Examining the Relationship Between Black Carbon and Soot in Flames and Engine Exhaust

This paper investigates the black carbon (BC) content of soot formed in premixed and diffusion flames and emitted by light duty gasoline and diesel vehicles. BC is measured photoacoustically and compared to particulate mass collected by filter and calculated from particle size distributions. The BC fraction of soot from rich premixed ethylene flames increases with height above the burner, but can remain well below unity in modestly sooting flames. The BC fraction produced by a propane diffusion flame soot generator (CAST) falls as the fuel is diluted with nitrogen, the principal means used to adjust the desired particle size. Thermally treating the soot to remove possible condensed semivolatile species does little to change these trends. TEM images show that despite low BC content, these particles display the characteristic fractal-like agglomerate morphology of soot. Particle mass spectra reveal PAH and fullerene fragments associated with low BC soot, which disappear as the BC fraction approaches unity. The results suggest that low BC content reflects immature solid soot which has not carbonized. Particulate matter (PM) measurements from current technology diesel and gasoline vehicles exhibit a high, >80% BC fraction. This is attributed to effective soot carbonization during the expansion and exhaust strokes of the engine, and to the substantial reductions of condensable hydrocarbons by catalytic aftertreatment. These results are discussed with respect to using light absorption based instruments to monitor engine exhaust PM and using flame generated soot for PM instrument calibration.