Nitrogen dioxide (NO2) concentrations in European city street air have not decreased after year 2000 in spite of stringent Euro 3 and Euro 4 NOx limits for diesel passenger cars.
NO2 emissions from modern diesel vehicles are caused by platinum catalysed Diesel Oxidation Catalysts (DOC) and platinum catalysed Diesel Particulate Filters (cDPF). The NO2 formed on DOC and cDPFs are used for passive soot regeneration but the excess of NO2 out of the filter is not controlled.
A Euro 4 diesel passenger car equipped with a DOC and a palladium base metal cDPF was compared with DOC plus a platinum based cDPF using NEDC test cycle with both cold and hot start, FTP-75, and Artemis test cycles. Emissions of NO2 and NOx were measured online in the raw exhaust, and with standard bag sampling method.
Relative to cold NEDC the NOx and NO2 levels increased with a warm engine. NOx emissions increased by a factor of 1.3 for the warm NEDC as well as for the FTP-75 tests, and by factors of 2 to 6 for the Artemis tests which represent real-world driving with high accelerations and high speed. In all tests, the NO2 levels for the Palladium cDPF were lower than for the Platinum cDPF, reduced by 30 to 80 % under similar test conditions. The Euro 4 regulated limits for CO, HC and PM were met for both types of cDPF. The platinum free catalytic coating on diesel filters can significantly help to reduce the high NO2 levels in city streets.