Evidence of increased mass fraction of NO2 within real-world NOx emissions of modern light vehicles — derived from a reliable online measuring method

Ambient roadside concentrations of nitrogen dioxide (NO2) have stabilized in recent years while concentrations of nitrogen oxides (NOx) decline. Oxidation catalytic converters of modern vehicles facilitating the formation of NO2 in the exhaust line, especially in diesel cars equipped with original equipment manufacturer (OEM) particle filters, are assumed to be responsible. NO2 is toxic and increased proportions of NO2 in total NOx in the atmosphere cause higher ambient ozone concentrations. These observations lead to a need for reliable emission factors for NO and NO2 for road vehicles, while only NOx is recorded in standard emission measurements. In this regard, it was recently shown that NO2 needs to be detected by an adequate online measuring method. The present work provides novel insight into these topics gained from an experimental campaign carried out with modern gasoline and diesel vehicles of certification categories Euro 3 and Euro 4. Reliable emission factors for NO and NO2 are presented for different driving situations, such as real-world driving, cold start and statutory tests, together with corresponding particle emission data. Highest emissions of NOx are recorded for diesel cars equipped with OEM particle filters with mass ratios of NO2 within NOx of up to 70%. The NOx emissions exceed the statutory emission limit and real-world emissions are even more pronounced, especially in urban driving conditions. Their particle emissions are greatly reduced, but the contribution of NO2 to soot oxidation is thought to be minor.