10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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RH Effect on the Oxidation of α-pinene and the Influence on New Particle Formation
XIAOXIAO LI, Sabrina Chee, Jingkun Jiang, James Smith, University of California, Irvine
Abstract Number: 1064 Working Group: Aerosol Chemistry
Abstract It is widely observed around the world that the frequency and intensity of new particle formation (NPF) events are reduced during periods of high relative humidity (RH) due to the combination of diminished solar radiation, which leads to low concentrations of precursors such as H2SO4 and low volatility organic compounds, together with increased condensation sink that leads to scavenging of precursors and clusters. However, NPF has also been observed in the free troposphere in vicinity of clouds, where RH is typically usually over 90%. NPF has also been observed when RH is above 80% in winter in Hyytiälä, Finland. Despite these observations, most chamber and modelling experiments related to the formation of new particles and its gas-phase precursors have been conducted in low or medium RH. Overall, laboratory, field and modeling studies disagree as to the potential impacts of RH on NPF. The current study focuses on how high RH affects NPF-related gas phase chemistry and new particle formation. These will be explored under a range of RH from 0-95% using a temperature-controlled flow tube and a continuous-flow reaction chamber. Results from flow tube studies show that high RH will increase the generated particle size, decrease particle number concentration and decrease the concentration of low-volatility gas-phase precursors. To identify and quantify the latter, we developed a new inlet for a chemical ionization mass spectrometer (CIMS) that significantly reduces water clustering on reagent ions, the latter of which confounds the interpretation of mass spectra. A comparison between the new CIMS inlet and a commercial inlet based on the design of Eisele and Tanner resulted in identical spectra when measuring precursors to NPF from α-pinene ozonolysis with an enhancement in sensitivity of more than a factor of 10. At RH above 90%, water clusters dominated the reagent ion spectra when using the commercial inlet, whereas for the new CIMS inlet water clustering is eliminated. Detailed chemical composition of low volatility NPF precursors from α-pinene ozonolysis will be presented in addition to measurements of nanoparticle composition using Thermal Desorption Chemical Ionization Mass Spectrometry.