Multi-Model Comparison in the Impact of Lateral Boundary Conditions on Simulated Surface Ozone across the United States Using Chemically Inert Tracers

Abstract

Abstract. This study represents an inter-comparison of four regional-scale air quality simulations, focused on understanding similarities and differences in the simulated impact of ozone lateral boundary conditions (LBCs) on the ground-level ozone predictions across the U.S. The chemically inert tracers were implemented in the simulations as a diagnostic tool to 15 understand the similarities and differences between models at process level. For all simulations, three chemically inert tracers (BC1 BC2 and BC3) are used to track the impact of ozone specified at different altitudes along the lateral boundaries of the modeling domain encompassing the contiguous U.S. The altitude ranges specified for BC1, BC2, and BC3 broadly represent the planetary boundary layer (PBL), free troposphere, and upper troposphere-lower stratosphere, respectively.The four simulations, namely WRF/CMAQ, WRF/CAMx, WRF/DEHM and COSMO-CLM/CMAQ, can have considerable 20 differences in the simulated inert tracers at surface, indicating their different estimates in the impact of lateral boundary on surface ozone within the U.S. due to the physical processes alone in chemical transport models. WRF/CMAQ is used as a base case, and the differences between WRF/CMAQ and the other three models are examined, respectively. The model pair of COSMO-CLM/CMAQ and WRF/CMAQ shows the smallest differences in inert tracers, with the difference in BCT (sum of BC1, BC2 and BC3) peak in winter to be 1.6 ppb averaged across all sites. The model pair of WRF/DEHM and 25 WRF/CMAQ shows the largest differences, with difference in BCT peak in summer to be 8.1 ppb averaged across all sites. Furthermore, the model differences in inert tracers are discussed with respect to the physical processes that inert tracers undergo. It is found that the process of vertical turbulent mixing between the PBL and the free troposphere is the main cause of the model differences in the simulated inert tracers, especially the relative contributions of BC1 and BC2 to the total inert tracers, in most seasons and regions of the U.S., although the processes of sub-grid cloud mixing and dry deposition can also 30 be important drivers for specific regions and seasons.Atmos. Chem. Phys. Discuss., https://doi

Modeling intercontinental transport of ozone in North America with CAMx for the Air Quality Model Evaluation International Initiative (AQMEII) Phase 3

“…Factors contributing to these differences may include fewer vertical layers in CAMx (26, compared to 35 in CMAQ), allowing greater transport of UTLS O 3 to ground level (Emery et al, 2012), omission of wet scavenging for the CAMx inert tracers, treatment of deep convective transport in CMAQ, or differences in model treatments of O 3 dry deposition. Liu et al (2017) performed multi-model process comparisons with four AQMEII models and draw similar conclusions regarding factors that can contribute to differences in tracer impacts.…”

Seasonal ozone vertical profiles over North America using the AQMEII3 group of air quality models: model inter-comparison and stratospheric intrusions

“…The two simulations using CMAQ (US3 and DE1) do not produce similar results at any of the sites, although they share the same BCs and emissions. In contrast, the CMAQ and CAMx simulations (US3 and US1 respectively) which share common meteorological fields, and thus the same PBL scheme (but use a different vertical resolution as noted by Liu et al, 2018), have rather similar results.…”

“…The seasonality of contributions seen in the LT and MT layers is, thus, directly related to planetary boundary layer processes and designates the significance of the influence that lateral boundary conditions have during each season. An in-depth multi-model comparison of the inert tracer mixing ratios at the surface is provided by Liu et al (2018).…”