10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
The Effects of Aerosol-Phase State and Chemical Composition on Multiphase Chemistry Leading to Isoprene-Derived Secondary Organic Aerosol Formation
YUE ZHANG, Yuzhi Chen, Andrew Lambe, Nicole Olson, Ziying Lei, Rebecca Craig, Manjula Canagaratna, Jordan Krechmer, Zhenfa Zhang, Avram Gold, Timothy Onasch, John Jayne, Douglas Worsnop, Cassandra Gaston, Joel A. Thornton, William Vizuete, Andrew Ault, Jason Surratt, Boston College; Aerodyne Research, Inc.
Abstract Number: 1639 Working Group: Aerosol Chemistry
Abstract Acid-catalyzed reactions between gas- and particle-phase constituents are an important formation mechanism for atmospheric secondary organic aerosol (SOA). Aerosol phase state, governed by aerosol composition, relative humidity (RH), and temperature, influences the reactive uptake process of gas-phase precursors by altering diffusion rates within particles. However, there is little experimental evidence to show the dependence of reactive uptake processes on particle-phase state with respect to these factors. This laboratory study systematically examines the reactive uptake probability of isoprene-derived epoxydiols (IEPOX) onto acidic ammonium sulfate particles with various types of pre-existing SOA coatings by coupling a flow tube reactor with an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). A uniform layer of organics is coated onto the acidic sulfate particles using the potential aerosol mass (PAM) oxidation flow reactor, confirmed via atomic force microscopy (AFM) and scanning electron microscopy (SEM). The measured reactive uptake probability is parameterized as a function of SOA coating thickness, oxidation state, and RH. Results show that certain pre-existing SOA coatings could significantly reduce the reactive uptake probability of IEPOX, in some cases by nearly an order of magnitude when the coating thickness is only 10 nm.
Particle composition is also analyzed by both online and offline analytical techniques, including an aerosol chemical speciation monitor (ACSM), ultra-performance liquid chromatography interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS), and ESI coupled to ion mobility spectrometry high-resolution time-of-flight mass spectrometry (ESI-IMS-HR-TOFMS). Results show that the oxidation state and composition (such as the oligomer content) of aerosol particles jointly contribute to their phase state, thereby altering the diffusion rate of IEPOX in the organic coating and the measured reactive uptake coefficient.
A box model combining experimental data with ambient measurements from the 2013 SOAS campaign is used to assess the effects of pre-existing organic coating on IEPOX-derived SOA formation. The diurnal trend of isoprene-derived SOA mass concentrations with and without coating effects is derived by modeling the experimental uptake coefficient with field-measured data from the southeastern U.S. IEPOX-derived SOA is estimated to be reduced by 16-27% due to pre-existing organic coatings during the afternoon (12-7 PM, local time), corresponding to its highest production period. Our study provides a potential explanation for the discrepancy between model predictions and field measurements of IEPOX-derived SOA reported by previous studies.
Our results suggest that the inorganic and organic components of aerosol particles, as well as their physical and chemical properties, jointly impact the formation, evolution, and fate of ambient SOA. For instance, pre-existing SOA constituents formed from the condensation of semi-volatile species, with certain compositions and phase states, can adversely affect the reactive uptake of gases leading to the formation of additional SOA. These results can be used to accurately characterize the formation and evolution of IEPOX-derived SOA. Moreover, the approach used in this study could be more widely applied to other multiphase chemical systems in regional and global scale models to better predict the impact of SOA on climate, human health, and visibility.