Vehicle Emissions are regulated by two main agencies in The United States:

The United States Environmental Protection Agency (EPA)

Here are some links from EPA's web site that might be useful for research.

Light Duty Info:

Consumer Information

Regulations and Related Documents

On-Board Diagnostics

Vehicle Service Information

Vehicle Programs and Compliance Guidance Letters

Certification and Fuel Economy Information System

Motor Vehicle and Engine Compliance Program Fees Information

Importing Vehicles and Engines

Light-Duty Automotive Technology and Fuel Economy Trends: 1975 Through Present 

Related Links

Heavy Duty Info:

Heavy Trucks, Buses, and Engines
This website presents general information about heavy - duty highway engines, such as trucks and buses, and their emissions.

Emission Standards Reference Guide for Heavy - Duty and Nonroad Engines
Provides information on emission standards for nonroad engines and heavy - duty engines.

Clean School Bus USA
Provides information about Clean School Bus USA.

California Air Resources Board:

Here are some links from CARB's website that might be useful for research.

What the ARB Does

Programs for cleaner air range from research and regulation to enforcement and education. The ARB:

Sets and enforces emission standards for motor vehicles, fuels and consumer products.

Sets health-based air quality standards.

Conducts research.

Monitors air quality.

Identifies and sets control measures for toxic air contaminants.

Provides compliance assistance for businesses.

Produces education and outreach programs and materials.

Oversees and assists local air quality districts which regulate most non-vehicular sources of air pollution.