Benzene, toluene and C2-benzene emissions of 4-stroke motorbikes: Benefits and risks of the current TWC technology

Chemical ionization mass spectrometry has been applied to determine benzene, toluene and C2-benzene emission rates of 4-stroke motorbikes. Extra emissions and duration of the cold start were deduced from the legislative urban driving cycle. The Common Artemis driving cycle was investigated to study the emission characteristics at transient driving from 0 to 135 km h−1. In addition, the benefits and risks of the currently available 3-way catalyst technology (TWC) are explored. Benzene, toluene and C2-benzene cold start emissions of 230–290, 920–980 and 950–1270 mg start−1 were obtained for the TWC motorbikes, exceeding those without catalyst by more than a factor of 3. At hot engine/catalyst, benzene, toluene and C2-benzene emission factors in the range of 10–140, 10–160 and 10–170 mg km−1 were found for the TWC motorbikes. Without catalyst, the corresponding emission factors were higher, varying from 40 to 260, 100 to 500 and 110 to 480 mg km−1, respectively. A comparison with the latest passenger car technology, with reported aromatic hydrocarbon (HC) emission factors of 0.2–3.0 mg km−1, revealed that the investigated 4-stroke motorbikes, indeed, are an important source of air pollution. Furthermore, cold start duration, driving distance under cold start influence and velocity dependence of aromatic HC emissions were deduced from time-resolved data. In addition, variations of aromatic HC mixing ratios were studied. Narrow and unimodal distributions of, e.g. benzene/C2-benzene mixing ratios with median values of 0.46–0.73 were found for all motorbikes but one. This motorcycle, equipped with a TWC, showed a broad and bimodal distribution with a median mixing ratio of 1.47. Catalyst-induced formation of benzene from alkylbenzenes is the assumed process, leading to increased benzene/alkylbenzene mixing ratios.