Secondary Organic Aerosols: Chemical Aging, Hygroscopicity, and Cloud Droplet Activation

Atmospheric aerosols have an important impact on the radiation balance, and thus, on the climate of the Earth. Aerosol particles scatter and absorb incoming solar and terrestrial radiation. Apart from this direct effect, aerosol particles act as cloud condensation nuclei (CCN), thereby greatly influencing the microphysics of clouds. Secondary organic aerosols (SOA) are an important fraction of the total aerosol mass. In many environments these organic compounds are mainly products of the oxidation of biogenic volatile organic compounds (VOC). In this study the hygroscopic growth and CCN activation of biogenic SOA were investigated which was formed by the oxidation of VOC with O$_{3}$ and photochemically formed OH radicals under low NO$_{x}$ conditions. For this purpose, a complex mixture of VOC emitted by boreal tree species as gas-phase precursors was used in the Jülich Plant Atmosphere Chamber (JPAC). In long-term studies in the atmosphere simulation chamber SAPHIR $\alpha$α-pinene or a defined mixture of $\alpha$-pinene, $\beta$-pinene, limonene, ocimene,$\Delta$-3-carene served as precursors. Initial precursor concentrations between 40 and 1000 ppbC were investigated. The observed SOA particles were slightly hygroscopic with an average hygroscopicity parameter $\kappa$(CCN) = 0.10 ± 0.02 and $\kappa$(90%RH) = 0.05 ± 0.01. Closure between hygroscopic growth and CCN activation data could be achieved allowing either surface tension reduction, limited solubility, or non-ideality of the solution in the droplet. The SOA solutions in equilibrium with RH