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The volatile and hygroscopic properties of am- monium sulphate seeded and unseeded secondary organic aerosol (SOA) derived from the photo-oxidation of atmo- spherically relevant concentrations of -pinene were studied. The seed particles were electrospray generated ammonium sulphate ((NH4)2SO4) having diameters of approximately 33 nm with a quasi-mono-disperse size distribution (geomet- ric standard deviation g=1.3). The volatile and hygroscopic properties of both seeded and unseeded SOA were simul- taneously measured with a VH-TDMA (volatility - hygro- scopicity tandem differential mobility analyzer). VH-TDMA measurements of unseeded SOA show a decrease in the hy- groscopic growth (HGF) factor for increased volatilisation temperatures such that the more volatile compounds appear to be more hygroscopic. This is opposite to the expected preferential evaporation of more volatile but less hygroscopic material, but could also be due to enhanced oligomerisation occurring at the higher temperature in the thermodenuder. In addition, HGF measurements of seeded SOA were mea- sured as a function of time at two relative humidities, below (RH 75%) and above (RH 85%) the deliquescence relative humidity (DRH) of the pure ammonium sulphate seeds. As these measurements were conducted during the onset phase of photo-oxidation, during particle growth, they enabled us to find the dependence of the HGF as a function of the volume fraction of the SOA coating. HGF's measured at RH of 85% showed a continuous decrease as the SOA coating thickness increased. The measured growth factors show good agree-

Analysis of the hygroscopic and volatile properties of ammonium sulphate seeded and unseeded SOA particles