Hygroscopic properties of aerosol formed by oxidation of limonene, alpha-pinene, and beta-pinene

The hygroscopic properties of aerosol formed by oxidation of three monoterp enes, limonene, alpha-pinene, and beta-pinene, were measured using a tandem differential mobility analyzer (TDMA). The experiments were performed in t he European Photoreactor (EUPHORE) in Valencia, Spain. The experiments incl uded ozonolysis and photooxidation with and without ammonium sulfate seed a erosol. Pure organic particles, formed by oxidation of the terpenes in the absence of the seed aerosol, proved to be slightly hygroscopic. The hygrosc opic growth factor (G) was close to 1.10 at relative humidity 84% +/- 1%, w hich is often observed as the G of the less hygroscopic mode of atmospheric aerosol in field measurements. In the experiments with ammonium sulfate se ed aerosol G decreased from approximately 1.5 before the start of terpene o xidation to approximately 1.1 as the oxidation products condensed on the pa rticles. G was not proportional to the organic layer thickness but decrease d with increasing organic volume fraction. Our analysis shows that in the i nternally mixed particles, ammonium sulfate and the organic products take u p water independently of one another.