Measurement of nanoparticles of organic carbon in non-sooting flame conditions

In this work we compare the results of several nanoparticle measurement techniques with the aim of investigating the formation of nanoparticles in non-sooting to slightly sooting flames. In slightly sooting conditions there is quite good agreement between Differential Mobility Analyser (DMA), Atomic Force Microscopy (AFM), and optical measurements on particle size and concentration. However, in rich flames below the onset of soot, DMA measures a strong drop-off in the total particle volume fraction at low fuel to air mixtures, which is not observed in optical or AFM measurements that detect a more gradual decrease in particle concentration with decreasing C/O and almost constant spectroscopic properties. The disagreement is significantly larger than experimental error and is only observed when the particle size distribution includes solely particles smaller than about 3 nm. Particle losses in the DMA sampling system does not seem to be the only possible reason for justifying the discrepancy with the other techniques. Further investigations are necessary in order to characterize chemically and physically this class of nanoparticles which constitute the earliest stage in the formation of particulate carbon.