See "Carcinogenicity of diesel-engine and gasoline-engine exhausts and some nitroarenes" at Lancet Oncology.
Diesel exhaust is elevated to Group 1 while gasoline exhaust is 2B. Most of the nitroarenes are 2B though 6-nitrochrysene was elevated to 2A. Clearly the thinking is that exhaust particulates from which tars can be extracted are what's driving the increased lung cancer risk as the exhaust gases alone showed little to no sign of genotoxicity. Regarding the epidemiology, some found cumulative dose to be associated with risk while others found length of exposure, irrespective of dose, to be driving the association. Only one found a big increase in risk and that was only among those most exposed.
Expect to see more lung cancer claims by truckers, railroad engineers and others routinely exposed to diesel exhaust in their work.