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Dry deposition could partially explain the observed response in ambient ozone to extreme hot and dry episodes. We examine the response of ozone deposition to heat and dry anomalies using three long‐term co‐located ecosystem‐scale carbon dioxide, water vapor and ozone flux measurement records. We find that, as expected, canopy stomatal conductance generally decreases during days with dry air or soil. However, during hot days, concurrent increases in non‐stomatal conductance are inferred at all three sites, which may be related to several temperature‐sensitive processes not represented in the current generation of big‐leaf models. This may offset the reduction in stomatal conductance, leading to smaller net reduction, or even net increase, in total deposition velocity. We find the response of deposition velocity to soil dryness may be related to its impact on photosynthetic activity, though considerable variability exists. Our findings emphasize the need for better understanding and representation of non‐stomatal ozone deposition.

 

Abstract. Ground-level ozone (O3) is a major air pollutant that adversely affects human health and ecosystem productivity. Removal of troposphericO3 by plant stomatal uptake can in turn cause damage to plant tissues with ramifications for ecosystem and crop health. In manyatmospheric and land surface models, the functionality of stomata opening is represented by a bulk stomatal conductance, which is oftensemi-empirically parameterized and highly fitted to historical observations. A lack of mechanistic linkage to ecophysiological processes such asphotosynthesis 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−atmosphereexchange of important gas species in GEOS-Chem, a chemical transport model widely used in atmospheric chemistry studies. The implementation not onlyallows for dry deposition to be coupled with plant ecophysiology but also enables plant and crop productivity and functions to respond dynamically toatmospheric chemical changes. We conduct simulations to evaluate the effects of the ecophysiology module on simulated dry deposition velocity andconcentration of surface O3 against an observation-derived dataset known as SynFlux. Our estimated stomatal conductance and dry depositionvelocity of O3 are close to SynFlux with root-mean-squared errors (RMSEs) below 0.3 cm s−1 across different plant functionaltypes (PFTs), despite an overall positive bias in surface O3 concentration (by up to 16 ppbv). Representing ecophysiology wasfound to reduce the simulated biases in deposition fluxes from the prior model but worsen the positive biases in simulated O3concentrations. The increase in positive concentration biases is mostly attributable to the ecophysiology-based stomatal conductance being generallysmaller (and closer to SynFlux values) than that estimated by the prior semi-empirical formulation, calling for further improvements in non-stomataldepositional and non-depositional processes relevant for O3 simulations. The estimated global O3 deposition flux is864 Tg O3 yr−1 with GEOS-Chem, and the new module decreases this estimate by 92 Tg O3 yr−1. Estimated global grossprimary 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 O3depositional sink (−3.3 %). Global isoprene emission simulated with a photosynthesis-based scheme is 317.9 Tg C yr−1, which is31.2 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 dynamicallyrepresents the two-way interactions between vegetation and air pollutants and thus provides a unique capability in evaluating vegetation-mediatedprocesses and feedbacks that can shape atmospheric chemistry and air quality, as well as pollutant impacts on vegetation health, especially for anytimescales shorter than the multidecadal timescale. 

Abstract. Dry deposition is a key process for surface ozone(O3) removal. Stomatal uptake is a major component of O3 drydeposition, which is parameterized differently in current land surfacemodels and chemical transport models. We developed and used a standaloneterrestrial biosphere model, driven by a unified set of prescribedmeteorology, to evaluate two widely used dry deposition modeling frameworks,Wesely (1989) and Zhang et al. (2003), with different configurations ofstomatal resistance: (1) the default multiplicative method in the Weselyscheme (W89) and Zhang et al. (2003) scheme (Z03), (2) the traditionalphotosynthesis-based Farquhar–Ball–Berry (FBB) stomatal algorithm, and (3) theMedlyn stomatal algorithm (MED) based on optimization theory. We found thatusing the FBB stomatal approach that captures ecophysiological responses toenvironmental factors, especially to water stress, can generally improve thesimulated dry deposition velocities compared with multiplicative schemes.The MED stomatal approach produces higher stomatal conductance than FBB andis likely to overestimate dry deposition velocities for major vegetationtypes, but its performance is greatly improved when spatially varying slopeparameters based on annual mean precipitation are used. Large discrepancieswere also found in stomatal responses to rising CO2 levels from 390to 550 ppm: the multiplicative stomatal method with an empirical CO2response function produces reduction (−35 %) in global stomatalconductance on average much larger than that with the photosynthesis-basedstomatal method (−14 %–19 %). Our results show the potential biases inO3 sink caused by errors in model structure especially in the Weselydry deposition scheme and the importance of using photosynthesis-basedrepresentation of stomatal resistance in dry deposition schemes under achanging climate and rising CO2 concentration. 

Abstract. We present in this technical note the research protocol for phase 4 of theAir Quality Model Evaluation International Initiative (AQMEII4). Thisresearch initiative is divided into two activities, collectively having threegoals: (i) to define the current state of the science with respect torepresentations of wet and especially dry deposition in regional models,(ii) to quantify the extent to which different dry depositionparameterizations influence retrospective air pollutant concentration andflux predictions, and (iii) to identify, through the use of a common set ofdetailed diagnostics, sensitivity simulations, model evaluation, andreduction of input uncertainty, the specific causes for the current range ofthese predictions. Activity 1 is dedicated to the diagnostic evaluation ofwet and dry deposition processes in regional air quality models (describedin this paper), and Activity 2 to the evaluation of dry deposition pointmodels against ozone flux measurements at multiple towers with multiyearobservations (to be described in future submissions as part of the specialissue on AQMEII4). The scope of this paper is to present the scientificprotocols for Activity 1, as well as to summarize the technical informationassociated with the different dry deposition approaches used by theparticipating research groups of AQMEII4. In addition to describing allcommon aspects and data used for this multi-model evaluation activity, mostimportantly, we present the strategy devised to allow a common process-levelcomparison of dry deposition obtained from models using sometimes verydifferent dry deposition schemes. The strategy is based on adding detaileddiagnostics to the algorithms used in the dry deposition modules of existingregional air quality models, in particular archiving diagnostics specific to land use–land cover(LULC) and creating standardized LULC categories tofacilitate cross-comparison of LULC-specific dry deposition parameters andprocesses, as well as archiving effective conductance and effective flux asmeans for comparing the relative influence of different pathways towards thenet or total dry deposition. This new approach, along with an analysis ofprecipitation and wet deposition fields, will provide an unprecedentedprocess-oriented comparison of deposition in regional air quality models.Examples of how specific dry deposition schemes used in participating modelshave been reduced to the common set of comparable diagnostics defined forAQMEII4 are also presented.