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

Abstract

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.

Large Eddy Simulation for Investigating Coupled Forest Canopy and Turbulence Influences on Atmospheric Chemistry

“…While observed HCHO mixing ratios are as low as 2 ppb during the afternoon (comparable to our simulated mixing ratios), the higher observed levels of HCHO mostly correspond to the higher isoprene levels simulated by our model. Better understanding of HCHO sources and sinks in the canopy will advance our ability to accurately represent HCHO in chemistry MLMs (e.g., Wei et al., 2021; Wolfe et al., 2011).…”

“…Thus, turbulent transport is often not accurately represented, and the impact of segregation cannot be quantified. While multilayer models of vegetation canopies (hereinafter, MLMs) have been coupled to complex chemical mechanisms to examine exchanges of reactive gases and the related in‐canopy chemistry for many years (e.g., Ashworth et al., 2015; Bryan et al., 2012; Forkel et al., 2006; Ganzeveld et al., 2002; Wei et al., 2021; Wolfe & Thornton, 2011), chemistry MLMs have yet to be coupled to turbulence‐resolving large eddy simulation (LES) with one exception (Fuentes et al., 2022). LES is a powerful tool for investigating turbulence including within and above canopies (e.g., Kanda & Hino, 1994; Ma & Liu, 2019; Shaw & Schumann, 1992; Su et al., 1998) yet can be computationally expensive.…”