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Abstract. Dry deposition is a major sink of tropospheric ozone.Increasing evidence has shown that ozone dry deposition actively linksmeteorology and hydrology with ozone air quality. However, there is littlesystematic investigation on the performance of different ozone drydeposition parameterizations at the global scale and how parameterizationchoice can impact surface ozone simulations. Here, we present the results ofthe first global, multidecadal modelling and evaluation of ozone drydeposition velocity (vd) using multiple ozone dry depositionparameterizations. We model ozone dry deposition velocities over 1982–2011using four ozone dry deposition parameterizations that are representative ofcurrent approaches in global ozone dry deposition modelling. We useconsistent assimilated meteorology, land cover, and satellite-derived leafarea index (LAI) across all four, such that the differences in simulatedvd are entirely due to differences in deposition model structures orassumptions about how land types are treated in each. In addition, we usethe surface ozone sensitivity to vd predicted by a chemical transportmodel to estimate the impact of mean and variability of ozone dry depositionvelocity on surface ozone. Our estimated vd values from four differentparameterizations are evaluated against field observations, and whileperformance varies considerably by land cover types, our results suggestthat none of the parameterizations are universally better than the others.Discrepancy in simulated mean vd among the parameterizations isestimated to cause 2 to 5 ppbv of discrepancy in surface ozone in theNorthern Hemisphere (NH) and up to 8 ppbv in tropical rainforests in July,and up to 8 ppbv in tropical rainforests and seasonally dry tropical forestsin Indochina in December. Parameterization-specific biases based onindividual land cover type and hydroclimate are found to be the two maindrivers of such discrepancies. We find statistically significant trends inthe multiannual time series of simulated July daytime vd in allparameterizations, driven by warming and drying (southern Amazonia, southernAfrican savannah, and Mongolia) or greening (high latitudes). The trend inJuly daytime vd is estimated to be 1 % yr−1 and leadsto up to 3 ppbv of surface ozone changes over 1982–2011. The interannual coefficient ofvariation (CV) of July daytime mean vd in NH is found to be5 %–15 %, with spatial distribution that varies with the dry depositionparameterization. Our sensitivity simulations suggest this can contributebetween 0.5 to 2 ppbv to interannual variability (IAV) in surface ozone, butall models tend to underestimate interannual CV when compared to long-termozone flux observations. We also find that IAV in some dry depositionparameterizations is more sensitive to LAI, while in others it is more sensitiveto climate. Comparisons with other published estimates of the IAV ofbackground ozone confirm that ozone dry deposition can be an important partof natural surface ozone variability. Our results demonstrate the importanceof ozone dry deposition parameterization choice on surface ozone modellingand the impact of IAV of vd on surface ozone, thus making a strong casefor further measurement, evaluation, and model–data integration of ozone drydeposition on different spatiotemporal scales. 

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.