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Volume 18, issue 16 | Copyright
Atmos. Chem. Phys., 18, 12433-12460, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 Aug 2018

Research article | 28 Aug 2018

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modeling

Anna L. Hodshire1, Brett B. Palm2,a, M. Lizabeth Alexander3, Qijing Bian1, Pedro Campuzano-Jost2, Eben S. Cross4,b, Douglas A. Day2, Suzane S. de Sá5, Alex B. Guenther6,7, Armin Hansel8, James F. Hunter4, Werner Jud8,c, Thomas Karl9, Saewung Kim6, Jesse H. Kroll3,10, Jeong-Hoo Park11,d, Zhe Peng2, Roger Seco6, James N. Smith12, Jose L. Jimenez2, and Jeffrey R. Pierce1 Anna L. Hodshire et al.
  • 1Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA
  • 2Dept. of Chemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
  • 3Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA
  • 4Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
  • 5School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
  • 6Department of Earth System Science, University of California, Irvine, Irvine, CA 92697, USA
  • 7Division of Atmospheric Sciences & Global Change, Pacific Northwest National Laboratory, Richland, WA 99352, USA
  • 8Institute of Ion and Applied Physics, University of Innsbruck, Innsbruck, 6020, Austria
  • 9Institute for Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, 6020, Austria
  • 10Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
  • 11National Center for Atmospheric Research, Boulder, CO 80305, USA
  • 12Department of Chemistry, University of California, Irvine, CA 92697, USA
  • anow at: Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
  • bnow at: Center for Aerosol and Cloud Chemistry, Aerodyne Research, Inc., Billerica, MA 01821, USA
  • cnow at: Institute of Biochemical Plant Pathology, Research Unit Environmental Simulation, Helmholtz Zentrum München, Munich, 85764, Germany
  • dnow at: Climate and Air Quality Research Department, National Institute of Environmental Research (NIER), Incheon, 22689, Republic of Korea

Abstract. Oxidation flow reactors (OFRs) allow the concentration of a given atmospheric oxidant to be increased beyond ambient levels in order to study secondary organic aerosol (SOA) formation and aging over varying periods of equivalent aging by that oxidant. Previous studies have used these reactors to determine the bulk OA mass and chemical evolution. To our knowledge, no OFR study has focused on the interpretation of the evolving aerosol size distributions. In this study, we use size-distribution measurements of the OFR and an aerosol microphysics model to learn about size-dependent processes in the OFR. Specifically, we use OFR exposures between 0.09 and 0.9 equivalent days of OH aging from the 2011 BEACHON-RoMBAS and GoAmazon2014/5 field campaigns. We use simulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics box model to constrain the following parameters in the OFR: (1) the rate constant of gas-phase functionalization reactions of organic compounds with OH, (2) the rate constant of gas-phase fragmentation reactions of organic compounds with OH, (3) the reactive uptake coefficient for heterogeneous fragmentation reactions with OH, (4) the nucleation rate constants for three different nucleation schemes, and (5) an effective accommodation coefficient that accounts for possible particle diffusion limitations of particles larger than 60nm in diameter.

We find the best model-to-measurement agreement when the accommodation coefficient of the larger particles (Dp>60nm) was 0.1 or lower (with an accommodation coefficient of 1 for smaller particles), which suggests a diffusion limitation in the larger particles. When using these low accommodation-coefficient values, the model agrees with measurements when using a published H2SO4-organics nucleation mechanism and previously published values of rate constants for gas-phase oxidation reactions. Further, gas-phase fragmentation was found to have a significant impact upon the size distribution, and including fragmentation was necessary for accurately simulating the distributions in the OFR. The model was insensitive to the value of the reactive uptake coefficient on these aging timescales. Monoterpenes and isoprene could explain 24%–95% of the observed change in total volume of aerosol in the OFR, with ambient semivolatile and intermediate-volatility organic compounds (S/IVOCs) appearing to explain the remainder of the change in total volume. These results provide support to the mass-based findings of previous OFR studies, give insight to important size-distribution dynamics in the OFR, and enable the design of future OFR studies focused on new particle formation and/or microphysical processes.

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We investigate the nucleation and growth processes that shape the aerosol size distribution inside oxidation flow reactors (OFRs) that sampled ambient air from Colorado and the Amazon rainforest. Results indicate that organics are important for both nucleation and growth, vapor uptake was limited to accumulation-mode particles, fragmentation reactions were important to limit particle growth at higher OH exposures, and an H2SO4-organics nucleation mechanism captured new particle formation well.
We investigate the nucleation and growth processes that shape the aerosol size distribution...