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

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.

Automated summary

The FORCAsT model has been upgraded to version 2.0, incorporating changes in operator splitting, vertical mixing, and dry deposition to improve estimation of isoprene oxidation products and iSOA formation. The inclusion of isoprene in the aerosol module in FORCAsT 2.0 leads to a 7% mass yield of iSOA, with tetrafunctionals and IEPOX identified as the most important iSOA precursors.