The improved steady state performance of engine control and aftertreatment makes the transient contribution to the total emissions increasingly important. As several control mechanism, e.g. the exhaust recirculation, are set for all cylinders at a time, misbalancing represents a practical limit to the achievable emission control. This paper tackles both issues at a time using a virtual soot sensor. The virtual soot sensor is based on piecewise principal components analysis (PCA) of the in-cylinder pressure measurements and on their representation by a polynomial structure which has been proposed in a previous work. The peaks detected by sensor during transient working condition cannot be confirmed as standard PM or soot measurement systems are too slow. In this paper we use an in-situ laser induced incandescence (LII) system to provide a fast soot reference and to validate the performance of the virtual soot sensor. The results confirm the ability of the proposed virtual sensor to recognize soot changes - also between individual cylinders - with sufficient speed without requiring any additional hardware.