Diesel soot mass calculation in real-time with a differential mobility spectrometer

This paper presents a methodology to allow a real-time particle size spectrometer to produce a mass concentration output by calculation from its electrical mobility response. As part of this, a Bayesian statistical algorithm for parametrising spectral data from the Cambustion DMS500 in terms of a number of lognormal functions is outlined, allowing the nucleation and accumulation modes of a Diesel aerosol to be treated separately and also to reduce mass calculation noise and improve spectral resolution. Previous literature is combined with new experimental results to develop a size:mass power-law relationship for this instrument. The effective density as a function of size for this instrument is found to be closer to that for water droplets than equivalent relationships for DMA/SMPS measurements in the literature, therefore making DMS500 mass calculation less susceptible to error from liquid adsorbed on agglomerates. The technique is validated with two Diesel engines against the gravimetric methods of filter paper and Diesel particulate filter (DPF) weighings. Good agreement is achieved over a variety of engine conditions, with the mean and standard deviation of the percentage difference of the calculated mass concentration from DPF weighings being - 12.1 ± 11.4 % and from filter paper weighings being - 15.1 ± 18.0 % . The calculated mass concentrations are systematically below those of the gravimetric methods.