At engine loads of 5% and 10% a perhaps more surprising behaviour
was observed. The negative conversions of NO to NO2 indicate
that, in fact, some of the NO2 present in the exhaust (Fig. 5) was
converted to NO. It is important to bear in mind that, while the percentage
conversions appear quite high, the absolute conversions
are in fact modest (Table 2). This type of behaviour has been previously
reported for the CRT used in diesel automotive applications,
and in fact corresponds to the reaction of NO2 with
carbonaceous particulate matter (PM) [48]. It should be noted that
PM concentrations were highest on engine start-up and at low
loads, which is consistent with the occurrence of this phenomenon
at low engine loads.