Characterizing the Stability and Composition of Size Resolved MSA-Amine Aerosol Nanoclusters from 1 – 2.5 nm

COLLEEN MILLER, Paulus Bauer, Véronique Perraud, Barbara Finlayson-Pitts, James Smith, University of California, Irvine

     Abstract Number: 512
     Working Group: Aerosol Chemistry

Abstract
New particle formation (NPF) is an important process in which new aerosols are generated and grow through various processes. There are many questions surrounding the exact mechanisms involved in the nucleation and growth of the smallest clusters (1 nm) to yield particles capable of influencing climate through direct and indirect effects. One mechanism of current interest is acid-base driven chemistry, in which acid and base molecules interact through intermolecular forces and/or proton transfer to stabilize the growing cluster. While sulfuric acid is recognized as the main contributing acid to this process, methanesulfonic acid is also a major nucleating species in the air, and is rising in importance as anthropogenic sources of H2SO4 are decreasing globally. In this work, we generated methanesulfonic acid – amine aerosol nanoclusters using a bipolar electrospray. The formed nanoclusters were subsequently mobility-selected with a half-mini differential mobility analyzer and analyzed using a Faraday cup electrometer (used for obtaining size distributions), a time-of-flight mass spectrometer (for cluster composition), and an Orbitrap mass spectrometer (for cluster composition and stability). Results from these experiments showed an overall neutral cluster composition (equal amounts of acid and base molecules) over all sizes for all systems except putrescine (PUT), which is a diamine. It was found that the MSA-PUT nanoclusters grow by addition of a 2-acid:1base cluster or a 1acid:1base cluster, thus giving rise to the presence of both neutral and acidic nanoclusters. Additional cluster stability analysis from collision-induced dissociation experiments revealed a linear correlation between the collision energy at which the survival yield of the parent cluster is 50% (CE50) and cluster size. Through these studies we directly observed the chemistry of particle nucleation and growth in its earliest stage (1-2.5 nm), furthering our understanding of aerosol growth mechanisms and informing our understanding of atmospheric NPF mechanisms.