot be resolved by conventional separation techniques … In this work, we determine the comprehensive chemical composition of POA for a much larger number of vehicles than is possible with dynamometer testing (N = 4000−7500 light duty, 50−200 medium, and 0−150 heavy duty) by analyzing filter samples collected in a highway tunnel using GC × GC/VUV-HRTOFMS. It is important to note that these data represent composites of a high number of vehicles and while they are representative of real-world fleet and driving conditions, high-emitting vehicles can make a disproportionate contribution to these composite samples. Here we use this comprehensive analysis to better determine the contributions of unburned gasoline fuel, unburned diesel fuel, lubricating oil, and combustion byproducts to motor vehicle POA compared to tracer based methods.

—Worton et al.

As in their earlier study on the formation of secondary organic aerosols (earlier post), the team collected filter samples at the Caldecott Tunnel on Highway 24 in the San Francisco Bay Area. The tunnel is 1 km long and eastbound traffic is uphill on a 4% grade—helpful for providing engine load for the purposes of emissions testing.

Averaged chemically characterized distributions of motor vehicle POA, separated into low heavy-duty (HD) (BC <15 μg m−3) and high HD influence (BC>15 μg m−3), and SAE-10W30 and SAE-15W40 lubricating oils. Distributions are shown as a function of carbon number (NC) and number of double bond equivalents (NDBE). The POA distributions are shown as mass concentrations in μg C m−3 while the lubricating oils are shown as mass fraction. Also shown is the average number of vehicles (±1 standard deviation) observed during filter collection periods separated by type (light duty, LD; medium duty, MD; and heavy duty, HD). Credit: ACS, Worton et al. Click to enlarge.

Filter samples were collected downstream of sharp cut cyclones at a flow rate of 16.7 L min−1 to remove particles with diameters greater than 2.5 μm from the sampled air.

Portions of all the filters (1.1 cm2) were analyzed for total organic carbon (OC) and black carbon (BC) using a thermal optical technique. A subset of 18 filter samples were analyzed by GC × GC/HRTOFMS (gas chromatography × GC/high resolution time-of-flight mass spectrometry).

The recently developed gas chromatography mass spectrometry approach utilizes “soft” vacuum ultraviolet photoionization to achieve unprecedented chemical characterization of the motor vehicle POA emissions with a mass closure of >60%.

The researchers characterized the POA by number of carbon atoms (NC); number of double bond equivalents (NDBE); and degree of molecular branching.

They observed that vehicular POA mass was dominated by branched cycloalkanes with one or more rings and one or more branched alkyl side chains (≥80%). The remaining mass comprised branched alkanes (<5%); n-alkanes (<3%); single ring aromatics (<3%); PAHs (<2%; and oxygenates (∼2%). High molecular weight combustion byproducts—i.e., alkenes, oxygenates, and aromatics—were not present in significant amounts.

Based on the similar observed carbon number (volatility) and chemical composition of POA and lubricating oil, and in the absence of significant high molecular weight material formed through the combustion process, we conclude that lubricating oil is the dominant constituent of vehicular POA emissions.

… The dominance of cycloalkanes in vehicular POA emissions has implications for the atmospheric behavior of POA, as a result of the structural dependence of hydrocarbon reaction rates and SOA yields. … Taken together, these data suggest that evaporated POA components, which are likely dominated by cyclohexane and cyclopentane rings, will react slower and form less fragmentation products than if it were assumed to be purely branched alkane material. Gas phase oxidation of relevant cycloalkanes is less well studied and further work to characterize the fate of branched cyclohexanes and cyclopentanes is important to improve our understanding of the processing of atmospheric POA in regions downwind of urban centers.

—Worton et al.


  • David R. Worton, Gabriel Isaacman, Drew R. Gentner, Timothy R. Dallmann, Arthur W. H. Chan, Christopher Ruehl, Thomas W. Kirchstetter, Kevin R. Wilson, Robert A. Harley, and Allen H. Goldstein (2014) “Lubricating Oil Dominates Primary Organic Aerosol Emissions from Motor Vehicles,” Environmental Science & Technology doi: 10.1021/es405375j



How many of those compounds can provoke cancers for people living close to main highways?


Basically noting. New HD vehicles have DPF+catalysts, which reducde POA and BC to levels that are not measurable.


Contribution from new HD vehicles, of course...


Long term studies on buses in Europe indicate that once the new HD vehicles are not new anymore the DPF and catalysts loose eficiency.


On the contrary, the technology works extremely well. In fact, the filtration efficiency of a DPF increases with increasing mileage.



I was just about to post the same thing. There at least 2 studies of which I'm familiar showing that DPF becomes MORE effective with age.

Furthermore, emissions in HD applications in the U.S. at least are officially certified at 435,000 miles, and in most cases, the certified PM levels are still at or very near zero (well below regulated limits).


Yeah, the evidence for durability is compelling for those who want to search. This also goes for health effects. For example, those who are interested in emission components from HD vehicles which could pose health hazards do not have to look any further than on this site. They can just search for the ACES study. It is also somewhat annoying to find that researchers in many cases prefer to carry out their work on older vehicles and mostly, without even bothering to mention about the fact that modern vehicles do have very good emission control.

About manufacturer obligations, I could add that also the EU has durability requirements. For HD vehicles over 16 tons it is 700 000 km. Vehicles in EU, US, etc… have on-board diagnostic (OBD) systems that should warn if the system is not working properly and potentially, shut down the engine. In addition, we have vehicle inspection in the EU, where both emissions and OBD are checked. If anything happens to the emission control during the durability period, the manufacturer has to repair the vehicle free of charge. Outside this period, it is up to the vehicle owner to handle this. Nevertheless, if the OBD system, yearly vehicle inspection, road check or any similar measure finds that the emission control is not working, the vehicle has to be repaired even if it the mileage is higher that the limit mentioned. This applies for the whole useful life of the vehicle.

In a few years, the next stage in on-board surveillance of the emission control system will be introduced. This is going to be on-board sensors that measure particle emissions in real time. This is one step further from current OBD, since particle emissions will be directly measured, not indirectly through other sensors. Needless to say, the requirements for such a particle sensor are rigorous. For example, it must have a detection limit just above ambient levels, it should be affordable and it must have a lifetime as long as the useful life of the vehicle (or else, be replaced at specified service intervals). Industry and government laboratories have been testing such sensors for some time now. Those who are interested can again turn to the literature, since much of this has already been published.


Just another one of the 10001 so called industrial non-polluting sources killing us.

With over 7 million dying a year from air pollution ONLY and 5 times more getting sick, we have been doing well to restrain world population.

Plagues, Spanish flu, WWI and WWII did even better in the past but modern industrial pollution will soon be the winner.

Roger Pham

Lubrication oil is polluting ONLY when an engine consumes oil. Modern engines are so precisely-built that there is practically no oil consumption between oil change intervals of 7,000 to 10,000 miles, when synthetic oil is used, unless the engine gets old and needs fixing. Therefore, non-synthetic oil should be outlawed.

Yearly state inspection for cars should also involve exhaust particulate check. Those that emit more than regulated amount of exhaust particulate should be forced to use exhaust particulate filters,or to get a tuneup until the problem fixed. Those that have blown piston ring or gaskets or damage to intake valve oil seal should not be allowed on the street until the problem fixed.

Those issues are easily solved!

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