Characterization of the Miniaturized Inverted Flame Burner as a Combustion Source to Generate a Nanoparticle Calibration Aerosol

Abstract

Soot generators are able to produce carbonaceous nanoparticles purposefully and can therefore play a vital role in the calibration of particle instruments with an actual combustion aerosol. Condensation particle counters (CPCs) have become the instrument of choice for legislative measurements of the particle number (PN) concentration. The Euro 5B standard introduced by the European Union in 2011 was the first regulation that introduced a PN limit for the exhaust emission from light duty diesel vehicles. Since then, several other PN regulations for soot-emitting vehicles and combustion engines have been introduced or are currently in draft stages, all with similar requirements for the periodic calibration of the particle counter’s measurement accuracy. It is for this reason that combustion particles produced under laboratory conditions have become an attractive choice of calibration aerosol. Nonetheless, it is often difficult to generate a large amount of particles in the small nanometer range from a laboratory combustion source. In this study, we evaluated the performance of a recently introduced soot generator and its suitability for calibrating the counting efficiency and lower cut-off size of CPCs in the nanometer size range. We first characterized the soot generator’s warm up time to determine when it reaches a stable output when using propane as its fuel. We then investigated the influence of a dozen propane-to-air flow settings on the resulting particle size distribution of the combustion aerosol. Finally, we determined the resulting nanoparticle concentrations for 13 size classes below 20 nm in order to achieve a high size resolution at or near the lower detection limit of common CPCs. We performed our measurements under low-pressure conditions as our group operates CPCs onboard commercial passenger aircraft that are used as an atmospheric measurement platform. Another consideration is that CPCs are also operated elsewhere at much less than standard sea-level pressure. Examples include high-altitude research stations as well as engine test rigs used for vehicle exhaust emission testing or certification operated at elevated locations, e.g., in the USA, Mexico, and China. From these experiments, we concluded that when operating this novel soot generator with 7.5 SLPM airflow and 62.5 SCPM propane flow, it is possible to generate a realistic combustion aerosol for tests and calibrations that is still adequate even in the small nanometer size range. The concentrations measured under these operating conditions were just sufficient for detection with an aerosol electrometer, which is the concentration reference typically used in CPC calibrations.