"Biogenic volatile organic compounds (BVOCs) play an important role in atmospheric chemistry and give rise to secondary organic aerosols (SOA), which have effects on climate and human health. A considerable lack of knowledge exists concerning the formation of new particle mass from BVOCs and the organic chemical composition of natural aerosols. Laboratory chamber experiments have been performed during several decades in an attempt to mimic atmospheric SOA formation. However, it is not entirely clear how close the aerosol particles generated in laboratory smog chamber experiments resemble atmospheric SOA particles with respect to their chemical composition. The objective of the current proposal includes determination of the elemental composition of secondary organic aerosols (SOAs) from both laboratory and ambient samples using novel ultrahigh resolution mass spectrometry. For reaching this objective, we will examine archived and newly collected aerosol samples from both laboratory experiments (i.e., performed at smog chamber facilities) and field campaigns at forested European sites (i.e., a boreal forest site, Hyytiälä, Finland, and a rural site, Auchencorth, United Kingdom) which exhibit large concentrations of biogenic SOA. Aerosol samples will be extracted in water and then directly analyzed with the ultra-high resolution Orbitrap MS. The elemental composition of detected compounds will be determined using Kendrick mass analysis methods. The influence of different smog chamber operation conditions (e.g., ozonolysis vs. photo-oxidation) on the elemental composition of SOA components will be evaluated. The elemental composition of the complex compound mixtures from SOA samples generated under various laboratory conditions will be compared with field samples using statistical data analysis methods and will enable to determine for the first time which laboratory SOA systems reproduce ambient aerosols most closely with respect to their detailed chemical composition."