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Multi-year remote-sensing measurements of gasoline light-duty vehicle emissions on a freeway ramp
Atmospheric Environment 34 (2000) 4657}4665
Multi-year remote-sensing measurements of gasoline light-duty vehicle emissions on a freeway ramp As ke SjoK din*, Kenth AndreH asson Swedish Environmental Research Institute, P.O. Box 47086, S-402 58 Gothenburg, Sweden Received 3 September 1999; accepted 2 March 2000
Abstract On-road optical remote-sensing measurements of gasoline light-duty vehicle (LDV) emissions } CO, HC, NO } were conducted on a freeway ramp in Gothenburg, Sweden, in 1991, 1995 and 1998. Based on almost 30,000 emission measurements, the results show that both catalyst cars and non-catalyst cars emissions deteriorate over time, but also that the emission performance of new TWC-cars has improved signi"cantly in recent years. Furthermore, it was found that #eet age rather than model year determines the rate of emission deterioration for TWC-cars for both CO and NO. The study demonstrates that remote sensing may constitute a powerful tool to evaluate real-world LDV emissions; however, daily "eld calibration procedures need to be developed in order to assure that the evolution in #eet average emissions can be accurately measured. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Light-duty vehicles; Emissions; On-road; Measurements; CO; HC; NO; Optical remote sensing; FEAT
1. Introduction Air quality is improving in many areas of the world, partly as a result of stricter emission standards for gasoline light-duty vehicles (LDVs), requiring the use of e!ective closed-loop three-way catalyst systems (TWCs). In North America, where such standards were introduced already in the early 1980s, the #eet turn-over to LDVs equipped with TWCs is already more or less complete. In Europe, on the other hand, where TWCs did not appear before on around 1990 model years, there is still a large fraction of the LDV #eet lacking e!ective emission control, in many cases more than 50%, thus leaving a great potential for further emission reductions and associated bene"ts in air quality. For instance, in Sweden the average 35% decrease in ambient NO levels observed in urban areas since the late 1980s coincides with an increase in the fraction of LDVs equipped with TWCs from 0% to about 50% (SjoK din et al., 1996). This demonstrates
* Corresponding author. E-mail address: [email protected] (As ke SjoK din).
the great importance of LDV emissions for urban air quality. However, the experience from the US is that air quality is not improving as fast as is predicted from the growth in the number of TWC-cars. It has been claimed that this may partly be due to existing inspection and maintenance (I/M) programs failing to maintain the optimum vehicle emission performance as vehicles age (Lawson, 1993, 1995; Bishop et al., 1996; Stedman et al., 1997). Furthermore, in general, knowledge of the actual emissions from in-use LDV #eets is still limited, since most available emission data arise from, e.g., FTPmeasurements on small-vehicle samples, that may be unrepresentative for the in-use #eet. An alternative approach to possibly overcome the problem with test #eet representativity in conventional emission measurements is to use the on-road optical remote-sensing device originally developed by the University of Denver in the late 1980s (Bishop et al., 1989). Through the years this technique has been further developed and re"ned (e.g. Guenther et al., 1995; Zhang et al., 1996; Popp et al., 1999) and applied in numerous locations around the world (e.g. Bishop et al., 1993; Lawson et al., 1990; Popp et al., 1998; SjoK din, 1994; SjoK din et al., 1997; Stephens et al., 1997; Zhang et al.,
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 1 5 8 - 8
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1995). However, so far few studies have been reported, where the same remote-sensing site has been revisited to study changes in #eet emission performance over time. The only exception is an o!-ramp in Denver, Colorado, where measurements have been carried out on several occasions since 1989 (Slott, 1996). This paper presents the results from three remotesensing monitoring campaigns conducted in 1991, 1995 and 1998 at the same freeway ramp site with the main objective to evaluate the change in emission performance over time of Swedish gasoline light-duty vehicles. The 1991 measurements at this site have been reported earlier (SjoK din, 1994).
2. Experimental Most of the background and experimental conditions have been described in detail elsewhere (SjoK din, 1994), and therefore only a brief description will be given here. 2.1. Fleet characteristics Emission standards requiring the use of closed-loop TWCs on all new light-duty gasoline vehicles were introduced in Sweden from model year 1989. TWC-equipped cars of model years 1987 and 1988 were sold on a voluntary basis with estimated fractions of about 25 and 85%, respectively. Model years before 1987 are considered to have uncontrolled emissions, with the only exception being the EGR-valve required from model year 1976. The Swedish light-duty #eet is dominated by largeand medium-sized cars of the two domestic makes Volvo and Saab, with displacement volumes of about 2.0}2.5 l, which together make up nearly 40% of the #eet. The 10 most common foreign makes in recent years are in decreasing order (range and estimated average displacement volumes in liters within parenthesis): Ford (1.1}2.9, avg. 2.0), Volkswagen (1.4}2.5, avg. 1.8), GM Opel (1.4}2.0, avg. 1.6), Audi (1.6}2.8, avg. 2.0), Mazda (1.3}2.5, avg. 1.8), Toyota (1.3}2.0, avg. 1.8), Nissan (1.3}2.0, avg. 1.6), Renault (1.4}2.0, avg. 1.6), Mitsubishi (1.3}2.0, avg. 1.8) and Mercedes (1.8}5.0, avg. 2.2). A centralized, test-only I/M program involving lowidle testing for CO has been in operation in Sweden since 1970. For catalyst cars, the test also involves HC. Additional high-idle CO and lambda testing was introduced for catalyst cars in 1997. Cars older than four years are inspected annually, whereas during the "rst four years the inspection is biennial. 2.2. Remote-sensing measurements Remote-sensing measurements were carried out at exactly the same site in 1991, 1995 and 1998 with remote sensors (FEAT) provided by the University of Denver.
The site is just at the exit of a sharp, 2703 curve of an uphill freeway interchange ramp, considered to carry only warmed-up vehicles since the nearest on-ramp is several minutes of driving away. Measurements in 1991 involved only CO and HC. The detector and "lter applied for the CO measurements were basically the same in all three years. The HC detector applied in 1991 utilized a "lter centered at 3030 cm\, which is sensitive to liquid water interference (SjoK din, 1994; Guenther et al., 1995), whereas the HC detector applied in 1995 and 1998 utilized a "lter centered at 2941 cm\, less responsive to water. NO measurements in 1995 were made by means of non-dispersive UV (Zhang et al., 1996), whereas NO measurements in 1998 were conducted by the newly developed high-speed UV diode}array spectrometer (Popp et al., 1999). The new NO detector has a much lower level of noise and is claimed to be not as sensitive to HC interference as the old detector. In 1995 and 1998, the remote-sensing measurements also involved speed and acceleration on an individual vehicle level. In each year the measurements comprised about three working days during the period mid-August to midSeptember, yielding a total of between 15,000 and 20,000 vehicle-induced beam blocks on the remote sensor in each campaign. During the campaigns the remote sensor was calibrated at least once every day by means of pu!s of certi"ed CO, HC, CO and NO gas mixtures from cylinders (1991 and 1998) or by a sealed gas}cell with known concentrations of the corresponding gases (1995).
3. Results and discussion 3.1. Fleet average emissions Overall #eet average emissions, expressed as average volume percent tailpipe exhaust concentrations, for the gasoline light-duty vehicles on the freeway ramp according to the 1991, 1995 and 1998 remote-sensing measurements are given in Table 1, along with corresponding averages of the measured speed and acceleration in 1995 and 1998. 10,284 valid CO and HC readings on gasoline light-duty vehicles, identi"ed from the video images of the license plates, were obtained in the 1991 measurements. The corresponding number of valid readings in the 1995 measurements were 8289 for CO, 7601 for HC and 5470 for NO, and in the 1998 measurements 10,672 for CO, 10,261 for HC and 9001 for NO. The errors given for the average tailpipe concentrations in Table 1 are standard errors derived by the same methodology used by Bishop et al. (1993) for the gamma-distributed remote sensing data. All pollutant emission data were randomly divided into 250 record samples, and the means of these samples were used to calculate the standard error reported in Table 1. The standard errors for vehicle speed and acceleration
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Table 1 Overall #eet average tailpipe emissions (in vol.%) and average speed and acceleration for gasoline light-duty vehicles on the freeway ramp in 1991, 1995 and 1998 according to the remote-sensing measurements. Errors are one standard deviation, calculated as described in the text Year
CO (%)
HC (%) (propane)
NO (%)
Speed (kmh\)
Acceleration (ms\)
1991 1995 1998
0.71$0.10 0.73$0.13 0.57$0.08
0.059$0.005 0.022$0.004 0.037$0.006
} 0.064$0.018 0.055$0.009
} 27.7$5.0 29.8$5.0
} 0.28$0.41 0.03$0.63
Di!erent detectors were used the "rst year compared to the succeeding year(s).
were calculated in the normal way assuming Gaussian statistics. Average vehicle speed and acceleration as well as distributions of these two parameters were similar in 1995 and 1998. Roughly, in both 1995 and 1998, more than 90% of all observed speeds were in the range 20}35 km h\, with slightly higher speeds in 1998, and more than 80% of all observed accelerations were in the range !0.5}0.5 m s\, with slightly higher accelerations in 1995 (apart from the highest percentiles, for which somewhat higher accelerations were observed in 1998). Only TWC-car HC and NO emissions exhibited a clear relationship with acceleration: HC increased with increasing deceleration, whereas NO increased with increasing acceleration. This is consistent with the results from a similar study (Popp et al., 1999). According to Table 1 there was no signi"cant change in the average tailpipe CO concentration for the gasoline light-duty #eet on the freeway ramp between 1991 and 1995. On the contrary, a slight, still however very signi"cant (p(0.001 according to a student-t distribution) decrease of about 20% in CO emissions was observed between 1995 and 1998. To a large extent, these observations can be explained from the model year distributions for the gasoline light-duty vehicle #eet in 1991, 1995 and 1998. The model year average of the #eet was 1986.0 in 1991, 1988.1 in 1995 and 1991.2 in 1998, corresponding to an estimated average age of the #eet of about 5.5, 7.4 and 7.3 yr, respectively. The increase in #eet age between 1991 and 1995 re#ects the general poor sales of new cars in Sweden in the early 1990s due to the recession in the economy, and, in terms of #eet average emissions, tends to counterbalance the increase in the fraction of TWCcars, estimated to 49% in 1991, 61% in the 1995 and 77% in 1998 from the observed model year distributions. According to Table 1, average HC emissions dropped markedly from 1991 to 1995, and then increased again in 1998. However, there is evidence for the accuracy of the remote-sensing HC measurements in 1991 being poor (SjoK din, 1994), and so was believed also to be the case in 1998 (see further analysis in Section 3.2.2). This means that any conclusions regarding possible changes in the
overall HC emissions from the Swedish light-duty gasoline #eet from the observations presented in Table 1 cannot be made. According to Table 1 average NO dropped signi"cantly (p(0.05 according to a student-t distribution) by 15% between 1995 and 1998. The NO average for 1995 reported in Table 1 is calculated after removing all data where HC exceeded 0.05% to eliminate the reported positive HC interference of the non-dispersive NO measurement technique used in 1995 (Popp et al., 1998), see further Section 3.2.3. The original average %NO in 1995 calculated for all data without discriminating for possible HC interference was 0.074$0.023, i.e. 15% higher. However, the 15% drop in NO between 1995 and 1998 according to Table 1, makes sense in light of the 20% drop observed for CO during the same period, mainly caused by the increase in the fraction of lowemitting three-way catalyst cars from 61% in 1995 to 77% in 1998. 3.2. Average emissions by model year 3.2.1. CO Fig. 1 displays average tailpipe CO concentrations by model year for the gasoline light-duty vehicle #eet on the freeway ramp in 1991, 1995 and 1998. The sharp drop in average emissions from pre-1987 to post-1987 model years re#ects the introduction of the tight new car emission standards compulsory from model year 1989, as observed already in the 1991 measurements (SjoK din, 1994). By comparing the average emissions by model year curves for all three years, it can clearly be seen from Fig. 1 that there is both a signi"cant deterioration of CO emissions as cars grow older, as well as an improvement in emission performance of new cars with time. For instance, freeway ramp CO emissions of the earliest model years of catalyst cars 1989, 1990 and 1991 measured in all three years increased roughly by a factor of 2 during the 7-yr period between 1991 and 1998. Furthermore, for model years 1992}1995, measured both in 1995 and 1998, three out of four exhibited higher-average CO emissions in 1998, indicating the signi"cance of emission
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Fig. 1. Average tailpipe CO concentrations (vol%) for gasoline light-duty vehicles at the freeway ramp in 1991, 1995 and 1998 according to the remote-sensing measurements.
deterioration also for the more recent model years of TWC-cars. A signi"cant improvement in emission performance due to improved emission control technology for new cars is indicated by the fact that CO emissions of model years 1995, 1994 and 1993 in 1995, i.e. cars of 0}3 yr of age in 1995, were roughly only half of those of model years 1991, 1990 and 1989 in 1991, i.e. the cars of corresponding age in 1991. However, taking into consideration also the 1998 measurements, results are contradictory, since according to the remote-sensing data truly new cars (i.e. 0}1 yr old, model year 1998) in 1998 had about twice as high emissions as truly new cars in 1995. Several explanations for this observation were sought for, i.e. di!erences in driving pattern or in #eet composition of new cars in 1995 compared to new cars in 1998 with regard to make, model or weight, but none of these could account for the observed di!erences. Note, however, that CO emissions of new cars, i.e. 0}3 yr old, in 1998 still were lower than cars of the corresponding age in 1991. Another observation from Fig. 1 is that a deterioration in CO emissions between 1991 and 1995 and between 1995 and 1998 is also apparent for non-catalyst cars. For instance, CO emissions were higher in 1995 compared to 1991 for all model years of non-catalyst cars displayed in Fig. 1, and a further increase between 1995 and 1998 can be seen for most model years. This indicates that the CO emissions of non-catalyst cars continue to deteriorate even above an age of 10 yr. However, the decreasing slope of the CO emission by model year curve with time for the
most recent model years of non-catalyst cars (1981}1986) in Fig. 1, indicates that the deterioration rate actually decreases with age. In 1998 this part of the emission by model year curve is nearly horizontal, indicating that any deterioration of non-catalyst cars CO emissions no longer takes place. From the remote-sensing measurements yearly rates of deterioration in CO emissions were estimated to be 17}30% for TWC-cars, and 5}8% for non-catalyst cars. More than 500 unique TWC-cars and about as many non-catalyst cars appearing in two consecutive measurement campaigns yielded additional opportunities to study the e!ect of aging on CO emissions. In all cases, the average CO emissions of #eets consisting of the same car individuals exhibited higher values in the succeeding campaign (1995 or 1998) compared to the preceding campaign (1991 or 1995). TWC-cars emissions were consistently higher in the second campaign compared to the "rst campaign about a factor of 2, whereas non-catalyst car emissions were higher about 15}20%. It was also found that the main reason to the observed increase in TWC-cars CO emissions was an increase in a small fraction of CO &&high}emitters''. 3.2.2. HC The average emission by model year curve for HC in all three years is given by Fig. 2. As already mentioned in Section 3.1 the results for HC were hard to interpretate, most likely due to measurements being less accurate or erroneous in one or more years. In the 1991
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Fig. 2. Average tailpipe HC concentrations (vol% as propane) for gasoline light-duty vehicles at the freeway ramp in 1991, 1995 and 1998 according to the remote-sensing measurements (measurements particularly in 1991 could be in#uenced from water interference and in 1998 from HC detector noise and non-linearity problems).
measurements it was found that particularly catalyst cars HC emissions were erroneously high due to water interference from the HC detector used at that time (SjoK din, 1994). 1995 and 1998 measurements involved another HC detector much less sensitive to water, however, when comparing only these two HC emissions by model year curves yet the results look strange. HC emissions for catalyst cars were consistently about 3 times higher and for non-catalyst cars about 50% higher in 1998 compared to in 1995. It was found not likely that measurements in both two years could be right. It was concluded that, of the two, most likely the HC measurements in 1998 were the most erroneous, based upon the following observations: (1) the high `o!-seta HC emission level for truly new cars in the 1998 measurements, only about 50% lower compared to the emission level for pre-control model year cars, (2) the much poorer agreement between average HC and CO emissions by model year in the 1998 measurements compared to the 1995 measurements, (3) the level of noise, represented by the number and magnitude of negative HC readings, in the 1998 HC measurements, being much higher than in the 1995 HC measurements, (4) the fact that when comparing emission distributions for HC in 1995 and 1998 for some model years of non-catalyst cars, for all percentiles except for the
highest ones, HC in 1998 were consistently a factor of about 1.5 higher compared to HC in 1995, whereas for the highest percentiles, corresponding to the HC concentration level at which the daily "eld calibrations of the remote sensor were carried out, HC in 1998 approached HC in 1995. Altogether this indicates a severe HC detector problem in the 1998 measurements, with a detector exhibiting excessive noise and non-linearity. It may be worth considering also that experimental evaluations have demonstrated that NDIR HC remotesensing exhibits di!erent response factors for various individual hydrocarbons as well as various mixtures of hydrocarbons (Stephens et al., 1996). Thus, a remote sensor will give a di!erent HC reading if the composition of the exhaust is altered, even if the overall HC emission or concentration is unchanged. Starting by the end of 1994, there have been major changes in the composition of the Swedish gasoline, mainly by reducing the content of benzene and other aromatic compounds in the liquid fuel, however, we have not been able to judge how this could have possibly a!ected the tailpipe HC composition and the comparison between the 1995 and the 1998 remote-sensor HC measurements. 3.2.3. NO Average tailpipe NO concentrations by model year in 1995 and 1998 are presented in Fig. 3. As a means of
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Fig. 3. Average tailpipe NO concentrations (vol%) for gasoline light-duty vehicles at the freeway ramp in 1995 and 1998 according to the remote-sensing measurements.
eliminating the e!ect of the posted HC interference on the NO detector used in 1995, three curves are presented for the 1995 NO data: All data with no discrimination, data after excluding NO values when HC exceeded 0.1% (propane) and "nally data after excluding NO values when HC exceeded 0.05%. It can be seen from Fig. 3 that the average NO emissions actually decrease with increasing discrimination of records with high HC values, however, for the non-TWC fraction of the #eet only. With the highest level of HC discrimination applied on the 1995 data, the "nal observation is that NO emissions of catalyst cars of all model years (except model year 1989) present in the 1995 measurements exhibit a signi"cant increase, whereas emissions of non-catalyst cars do not appear to have changed at all from 1995 to 1998. However, if all records in 1995 with high HC emissions, which might be synonymous with all cars operating rich, i.e. those cars for which NO emissions may be suppressed, are excluded systematically, it cannot be completely ruled out that by such an approach non-catalyst cars NO emissions in 1995 to some extent may be underestimated, possibly hiding a real `negativea deterioration e!ect for NO for the non-catalyst fraction of the #eet. Note that the relative deterioration tends to be higher for later model years than for earlier model years of catalyst cars. Note also that new cars NO emissions in 1998 are not signi"cantly lower than new cars emissions in 1995. It thus seems as the big leap in TWC technology as regards improved emission performance for NO was from V
model year 1991 to model year 1994. However, further measurements are needed to prove the observed trends and the reliability of the new NO detector. 3.3. Emission distributions As observed in many other remote sensing studies, emission distributions were skewed, particularly for the TWC-fraction of the #eet and for CO. For the latest model years, CO emissions were below 1% CO for more than 99% of the #eet, and even for the oldest model years of catalyst cars, as well as for non-catalyst cars, the fraction of cars with very high emissions was low, although contributing typically as much as 50% of the overall #eet CO emissions. Lower percentiles, median and higher percentiles for tailpipe %CO all increased with increasing age, indicating that the observed deterioration in CO emissions has two components, a slower deterioration which might be synonymous with a slow deterioration of the catalyst itself, and one resulting from an increase in the fraction of high}emitters within each model year, which may be due to malfunctions of other parts of the emission control system than the catalyst, i.e. the EGR or the lambda-sond or other critical components of the emission control system. The fraction of TWC-cars exceeding any given cutpoint for CO appeared to be a linear function of vehicle age, as seen in Fig. 4. A very similar pattern was observed for NO. Such observations have also been made for US
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Fig. 4. Percent fraction of the overall gasoline light-duty vehicle #eet of a given age exceeding various cutpoints for tailpipe %CO on the freeway ramp in 1998. Table 2 Linear relationships to estimate the #eet fractions of TWC cars exceeding a given cutpoint i as a function of vehicle age. Coe$cients a and b and R values derived according to a linear regression analysis for the relationship F "ax#b, see further description in the G text Cutpoint
Measurements in 1998 b
R
a
3.49 2.96 1.61 0.75 0.31 0.07 0.02
16.4 3.9 !1.8 !1.3 !0.7 !0.1 !0.1
0.91 0.84 0.80 0.87 0.63 0.37 0.30
4.62 3.31 1.89 1.13 0.47 0.13 0.05
4.02 4.02 2.95 2.01 0.81 0.22
20.3 9.1 2.1 !0.2 !1.0 !0.5
0.99 0.97 0.93 0.93 0.87 0.65
4.94 5.06 3.92 2.58 0.82 0.22
a CO '0.1% '0.2% '0.5% '1% '2% '4% '6% NO '0.01% '0.02% '0.05% '0.1% '0.2% '0.3%
Measurements in 1995
light-duty #eets from remote sensing measurements (Stephens, 1994). Therefore, linear relationships could be established by means of linear regression analysis of the plots represented by Fig. 4, to estimate the #eet fractions exceeding a given cutpoint i as a function of vehicle age, according to the formula F "ax#b, G
Measurements in 1991 R
a
2.9 !1.5 !3.0 !2.5 !1.0 !0.2 !0.1
0.96 0.91 0.91 0.93 0.92 0.52 0.17
3.16 2.20 1.23 0.96 0.27 0.09 0.08
29.4 19.2 8.1 0.6 !0.0 !0.1
0.86 0.87 0.84 0.63 0.43 0.49
} } } } } }
B
b
R
12.7 5.2 1.0 !0.5 0.4 0.4 !0.1
0.75 0.63 0.50 0.46 0.32 0.39 0.85
} } } } } }
} } } } } }
where F is the fraction of the #eet exceeding cutpoint i, G x is the #eet age and a and b are the coe$cients derived from the linear regression analysis. These coe$cients are given in Table 2 for various cutpoints for CO and NO for TWC-cars for the measurements in 1991, 1995 and 1998, respectively. Note that regardless of what year the measurements were made, the relationships, particularly represented by
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the slope coe$cients (a), for a given pollutant (CO or NO) and a given cutpoint, are fairly uniform, indicating that for the Swedish TWC-#eet vehicle age rather than model year is the most important factor determining CO and NO emissions. For non-catalyst cars the fraction exceeding a given cutpoint tends to be independent of age, cf. Fig. 4.
4. Conclusions This study has demonstrated that remote sensing, by revisiting sites, may constitute an interesting approach to evaluate the real-world emission performance of gasoline light-duty vehicles, e.g. analyzing and quantifying realworld emission deterioration with components such as the slow, thermal aging of the catalyst and the increase in the number of incidents of more severe failures of the emission control system (increased frequency of `highemittersa) occurring over time, as well as the improvement in emission performance due to improved emission control technology on new vehicles. However, some precautions in the use of remote sensing for such purposes are also obvious from this work. To assure that the evolution of #eet average emissions can be accurately tracked the present standard "eld calibration procedure, with only a single-point calibration representing a highemitting vehicle, does not seem su$cient. Daily multipoint calibrations, ideally with real wet exhaust, to check remote sensor performance over the entire range of emissions which are representative for the in-use #eet are therefore strongly recommended in future work.
Acknowledgements Don Stedman is deeply acknowledged for providing remote sensors and for big support in the measurements in 1991, 1995 and 1998, as well as for fruitful discussions on the results. This work was funded by the Environmental Research Foundation of the Preem Petroleum Co., the Swedish Communications Research Board, the Swedish National Road Administration, the Swedish Environmental Protection Agency, Volvo, Saab Automobile, Scania, the Association of the Swedish Car Manufacturers and Whole Salers, the Swedish Motor Vehicle Inspection Company and the GoK teborg Region Association of Local Authorities.
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A. Sjo( din, K. Andre& asson / Atmospheric Environment 34 (2000) 4657}4665 Stephens, R.D., Giles, M., McAlinden, K., Gorse Jr., R.A., Ho!man, D., James, R., 1997. An analysis of Michigan and California CO remote sensing measurements. Journal of Air and Waste Management Association 47, 601}607. Zhang, Y., Stedman, D.H., Bishop, G.A., 1995. Worldwide on-road vehicle exhaust emissions study by remote
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Multi-year remote-sensing measurements of gasoline light-duty vehicle emissions on a freeway ramp As ke SjoK din*, Kenth AndreH asson Swedish Environmental Research Institute, P.O. Box 47086, S-402 58 Gothenburg, Sweden Received 3 September 1999; accepted 2 March 2000
Abstract On-road optical remote-sensing measurements of gasoline light-duty vehicle (LDV) emissions } CO, HC, NO } were conducted on a freeway ramp in Gothenburg, Sweden, in 1991, 1995 and 1998. Based on almost 30,000 emission measurements, the results show that both catalyst cars and non-catalyst cars emissions deteriorate over time, but also that the emission performance of new TWC-cars has improved signi"cantly in recent years. Furthermore, it was found that #eet age rather than model year determines the rate of emission deterioration for TWC-cars for both CO and NO. The study demonstrates that remote sensing may constitute a powerful tool to evaluate real-world LDV emissions; however, daily "eld calibration procedures need to be developed in order to assure that the evolution in #eet average emissions can be accurately measured. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Light-duty vehicles; Emissions; On-road; Measurements; CO; HC; NO; Optical remote sensing; FEAT
1. Introduction Air quality is improving in many areas of the world, partly as a result of stricter emission standards for gasoline light-duty vehicles (LDVs), requiring the use of e!ective closed-loop three-way catalyst systems (TWCs). In North America, where such standards were introduced already in the early 1980s, the #eet turn-over to LDVs equipped with TWCs is already more or less complete. In Europe, on the other hand, where TWCs did not appear before on around 1990 model years, there is still a large fraction of the LDV #eet lacking e!ective emission control, in many cases more than 50%, thus leaving a great potential for further emission reductions and associated bene"ts in air quality. For instance, in Sweden the average 35% decrease in ambient NO levels observed in urban areas since the late 1980s coincides with an increase in the fraction of LDVs equipped with TWCs from 0% to about 50% (SjoK din et al., 1996). This demonstrates
* Corresponding author. E-mail address: [email protected] (As ke SjoK din).
the great importance of LDV emissions for urban air quality. However, the experience from the US is that air quality is not improving as fast as is predicted from the growth in the number of TWC-cars. It has been claimed that this may partly be due to existing inspection and maintenance (I/M) programs failing to maintain the optimum vehicle emission performance as vehicles age (Lawson, 1993, 1995; Bishop et al., 1996; Stedman et al., 1997). Furthermore, in general, knowledge of the actual emissions from in-use LDV #eets is still limited, since most available emission data arise from, e.g., FTPmeasurements on small-vehicle samples, that may be unrepresentative for the in-use #eet. An alternative approach to possibly overcome the problem with test #eet representativity in conventional emission measurements is to use the on-road optical remote-sensing device originally developed by the University of Denver in the late 1980s (Bishop et al., 1989). Through the years this technique has been further developed and re"ned (e.g. Guenther et al., 1995; Zhang et al., 1996; Popp et al., 1999) and applied in numerous locations around the world (e.g. Bishop et al., 1993; Lawson et al., 1990; Popp et al., 1998; SjoK din, 1994; SjoK din et al., 1997; Stephens et al., 1997; Zhang et al.,
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1995). However, so far few studies have been reported, where the same remote-sensing site has been revisited to study changes in #eet emission performance over time. The only exception is an o!-ramp in Denver, Colorado, where measurements have been carried out on several occasions since 1989 (Slott, 1996). This paper presents the results from three remotesensing monitoring campaigns conducted in 1991, 1995 and 1998 at the same freeway ramp site with the main objective to evaluate the change in emission performance over time of Swedish gasoline light-duty vehicles. The 1991 measurements at this site have been reported earlier (SjoK din, 1994).
2. Experimental Most of the background and experimental conditions have been described in detail elsewhere (SjoK din, 1994), and therefore only a brief description will be given here. 2.1. Fleet characteristics Emission standards requiring the use of closed-loop TWCs on all new light-duty gasoline vehicles were introduced in Sweden from model year 1989. TWC-equipped cars of model years 1987 and 1988 were sold on a voluntary basis with estimated fractions of about 25 and 85%, respectively. Model years before 1987 are considered to have uncontrolled emissions, with the only exception being the EGR-valve required from model year 1976. The Swedish light-duty #eet is dominated by largeand medium-sized cars of the two domestic makes Volvo and Saab, with displacement volumes of about 2.0}2.5 l, which together make up nearly 40% of the #eet. The 10 most common foreign makes in recent years are in decreasing order (range and estimated average displacement volumes in liters within parenthesis): Ford (1.1}2.9, avg. 2.0), Volkswagen (1.4}2.5, avg. 1.8), GM Opel (1.4}2.0, avg. 1.6), Audi (1.6}2.8, avg. 2.0), Mazda (1.3}2.5, avg. 1.8), Toyota (1.3}2.0, avg. 1.8), Nissan (1.3}2.0, avg. 1.6), Renault (1.4}2.0, avg. 1.6), Mitsubishi (1.3}2.0, avg. 1.8) and Mercedes (1.8}5.0, avg. 2.2). A centralized, test-only I/M program involving lowidle testing for CO has been in operation in Sweden since 1970. For catalyst cars, the test also involves HC. Additional high-idle CO and lambda testing was introduced for catalyst cars in 1997. Cars older than four years are inspected annually, whereas during the "rst four years the inspection is biennial. 2.2. Remote-sensing measurements Remote-sensing measurements were carried out at exactly the same site in 1991, 1995 and 1998 with remote sensors (FEAT) provided by the University of Denver.
The site is just at the exit of a sharp, 2703 curve of an uphill freeway interchange ramp, considered to carry only warmed-up vehicles since the nearest on-ramp is several minutes of driving away. Measurements in 1991 involved only CO and HC. The detector and "lter applied for the CO measurements were basically the same in all three years. The HC detector applied in 1991 utilized a "lter centered at 3030 cm\, which is sensitive to liquid water interference (SjoK din, 1994; Guenther et al., 1995), whereas the HC detector applied in 1995 and 1998 utilized a "lter centered at 2941 cm\, less responsive to water. NO measurements in 1995 were made by means of non-dispersive UV (Zhang et al., 1996), whereas NO measurements in 1998 were conducted by the newly developed high-speed UV diode}array spectrometer (Popp et al., 1999). The new NO detector has a much lower level of noise and is claimed to be not as sensitive to HC interference as the old detector. In 1995 and 1998, the remote-sensing measurements also involved speed and acceleration on an individual vehicle level. In each year the measurements comprised about three working days during the period mid-August to midSeptember, yielding a total of between 15,000 and 20,000 vehicle-induced beam blocks on the remote sensor in each campaign. During the campaigns the remote sensor was calibrated at least once every day by means of pu!s of certi"ed CO, HC, CO and NO gas mixtures from cylinders (1991 and 1998) or by a sealed gas}cell with known concentrations of the corresponding gases (1995).
3. Results and discussion 3.1. Fleet average emissions Overall #eet average emissions, expressed as average volume percent tailpipe exhaust concentrations, for the gasoline light-duty vehicles on the freeway ramp according to the 1991, 1995 and 1998 remote-sensing measurements are given in Table 1, along with corresponding averages of the measured speed and acceleration in 1995 and 1998. 10,284 valid CO and HC readings on gasoline light-duty vehicles, identi"ed from the video images of the license plates, were obtained in the 1991 measurements. The corresponding number of valid readings in the 1995 measurements were 8289 for CO, 7601 for HC and 5470 for NO, and in the 1998 measurements 10,672 for CO, 10,261 for HC and 9001 for NO. The errors given for the average tailpipe concentrations in Table 1 are standard errors derived by the same methodology used by Bishop et al. (1993) for the gamma-distributed remote sensing data. All pollutant emission data were randomly divided into 250 record samples, and the means of these samples were used to calculate the standard error reported in Table 1. The standard errors for vehicle speed and acceleration
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Table 1 Overall #eet average tailpipe emissions (in vol.%) and average speed and acceleration for gasoline light-duty vehicles on the freeway ramp in 1991, 1995 and 1998 according to the remote-sensing measurements. Errors are one standard deviation, calculated as described in the text Year
CO (%)
HC (%) (propane)
NO (%)
Speed (kmh\)
Acceleration (ms\)
1991 1995 1998
0.71$0.10 0.73$0.13 0.57$0.08
0.059$0.005 0.022$0.004 0.037$0.006
} 0.064$0.018 0.055$0.009
} 27.7$5.0 29.8$5.0
} 0.28$0.41 0.03$0.63
Di!erent detectors were used the "rst year compared to the succeeding year(s).
were calculated in the normal way assuming Gaussian statistics. Average vehicle speed and acceleration as well as distributions of these two parameters were similar in 1995 and 1998. Roughly, in both 1995 and 1998, more than 90% of all observed speeds were in the range 20}35 km h\, with slightly higher speeds in 1998, and more than 80% of all observed accelerations were in the range !0.5}0.5 m s\, with slightly higher accelerations in 1995 (apart from the highest percentiles, for which somewhat higher accelerations were observed in 1998). Only TWC-car HC and NO emissions exhibited a clear relationship with acceleration: HC increased with increasing deceleration, whereas NO increased with increasing acceleration. This is consistent with the results from a similar study (Popp et al., 1999). According to Table 1 there was no signi"cant change in the average tailpipe CO concentration for the gasoline light-duty #eet on the freeway ramp between 1991 and 1995. On the contrary, a slight, still however very signi"cant (p(0.001 according to a student-t distribution) decrease of about 20% in CO emissions was observed between 1995 and 1998. To a large extent, these observations can be explained from the model year distributions for the gasoline light-duty vehicle #eet in 1991, 1995 and 1998. The model year average of the #eet was 1986.0 in 1991, 1988.1 in 1995 and 1991.2 in 1998, corresponding to an estimated average age of the #eet of about 5.5, 7.4 and 7.3 yr, respectively. The increase in #eet age between 1991 and 1995 re#ects the general poor sales of new cars in Sweden in the early 1990s due to the recession in the economy, and, in terms of #eet average emissions, tends to counterbalance the increase in the fraction of TWCcars, estimated to 49% in 1991, 61% in the 1995 and 77% in 1998 from the observed model year distributions. According to Table 1, average HC emissions dropped markedly from 1991 to 1995, and then increased again in 1998. However, there is evidence for the accuracy of the remote-sensing HC measurements in 1991 being poor (SjoK din, 1994), and so was believed also to be the case in 1998 (see further analysis in Section 3.2.2). This means that any conclusions regarding possible changes in the
overall HC emissions from the Swedish light-duty gasoline #eet from the observations presented in Table 1 cannot be made. According to Table 1 average NO dropped signi"cantly (p(0.05 according to a student-t distribution) by 15% between 1995 and 1998. The NO average for 1995 reported in Table 1 is calculated after removing all data where HC exceeded 0.05% to eliminate the reported positive HC interference of the non-dispersive NO measurement technique used in 1995 (Popp et al., 1998), see further Section 3.2.3. The original average %NO in 1995 calculated for all data without discriminating for possible HC interference was 0.074$0.023, i.e. 15% higher. However, the 15% drop in NO between 1995 and 1998 according to Table 1, makes sense in light of the 20% drop observed for CO during the same period, mainly caused by the increase in the fraction of lowemitting three-way catalyst cars from 61% in 1995 to 77% in 1998. 3.2. Average emissions by model year 3.2.1. CO Fig. 1 displays average tailpipe CO concentrations by model year for the gasoline light-duty vehicle #eet on the freeway ramp in 1991, 1995 and 1998. The sharp drop in average emissions from pre-1987 to post-1987 model years re#ects the introduction of the tight new car emission standards compulsory from model year 1989, as observed already in the 1991 measurements (SjoK din, 1994). By comparing the average emissions by model year curves for all three years, it can clearly be seen from Fig. 1 that there is both a signi"cant deterioration of CO emissions as cars grow older, as well as an improvement in emission performance of new cars with time. For instance, freeway ramp CO emissions of the earliest model years of catalyst cars 1989, 1990 and 1991 measured in all three years increased roughly by a factor of 2 during the 7-yr period between 1991 and 1998. Furthermore, for model years 1992}1995, measured both in 1995 and 1998, three out of four exhibited higher-average CO emissions in 1998, indicating the signi"cance of emission
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Fig. 1. Average tailpipe CO concentrations (vol%) for gasoline light-duty vehicles at the freeway ramp in 1991, 1995 and 1998 according to the remote-sensing measurements.
deterioration also for the more recent model years of TWC-cars. A signi"cant improvement in emission performance due to improved emission control technology for new cars is indicated by the fact that CO emissions of model years 1995, 1994 and 1993 in 1995, i.e. cars of 0}3 yr of age in 1995, were roughly only half of those of model years 1991, 1990 and 1989 in 1991, i.e. the cars of corresponding age in 1991. However, taking into consideration also the 1998 measurements, results are contradictory, since according to the remote-sensing data truly new cars (i.e. 0}1 yr old, model year 1998) in 1998 had about twice as high emissions as truly new cars in 1995. Several explanations for this observation were sought for, i.e. di!erences in driving pattern or in #eet composition of new cars in 1995 compared to new cars in 1998 with regard to make, model or weight, but none of these could account for the observed di!erences. Note, however, that CO emissions of new cars, i.e. 0}3 yr old, in 1998 still were lower than cars of the corresponding age in 1991. Another observation from Fig. 1 is that a deterioration in CO emissions between 1991 and 1995 and between 1995 and 1998 is also apparent for non-catalyst cars. For instance, CO emissions were higher in 1995 compared to 1991 for all model years of non-catalyst cars displayed in Fig. 1, and a further increase between 1995 and 1998 can be seen for most model years. This indicates that the CO emissions of non-catalyst cars continue to deteriorate even above an age of 10 yr. However, the decreasing slope of the CO emission by model year curve with time for the
most recent model years of non-catalyst cars (1981}1986) in Fig. 1, indicates that the deterioration rate actually decreases with age. In 1998 this part of the emission by model year curve is nearly horizontal, indicating that any deterioration of non-catalyst cars CO emissions no longer takes place. From the remote-sensing measurements yearly rates of deterioration in CO emissions were estimated to be 17}30% for TWC-cars, and 5}8% for non-catalyst cars. More than 500 unique TWC-cars and about as many non-catalyst cars appearing in two consecutive measurement campaigns yielded additional opportunities to study the e!ect of aging on CO emissions. In all cases, the average CO emissions of #eets consisting of the same car individuals exhibited higher values in the succeeding campaign (1995 or 1998) compared to the preceding campaign (1991 or 1995). TWC-cars emissions were consistently higher in the second campaign compared to the "rst campaign about a factor of 2, whereas non-catalyst car emissions were higher about 15}20%. It was also found that the main reason to the observed increase in TWC-cars CO emissions was an increase in a small fraction of CO &&high}emitters''. 3.2.2. HC The average emission by model year curve for HC in all three years is given by Fig. 2. As already mentioned in Section 3.1 the results for HC were hard to interpretate, most likely due to measurements being less accurate or erroneous in one or more years. In the 1991
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Fig. 2. Average tailpipe HC concentrations (vol% as propane) for gasoline light-duty vehicles at the freeway ramp in 1991, 1995 and 1998 according to the remote-sensing measurements (measurements particularly in 1991 could be in#uenced from water interference and in 1998 from HC detector noise and non-linearity problems).
measurements it was found that particularly catalyst cars HC emissions were erroneously high due to water interference from the HC detector used at that time (SjoK din, 1994). 1995 and 1998 measurements involved another HC detector much less sensitive to water, however, when comparing only these two HC emissions by model year curves yet the results look strange. HC emissions for catalyst cars were consistently about 3 times higher and for non-catalyst cars about 50% higher in 1998 compared to in 1995. It was found not likely that measurements in both two years could be right. It was concluded that, of the two, most likely the HC measurements in 1998 were the most erroneous, based upon the following observations: (1) the high `o!-seta HC emission level for truly new cars in the 1998 measurements, only about 50% lower compared to the emission level for pre-control model year cars, (2) the much poorer agreement between average HC and CO emissions by model year in the 1998 measurements compared to the 1995 measurements, (3) the level of noise, represented by the number and magnitude of negative HC readings, in the 1998 HC measurements, being much higher than in the 1995 HC measurements, (4) the fact that when comparing emission distributions for HC in 1995 and 1998 for some model years of non-catalyst cars, for all percentiles except for the
highest ones, HC in 1998 were consistently a factor of about 1.5 higher compared to HC in 1995, whereas for the highest percentiles, corresponding to the HC concentration level at which the daily "eld calibrations of the remote sensor were carried out, HC in 1998 approached HC in 1995. Altogether this indicates a severe HC detector problem in the 1998 measurements, with a detector exhibiting excessive noise and non-linearity. It may be worth considering also that experimental evaluations have demonstrated that NDIR HC remotesensing exhibits di!erent response factors for various individual hydrocarbons as well as various mixtures of hydrocarbons (Stephens et al., 1996). Thus, a remote sensor will give a di!erent HC reading if the composition of the exhaust is altered, even if the overall HC emission or concentration is unchanged. Starting by the end of 1994, there have been major changes in the composition of the Swedish gasoline, mainly by reducing the content of benzene and other aromatic compounds in the liquid fuel, however, we have not been able to judge how this could have possibly a!ected the tailpipe HC composition and the comparison between the 1995 and the 1998 remote-sensor HC measurements. 3.2.3. NO Average tailpipe NO concentrations by model year in 1995 and 1998 are presented in Fig. 3. As a means of
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Fig. 3. Average tailpipe NO concentrations (vol%) for gasoline light-duty vehicles at the freeway ramp in 1995 and 1998 according to the remote-sensing measurements.
eliminating the e!ect of the posted HC interference on the NO detector used in 1995, three curves are presented for the 1995 NO data: All data with no discrimination, data after excluding NO values when HC exceeded 0.1% (propane) and "nally data after excluding NO values when HC exceeded 0.05%. It can be seen from Fig. 3 that the average NO emissions actually decrease with increasing discrimination of records with high HC values, however, for the non-TWC fraction of the #eet only. With the highest level of HC discrimination applied on the 1995 data, the "nal observation is that NO emissions of catalyst cars of all model years (except model year 1989) present in the 1995 measurements exhibit a signi"cant increase, whereas emissions of non-catalyst cars do not appear to have changed at all from 1995 to 1998. However, if all records in 1995 with high HC emissions, which might be synonymous with all cars operating rich, i.e. those cars for which NO emissions may be suppressed, are excluded systematically, it cannot be completely ruled out that by such an approach non-catalyst cars NO emissions in 1995 to some extent may be underestimated, possibly hiding a real `negativea deterioration e!ect for NO for the non-catalyst fraction of the #eet. Note that the relative deterioration tends to be higher for later model years than for earlier model years of catalyst cars. Note also that new cars NO emissions in 1998 are not signi"cantly lower than new cars emissions in 1995. It thus seems as the big leap in TWC technology as regards improved emission performance for NO was from V
model year 1991 to model year 1994. However, further measurements are needed to prove the observed trends and the reliability of the new NO detector. 3.3. Emission distributions As observed in many other remote sensing studies, emission distributions were skewed, particularly for the TWC-fraction of the #eet and for CO. For the latest model years, CO emissions were below 1% CO for more than 99% of the #eet, and even for the oldest model years of catalyst cars, as well as for non-catalyst cars, the fraction of cars with very high emissions was low, although contributing typically as much as 50% of the overall #eet CO emissions. Lower percentiles, median and higher percentiles for tailpipe %CO all increased with increasing age, indicating that the observed deterioration in CO emissions has two components, a slower deterioration which might be synonymous with a slow deterioration of the catalyst itself, and one resulting from an increase in the fraction of high}emitters within each model year, which may be due to malfunctions of other parts of the emission control system than the catalyst, i.e. the EGR or the lambda-sond or other critical components of the emission control system. The fraction of TWC-cars exceeding any given cutpoint for CO appeared to be a linear function of vehicle age, as seen in Fig. 4. A very similar pattern was observed for NO. Such observations have also been made for US
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Fig. 4. Percent fraction of the overall gasoline light-duty vehicle #eet of a given age exceeding various cutpoints for tailpipe %CO on the freeway ramp in 1998. Table 2 Linear relationships to estimate the #eet fractions of TWC cars exceeding a given cutpoint i as a function of vehicle age. Coe$cients a and b and R values derived according to a linear regression analysis for the relationship F "ax#b, see further description in the G text Cutpoint
Measurements in 1998 b
R
a
3.49 2.96 1.61 0.75 0.31 0.07 0.02
16.4 3.9 !1.8 !1.3 !0.7 !0.1 !0.1
0.91 0.84 0.80 0.87 0.63 0.37 0.30
4.62 3.31 1.89 1.13 0.47 0.13 0.05
4.02 4.02 2.95 2.01 0.81 0.22
20.3 9.1 2.1 !0.2 !1.0 !0.5
0.99 0.97 0.93 0.93 0.87 0.65
4.94 5.06 3.92 2.58 0.82 0.22
a CO '0.1% '0.2% '0.5% '1% '2% '4% '6% NO '0.01% '0.02% '0.05% '0.1% '0.2% '0.3%
Measurements in 1995
light-duty #eets from remote sensing measurements (Stephens, 1994). Therefore, linear relationships could be established by means of linear regression analysis of the plots represented by Fig. 4, to estimate the #eet fractions exceeding a given cutpoint i as a function of vehicle age, according to the formula F "ax#b, G
Measurements in 1991 R
a
2.9 !1.5 !3.0 !2.5 !1.0 !0.2 !0.1
0.96 0.91 0.91 0.93 0.92 0.52 0.17
3.16 2.20 1.23 0.96 0.27 0.09 0.08
29.4 19.2 8.1 0.6 !0.0 !0.1
0.86 0.87 0.84 0.63 0.43 0.49
} } } } } }
B
b
R
12.7 5.2 1.0 !0.5 0.4 0.4 !0.1
0.75 0.63 0.50 0.46 0.32 0.39 0.85
} } } } } }
} } } } } }
where F is the fraction of the #eet exceeding cutpoint i, G x is the #eet age and a and b are the coe$cients derived from the linear regression analysis. These coe$cients are given in Table 2 for various cutpoints for CO and NO for TWC-cars for the measurements in 1991, 1995 and 1998, respectively. Note that regardless of what year the measurements were made, the relationships, particularly represented by
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the slope coe$cients (a), for a given pollutant (CO or NO) and a given cutpoint, are fairly uniform, indicating that for the Swedish TWC-#eet vehicle age rather than model year is the most important factor determining CO and NO emissions. For non-catalyst cars the fraction exceeding a given cutpoint tends to be independent of age, cf. Fig. 4.
4. Conclusions This study has demonstrated that remote sensing, by revisiting sites, may constitute an interesting approach to evaluate the real-world emission performance of gasoline light-duty vehicles, e.g. analyzing and quantifying realworld emission deterioration with components such as the slow, thermal aging of the catalyst and the increase in the number of incidents of more severe failures of the emission control system (increased frequency of `highemittersa) occurring over time, as well as the improvement in emission performance due to improved emission control technology on new vehicles. However, some precautions in the use of remote sensing for such purposes are also obvious from this work. To assure that the evolution of #eet average emissions can be accurately tracked the present standard "eld calibration procedure, with only a single-point calibration representing a highemitting vehicle, does not seem su$cient. Daily multipoint calibrations, ideally with real wet exhaust, to check remote sensor performance over the entire range of emissions which are representative for the in-use #eet are therefore strongly recommended in future work.
Acknowledgements Don Stedman is deeply acknowledged for providing remote sensors and for big support in the measurements in 1991, 1995 and 1998, as well as for fruitful discussions on the results. This work was funded by the Environmental Research Foundation of the Preem Petroleum Co., the Swedish Communications Research Board, the Swedish National Road Administration, the Swedish Environmental Protection Agency, Volvo, Saab Automobile, Scania, the Association of the Swedish Car Manufacturers and Whole Salers, the Swedish Motor Vehicle Inspection Company and the GoK teborg Region Association of Local Authorities.
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