I'm not a chemist but a physicist. Can you tell me the chemistry of soot (carbon) removal from the DPF? Is the end product of regeneration not carbon dioxide? If so, are we not swapping one problem for another, or is the carbon dioxide yield trivial compared to that produced elsewhere in the engine?
Stan, Stroud, Gloucestershire
The exact chemical pathways may be complicated, but essentially you are right that regeneration involves the combustion of carbon-based soot to produce carbon dioxide (which is a greenhouse gas). However, it is not really swapping one problem for another because (a) the amount of carbon dioxide produced is tiny compared with that from the useful fuel combustion in the engine and (b) soot pollution can also contribute to global warming anyway.
But more importantly, it should be stressed that exhaust aftertreatment devices such as DPFs are primarily concerned with reducing local pollutants (those which are damaging to human health) rather than greenhouse gases. The DPF in particular is used to prevent particulates (soot) being emitted from the vehicle, which are implicated in a number of health concerns including asthma, lung problems and increased cancer risk.