Reducing nitrogen oxides in the exhaust gas of an internal combustion engine using a selective catalytic reduction (SCR) catalyst comprises injecting a urea-water solution as reducing agent upstream of the SCR catalyst, measuring the carbon dioxide concentration upstream and downstream of the injection point and adjusting the amount of reducing agent injected as a function of the difference between the two carbon dioxide concentrations. An independent claim is also included for apparatus for reducing nitrogen oxides in the exhaust gas of an internal combustion engine, comprising a selective catalytic reduction (SCR) catalyst (30), a metering device (15) for injecting a urea-water solution as reducing agent, a first carbon dioxide (CO2) sensor (32) upstream of the metering device, a second CO2 sensor (34, 34') downstream of the metering device, and a control unit (38) for controlling the metering device in response to a CO2 concentration difference measured by the two sensors.
