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 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.
Highlights
Forests cover 30 % of the land surface and play an important role in the Earth system through exchanges of energy, water, carbon dioxide, and reactive chemical species with the atmosphere (Bonan, 2008)
Major updates include (i) separating the integration of chemistry from the emission and dry deposition to provide more realistic representations of vertical gradients in the forest canopy and to make the chemical module more flexible with future chemical mechanism updates, (ii) improving the vertical mixing parameterization in the boundary layer, (iii) updating the dry deposition velocities for chemical species with available measurements (Nguyen et al, 2015), (iv) implementing the Reduced Caltech Isoprene Mechanism (RCIM) to reflect the current understanding of isoprene fate under low-NOx conditions (Wennberg et al, 2018), and (v) extending the MPMPO (Model to Predict the Multiphase Partitioning of Organics) aerosol module (Griffin et al, 2005; Ashworth et al, 2015) to include isoprene-derived secondary organic aerosol (SOA) formation
The results presented in this study are based on a 2 d model simulation for the 2 sunny days of 22–23 July 2016; the first day (22 July) is a well-mixed day and the second day (23 July) is relatively stagnant based on micrometeorological analysis (Wei et al, 2020)
Summary
Forests cover 30 % of the land surface and play an important role in the Earth system through exchanges of energy, water, carbon dioxide, and reactive chemical species with the atmosphere (Bonan, 2008). Major updates include (i) separating the integration of chemistry from the emission and dry deposition ( called operator splitting) to provide more realistic representations of vertical gradients in the forest canopy and to make the chemical module more flexible with future chemical mechanism updates, (ii) improving the vertical mixing parameterization in the boundary layer, (iii) updating the dry deposition velocities for chemical species with available measurements (Nguyen et al, 2015), (iv) implementing the RCIM to reflect the current understanding of isoprene fate under low-NOx conditions (Wennberg et al, 2018), and (v) extending the MPMPO (Model to Predict the Multiphase Partitioning of Organics) aerosol module (Griffin et al, 2005; Ashworth et al, 2015) to include isoprene-derived SOA formation. We evaluate FORCAsT 2.0’s performance against FORCAsT 1.0 (Ashworth et al, 2015) and the observations from the AMOS (Atmospheric Measurements of Oxidants in Summer) field campaign conducted at the University of Michigan Biological Station (UMBS) during the summer of 2016
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