Excimer laser fragmentation fluorescence spectroscopy for real-time monitoring of combustion generated pollutants.

摘要

Toxic pollutant emissions from combustion pose a hazard to public and environmental health. Better diagnostic techniques would benefit emissions monitoring programs and aid research aimed at understanding toxic pollutant formation and behavior. Excimer Laser Fragmentation Fluorescence Spectroscopy (ELFFS) provides sensitive, real-time, in situ measurements of several important combustion related pollutants. This thesis demonstrates the capabilities of ELFFS for detecting amines in combustion exhausts and carbonaceous particulate matter from engines.; ELFFS photofragments target species using a 193 nm excimer laser to form fluorescent signature species. The NH (A–X) band at 336 nm is used to monitor ammonia, ammonium nitrate and ammonium sulfate. There are no major interferences in this spectral region. The sensitivity is approximately 100 ppb (1 second measurement) for ammonia in post flame gases and 100 ppb (mole fraction) for ammonium nitrate/sulfate in ambient air. Quenching of NH by the major combustion products does not limit the applicability of the detection method.; Fluorescence from excited carbon atoms at 248 nm (1P 0 → 1S₀) following photofragmentation measures particulate matter in a two-stroke gasoline engine and a four-stroke diesel engine. Fluorescence from CH (A2Δ → X 2Π, 431 nm) C₂ (d3Π_g → a3Π_u, 468 nm) fragments is also observed. The atomic carbon fluorescence signal is proportional to the mass concentration of particles in the laser interrogation region. The 100-shot (1 second) detection limit for particles in the two-stroke gasoline engine exhaust is 0.5 ppb (volume fraction). The 100-shot detection limit for four-stroke diesel particulate matter is 0.2 ppb. Interferences from carbon monoxide and carbon dioxide are negligible. The ratios of atomic carbon, C₂, and CH peaks provide information on the molecular forms of compounds condensed on or contained within the particles measured. The C/C₂ signal ratio can be used to distinguish between two-stroke gasoline particulate matter and four-stroke diesel particulate matter.; ELFFS was used to investigate the effect of fuel type on particulate emissions from a four-stroke diesel engine. Both a low-sulfur and a Fischer-Tropsch fuel significantly reduced particulate emissions. An oxygenated diesel blend and an oxygenated Fischer-Tropsch fuel reduced particulate matter emissions by 30–70%.