An apparatus for determining particle characteristics, comprising: an inlet and an outlet; a flow system for generating a flow path between the inlet and the outlet along which an accelerating flow is to be provided containing particles to be analyzed, wherein the apparatus is adapted to provide said accelerating flow along the flow path; a light source for providing light to the flow path, wherein the envelope of the light defines a measurement zone of the flow path with continuous illumination, the measurement zone having a path length of more than 1cm; a light detector for detecting light received from the measurement zone which has been scattered by the particles; and a controller for analyzing the detected received light, wherein the controller is adapted, for a particle of the particles, to: determine a time duration for which the particle remains in the measurement zone based on a width of a detected received light intensity pulse relating to the transit of the particle through the measurement zone, and thereby determine an effective aerodynamic particle diameter of the particle; determine a peak detected received light intensity and thereby determine an effective optical particle diameter of the particle; analyze a level of variation of the detected received intensity over time and thereby derive a particle shape parameter of the particle as a first further particle parameter; and determine a particle density of the particle from the effective aerodynamic particle diameter, the effective optical particle diameter and the particle shape parameter, as a second further particle parameter. an inlet and an outlet; a flow system for generating a flow path between the inlet and the outlet along which an accelerating flow is to be provided containing particles to be analyzed, wherein the apparatus is adapted to provide said accelerating flow along the flow path; a light source for providing light to the flow path, wherein the envelope of the light defines a measurement zone of the flow path with continuous illumination, the measurement zone having a path length of more than 1cm; a light detector for detecting light received from the measurement zone which has been scattered by the particles; and a controller for analyzing the detected received light, wherein the controller is adapted, for a particle of the particles, to: determine a time duration for which the particle remains in the measurement zone based on a width of a detected received light intensity pulse relating to the transit of the particle through the measurement zone, and thereby determine an effective aerodynamic particle diameter of the particle; determine a peak detected received light intensity and thereby determine an effective optical particle diameter of the particle; analyze a level of variation of the detected received intensity over time and thereby derive a particle shape parameter of the particle as a first further particle parameter; and determine a particle density of the particle from the effective aerodynamic particle diameter, the effective optical particle diameter and the particle shape parameter, as a second further particle parameter. determine a time duration for which the particle remains in the measurement zone based on a width of a detected received light intensity pulse relating to the transit of the particle through the measurement zone, and thereby determine an effective aerodynamic particle diameter of the particle; determine a peak detected received light intensity and thereby determine an effective optical particle diameter of the particle; analyze a level of variation of the detected received intensity over time and thereby derive a particle shape parameter of the particle as a first further particle parameter; and determine a particle density of the particle from the effective aerodynamic particle diameter, the effective optical particle diameter and the particle shape parameter, as a second further particle parameter.