A study led by researchers at UC Berkeley has found that diesel exhaust forms about seven times more secondary organic aerosols (SOA) than gasoline exhaust for the same mass of unburned fuel emissions and, given emission factors, can be expected to form 15 times more SOA than gasoline per liter of fuel burned. The study determined that, depending on a region’s fuel use, diesel exhaust is responsible for 65% to 90% of vehicular-derived SOA, with substantial contributions from aromatic and aliphatic hydrocarbons.
The new findings contradict previous research focused on the LA Basin which concluded that gasoline vehicles contributed more to the production of secondary organic aerosols (SOA) than exhaust from diesel vehicles. (Earlier post.)
The Berkeley study, published in the Proceedings of the National Academy of Sciences, comprehensively characterizes the chemical composition, mass distribution, and organic aerosol formation potential of emissions from gasoline and diesel vehicles. (As part of the work, they also presented what they called “the most comprehensive chemical speciation of diesel fuel to date” with more than 90% mass closure as part of an overall assessment of gasoline and diesel fuel.) SOAs are tiny particles that are formed in air and make up typically 40% to 60% of the aerosol mass in urban environments.
Organic aerosol (OA) in the atmosphere is detrimental to human health and represents a highly uncertain forcing of climate change. The use of petroleum-derived fuels is an important source of reactive gas-phase organic carbon that provides key precursors to the formation of secondary OA (SOA) and tropospheric ozone. Controlling these emissions from gasoline and diesel vehicles is central to air quality mitigation policies in urban areas Previous work has concluded that further research is necessary to elucidate all organic sources of SOA precursors. Significant controversy exists over the contributions of precursors from gasoline and diesel vehicles, and the relative importance of each for SOA formation remains in question, in part because of insufficient chemical characterization of fuels and emissions, and the difficulty of ambient measurements of gas-phase compounds emitted from diesel sources.
...Noncombusted hydrocarbons from the fuels are emitted in the exhaust of gasoline and diesel engines, and also via evaporation from gasoline vehicles and service stations. These compounds in un-burned gasoline and diesel fuel dominate vehicular emissions of reactive gas-phase carbon that have the potential to form SOA. Previous work has shown nontailpipe emissions account for ∼30% of gasoline-related emissions in urban regions, but limited work exists constraining the emissions and SOA formation potential of gas-phase organic carbon from gasoline and diesel sources.
By using extensive fuel analyses and field data from two sites that include many compounds with no previous in situ measurements, we present the most comprehensive data to date on the chemical composition, mass distribution, emissions, and SOA formation potential of non-tailpipe gasoline, gasoline exhaust, and diesel exhaust. We determine the relative importance of gasoline and diesel sources for SOA formation in, and downwind of, urban regions. We assess these results in the context of other studies during the past decade and discuss their significant implications for air pollution measurement, modeling, and control.—Gentner et al.
The researchers collected a total of 40 gasoline and 12 diesel fuel samples from California and characterized them using several gas-chromatography methods, yielding comprehensive speciation of the “unresolved complex mixture” in diesel fuel. This was accomplished, they said, by using soft photoionization techniques. The results provide “unprecedented” detail on the molecular identification and mass distribution of hydrocarbons in diesel fuel.
The team used a chemical mass balance model to examine the contributions from each source to reactive gas-phase organic carbon. The model uses a subset of measured compounds and exploits differences in the chemical composition of sources to assess the magnitude of total noncombusted hydrocarbon emissions from each source.
To assess the importance of gasoline and diesel sources for SOA in urban areas, they calculated bulk SOA yields for all three sources and compared them in context of emission factors for noncombusted gas-phase organic carbon in exhaust (0.38 ± 0.11 gC·L−1 for gasoline and 0.86 ± 0.25 gC·L−1 for diesel), and source contributions. Data on SOA yields are limited for many of the hydrocarbons, they noted; the mass fraction of diesel, gasoline, and non-tailpipe gasoline emissions that have unknown yields are 66%, 25%, and 7%, respectively. They modeled high-NOx SOA yields by using published data (where available) and an estimation of yields and uncertainties for unknown values based on best estimates from various plausible scenarios.
For the same mass of unburned fuel emissions reacted, diesel exhaust forms 6.7 ± 2.9 times more SOA than gasoline exhaust (bulk SOA yields of 0.15 ± 0.05 and 0.023 ± 0.007 μgSOA·μg−1, respectively). Considering differences in emission factors, diesel exhaust is expected to form 15 times more SOA than gasoline per liter of fuel burned. For populated regions with 10% to 30% diesel fuel use, this implies that diesel exhaust is responsible for two to seven times more SOA than gasoline exhaust. Nontailpipe gasoline emissions were 39% to 77% lower than gasoline exhaust emissions and produce negligible SOA as a result of a substantially lower yield (0.0024 ± 0.0001).—Gentner et al.
They also determined that the vast majority of SOA from gasoline sources is a result of its aromatic content, whereas diesel SOA is predicted to be 47 ± 7% from aliphatics, with the remainder from aromatics.
Their data also suggest that the vast majority of diesel fuels sold in California are certified alternative formulations that contain nearly double the aromatic content than initial regulations intended. By way of background, in 1993, California regulated diesel fuel to have less than 10% single-ring aromatics and 1.4% polycyclic aromatic hydrocarbons in an effort to mitigate emissions of particulates and nitrogen oxides. However, concerns about engine performance and the cost of fuel production led the state to allow higher aromatic levels in diesel fuel.
Although the regulations help control primary particulate emissions, the Berkeley team suggests that this enhancement of aromatic content in diesel fuel increases the SOA potential of diesel emissions, especially for hydrocarbons with 9 to 17 carbon atoms.
Significant progress is being made to improve heavy-duty diesel engine performance with postcombustion control technology, which may affect emissions of gas-phase organic carbon, but it is clear that attention to gasoline and diesel fuel composition and emissions of reactive organic gases is necessary to control SOA precursor contributions from all vehicle classes. Furthermore, this work has focused on organic carbon emissions originating from fuels, but emissions of unburned motor oil from both gasoline and diesel vehicles represent an additional source of organic carbon. Although total consumption of oil is minor relative to fuel, oil contributes gas and particle-phase compounds with lower volatilities than diesel fuel and should continue to be monitored in field, laboratory, and modeling studies.
...Our expanded measurement capabilities for gasoline and diesel compounds in the liquid fuels and the ambient atmosphere produce a more complete picture of SOA formation from motor vehicles. We provide the ability to predict emissions of SOA precursors and SOA formation that is consistent with fuel use data and ambient measurements. SOA from diesel sources outweighs gasoline contributions, and other sources provide significant precursors in many urban regions. The inclusion of our insights will allow for the development of more effective pollution control policies and inform the design of future studies in the ambient atmosphere, laboratory experiments, and modeling efforts.—Gentner et al.
The California Air Resources Board and the US Environmental Protection Agency helped support this research.
|Gasoline or diesel?
|In March, Bahreini et al. reported in Geophys. Res. Lett. their findings that exhaust from gasoline vehicles contributes more to the production of secondary organic aerosols (SOA) than exhaust from diesel vehicles—a finding they called contrary to what the scientific community expected. (Earlier post.)
|The team used airborne and ground-based measurements of organic aerosol (OA) in the Los Angeles (LA) Basin, California during May and June 2010. From their measurements, the scientists confirmed that diesel trucks were used less during weekends, while the use of gasoline vehicles remained nearly constant throughout the week. The team expected that the weekend levels of SOAs would take a dive from their weekday levels.
|Instead they found that 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, they concluded that the results pointed directly to gasoline as the key source.
|Addressing this, the Berkeley team noted:
|Examining differences between weekdays and weekends is another common and insightful metric for assessing emissions and chemical processes. We observed no weekday/weekend difference in the distribution of emissions between gasoline and diesel exhaust in Bakersfield, as daytime values of both decreased by ∼40% over the weekend. However, weekend OA concentrations (total and vehicular) were greater as a result of increased photochemical aging evidenced by higher ΔOA/ΔCO ratios.
|Recent work focused on Los Angeles reported that gasoline is vastly more important than diesel as a source of SOA precursors based on the observation that weekend ΔOA/ΔCO slopes were marginally similar to weekday slopes, with similar photochemical ages despite large differences in diesel activity. Similar to Los Angeles, OA concentrations and ΔOA/ΔCO ratios are higher in Bakersfield over the weekend, but this occurs despite no change in the relative use of gasoline and diesel, suggesting that increased OA at both locations over the weekend is a function of decreased diesel NOx emissions leading to faster photochemical processing, and is independent of changes in the mix of fuel use.
|The ubiquitous increase in ΔOA/ΔCO ratios with increased processing for vehicular and total OA is independent of the mixture of gasoline and diesel, and ΔOA/ΔCO slopes alone are insufficient to discern organic SOA precursor contributions from gasoline vs. diesel given the variability in Los Angeles measurements.
—Gentner et al.
Drew R. Gentner, Gabriel Isaacman, David R. Worton, Arthur W. H. Chan, Timothy R. Dallmann, Laura Davis, Shang Liu, Douglas A. Day, Lynn M. Russell, Kevin R. Wilson, Robin Weber, Abhinav Guha, Robert A. Harley, and Allen H. Goldstein (2012) Elucidating secondary organic aerosol from diesel and gasoline vehicles through detailed characterization of organic carbon emissions PNAS doi: 10.1073/pnas.1212272109