Approaches and methodologies for measurement of diesel exhaust self-pollution in school buses

Diesel exhaust self-pollution - the re-entrainment of exhaust gases and particulate from the tailpipe into the bus cabin - has been a controversial topic since the first measurements of this phenomenon were reported several years ago. Since then, a number of studies have been conducted around the US and Canada, with varied results, ranging from little or no self-pollution to very high levels.. A number of measurement approaches have been utilized, and it appears that some of the range in reported results may be a function of those methods. In general, direct measurement of diesel PM surrogates such as black carbon have shown high amounts of self pollution on older diesel buses when the windows were closed, and fuel tracer methods such as iridium have shown very low levels of self pollution, although generally this has been used for newer buses. However, eight buses were recently tested using both these different methods, with surprisingly similar results. This paper will examine these recent results in the context of other research tests that have been conducted over the last few years. A comparison of the methods used and results obtained will be made.