The evolution of US NOx standards for cars

Dan Farber at Legal Planet recently posted on "Cars, Smog, and EPA". An excerpt:

For the first 20 years of federal regulation, Congress set the NOx [nitrogen oxides] standards for new cars itself. That’s quite different from the standards for industrial pollution sources, which Congress has always delegated to EPA. The reason may have been the high political stakes in the car industry or the relatively easier task of setting standards for new products in a single industry using a single energy process.

East River and Manhattan Skyline in Heavy Smog (Chester Higgins, Jr., EPA, 1973)

... The initial standard, set in the 1970 Clean Air Act, was 3.1 grams per mile (gpm) for NOx. Achieving that standard was thought to be nearly impossible when Congress created this mandate, but the mandate forced the car companies to make technological breakthroughs with catalytic converters.

Note that the standard is set in terms of pollution per mile rather than pollution per gallon. That means that any increase in fuel efficiency automatically helps a company meet the pollution standards as well. Theoretically, a car could have no pollution control at all but get such phenomenal mileage that it met the pollution standard. Actual mileage has improved but not enough to obviate the need for pollution controls.

Congress adjusted the standards twice. A 1977 amendment reset the limit to 1 gpm in 1981. The 1990 amendments changed the standard to 0.6 gpm, effective in 1994. These are called Tier 1 standards. Apparently Congress did not relish the task of periodically resetting the standards itself  The 1990 Amendments authorized EPA to set standards for 2004 and beyond.

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This has to be considered a successful regulatory program. The Biden proposed standard  [of 2023] allow less than 1% of the pollution levels that Congress mandated in 1970.

Still, the program has an inherent limitation: it addresses pollution per mile, but not the number of miles driven.  That has approximately tripled since 1970....

As a result of the growth in vehicle use, the improvements in vehicle pollution control have not resulted in proportional improvements in air quality. Smog has definitely gotten better in urban areas, but not nearly as much as it might have, if we had more compact cities and heavier use of public transportation. Given the limited tools that Congress gave it, however, EPA has achieved impressive results.