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 spectrometry has provided important chemical data for particles above about 10 nm in diameter, data for smaller particle sizes is lacking. The problem is the low efficiency for placing an electrical charge on these small particles, as is required for their mobility-based size-selection. 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. Instead of several seconds, our approach condenses and dries the particles within about 40 milliseconds, thereby minimizing the opportunity for chemical artifacts. Preliminary TDCIMS mass spectrometry data obtained using our condensation-evaporation system show clean negative ion spectra. Organic acids, thought to be important in the growth process, are seen in negative ion spectra. Commercial Applications and Other Benefits: This technique will enable measurement of the chemical composition of newly formed particles. Such data will have important atmospheric implications, and for improving 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 particles is critical.