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 ecosystem productivity. Removal of tropospheric O3 by plant stomatal uptake can in turn cause damage to plant tissues with ramifications for ecosystem and crop health. In many atmospheric and land surface models, the functionality of stomata opening 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 inadequate 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. The implementation not only allows for dry deposition to be coupled with plant ecophysiology but also enables plant and crop productivity and functions to respond dynamically to atmospheric chemical changes. 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 stomatal conductance and dry deposition velocity of O3 are close to SynFlux with root-mean-squared errors (RMSEs) below 0.3 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-stomatal depositional and non-depositional processes relevant for O3 simulations. The 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 production (GPP) without O3 damage is 119 Pg C yr−1. O3-induced reduction in GPP is 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 317.9 Tg C yr−1, which is 31.2 Tg C yr−1 (−8.9 %) less than that calculated using the MEGAN (Model of Emissions of Gases and Aerosols from Nature) 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 vegetation-mediated processes and feedbacks that can shape atmospheric chemistry and air quality, as well as pollutant impacts on vegetation health, 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).…”

“…Similarly, Proietti et al (2016) found a GPP reduction up to 30% due to O 3 during 2000-2010 combining data from 37 European forest sites with satellite retrievals. 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.…”

Impacts of terrestrial vegetation on surface ozone in China: from present to carbon neutrality

“…The control of O 3 pollution requires a clear understanding of the contribution of vegetation processes to O 3 source and sink, especially in the context of China's potential efforts to achieve carbon neutrality by 2060 through afforestation in the future. However, accurately quantifying the impact of vegetation processes on surface O 3 has always remained a challenge, mainly due to the significant uncertainty in the description of complex vegetation processes in current atmospheric chemical transport models (Wong et al 2019, Lei et al 2020, Lam et al 2023. In this study, using a newly coupled atmospheric chemistry-vegetation model, which accounts for dry deposition and BVOC emissions through the photosynthesis-dependent schemes, we conduct eight simulations to assess the net impacts of terrestrial vegetation on surface O 3 in China.…”

“…To improve the model skill in simulating the O 3 dry deposition velocity and BVOC emissions, several scholars began to couple vegetation models with chemical transport models. For example, Lei et al (2020) and Lam et al (2023) have coupled the vegetation modules to the GEOS-Chem chemical transport model, which provide an important tool to quantify the interactions between atmospheric chemistry and terrestrial vegetation.…”