The chemical composition of secondary organic sub-50 nm diameter particles (nanoparticles) is among the key measurements required for understanding the processes responsible for atmospheric nucleation and subsequent growth. While aerosol mass spectrometry has typically been restricted to particle sizes greater than 50 nm due to sampling challenges, few techniques exist for investigating particle composition in a size range close to where nucleation and growth by organic vapours takes place. With the newly developed high-resolution time-of-flight (HTOF) Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS, e.g., Voisin et al., 2003) we are now able to obtain highly resolved mass spectra from sampled nanoparticles in the size range from 10 nm to 80 nm. Thereby we can directly measure chemical compounds responsible for the growth of particles formed from atmospheric nucleation. HTOF-TDCIMS comprises of a particle collection and thermal desorption unit coupled to a chemical ionization HTOF mass spectrometer. At the inlet, the aerosol is charged and size classified by two DMAs (McMurry et al., 2009) and subsequently collected on a charged platinum filament. After collection (typically in the order of ~15 minutes, depending on particle size) the filament is translated into the ion source region, where the collected material gets thermally desorbed by resistively heating the filament. Neutral species desorbed from the particles are ionized by both positively and negatively-charged reagent ions; the resulting ions are alternately analysed in the mass spectrometer. It is notable that by ramping the temperature of the wire valuable information on the volatility of the substances is obtained (Smith and Rathbone, 2008). In this study we investigated biogenic nanoparticles in the atmosphere and laboratory. During summer 2011 we participated in the BEACHON-RoMBAS (Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen - Rocky Mountain Biogenic Aerosol Study) field campaign at the Manitou Forest Observatory (MFO) near Woodland Park, CO. The site is located in a ponderosa pine dominated forest at an elevation of roughly 2400 m above sea level. The remote location relatively far from direct emission sources typically provides clean conditions with substantial monoterpene and sesquiterpene concentrations. New particle formation events starting around noon were observed frequently and particle diameters around 10-15 nm were found at the low end of the size distribution. Correspondingly, we focused on 20 nm particles as the smallest size for the TDCIMS measurements. At this size ambient nanoparticles contained detectable levels of inorganic species such as sulphate, nitrate, and ammonium. In addition, several organic substances were found in significant amounts. The high mass resolution of the HTOF-TDCIMS allows for the identification of the molecular formulae of detected ions. In order to be able to attribute the organic signals observed at MFO to corresponding precursor gases we conducted laboratory SOA formation studies using both the NCAR biogenic aerosol chamber as well as a flow tube. Different kinds of monoterpenes and sesquiterpenes representative for MFO conditions were oxidized by ozone under dark conditions. Little difference was found between the particle compositions in the chamber and flow tube experiments, respectively, although reaction rates varied considerably. Figure 1 illustrates a mass spectrum of positive ions obtained from 10 nm particles formed from the oxidation of a-pinene in the flow tube. As can be seen a series of peaks separated by m/z-differences of typically 14 amu is observed. While many species as well as a similar pattern were found from the ambient measurements it is likely that compounds from other precursor gases contribute to the ambient signal as well. Current work is focused on identifying these ions, and ultimately the precursors and mechanisms responsible for the observed growth. Voisin et al., Aerosol Sci. Technol. 37, 471-475 (2003). McMurry et al., Environ. Sci. Technol. 43, 4653 (2009). Smith & Rathbone, Int. J. Mass. Spectrom. 274, 8 (2008).