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

• Published in: Geoscientific Model Development 2021-10, Vol.14 (10), p.6309-6329
• Main Authors: Wei, Dandan, Alwe, Hariprasad D, Millet, Dylan B, Bottorff, Brandon, Lew, Michelle, Stevens, Philip S, Shutter, Joshua D, Cox, Joshua L, Keutsch, Frank N, Shi, Qianwen, Kavassalis, Sarah C, Murphy, Jennifer G, Vasquez, Krystal T, Allen, Hannah M, Praske, Eric, Crounse, John D, Wennberg, Paul O, Shepson, Paul B, Bui, Alexander A. T, Wallace, Henry W, Griffin, Robert J, May, Nathaniel W, Connor, Megan, Slade, Jonathan H, Pratt, Kerri A, Wood, Ezra C, Rollings, Mathew, Deming, Benjamin L, Anderson, Daniel C, Steiner, Allison L
• Format: Article
• Language: English
• Subjects: Aerosols; Air temperature; Atmosphere; Atmospheric boundary layer; Atmospheric chemistry; Boundary layers; Canopies; Canopy; Chemical reactions; Chemistry; Deposition; Dry deposition; Eddy diffusion; Eddy diffusivity; Forests; Gases; Isoprene; Ketones; Modules; Nitrates; Organic nitrates; Oxidation; Parameterization; Plant cover; Secondary aerosols; Splitting; Vertical mixing; VOCs; Volatile organic compounds
• ISSN: 1991-9603; 1991-959X; 1991-962X; 1991-9603; 1991-962X
• DOI: 10.5194/gmd-14-6309-2021

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.