Chemical composition, Hygroscopicity and CCN properties of biogenic secondary organic aerosols

Studies using photochemical ?smog?, or aerosol, chambers can provide valuable insights into the complex multiphase processes leading to the formation and transformation of atmospheric particulates. As part of the Aerosol Coupling in the Earth System (ACES) project, a series of novel experiments were carried out at the Manchester Aerosol Chamber in order to investigate the chemistry and microphysics of the formation and transformation of biogenic secondary aerosols under realistic conditions. A selection of compounds covering a wide range of reactivity including isoprene (C5 H8), monoterpenes (isomeric formula C10 H16), sesquiterpenes (isomeric formula C15H24) and oxygenated VOCs have been studied in detail. The chemical composition of the formed SOA was measured on-line using an Aerodyne Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS). A hygroscopicity tandem differential mobility analyser (HTDMA) and a cloud condensation nuclei (CCN) counter were used to probe the hygroscopic properties and of the aerosols in the sub- and super-saturated regimes, respectively. A proton transfer mass spectrometer was used to study the evolution of the gas phase oxidation products. This paper presents a comprehensive overview of the chemical composition, hygroscopicity and cloud condensation nuclei (CCN) properties secondary organic aerosols formed from five structurally different biogenic VOCs (?-caryophyllene, limonene, myrcene, linalool, ?-pinene), and compares them to those of SOA formed from the photooxidation of real plant emissions (Silver Birch). A discussion of results of the use of organic (Hamilton et al., 2010) and inorganic seed will be presented and compared to those obtained from nucleation experiments.Results obtained using the ToF-AMS showed that SOA formed from the above precursors have a wide range of chemical properties, as expressed by the fraction of mass fragment 44 (a typical marker for highly oxygenated organic molecules) and 43 (a possible marker for less oxygenated organic molecules) to the total organic signal. Furthermore, the effect of photochemical ageing on those properties was not uniform across all five precursors. A link of these results to the findings of the hygroscopic properties of the same SOA particles and their CCN behavior will be presented and discussed.This work was supported by the UK Natural Environment Research Council (NERC) through the Aerosol Properties, PRocesses And InfluenceS on the Earth's climate (APPRAISE) programme. M. Rami Alfarra is supported by NERC?s National Centre for Atmospheric Science (NCAS). Hamilton, J.F., Alfarra, M.R., Wyche, K.P., Ward, M.W., Lewis, A.C., McFiggans, G.B., Good, N., Monks, P.S., Carr, T., White, I.R. and Purvis, R.P. (2010) Atmos. Chem. Phys. Discuss., 10(10), 25117-25151.