So the big deal with the Diesel[scandal] is that Nox emissions are off the chart. Nox, or Mono-Nitrogen Oxide, is a byproduct of the lean oxygen free combustion process of diesels generally and its pretty bad stuff. It forms smog and ozone and has been linked to health effects from aspiration issues (brochitus, athsma, etc) to mutation...yeah not to mention the formation of Nirtic Acid (i.e. acid rain). BUT, it does have upside

“NOx emissions also causes global cooling through the formation of OH groups that destroy methane molecules, countering the effect of greenhouse gases. The effect can be significant. For instance, according to the OECD “the large NOx emissions from ship traffic lead to significant increases in hydroxyl (OH), which is the major oxidant in the lower atmosphere. Since reaction with OH is a major way of removing methane from the atmosphere, ship emissions decrease methane concentrations. (Reductions in methane lifetimes due to shipping-based NOx emissions vary between 1.5% and 5% in different calculations).” “In summary, most studies so far indicate that ship emissions actually lead to a net global cooling. This net global cooling effect is not being experienced in other transport sectors....”

“However, it should be stressed that the uncertainties with this conclusion are large, in particular for indirect effects, and global temperature is only a first measure of the extent of climate change in any event.”[13]

So all in all, Nox = bad. But it is does have some upsides.