Highly oxygenated organic molecules (HOMs) are important sources of atmospheric aerosols. Resolving the molecular-level formation mechanisms of these HOMs from freshly emitted hydrocarbons improves the understanding of aerosol properties and their influence on the climate. In this study, we measure the electrical mobility and mass-to-charge ratio of α-pinene oxidation products using a secondary electrospray-differential mobility analyzer-mass spectrometer (SESI-DMA-MS). The mass-mobility spectrum of the oxidation products is measured with seven different reagent ions generated by the electrospray. We analyzed the mobility-mass spectra of the oxidation products C 9–10 H 14–18 O 2–6 . Our results show that acetate and chloride yield the highest charging efficiencies. Analysis of the mobility spectra suggests that the clusters have 1–5 isomeric structures (i.e., ion-molecule cluster structures with distinct mobilities), and the number is affected by the reagent ion. Most of the isomers are likely cluster isomers originating from binding of the reagent ion to different sites of the molecule. By comparing the number of observed isomers and measured mobilities and collision cross sections between standard pinanediol and pinonic acid to the values observed for C 10 H 18 O 2 and C 10 H 16 O 3 produced from oxidation of α-pinene, we confirm that pinanediol and pinonic acid are the only isomers for these elemental compositions in our experimental conditions. Our study shows that the SESI-DMA-MS produces new information from the first steps of oxidation of α-pinene.

<p>OXIDATION PRODUCTS OF ALPHA-PINENE AND THEIR ELECTRICAL MOBILITIES<br>A. SKYTTÄ 1 , L. AHONEN 1 , R. CAI 1 and J. KANGASLUOMA 1<br>1 Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of<br>Helsinki, Helsinki, 00140, Finland</p><p>α-pinene C10H16 is a monoterpene emitted by vegetation and its low volatile oxidation products are important source for secondary organic aerosols (SOA) in the atmosphere (Ehn et al., 2014). Because of the significant amount of α-pinene in the atmosphere, we investigated the oxidation<br>products of α-pinene.</p><p>In our setup we used parallel plate DMA (SEADM; (de la Mora et al., 2006)) at mobility resolution of about 80 coupled with APITOF-MS (Tofwerk AG; (Junninen et al., 2010)) and a flow tube system. A DMA can be used to measure the electrical mobility of the molecule or cluster and mass<br>spectrometer to measure the mass of those clusters. Based on the mass the chemical composition of the cluster can be determined.</p><p><br>The electrospray solution is sprayed through a thin capillary into the chamber through which neutral<br>sample is passed through. As a solute we used NaNO3 , NaI, LiCl and CH3CO2K dissolved in<br>methanol all charged in positive and negative mode. Particles that are charged by reagent ions are<br>led into the DMA via narrow inlet slit.</p><p><br>α-pinene was evaporated into a carrier gas flow and then oxidized using ozone produced from synthetic air with UV-light. The oxidation products are detected by charging them with ions sprayed from the electrospray solution and then directed into the DMA chamber. α-pinene oxidation products of oxidation state C10H16O2−7 were detected with almost all charger ions. Also, other products with different amounts of carbon and hydrogen were detected. Measurements made in negative mode were much more clear and because of this concentrated to examine them.</p><p><br>Mobility provides information on the structure of the compound. One cluster can have multiple peaks in the mobility spectrum if it has multiple different structures. In the mobility spectrum of C10H16O3 charged with NO3− we observe two peaks clearly separate mobility peaks that likely<br>correspond to two different structural isomers of the compound. We will present analysis of the mobility-mass measurements of α-pinene oxidation products, from where structural information will be obtained when combined to chemical reaction pathways and modeling of the electrical mobilities from the calculated structures.</p><p><br>REFERENCES<br>Ehn, M. et al, (2014). A large source of low-volatility secondary organic aerosol. (Nature, 506(7489), 476-+.<br>doi:10.1038/nature13032).</p><p><br>Fernández de la Mora et al, (2006). The potential of differen-<br>tial mobility analysis coupled to MS for the study of very large singly and multiply chargedproteins and protein complexes in the gas phase.<br>doi:10.1002/biot.200600070). (Biotechnology Journal, 1(9), 988-997.</p><p><br>Junninen, H. et al, (2010). A high-resolution mass spectrometer<br>to measure atmospheric ion composition. (Atmospheric Measurement Techniques, 3(4), 1039-<br>1053. doi:10.5194/amt-3-1039-2010).</p>

Condensation and evaporation of vapor species on nanoparticle surfaces drive the aerosol evolution in various industrial/atmospheric systems, but probing these transient processes is challenging due to related time and length scales. Herein, we present a novel methodology for deducing nanoparticle evaporation kinetics using electrical mobility as a natural size indicator. Monodispersed nanoparticles are fed to a differential mobility analyzer which serves simultaneously as an evaporation flowtube and an instrument for measuring the electrical mobility, realizing measurements of evaporation processes with time scales comparable to the instrument response time. A theoretical framework is derived for deducing the evaporation kinetics from instrument responses through analyzing the nanoparticle trajectory and size–mobility relationship, which considers the coupled mass and heat transfer effect and is applicable to the whole Knudsen number range. The methodology is demonstrated against evaporation but can potentially be extended to condensation and other industrial/atmospheric processes involving rapid size change of nanoparticles.