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 similar to0.1 ppb soot volume fraction. To verify the visible laser DFWM system, NO2 concentrations from 2 to 200 ppm were measured with a sensitivity of 2 ppm of NO2. Both smoke and NO2 species were resonantly pumped at 500 nm by an excimer-pumped dye laser (similar to2 mJ/pulse). The DFWM signal from both NO2 and smoke showed a squared dependence on concentration. Single-pulse measurements of NO2 and smoke in sample cells indicate that temporally and spatially resolved in situ DFWM measurements are feasible for engine exhausts.