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

Wei, Dandan and Alwe, Hariprasad D. and Millet, Dylan B. and Bottorff, Brandon and Lew, Michelle and Stevens, Philip S. and Shutter, Joshua D. and Cox, Joshua L. and Keutsch, Frank N. and Shi, Qianwen and Kavassalis, Sarah C. and Murphy, Jennifer G. and Vasquez, Krystal T. and Allen, Hannah M. and Praske, Eric and Crounse, John D. and Wennberg, Paul O. and Shepson, Paul B. and Bui, Alexander A. T. and Wallace, Henry W. and Griffin, Robert J. and May, Nathaniel W. and Connor, Megan and Slade, Jonathan H. and Pratt, Kerri A. and Wood, Ezra C. and Rollings, Mathew and Deming, Benjamin L. and Anderson, Daniel C. and Steiner, Allison L. (2021) FORest Canopy Atmosphere Transfer (FORCAsT) 2.0: model updates and evaluation with observations at a mixed forest site. Geoscientific Model Development, 14 (10). pp. 6309-6329. ISSN 1991-9603. doi:10.5194/gmd-14-6309-2021.

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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 secondary organic 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 is 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.

Item Type:Article
Related URLs:
URLURL TypeDescription version 2.0, monoterpenes, and MVK+MACR ISOPOOH, IEPOX, and IHN
Wei, Dandan0000-0002-5597-6233
Alwe, Hariprasad D.0000-0003-3851-4106
Millet, Dylan B.0000-0003-3076-125X
Stevens, Philip S.0000-0001-9899-4215
Shutter, Joshua D.0000-0001-8291-8242
Shi, Qianwen0000-0001-5000-1499
Kavassalis, Sarah C.000-0001-5036-9138
Murphy, Jennifer G.0000-0001-8865-5463
Vasquez, Krystal T.0000-0003-4540-4212
Praske, Eric0000-0001-7169-4423
Crounse, John D.0000-0001-5443-729X
Wennberg, Paul O.0000-0002-6126-3854
Shepson, Paul B.0000-0002-1726-3291b
Bui, Alexander A. T.0000-0002-1205-1564
Griffin, Robert J.0000-0001-7682-8769
May, Nathaniel W.0000-0002-8913-4827
Connor, Megan0000-0003-3266-4162
Pratt, Kerri A.0000-0003-4707-2290
Wood, Ezra C.0000-0002-9533-215X
Deming, Benjamin L.0000-0003-3690-0046
Anderson, Daniel C.0000-0002-9826-9811
Steiner, Allison L.000-0002-3823-1512
Additional Information:© Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 31 Mar 2021 – Discussion started: 23 Apr 2021 – Revised: 27 Jul 2021 – Accepted: 20 Sep 2021 – Published: 21 Oct 2021. Andrew P. Ault and Ryan Cook (University of Michigan) are acknowledged for field campaign assistance. Kerri A. Pratt, Nathaniel W. May, and Megan Connor acknowledge support from the University of Michigan MCubed 2.0 program and the Marian P. and David M. Gates Graduate Student Endowment Fund. The construction of the GC-HR-ToF-CIMS instrument was supported by the National Science Foundation (AGS-1428482), with additional NSF support (AGS-1240604) provided for the instrument field deployment to UMBS. Work performed by Krystal T. Vasquez and Hannah M. Allen was supported by the National Science Foundation Graduate Research Fellowship (NSF GRFP). Krystal T. Vasquez also acknowledges support from an Earl C. Anthony Fellowship in chemistry during an early portion of this study. The measurements of peroxy radicals were supported by NSF AGS-1443842 to the University of Massachusetts Amherst and AGS-1719918 to Drexel University. Work performed by Joshua D. Shutter and Joshua L. Cox was supported by the National Science Foundation Graduate Research Fellowship (NSF GRFP). Frank N. Keutsch acknowledges support by the National Science Foundation (AGS-1643306). This research has been supported by the National Oceanic and Atmospheric Administration (grant no. NA18OAR4310116). Author contributions. ALS designed the study. DW implemented the model updates and performed the simulations and analysis. HDA and DBM provided the VOCs and turbulence data. BB, ML, and PSS provided the OH and HO2 data. JDS, JLC, and FNK provided the HCHO data. QS, SCK, and JGM provided the NOx data. KTV, HMA, EP, JDC, and POW provided above-canopy ISOPOOH and IHN data. JDC and POW provided a critical review on model evaluation regarding IHN. PBS provided the in-canopy IHN and MTN data and a critical review on the inclusion of liquid water content associated with organics. AATB, HWW, and RJG provided the AMS data. RJG provided a critical review on the MPMPO module. NWM, MC, JHS, and KAP provided the AIM data. KAP contributed to the calculation of liquid water content associated with inorganic species. ECW, MR, BLD, and DCA provided the ROx data. All authors contributed to model evaluation and the paper preparation. Code availability The codes of FORCAsT version 2.0 are available on Zenodo (, Steiner, 2021). Data availability. Isoprene, monoterpenes, and MVK+MACR are available online at (Alwe et al., 2019). The GC-HR-ToF-CIMS ISOPOOH, IEPOX, and IHN are available through (Vasquez et al., 2018a, The authors declare that they have no conflict of interests. This paper was edited by Gerd A. Folberth and reviewed by two anonymous referees.
Funding AgencyGrant Number
University of MichiganUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Earl C. Anthony FellowshipUNSPECIFIED
National Oceanic and Atmospheric Administration (NOAA)NA18OAR4310116
Issue or Number:10
Record Number:CaltechAUTHORS:20211208-951475000
Persistent URL:
Official Citation:Wei, D., Alwe, H. D., Millet, D. B., Bottorff, B., Lew, M., Stevens, P. S., Shutter, J. D., Cox, J. L., Keutsch, F. N., Shi, Q., Kavassalis, S. C., Murphy, J. G., Vasquez, K. T., Allen, H. M., Praske, E., Crounse, J. D., Wennberg, P. O., Shepson, P. B., Bui, A. A. T., Wallace, H. W., Griffin, R. J., May, N. W., Connor, M., Slade, J. H., Pratt, K. A., Wood, E. C., Rollings, M., Deming, B. L., Anderson, D. C., and Steiner, A. L.: FORest Canopy Atmosphere Transfer (FORCAsT) 2.0: model updates and evaluation with observations at a mixed forest site, Geosci. Model Dev., 14, 6309–6329,, 2021.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112314
Deposited By: George Porter
Deposited On:10 Dec 2021 20:51
Last Modified:10 Dec 2021 20:51

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