The purpose of any catalytic converter [local] is to reduce harmful emissions from the exhaust of a combustion engine. A Two-Way converter oxidizes unburned harmful hydrocarbons and carbon monoxide into water and carbon dioxide. A Three-Way converter also reduces nitrous oxides into nitrogen and oxygen. A Three-Way + Air converter adds secondary air between the two internal catalyst substrates that improves the oxidation capabilities of the converter.

Converters may be leading to increase in ammonia [local] in the atmosphere, an undesirable outcome.