Measurement of smoke concentration using degenerate four-wave mixing

Degenerate four-wave mixing (DFWM) was successfully used to monitor a wide range of smoke concentrations (0.1-10 mg m -3 ) in sample cells. To the authors' knowledge, this is the first measurement of smoke by DFWM. The DFWM method is very sensitive, measuring down to ~0.1 ppb soot volume fraction. To verify the visible laser DFWM system, NO 2 concentrations from 2 to 200 ppm were measured with a sensitivity of 2 ppm of NO 2 . Both smoke and NO 2 species were resonantly pumped at 500 nm by an excimer-pumped dye laser (~2 mJ/pulse). The DFWM signal from both NO 2 and smoke showed a squared dependence on concentration. Single-pulse measurements of NO 2 and smoke in sample cells indicate that temporally and spatially resolved in situ DFWM measurements are feasible for engine exhausts.