1. There is no adding up constraint, or cap, in attainment areas. Even if very source is in compliance with applicable rules, growth and the addition of sources could increase the number of standards violations (exceedance) and push a region into non-attainment. The air quality can even deteriorate further in non-attainment areas. New stationary sources must buy offsets exceeding their own emissions, but new mobile sources (just additional cars in many cases) dont have to buy offsets.
4. Some clunkers would not have survived the 2-3 year life of a MERC even if they had not been retired early. A guestimate of that natural attrition rate is used to shrink a MERC with the passage of time since the EMVR that created it.
5. According to Bishop et al (1993), selection bias - encouraging the retirement of the least valued (least driven) cars - is "the fatal flaw in conventional scrappage programs." Alberini et al (1994) found that while selection bias was a serious liability, EMVRPs could still produce emission reductions more cheaply than many existing and proposed policies.
7. The US EPA appears to have such a model, and they used it for their guidance document. However, predicted emissions vary only by model year. All cars of the same model year yield the same emissions. Since substantial variability exists, and has been documented by IEPA (1993), more detailed prediction capabilities would be necessary to permit the cessation of vehicle testing.
9. Emissions fees produce cost-effective results. The marginal control costs end up the same for each emitter. However, an emissions fee produces an efficient result only if the authorities keep the fee at the level where the marginal benefit of resulting emission reductions equals the marginal cost of those reductions. See Milliman and Prince (1989), Nelson (1987), and Welch (1983) for additional discussion of political (adoption), implementation, and dynamic properties of emission fees.