Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Damages and expected deaths due to excess NOx emissions from 2009−2015 Volkswagen diesel vehicles” The final potential benefit is that there is a trade-off between NOx and PM emissions, so that excess NOx emissions necessarily imply reduced PM emissions. The AP2 model evaluates damages from PM, so we can do a simple calculation of this benefit. Suppose that, during EPA emission testing, the VW vehicles emitted PM at a rate equal to the EPA Tier 2 Bin 5 standard of 0.01 g/mile. Also suppose that, during normal use, the PM emissions were completely eliminated. Under these conditions, the benefit of reduced PM emissions would be equal to approximately $38 million. This is about the same as the CO2 benefit, and much lower than the damages from excess NOx emissions ($430 million.)
n “Damages and Expected Deaths Due to Excess NOx Emissions from 2009 to 2015 Volkswagen Diesel Vehicles” (Holland et al., 2016), we estimated mean damages of $430 million and 46 expected deaths in the United States due to excess emissions of NOx from Volkswagen (VW) diesel vehicles that were equipped with defeat devices. These devices lowered emissions of NOx when the vehicles were being emission tested by the EPA but allowed excess emissions during normal operation. Hoekman (2016) presents three main criticisms. First, some of our assumptions are questioned. Second, a list of potential benefits from the defeat devices is given. Third, it is suggested that we should report entire probability distributions, rather than just the expected value of these distributions. We address each of these criticisms in turn, and find little to no effects on our results.
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REPORT PROBABILITY DISTRIBUTIONS We elected to report expected values for damages and deaths and then analyze the sensitivity of these values to changes in the underlying variables. Hoekman (2016) suggests that it would have been better to assign probability distributions to each of the variables, and then conduct a Monte Carlo analysis to determine a summary probability distribution for damages and deaths. It is not obvious that one procedure is better than the other. Both sensitivity analysis and Monte Carlo analysis are widely used and broadly accepted ways to report model uncertainty. We agree that presenting probability distributions has an intuitive appeal. But, to generate them, one has to make additional assumptions about the underlying distribution for each variable. In many cases there will not be sound basis for selecting these distributions. Hoekman (2016) also critiques our comparison of expected deaths to the deaths associated with accidents due to other defective vehicles. We note that there is often uncertainty about the causes of accidents as well as the causes of deaths that occur in the accidents. For example, in the General Motors pickup truck case, it may be difficult to untangle whether a given death was due to the collision itself or due to the placement of the gas tank and resulting fire.
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ASSUMPTIONS The first criticized assumption is that we use the results from a small-sample study to estimate excess emissions. We discuss this assumption and its potential implications in our paper. In particular, our sensitivity analysis shows the effects of using higher and lower emission rates. The second criticized assumption is that excess NOx emissions are calculated relative to the EPA emission standard of 0.043 g/km. Hoekman (2016) suggests that this is not appropriate because other vehicles typically exceed the emission standard in actual use. But our research question was how much VW was out of compliance, not how much VW was worse than other vehicles. Our results may actually understate the effect of the defeat devices, because NOx emissions may have been less than the standard during EPA testing. The final criticized assumption is that only NOx emissions were affected, not the NO/NO2 ratio. Once again, we discuss this assumption and its potential implications in our paper.
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POTENTIAL BENEFITS In addition to the damages and expected deaths attributable to the defeat devices, we also calculated the benefits from reduced CO2 emissions. Hoekman (2016) suggests that there are potentially other benefits. The first two potential benefits are reduced nitrous oxide and ammonia emissions from vehicles with SCR pollution control systems. We cannot evaluate these benefits in the manner that we did for CO2, because there are no studies that quantify the degree to which the defeat devices lowered these emissions. However, our data does give us some insight. SCR systems were used on 27% of the vehicles, and these vehicles tended to be the newer model years, so they account for only 17% of the total miles driven. Thus, the magnitude of any such benefits are likely to be small. The next potential benefit is due to the fact that, in some instances, NOx emissions may actually decrease ozone concentrations. This effect is fully accounted for by the AP2 model, thus it is already included in our estimates. © XXXX American Chemical Society
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ERRATUM The scale given in Figure 1 of Holland et al., (2016) is incorrect. The numbers should have been divided by 1.37. This error does not effect any of the calculations in the paper. Stephen P. Holland* University of North Carolina at Greensboro, Greensboro, North Carolina 27402, United States National Bureau of Economic Research, Cambridge, Massachusetts 02138, United States Erin T. Mansur Tuck School of Business at Dartmouth, Hanover, New Hampshire 03755, United States
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DOI: 10.1021/acs.est.6b01157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Correspondence/Rebuttal
National Bureau of Economic Research, Cambridge, Massachusetts 02138, United States
Nicholas Z. Muller Middlebury College, Middlebury, Vermont 05753, United States National Bureau of Economic Research, Cambridge, Massachusetts 02138, United States
Andrew J. Yates†
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University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
AUTHOR INFORMATION
Corresponding Author
†Phone 919-966-2383; fax: 919-966-4986; e-mail: ajyates@ email.unc.edu. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.est.6b01157 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
