AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Observation of Water Vapor Uptake by Dimethylamine-Sulfuric Acid Cluster Ions in the Sub 2 nm Size Range via Ion Mobility Spectrometry-Mass Spectrometry
Jikku Thomas, Siqin He, Joseph DePalma, Carlos Larriba-Andaluz, Murray Johnston, CHRISTOPHER HOGAN JR., University of Minnesota
Abstract Number: 158 Working Group: Advances in the Physics and Chemistry of New Particle Formation and Growth
Abstract A number recent studies suggest that the combination of amines and sulfuric acid in the vapor phase lead to the formation of stable clusters, and hence can lead to new particle formation in the atmosphere. Using high resolution ion mobility spectrometry-mass spectrometry, we have determined the mobilities of singly charged ions of the type (H$^+)(DMA)$_x(H2SO4)$_y (DMA = dimethylamine; H2SO4 = sulfuric acid), where x ranges from 4 to 8 and y ranges from x-1 to x+5, and further examined water uptake by these clusters near atmospheric pressure and room temperature, at relative humidities ranging from 0 to 25%. For measurements we generated ions via positive electrosprays of dimethylamine-sulfuric acid solutions, and electrostatically directed ions into a parallel-plate differential mobility analyzer, operated with a resolution near 50. Water vapor uptake was monitored by humidifying the sheath flow of the differential mobility analyzer. Measurements clearly show that for a given number of dimethylamine molecules in a cluster, the extent of water vapor uptake increases with increasing number of sulfuric acid molecules, with a maximum 10% shift in cluster ion inverse mobility apparent at the highest examined relative humidities. Selected cluster structures, predicted via density functional theory calculations, were used in gas model scattering calculations to compare measured mobilities to predictions. Further, classical theories of heterogeneous uptake have been compared to measurements. Overall, we show that even at modest relative humidities (H$^+)(DMA)$_x(H2SO4)$_y clusters clearly uptake several molecules, which needs to be considered in models of cluster growth and new particle formation.