A particle sensing device is disclosed for sensing particles entrained in a gas, distribution of particle sizes including a first size range (e.g. PM2.5) and a second size range (e.g. PM10), larger than the first size range. A thermophoretic impulse is applied to the entrained particles. The device has a a first sensor, downstream of the thermophoretic impulse region. The thermophoretic impulse, the flow of gas and gravity combine to cause at least some of the particles to follow respective trajectories within the sampling volume. An interception unit is interposed between the thermophoretic impulse region and the first sensor, to intercept the respective trajectories of particles of the second size range but not respective trajectories of particles of the first size range. The first sensor is located to intercept and detect the respective trajectories of particles of the first size range.