Final answer:
The decrease in NOx emissions in the U.S. from 1970 to 2014 can be partially attributed to anti-pollution policies and technological advancements, including the implementation of the Clean Air Act and the evolution of automotive pollution control devices.
Step-by-step explanation:
The decrease in NOx emissions in the U.S. from 1970 to 2014 can be partially attributed to a combination of anti-pollution policies and technological advancements. One significant factor is the implementation of the Clean Air Act signed by President Nixon in 1970, which set new pollution standards and required industries to meet them within five years. This law also mandated automakers to reduce vehicle emissions by 90% and led to the inclusion of catalytic converters in cars, resulting in a reduction in smog. The use of catalysts in these devices helped alter the chemical properties of exhaust and decrease NOx emissions. Another contributing factor is the evolution of automotive pollution control devices over the years, which has led to more sophisticated and efficient engines that emit fewer air pollutants, including NOx.