Submitted as: model evaluation paper 23 Apr 2021

Submitted as: model evaluation paper | 23 Apr 2021

Review status: a revised version of this preprint is currently under review for the journal GMD.

FORest Canopy Atmosphere Transfer (FORCAsT) 2.0: model updates and evaluation with observations at a mixed forest site

Dandan Wei1, Hariprasad D. Alwe2, Dylan B. Millet2, Brandon Bottorff3, Michelle Lew3, Philip S. Stevens4, Joshua D. Shutter5, Joshua L. Cox5, Frank N. Keutsch5, Qianwen Shi6, Sarah C. Kavassalis6, Jennifer G. Murphy6, Krystal T. Vasquez7, Hannah M. Allen7, Eric Praske7, John D. Crounse8, Paul O. Wennberg9, Paul B. Shepson10, Alexander A. T. Bui11, Henry W. Wallace11, Robert J. Griffin11, Nathaniel W. May12, Megan Connor12, Jonathan H. Slade12, Kerri A. Pratt12, Ezra C. Wood13, Mathew Rollings14,a, Benjamin L. Deming14,b, Daniel C. Anderson13,c, and Allison L. Steiner1 Dandan Wei et al.
  • 1Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA
  • 2Department of Soil, Water and Climate, University of Minnesota, Twin Cities, St. Paul, MN, USA
  • 3Department of Chemistry, Indiana University, Bloomington, IN, USA
  • 4School of Public and Environmental Affairs, Indiana University, Bloomington, IN, USA
  • 5Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA
  • 6Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
  • 7Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
  • 8Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
  • 9Divisions of Engineering and Applied Science and Geological and Planetary Science, California Institute of Technology, Pasadena, CA, USA
  • 10Department of Chemistry, Purdue University, West Lafayette, IN, USA and School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, USA
  • 11Department of Civil and Environmental Engineering, Rice University, Houston, TX, USA
  • 12Department of Chemistry, University of Michigan, Ann Arbor, MI, USA
  • 13Department of Chemistry, Drexel University, Philadelphia, PA, USA
  • 14Department of Chemistry, University of Massachusetts, Amherst, MA, USA
  • anow at: Department of Chemistry, University of California, Berkeley, CA, USA
  • bnow at: Department of Chemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • cnow at: Universities Space Research Association, Columbia, MD and NASA Goddard Space Flight Center, Greenbelt, MD USA

Abstract. The FORCAsT (FORest Canopy Atmosphere Transfer) model version 1.0 is updated to FORCAsT 2.0 by implementing five major changes, including (1) a change to the operator splitting, separating chemistry from emission and dry deposition, which reduces the run time of the gas-phase chemistry by 70 % and produces a more realistic in-canopy profile for isoprene; (2) a modification of the eddy diffusivity parameterization to produce greater and more realistic vertical mixing in the boundary layer, which ameliorates the unrealistic simulated end-of-day peaks in isoprene under well-mixed conditions and improves daytime air temperature; (3) updates to dry deposition velocities with available measurements; (4) implementation of the Reduced Caltech isoprene mechanism (RCIM) to reflect the current knowledge of isoprene oxidation; and (5) extension of the aerosol module to include isoprene-derived aerosol (iSOA) formation. Along with the operator splitting, modified vertical mixing and dry deposition, RCIM improves the estimation of first generation isoprene oxidation products (methyl vinyl ketone and methacrolein) and some second generation products (such as isoprene epoxydiols). Inclusion of isoprene in the aerosol module in FORCAsT 2.0 leads to a 7 % mass yield of iSOA. The most important iSOA precursors are IEPOX and tetrafunctionals, which together account for > 86 % of total iSOA. The iSOA formed from organic nitrates are more important in the canopy, accounting for 11 % of the total iSOA. The tetrafunctionals compose up to 23 % of the total iSOA formation, highlighting the importance of the fate (i.e. dry deposition and gas-phase chemistry) of later-generation isoprene oxidation products in estimating iSOA formation.

Dandan Wei et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CEC1: 'Comment on gmd-2021-104', Juan Antonio Añel, 17 May 2021
    • AC1: 'Reply on CEC1', Dandan Wei, 20 May 2021
      • CEC2: 'Reply on AC1', Juan Antonio Añel, 21 May 2021
  • RC1: 'Comment on gmd-2021-104', Anonymous Referee #1, 25 May 2021
    • AC2: 'Reply on RC1', Dandan Wei, 16 Jul 2021
  • RC2: 'Review of D. Wei et al.', Anonymous Referee #2, 07 Jul 2021
    • AC3: 'Reply on RC2', Dandan Wei, 16 Jul 2021

Dandan Wei et al.

Dandan Wei et al.


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Short summary
Over the past decade, understanding of isoprene oxidation has improved and proper representation of isoprene oxidation and isoprene-derived SOA (iSOA) formation in canopy-chemistry models is now recognized to be important for an accurate understanding of forest-atmosphere exchange. The updated FORCAsT version 2.0 improves the estimation of some isoprene oxidation products, and is one of the few canopy models currently capable of simulating SOA formation from monoterpenes and isoprene.