A vertically-resolved canopy improves chemical transport model
predictions of ozone deposition to north temperate forests
Abstract
Dry deposition is the second-largest tropospheric ozone (O3) sink and
occurs through stomatal and nonstomatal pathways. Current O3 uptake
predictions are limited by the simplistic big-leaf schemes commonly used
in chemical transport models (CTMs) to parameterize deposition. Such
schemes fail to reproduce observed O3 fluxes over terrestrial
ecosystems, highlighting the need for more realistic treatment of
surface-atmosphere exchange in CTMs. We address this need by linking a
resolved canopy model (1D Multi-Layer Canopy CHemistry and Exchange
Model, MLC-CHEM) to the GEOS-Chem CTM, and use this new framework to
simulate O3 fluxes over three north temperate forests. We compare
results with in-situ measurements from four field studies and with
standalone, observationally-constrained MLC-CHEM runs to test current
knowledge of O3 deposition and its drivers. We show that GEOS-Chem
overpredicts observed O3 fluxes across all four studies by up to 2×,
whereas the resolved-canopy models capture observed diel profiles of O3
deposition and in-canopy concentrations to within 10%. Relative
humidity and solar irradiance are strong O3 flux drivers over these
forests, and uncertainties in those fields provide the largest remaining
source of model deposition biases. Flux partitioning analysis shows
that: 1) nonstomatal loss accounts for 60% of O3 deposition on average;
2) in-canopy chemistry makes only a small contribution to total O3
fluxes; and 3) the CTM big-leaf treatment overestimates O3-driven
stomatal loss and plant phytotoxicity in these temperate forests by up
to 7×. Results motivate the application of fully-online, vertically
explicit canopy schemes in CTMs for improved O3 predictions.