Automotive exhaust emissions of polynuclear aromatic (C16+) hydrocarbons (PNA) were reduced by 65-70% by current emissions control systems and by about 99% by two experimental advanced emission control systems. At a given level of emission control, PNA emission was primarily controlled by fuel PNA content through the transient storage of PNA in engine deposits and their later emission under more severe engine operating conditions. A relatively minor contribution to PNA emission was made by PNA synthesized from lower molecular weight fuel aromatics, particularly C10-C14 aromatics.Deposit-related PNA emissions were linearly correlated with the PNA content of the deposit formation fuel. In comparison with a fuel of field-average PNA content (0.5 ppm benzo(a)pyrene), a field-maximum fuel (3 ppm) contained 4 to 7 times as much of three major PNA species and caused 3 to 5 times higher emissions of these species.PNA equivalent to 4 to 12% of the PNA present in consumed fuel accumulated in used engine oil, but the accumulation of PNA in engine oil did not cause a measurable increase in exhaust PNA emissions under normal oil consumption conditions. Large increases in PNA emission occurred when oil consumption was artifically increased to very high levels.Partial loss of some PNA species occurred by chemical reactions in the collection system. From 52% to 58% of the emitted benzo(a)pyrene and benz(a)anthracene and their alkyl derivatives were lost, but other, less reactive PNA species were not lost. Conclusions as to the relative effects of variables on PNA emission are not affected.