AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Size-resolved Chemical Composition of Nanoparticles from Reactions of Sulfuric Acid with Ammonia and Dimethlylamine
HAIHAN CHEN, Michael Lawler, Sabrina Chee, James Smith, University of California, Irvine
Abstract Number: 509 Working Group: Aerosol Chemistry
Abstract Sulfuric acid (H2SO4) is widely accepted as the key species driving new particle formation in the atmosphere. Ammonia and amines contribute to nanoparticle growth by forming ammonium/aminium salts with sulfate and bisulfate ions. Measurements of molecular clusters up to ~2 nm in the sulfuric acid-base system at the CLOUD chamber show a nucleation process involving stepwise addition of H2SO4 and base in an acid-to-base ratio close to 1:1. But in another recent study at the CLOUD chamber, we observed highly acidic 10-30 nm nanoparticles from reactions of sulfuric acid with excess base. To better understand the growth mechanism of nanoparticles by acid-base chemistry, we measured the chemical composition of size-selected particles in the diameter range of 8-20 nm produced from reactions of H2SO4 with ammonia and dimethylamine in a borosilicate glass flow reactor. Those measurements were performed with Thermal Desorption Chemical Ionization Mass Spectrometry (TDCIMS). The concentration of base and relative humidity were varied to explore their effects on particle growth and chemical composition. The results suggest that particle growth by acid-base chemistry is very dynamic and complex. The acid-to-base ratio in nanoparticles shows a strong dependence on particle size and relative humidity. Smaller particles are more acidic, consistent with our previous observations from CLOUD, with higher bisulfate present, while larger particles are more neutralized and contain more sulfate. Possible explanations of this dynamic and complex growth mechanism are discussed.