Hygroscopic and Volatile Properties of Ultrafine Particles in the Eucalypt Forests: Comparison with Chamber Experiments and the Role of Sulphates in New Particle Formation

Simultaneous measurements of the volatile and hygroscopic properties of ultrafine particles were conducted in a Eucalypt forest in Tumbarumba, South-East Australia, in November 2006. These measurements were part of an intensive field campaign EUCAP 2006 (Eucalypt Forest Aerosols and Precursors). The particles exhibited a 2 step volatilisation with the first component starting to evaporate at temperatures above 50 degrees Celsius. With the onset of evaporation of the first component the hygroscopic growth factor increased. This indicated that the particle was composed of a less volatile, but more hygroscopic core, which was coated with a more volatile, but less hygroscopic, coating. The fraction of the more hygroscopic component was proportional to the measured maximum SO2 concentration indicating the role of gaseous H2SO4 in new particle formation. As the volatilisation temperature of the second more hygroscopic component was above that for H2SO4 it is likely that this component is partially or fully neutralised H2SO4. Comparison with pinene smog chamber experiments shows an excellent agreement with the first step volatilisation indicating its origin in the photooxidation of a monoterpene precursor.