Bounce characteristics of α-pinene-derived SOA particles with implications to physical phase.

Electrical Low Pressure Impactor (ELPI) was used to study the physical phase of fresh secondary-organic-aerosol (SOA) particles formed by the ozonolysis of pure α-pinene in a chamber. The particles bounced from smooth impactor plates towards lower impactor stages, indicating a solid physical state. These particles had a similar bouncing ability as Scots pine-derived particles in previous studies. The measured bounce factor of large particles (diameter > 40 nm) did not significantly change during the particle growth, indicating no changes in the particle solidity. For the smallest particles, the calculated bounce factor increased as the particles grew. The smallest particles were less solid (i.e. having lower viscosity) than the larger ones. The maximum value of the bounce factor decreased for consequent impactor stages. According to a simplified model, this can be explained by a combination of bounce probability and charge transfer between the particles and the impactor surface if at least 50% particles are bouncing. [ABSTRACT FROM AUTHOR]