Final answer:
Between 1970 and 1996, Nitrogen oxides (NOx) saw increased emissions in the United States due to the growth in vehicle use and industrial activities, despite advancements in pollution control policies.
Step-by-step explanation:
Between 1970 and 1996, the pollutant that saw increased emissions in the United States was Nitrogen oxides (NOx). Nitrogen oxides, including nitrogen dioxide (NO2), are part of a group of highly reactive gases emitted from combustion engines found in vehicles and power plants. While there was significant progress made in reducing other pollutants during this time, NOx was one of the notable exceptions.
Nitrogen oxides are primary pollutants, which play a significant role in the formation of secondary pollutants like ground-level ozone. Ozone itself is not emitted directly but is formed through chemical reactions between Nitrogen oxides and volatile organic compounds (VOCs) in the presence of sunlight. This reaction leads to the production of ozone, which is a major component of smog and can cause various health problems, especially on hot sunny days in urban environments.
The increase in Nitrogen oxide emissions during the specified period can be attributed to the growth in vehicle use and industrial activities. Despite advancements in pollution control policies and technologies, the emissions of NOx increased as the number of combustion sources, such as vehicles and industrial machinery, grew. The regulatory efforts in reducing lead and other emissions have been successful, but the struggle with NOx remains significant due to its formation mechanisms and the challenges associated with controlling a larger number of dispersed sources.