Characterization of Acid-Base Aerosol Nanoclusters Using a Thermal Desorption Chemical Ionization Mass Spectrometer with a Half-Mini Differential Mobility Analyzer

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

     Abstract Number: 374
     Working Group: Aerosol Chemistry

Abstract
New particle formation (NPF) events are a significant source of ambient aerosols, but uncertainty exists in the understanding of how newly formed clusters grow from ~1 nm to 10 nm in diameter, which we refer to here as the aerosol nanocluster (AN) size range. The growth process is thought to be dependent on the chemical composition of the AN, with acid-base ratios, oxidation state, and other physicochemical properties influencing their development. A gap currently exists in our ability to measure AN composition, with current instruments capable of measuring gas-phase clusters up to about 2 nm and aerosol online instruments capable of chemically characterizing nanoparticles greater than 10 nm. This is a challenging size range to measure due to their small sizes and mass as well as the short lifetimes associated with these particles.

Here, we used a Thermal Desorption Chemical Ionization Mass Spectrometer to measure the composition of ANs size-selected using a Half-Mini Differential Mobility Analyzer (SEADM S.L.). ANs were generated using a bipolar electrospray, which enabled the formation of mostly singly charged clusters. For initial proof of concept, tetraheptylammonium bromide particles were first investigated followed by atmospherically relevant acid-base salt ANs such as those comprised of 4-aminobutaol and methanesulfonic acid. Results from bipolar electrospray generated ANs are compared with ANs generated from acid-base reactions in a flow reactor. Through these studies, we directly observe the chemistry of particle growth in its earliest stage, thereby contributing substantially to our understanding of atmospheric NPF events.