Nitrogen oxides are the primary air pollutant emitted by mobile sources, particularly diesel engines.

The Mobile Source Air Toxics (MSAT) Rule is the primary air quality standard that regulates emissions from mobile sources.

The primary goal of the MSAT Rule is to reduce emissions of hazardous air pollutants (HAPs) from mobile sources, such as benzene, formaldehyde, and diesel particulate matter.

Heavy-duty vehicles, such as trucks and buses, are subject to the most stringent emissions standards under the MSAT Rule due to their higher emissions of HAPs.

The Tier 2 Motor Vehicle Emissions Standards program is a federal program that aims to reduce emissions from new vehicles by setting stricter emissions standards for criteria pollutants, such as nitrogen oxides and particulate matter.

Methane is not a criteria pollutant regulated under the Clean Air Act, as it is not considered to be a major contributor to air pollution.

The Clean Air Act is a federal law that aims to protect human health and the environment from air pollution by regulating emissions from various sources, including mobile sources.

Oxygen sensors are not considered to be a type of mobile source emission control technology, as they primarily serve to monitor the oxygen content in the exhaust gases to optimize engine performance and fuel efficiency.

Retrofitting is the process of modifying existing vehicles to reduce their emissions, typically by installing emission control devices or upgrading to cleaner engine technologies.

While reducing air pollution from mobile sources can have many benefits, increased fuel efficiency is not directly related to air quality improvements.

Conversion refers to the process of modifying a vehicle to run on alternative fuels, such as electricity, natural gas, or hydrogen.

Gasoline is a traditional fossil fuel and not considered an alternative fuel in the context of reducing air pollution from mobile sources.

A hybrid electric vehicle (HEV) is a vehicle that uses both a traditional fuel engine and an electric motor to power the vehicle.

Hybrid electric vehicles (HEVs) are not considered to be fully electric vehicles, as they still rely on a traditional fuel engine in addition to an electric motor.

A battery electric vehicle (BEV) is a vehicle that runs solely on electricity stored in a battery.