Correspondence/Rebuttal pubs.acs.org/est
Comment on “Damages and Expected Deaths Due to Excess NOx Emissions from 2009 to 2015 Volkswagen Diesel Vehicles”
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achieve somewhat lower fuel economy during real-world operation as compared to EPA’s certification test conditions. Also, it has been reported recently that SCR systems can emit nitrous oxide (N2O), a very potent GHG.3 Presumably, this problem would be reduced if the SCR system were deactivated. Another benefit is reduced ammonia emissions. SCR systems require addition of urea (which is converted to ammonia in situ) to reduce NOx. A recognized problem with SCR systems is “ammonia slip” emissions resulting from periodic excess injection of urea.4,5 With improperly functioning SCR systems, it is likely that less urea was being injected, resulting in less ammonia slip. The health benefit of this malfunction could be significant, as the simultaneous emission of NOx and ammonia at the roadway leads to formation of PM2.5 at “nose level.” A third benefit stems from the complex chemistry of ozone formation. Under certain atmospheric conditions, increases in NOx are known to decrease ozone concentrations. The relative deficit of vehicular NOx emissions during weekends is recognized as a cause of the so-called “weekend ozone effect,” whereby ozone pollution levels in some urban areas are higher on weekends, despite overall reductions in precursor emissions.6,7 A final benefit is reduced diesel PM. With diesel combustion, a trade-off exists between NOx and PM emissions. High NOx emissions from the VW vehicles suggests that engine-out PM emissions may have been reduced. As these vehicles are equipped with diesel particulate filters (DPFs), tailpipe-out PM emissions are expected to be very low. However, DPFs require periodic regeneration to burnoff the accumulated diesel soot. During regeneration events, tailpipe-out PM emissions can be very high.8 More frequent regeneration events would lead to higher overall PM emissions. If the defeat devices lowered engine-out PM, these vehicles would not require regeneration as frequently, thus giving lower overall PM emissions. Due to the extremely high risk factor applied to diesel PM (in California), it might even be argued that use of these defeat devices reduced loss of statistical lives.
he recent report on the impacts of VW’s emissions scandal by Holland et al.1 and the similar work previously reported by Barrett et al.2 strike me as “ambulance chasing,” in striving to be first on the scene to report body counts following some environmental disaster. In both studies, complex models were used to calculate health damages and “statistical deaths” based on assumed increases in NOx emissions from vehicles equipped with so-called “defeat devices.” Because similar approaches, emissions assumptions, and risk factors were used, the results by Holland et al. and Barrett et al. are also very similar. However, these similarities do not add confidence to the results; they simply confirm that modeling the same thing in the same way gives the same outcome. Clearly, the real world is much more complex than depicted in any modeled scenario. Therefore, simplifying assumptions are used in modelbased approaches, which increases uncertainty and may lead to erroneous conclusions. This should be remembered when considering statistical death values reported to three significant figures!
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USE OF SIMPLIFYING ASSUMPTIONS A critical assumption used by Holland et al. is that two test vehicles adequately represent the entire VW diesel fleet equipped with either lean NOx trap (LNT) or selective catalytic reduction (SCR) emission control systems. However, it is well-known that vehicle fleets have broad emission distributions, and therefore cannot be accurately represented by just a few vehicles. It is also assumed that during real-world use, normal vehicles emit NOx at levels equivalent to the EPA certification standard of 0.043 g/km. For many vehicles, this is not valid. The problem of in-use vehicles exceeding emission standards is well recognized, and is the reason for Smog Check programs in many locations. Thus, the “excess emissions” calculated by Holland et al. are probably overestimated. A third assumption is that only NOx emissions were affected in these VW vehicles. However, modern vehicle emission control systems involve complex integration of numerous devices, managed through a network of sensors and controllers that respond in real-time to changes in engine operating conditions. In addition, the performance of these systems varies dynamically with temperature, space velocity, and exhaust gas composition. Given this complexity, it is improbable that use of defeat devices would affect only total NOx emissions. Another important parameter that likely changed is the NO/NO2 ratio. This is quite important, as NO and NO2 have very different impacts with respect to air quality and health effects.
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MODEL UNCERTAINTY AND PRESENTATION OF RESULTS Numeric results computed from model simulations can be deceptive, particularly when presented without a full understanding of their uncertainty. Holland et al. state “a range for excess expected deaths from 40 to 59,” but this range is simply derived by adjusting the vehicle emission rates up and down by one standard error. A rigorous uncertainty analysis would consider many other factors−including fleet-wide NO x distributions of baseline and tampered vehicles, changes in emissions composition (especially NO/NO2 ratios), uncertainties in health risk factors, and impacts of offsetting benefits. (A good example of a comprehensive uncertainty analysis of
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POTENTIAL BENEFITS OF HIGH NOX EMISSIONS Besides the adverse effects of increased NOx emissions, which were used by Holland et al. to compute statistical deaths and costs, several benefits are likely associated with use of VW’s defeat devices. The benefit of reduced GHG emissions due to improved fuel economy is mentioned, though probably underestimated, since properly functioning vehicles typically © XXXX American Chemical Society
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DOI: 10.1021/acs.est.6b00856 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
model-derived environmental outputs was recently published in ES&T.9) It is probable that if a rigorous uncertainty analysis were performed here, the estimated number of excess statistical deaths could not be distinguished from zero. To provide a better understanding, model results should be presented as probability distributions, not singular numerical values. Such depictions would also help illustrate the inappropriateness of comparing model-predicted excess statistical deaths with specific, identifiable deaths due to vehicle accidents. Probabilistic models such as those applied by Holland et al. are extremely useful, and probably essential in assessing relative risks and defining policy. However, because such models are incomplete and simplified representations of the real-world, their results have high uncertainties, and should not be compared directly with real-world outcomes. It seems that the scientific community is placing everincreasing confidence in model results, without adequate recognition of their limitations. The traditional approach of “hypothesize, test, and confirm” is being replaced by “model, visualize, and believe.” This is not a healthy development for science.
S. Kent Hoekman*
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Desert Research Institute Reno, Nevada 89512, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest.
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REFERENCES
(1) Holland, S. P.; Mansur, E. T.; Muller, N. Z.; Yates, A. J. Damages and Expected Deaths Due to Excess NOx Emissions from 2009 to 2015 Volkswagen Diesel Vehicles. Environ. Sci. Technol. 2016, 50, 1111−1117. (2) Barrett, S.; Speth, R.; Eastham, S.; Dedoussi, I.; Ashok, A.; Malina, R.; Keith, D. Impact of the Volkswagen emissions control defeat device on US public health. Environ. Res. Lett. 2015, 10, 114005. (3) Bartley, G.; Sharp, C. Brief investigation of SCR high temperature N2O production. SAE Int. J. Engines 2012, 5 (2), 683−687. (4) Theis, J. Selective catalytic reduction for treating the NOx emissions from lean engines. Performance assessment. SAE Int. J. Fuels Lubr. 2009, 1 (1), 364−375. (5) Tang, W.; Cai, Y.; Wang, J. Experimental studies on the diesel engine urea-SCR system using a double NOx sensor system. Environ. Engineering. Res. 2015, 20 (4), 397−402. (6) Pollack, I. B.; Ryerson, T. B.; Trainer, M.; et al. Airborne and ground-based observations of a weekend effect in ozone, precursors, and oxidation products in the California South Coast Air Basin. J. Geo. Res., Atmos. 2012, 117, D00V05. (7) Heuss, J. M.; Kahlbaum, D. F.; Wolff, G. T. Weekday/weekend ozone differences. What can we learn from them? J. Air Waste Manage. Assoc. 2003, 53, 772−788. (8) Dwyer, H.; Ayala, A.; Zhang, S.; Collins, J.; Huai, T.; Herner, J.; Chau, W. Emissions from a diesel car during regeneration of an active diesel particulate filter. J. Aerosol Sci. 2010, 41, 541−552. (9) Plevin, R. J.; Beckman, J.; Golub, A. A.; Witcover, J.; O’Hare, M. Carbon accounting and economic model uncertainty of emissions from biofuels-induced land use change. Environ. Sci. Technol. 2015, 49, 2656−2664.
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DOI: 10.1021/acs.est.6b00856 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
