Particle size distribution functions (PSDF) and mean particle sizes have been determined in a laminar premixed ethylene/air flame with three different experimental approaches: photo-ionization mass spectrometry (PIMS), scanning mobility particle sizing (SMPS), and laser-induced incandescence (LII). The main goal of this investigation was the crossvalidation of these three methods used at our institute for the determination of particle sizes in a great variety of flames or exhaust gases. We found good agreement between the three methods in the ranges where they are comparable as well as a complementary behavior for the different size ranges. PIMS and SMPS are able to measure the particle size distribution functions with good resolution. PIMS is favorable in detecting the smallest particles (<6 nm) and thereby able to detect even bimodal distributions of the soot precursor particles. SMPS and LII are suitable in the mid-and upper range of the particle sizes (>2 nm and >3 nm, respectively). LII offers the particular advantage of being a non-intrusive method. This makes it applicable in extreme environments, such as high pressure flames, as well as in very sensitive flames because no probe is needed.
The soot precursor particles reported in the literature to date may be roughly divided into two classes. Some of their features are very different. Using photoionization mass spectrometry behind premixed atmospheric ethylene/air flames, particle mass distribution functions were measured for these precursor particles. Within a limited C/O range bimodality was found, i.e. two types of particles are formed simultaneously. Through analysis of the photoionization behaviour it was found that the ionization order (IO) is different for these two modes as is the stability of the respective particles. In accordance with earlier measurements, particles with IO = 1 are interpreted as polyaromatic hydrocarbon (PAH) stacks whereas the IO = 2 particles rather seem to be large molecules. This is consistent with the different particle classes mentioned above. Their potential role in soot formation is briefly addressed.
This series aims at the measurement of flame generated nanoparticles and an assessment of their biological effects. In the present part a brief review is given on previous papers dealing with nanoparticles with emphasis on detection through various methods. A more detailed account is provided of own measurements using photo ionization mass spectrometry. This includes photo fragmentation, spectra of low pressure flames, coagulation effects, and nanoparticle measurements in the exhaust gas of a sooting normal pressure premixed flame well outside of the visible zone. A model on nanoparticle formation is discussed as are its implications on emissions of vehicle engines.
Abstract. Based on photoionisation mass spectrometry two types of experiments were carried out. (i) In a fast flow reactor coupled to a low pressure flame as a particle source, rate coefficients for the coagulation of primary nanoparticles were measured through variation of the reactor residence time. The results are kc (350K) = 3.5x10-10cm3/s and kc (573K) = 1.1x10-9cm3/s, i.e. very high rate coefficients. It was also shown that coagulated nanoparticles can have masses beyond 50ku, corresponding to equivalent diameters between 4 to 5nm. These particles are easily fragmented during photoionisation. (ii) Using a second and mobile photoionisation mass spectrometer equipped with a fast flow inlet system, measurements were carried out behind three different vehicle engines, a two-stroke scooter engine, a four-stroke motorbike engine and a DI (direct injection) gasoline research engine. In all cases ion signals around 1000u were found that are clearly dependent on engine conditions. In the case of the DI engine, they correlate with the smoke number. These signals cannot be explained by PAHs due to their low volatility at the respective masses. Major contributions of soot or droplet fragmentation were ruled out through additional experiments using a heated inlet line and a filter. Consequently, these signals are interpreted as fragments of coagulated nanoparticles.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.