Selective catalytic reduction (SCR) uses urea injection to reduce which type of emissions?

Selective catalytic reduction (SCR) is a technology used to reduce nitrogen oxides (NOx) emissions from the exhaust of diesel engines. The process involves the injection of urea, often referred to as diesel exhaust fluid (DEF), into the exhaust stream. When the urea is introduced, it reacts with the NOx gases in the presence of a catalyst, converting them into nitrogen and water—products that are far less harmful to the environment.

In the context of diesel engine emissions, NOx gases are a significant concern because they contribute to air pollution and the formation of smog. By effectively reducing these emissions, SCR systems play a crucial role in helping diesel vehicles meet stringent emissions regulations while improving overall air quality.

The other types of emissions listed—carbon dioxide (CO2), sulfur dioxide (SO2), and particulates—are addressed by different technologies or methods. CO2 is a greenhouse gas and is typically reduced through improved fuel efficiency rather than SCR. Sulfur dioxide emissions are usually managed through the use of low-sulfur fuels and oxidation catalysts, while particulate emissions are often reduced with particulate filters. Hence, SCR is specifically designed for the reduction of NOx emissions.