Size-Resolved Chemistry of Newly-Formed Atmospheric Particles

Atmospheric nucleation processes produce large numbers of particles. Once formed, these particles grow rapidly and may alter the formation and lifetime of clouds, and thereby influence the earths radiation balance. Rapid growth of newly formed particles has been observed in many locations, but it is not known what chemical constituents contribute to this growth. While mobility-selection mass combined with spectrometry has provided important chemical data for particles above about 10 nm in diameter, data for smaller particle sizes are lacking. The problem is the low efficiency for placing a single electrical charge on these small particles, without significant multiple charging, as is required for their mobility-based size-selection. These data are required to validate models for nanoparticle growth. This work aims to improve the electrical charging, and hence the efficiency of the mobility size selection and particle collection process. Even with a unipolar charger, the fraction of particles that carry an electrical charge is small (a few percent), and this fraction decreases rapidly with decreasing particle diameter. Our approach is a condensationally-enhanced charging and evaporation method for increased efficiency of particle charging. In contrast to other condensation approaches, our method greatly reduces the time for the entire condensation-charging-evaporation process to a few tens of milliseconds, thereby minimizing the opportunity for chemical artifacts. Thermal desorption chemical ionization mass spectrometry data obtained using our condensation-evaporation system show clean ion spectra for particles sampled in chemically reacting atmospheres. This project will optimize this technique for placing a single electrical charge on particles in the 3 10 nm size range. It will also integrate the new charger with the Thermal Desorption Chemical Ionization Mass Spectrometer, providing critically-needed information on the species that are responsible for the growth of nascent atmospheric aerosol. Commercial Applications and OtherBenefits: This technique will enable measurement of the chemical composition of newly formed particles. Such data will have important atmospheric implications, and will improve understanding of cloud formation and global climate. Commercial applications extend beyond the atmospheric research community to the nanofabrication industry, where size-selective characterization of nanometer-sized particles is critical, and to the emerging field of ion trap mass spectrometery where controlled charging of large molecules is needed.