Development of an ecophysiology module in the GEOS-Chem chemical transport model version 12.2.0 to represent biosphere−atmosphere fluxes relevant for ozone air quality

Abstract

Abstract. Ground-level ozone (O3) is a major air pollutant that adversely affects human health and agricultural productivity. Removal of air pollutants including tropospheric O3 from the atmosphere by vegetation is controlled mostly by the process of dry deposition, an important component of which is plant stomatal uptake that can in turn cause damage to plant tissues with ramifications for ecosystem and crop health. In many atmospheric and land surface models, the openness of plant stomata is represented by a bulk stomatal conductance, which is often semi-empirically parameterized, and highly fitted to historical observations. A lack of mechanistic linkage to ecophysiological processes such as photosynthesis may render models insufficient to represent plant-mediated responses of atmospheric chemistry to long-term changes in CO2, climate and short-lived air pollutant concentrations. A new ecophysiology module was thus developed to mechanistically simulate land−atmosphere exchange of important gas species in GEOS-Chem, a chemical transport model widely used in atmospheric chemistry studies. We adopted the formulations from the Joint UK Land Environmental Simulator (JULES) to couple photosynthesis rate, bulk stomatal conductance and isoprene emission rate dynamically. The implementation not only allows dry deposition to be coupled with plant ecophysiology, but also enables plant and crop productivity and functions to respond dynamically to atmospheric chemical changes. The research questions of this study include: 1) how the new ecophysiology module compares with the prior, semi-empirical parameterization in terms of simulating concentration and dry deposition velocity of O3 with respect to site measurement-based estimates; and 2) whether the ecophysiology module simulates vegetation productivity, dry deposition, isoprene emission rate and O3–vegetation interactions reasonably under a present-day and an elevated CO2 concentration. We conduct simulations to evaluate the effects of the ecophysiology module on simulated dry deposition velocity and concentration of surface O3 against an observation-derived dataset known as SynFlux. Our estimated dry deposition velocity of O3 is close to SynFlux dry deposition velocity with root-mean-squared errors (RMSE) ranging from 0.1 to 0.2 cm s–1 across different plant functional types (PFTs), despite an overall positive bias in surface O3 concentration (by up to 16 ppbv). Representing ecophysiology was found to reduce the simulated biases in deposition fluxes from the prior model, but worsen the positive biases in simulated O3 concentrations. The increase in positive concentration biases is mostly attributable to the ecophysiology-based stomatal conductance being generally smaller (and closer to SynFlux values) than that estimated by the prior semi-empirical formulation, calling for further improvements in non-depositional processes relevant for O3 simulations. Estimated global O3 deposition flux is 864 Tg O3 yr–1 with GEOS-Chem, and the new module decreases this estimate by 92 Tg O3 yr–1. Estimated global gross primary product (GPP) is 119 Pg C yr–1, with an O3-induced damage of 4.2 Pg C yr–1 (3.5 %). An elevated CO2 scenario (580 ppm) yields higher global GPP (+16.8 %) and lower global O3 depositional sink (–3.3 %). Global isoprene emission simulated with a photosynthesis-based scheme is 318 Tg C yr–1, which is 31 Tg C yr−1 (−8.9 %) less than that calculated using the MEGAN emission algorithm. This new model development dynamically represents the two-way interactions between vegetation and air pollutants, and thus provides a unique capability in evaluating pollutant impacts on vegetation health and feedback processes that can shape atmospheric chemistry and air quality especially for any timescales shorter than the multidecadal timescale.

Responses of Ecosystem Productivity to Anthropogenic Ozone and Aerosols at the 2060

“…Globally, GPP increases by 38.7% under SSP5‐8.5 (Figure 7b) and 15.9% under SSP1‐2.6 (Figure 8b) scenarios by CO 2 and climate, suggesting an increase of 0.16%–0.24% in GPP per ppm CO 2 given limited effects by climate (Tian et al., 2021). Such efficiency is consistent with an increase of 0.08% ppm −1 from a modeling study conducted using GEOS‐Chem (Lam et al., 2023) and within the range of 0.01%–0.32% ppm −1 from multiple free‐air CO 2 enrichment experiments (Ainsworth & Long, 2005). The O 3 ‐induced GPP recovery of 0.3 Pg [C] in eastern China at 2060 under SSP1‐2.6 (Figure 8c) is higher than the estimate of 0.1 ± 0.03 Pg [C] yr −1 in Yue et al.…”

“…Lam et al. (2023) pointed out that surface O 3 caused GPP reductions exceeding 20% over eastern United States and China. In contrast, the aerosol‐induced increase in diffuse radiation helps promote the light use efficiency of canopy, leading to enhanced GPP for the whole ecosystem especially under the clear‐sky conditions (Gu et al., 2002; X. Wang et al., 2018; Zhou et al., 2021).…”

Satellite-derived constraints on the effect of drought stress on biogenic isoprene emissions in the southeastern US

“…As the default GEOS-Chem does not have these photosynthetic parameters, we adopted the ecophysiology module created by Lam et al (2022) that is based on the photosynthesis calculation in the Joint UK Land Environmental Simulator (JULES; Best et al, 2011;Clark et al, 2011) as an online component in GEOS-Chem so that it simulates photosynthesis rate and bulk stomatal conductance dynamically and consistently with the underlying meteorology that drives GEOS-Chem. The outputs of V cmax and β t from the ecophysiology module were passed to MEGAN2.1 in GEOS-Chem to parameterize the drought stress according to Eq.…”

“…In general, the implementation of the ecophysiology module much improved the simulated stomatal conductance and dry-deposition velocity relative to site observations on average for seasonal timescales, but the β t computed has not been calibrated to intermittent drought conditions. Instead of adopting the values of V cmax and β t from Jiang et al (2018), which were based on the Community Land Model, we need to determine the β t threshold and the α value specific to GEOS-Chem with the ecophysiology module of Lam et al (2022). To calibrate β t , we first examined the statistical distribution of β t at the MOFLUX grid (Fig.…”