PNNL

Abstract

Particle volumes are most often obtained by measuring particle mobility size distributions and assuming that the particles are spherical. These volumes are then converted to mass loads by using particle densities that are commonly either assumed or estimated from the measured mobility and vacuum aerodynamic diameters assuming again that the particles are spherical. Depending on the system, these assumptions can introduce significant errors. We present a new method that can be applied to any particle system to determine in real-time whether the particles are spherical or not. We use our 2nd generation single particle mass spectrometer (SPLAT II) to measure with extremely high precision the vacuum aerodynamic size distributions of particles classified by differential mobility analyzer (DMA) and demonstrate that the line shape of these distributions provide a way to unambiguously distinguish between spherical and aspherical particles. Moreover, the very same experimental system is used to obtain in addition to individual particle size, its density, composition and dynamic shape factor. We illustrate the application of this method to secondary organic aerosols formed as a result of ozonolysis of a-pinene in the presence and absence of an OH scavenger and find these particles to be spherical with densities of 1.198±0.004 gcm-3 and 1.213±0.003 gcm-3 respectively.