Ion dynamics and aerosol mass spectrometry during atmospheric new particle formation

New particle formation (NPF) is a frequently occurring phenomenon in the Earth’s atmosphere, thereby contributing to the total number concentration of aerosol particles. The formed particles, originating from gas-to-particle conversion processes, typically grow to diameters relevant for the Earth’s climate. Climate effects of particles include the direct and indirect interaction with solar shortwave radiation and thermal radiation emitted by the Earth’s surface. In the past 20 years a lot of effort has been put into investigating the mechanisms governing atmospheric NPF. Only in the course of the past few years a general picture of the mechanisms dominating atmospheric NPF was put together. To date, NPF is known to occur due to clustering of precursor gas molecules (e.g. sulfuric acid, amines, ammonia, organic compounds) to form stable initial particle nuclei (diameter between 1.5 and 1.9 nm). Further, condensation of predominantly organic molecules causes particle growth. Nevertheless, a lot of open questions remain, regarding the initial particle formation, as well as the processes dominating particle growth. In this work, the role of ions on particle formation and growth is assessed, and a novel instrument to analyze the chemical composition of the growing particles is presented. Ionized air molecules, permanently available in the Earth’s atmosphere, are thought to considerably influence NPF. During the initial clustering process the presence of a charge stabilizes the clusters and enhances condensational growth by gas molecules with an electrical dipole moment (e.g. sulfuric acid). To assess the role of ions in NPF, ion and neutral particle number size distributions from four different field sites were analyzed. At all four locations, the same type of instrument was deployed (neutral cluster and air ion spectrometer – NAIS). The NAIS exhibits detection limits in the diameter range of molecular clusters for the ion fraction (~0.8 nm) and in the diameter range of ~2 nm for neutral particles. Data from two locations, with presumably high precursor gas concentrations, showed a clear earlier occurrence of 2 nm ions compared to 2 nm neutral particles. This initial advance of the ion fraction vanished during the consecutive growth process. At diameters between 10 and 20 nm, neutral particles exhibited the same diameter as the ion fraction. Neutral particles with diameters below 10 to 20 nm were observed to experience an enhanced growth rate (GR) in comparison to the ion fraction. On the other hand, at locations with presumably low precursor gas concentrations an advance of the ion fraction was not as pronounced, and GRs of ions and neutral particles exhibited similar values. A special focus was put on the Waldstein site, NE Bavaria, Germany, where relatively high precursor gas concentrations and elevated ion concentrations are thought to be present. For this field site, the influence of ion recombination on neutral particle formation was assessed. Therefore, neutral particle distributions arising from recombination of oppositely charged ions were calculated. GRs of the recombined neutral particles were elevated in comparison to ion GRs. Consequently, an ion accelerated growth mechanism including recombination and attachment of ions is proposed in order to explain the observed elevated neutral GRs. At field sites with low precursor gas concentrations, the ion accelerated growth mechanism does not contribute significantly to neutral particle growth. Nevertheless, ions are likely to contribute largely to the initial formation of neutral particles at sites with low precursor gas concentrations. To shed more light on the chemical compounds contributing to particle growth, a particle interface for a time of flight mass spectrometer (ToF-MS) was developed in this work. The developed chemical analyzer for charged ultrafine particles (CAChUP) is capable of analyzing particles with diameters below 30 nm for their chemical composition. Charged particles of a defined diameter interval are electrostatically collected on a metal filament. Following the collection, the filament is resistively heated to evaporate the sample. The resultant gas phase is eventually transferred to the ToF-MS, ionized via electron impact ionization and analyzed for its molecular composition. To characterize CAChUP in the laboratory, two main experiments were performed: (1) the application of known masses of camphene to the metal filament and (2) the collection and analysis of laboratory-generated secondary organic aerosol (SOA) particles with diameters of 25 nm. Direct application, evaporation and analysis of known camphene masses to/from the metal filament resulted in a detection limit of CAChUP below 5 ng. SOA was produced by dark ozonolysis of alpha-pinene in a flow tube reactor. Particles with diameters of 25 nm exiting the flow tube reactor were selected in a radial differential mobility analyzer (rDMA). The analysis of the samples in the ToF-MS resulted in typical mass spectra for SOA generated by dark ozonolysis of alpha-pinene. Collected masses for the 0.5 hour experiment were estimated to be 2.1 to 2.6 ng. Sampling times below 0.5 hours did not yield quantifiable mass spectra. CAChUP performed well in the laboratory, and is ready for further laboratory experiments. A sensitivity analysis regarding the deployment of CAChUP to measure atmospheric NPF in the field, showed that extended collection periods (8 hours for 20 nm particles) are required with the current setup. Therefore, field measurements are assumed to be possible for NPF events with continuous particle formation and high particle number concentrations.