High emitters are defined as having emissions with more than 3. The number on each bar segment refers to the number of cars recorded in the sample of vehicles measured at a specific location in the Denver area. Because catalytic converters will operate at reduced effectiveness under cold-start conditions, the relative importance of high emitters under these conditions might be less.
If cold-start emissions represent a sizable fraction of the total exhaust emissions, the benefits of oxygenated fuels may be more uniformly spread across the light-duty vehicle fleet. The tunnel studies and remote-sensing measurements discussed in the previous section have provided useful information concerning the effects of RFG blends on the emissions of a variety of ozone precursors, including CO from LDVs.
However, it is difficult to relate these snapshot assessments of LDV exhaust and running losses to the actual net effect. To accomplish this, the use of atmospheric measurements as an assessment must be examined. The attribution of the trends of ambient ozone concentrations and those of its precursors to specific control policies is complicated by the presence of confounding influences such as meteorological fluctuations NRC ; Rao et al.
Therefore, answering questions such as "what portion of the change in ambient ozone concentrations can reasonably be attributed to a particular emissions control policy? Therefore, it is critical that a spatially well-designed monitoring network be in place to measure precursors as soon as possible.
When time series of ozone and precursor data covering the pre- and post-implementation time periods are available for the regions where the control program is in effect and where it is not in effect, one can apply space-time analyses and change-point detection techniques as suggested by Rao et al. During the last 30 years, there have been extensive data sets acquired from integrated field measurements and the estimation of long-term trends from those measurements. However, these measurements were not aimed at determining specifically the effectiveness of particular air-quality regulations.
The measurements were aimed at assessing the reductions in concentrations of criteria pollutants or w determine the processes or sources of the primary emissions that limit these reductions. In the case of ozone, which is formed by secondary photochemical reactions, these measurements were not designed to determine the alteration of ozone concentrations that results from the RFGs.
Unfortunately, when the planning of integrated field measurements or monitoring fails to include directed observations to document a particular aspect of air-quality regulations, it is generally not possible to isolate these effects from the data that have been acquired for other purposes.
For these reasons, the ability to discern an "RFG signal" in the ambient data sets is quite limited. At this time, researchers are only able to even attempt such an analysis for a limited set of relevant species: RFG oxygenates, toxics, CO, and ozone.
Rao et al. Benzene is both an ozone precursor and an air toxic and, as a result, regulations have specifically targeted its reduction. LDV emissions of benzene are derived directly from benzene and from higher aromatics in the fuel. The RFG programs, with their prescribed reductions in benzene and other aromatics see Table , are intended to reduce ambient benzene concentrations. Although reductions have been observed in the atmospheric concentrations of benzene over the past several years EPA , the observations are not capable of attributing these reductions to a particular control strategy or to differentiate between different oxygenates used in fuels.
Because oxygenated compounds were added to RFGs specifically to replace benzene and other aromatic compounds, it is reasonable to assume that at least part of the observed reduction in ambient concentrations is associated with the reduction in vehicular emissions as a source. To date, although reductions are observed at many locations in various VOC concentrations including larger aromatic compounds, the trends are not sufficiently consistent to draw definite conclusions.
Both MTBE and ethanol have been observed to be present in the atmosphere. These compounds can serve as ozone precursors, but because their atmospheric reactivity is low, they are not expected to be as effective as more-reactive VOCs in generating ozone in urban environments. However, like benzene and CO, they might be more effective in ozone formation farther downwind of the source of their emission. Because the only identified use for MTBE is as a motor-fuel additive, it is reasonable to assume that its presence in the atmosphere is associated with the emissions from LDVs using fuels with an MTBE additive.
In this connection, MTBE could serve as an important tracer to determine the influence of its addition to motor fuel on the other compounds of interest. By contrast, ethanol has many natural and anthropogenic sources.
To date, no analysis has yet been carried out to determine if or how. Because motor vehicles are the primary source for CO, the U. Environmental Protection Agency EPA required that urban areas classified as nonattainment for CO use oxygenated fuels in gasoline-fueled engines during the winter season beginning in CO is primarily a winter problem because low surface temperatures limit the dispersion of the pollutant and enhance its emissions from cold engines.
As outlined in Table , oxygenated fuels in most CO nonattainment areas are blended to contain a minimum of 2. The Oxygenated Fuels Program has now been in effect for at least five winters in several different metropolitan areas, a time interval that might be long enough to begin an assessment of whether or not this program has or has not been effective.
In fact, recently, a number of researchers have attempted to assess the impact of fuels on ambient CO concentrations Mannino and Etzel ; Cook et al. Those studies have generally concluded that the oxygenated fuels program has resulted in a discernable downward trend in ambient CO concentrations. However, in the committee's view, the studies are not conclusive. The Oxygenated Fuels Program was initiated in the midst of other control programs and technological improvements designed to lower CO emissions.
Colorado, for example, places restrictions on both wood-burning stoves and driving times when CO concentrations are likely to be high. Most likely, such other programs and improvements have had some downward effects on CO emissions. Discerning the portion of the downward CO trend in an area that is specifically attributable to oxygenated-fuel use is a challenging problem. Another problem arises from inhomogeneities and discontinuities in the way in which CO data are reported. During the s, the reporting of CO data in the United States was changed from rounding to the nearest 1 ppm to the nearest 0.
Such discontinuities can produce an artifact in a trend analysis that confounds identification of an impact of a control program. To illustrate such problems, CO data from areas using oxygenated fuels were analyzed. It is important to note that this analysis is not intended to be a comprehensive assessment of the relationship between oxygenated fuel and ambient CO but just an illustration of the difficulties such analyses can encounter.
Colorado implemented a statewide oxygenated fuels program in , and California during the winter of The monitoring sites considered are listed in Table With the exception of California, these sites were chosen because of the length of their CO time-series records. The sites in California are the same as those used by Dolislager , and are sites which have reasonably complete records that include violations of the 8-hr standard for CO during There are some problems with the raw hourly data because of the way in which the lowest values detection limits were reported.
For the northeastern-states sites and Colorado sites, retaining only the daily maxima of 1-hr concentrations eliminates this problem. However, for the sites in California, the hourly data were first rounded to the nearest part per million prior to extracting the maxima. Rather than examining the reported daily maxima, their logarithms were examined for this report to help stabilize the variability due to seasonal variation.
The various physical processes reflected in each of the CO time-series were separated into three components that contribute independently to the overall trend. These components are a short-term component attributable to fluctuations in weather and day-to-day emissions ; a seasonal-variation component attributable to the Earth's rotation around the Sun ; and a long-term component attributable to secular, or lasting, changes in climate or emissions.
It is this last component that is most important here, because the effect of control policies must be manifested in this component. Table shows the amount contributed by each component to the total variance of the data at each monitoring site. The contribution of. Note that Riverside is not included in Table A change in the detection limits and resolution or dam-reporting practices around is apparent from an inspection of the lower values shown in Figure A.
The strongest decline in CO levels has occurred since see Figure D. The presence of strong downward trends in CO throughout the time series in the post period complicates evaluation of mid-series changes to regulatory policy. Examination of CO concentrations before and after implementation of the oxygenated fuels program might very well indicate a decrease in CO, but this decrease may be indicative of the overall downward trend that began well prior to the implementation of the program as opposed to the program itself.
To discern the contribution of oxygenated fuels to a trend such as that depicted in Figure , an analytical approach is needed that attempts to identify an abrupt "break" or change in the trend line at the time the program was first implemented. One such approach uses a linear regression on the long-term component i. That linear trend, prevailing prior to implementation of the program, is removed from the long-term component of the entire time series.
Linear trends are then estimated for the detrended data for the pre- and post-implementation time periods. By definition, the slope and intercept of the trend for the pre-implementation time period are zero. Three components of the overall trend are seasonal B , short term C , and long term D. Thus, while this analysis suggests that the oxygenated fuels program probably has had some small ameliorative effect on CO concentrations, its impact does not appear to be spatially uniform and in many cases is too small to discern with a high degree of statistical confidence.
The NSTC report reviewed various studies relating to the ambient air-quality effects of oxygenated fuels. It concluded that CO concentrations in urban areas have been decreasing at a rate of 2. This decrease is attributable primarily to EPA-mandated motor-vehicle emissions standards and improved vehicular emissions control technology. However, the NSTC report concluded that the benefits of oxygenated fuels on ambient air quality in cold climate areas could not be confirmed.
See Anderson et al. Assessing the effects of RFG on ambient ozone air quality involves challenges similar to those discussed above for CO. For example, Larsen and Brisby attempted to assess the effect of California's cleaner-burning gasoline program on ozone concentrations. However, the contribution of cleaner-burning gasoline to this decrease is uncertain because of the presence of many other ongoing ozone-mitigation efforts.
To address this problem, Larsen and Brisby assumed that the contribution of the cleaner-burning fuels program to the overall ozone decrease was proportional to the estimated percent reduction in the precursor emission inventory resulting from the program.
Thus, even though the Larsen and Brisby study was based on ambient ozone concentrations, the attribution of a portion of the observed ozone decrease to the use of cleaner-burning gasoline was derived from an emission inventory and does not constitute empirical verification of program effectiveness.
To further illustrate some of the difficulties with applying trend analysis to ambient ozone data, consider the log-transformed ozone concentrations from Riverside, California, presented in Figure Because the information from the moving-average filter Zurbenko et al.
To examine whether the use of RFGs in California had an impact on ambient ozone concentrations, data during the period from several locations in the Los Angeles Air Basin of California were also analyzed.
As was the case for the CO analysis in the previous section, an overall downward trend in ozone over the past year period is evident in the long-term component at Riverside Figure D. Ozone then increased slightly in , and then decreased again in To these blendstocks other liquids are added to make the substances that fuel our carpools, take us to grocery stores and get our families to their summer vacations.
And, mostly, that final mixology does not happen at the refinery level. This blog explores the basics of the gasoline recipe. OPIS just recorded a new Crash Course podcast taking you through a of refining gasoline and distillates and explaining the ins and out of major refining units. Check it out right here. To reiterate, most of the gasoline produced by refineries is actually unfinished gasoline or gasoline blendstock.
Blendstocks are blended with other liquids, such as ethanol, to make finished gasoline. The blendstocks are a mix of components such as butane, reformate and FCC gasoline, which can be combined in different ways to reach needed specifications.
Considerable in-use experience see below with oxygenate additives prior to the RFG program suggests that vehicle performance should not be affected. Changes other than those discussed above are due primarily to dilution from oxygenates. It may contain oxygenates, which increase the combustion efficiency of gasoline and reduce carbon monoxide emissions. The exhaust emissions of vehicles using RFG will have less air toxins and ozone-forming pollutants, and the use of RFG should have no adverse effects on vehicle performance or the durability of engine and fuel system components.
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