Abstract

Increasing emphasis has been placed on characterizing the contributions and the uncertainties of ozone imported from outside the US. In chemical transport models (CTMs), the ozone transported through lateral boundaries (referred to as LB ozone hereafter) undergoes a series of physical and chemical processes in CTMs, which are important sources of the uncertainty in estimating the impact of LB ozone on ozone levels at the surface. By implementing inert tracers for LB ozone, the study seeks to better understand how differing representations of physical processes in regional CTMs may lead to differences in the simulated LB ozone that eventually reaches the surface across the US. For all the simulations in this study (including WRF/CMAQ, WRF/CAMx, COSMO-CLM/CMAQ, and WRF/DEHM), three chemically inert tracers that generally represent the altitude ranges of the planetary boundary layer (BC1), free troposphere (BC2), and upper troposphere-lower stratosphere (BC3) are tracked to assess the simulated impact of LB specification. Comparing WRF/CAMx with WRF/CMAQ, their differences in vertical grid structure explain 10 %-60 % of their seasonally averaged differences in inert tracers at the surface. Vertical turbulent mixing is the primary contributor to the remaining differences in inert tracers across the US in all seasons. Stronger vertical mixing in WRF/CAMx brings more BC2 downward, leading to higher BCT (BCT = BC1+BC2+BC3) and BC2/BCT at the surface in WRF/CAMx. Meanwhile, the differences in inert tracers due to vertical mixing are partially counteracted by their difference in sub-grid cloud mixing over the southeastern US and the Gulf Coast region during summer. The process of dry deposition adds extra gradients to the spatial distribution of the differences in DM8A BCT by 5-10 ppb during winter and summer. COSMO-CLM/CMAQ and WRF/CMAQ show similar performance in inert tracers both at the surface and aloft through most seasons, which suggests similarity between the two models at process level. The largest difference is found in summer. Sub-grid cloud mixing plays a primary role in their differences in inert tracers over the southeastern US and the oceans in summer. Our analysis of the vertical profiles of inert tracers also suggests that the model differences in dry deposition over certain regions are offset by the model differences in vertical turbulent mixing, leading to small differences in inert tracers at the surface in these regions.

Highlights

  • Studies based on chemical transport models (CTMs) have shown that air quality in the US can be considerably influenced by pollutants beyond the US boundaries, such as through intercontinental transport and through stratosphereto-troposphere exchange

  • WRF/CMAQ is used as a base case, and the impact of a variety of physical processes on the inert tracers at the surface is investigated by comparing WRF/CMAQ with sensitivity simulations

  • This study investigated the impact of physical treatment in CTMs on lateral boundary (LB) ozone reaching the surface across the US with the implementation of inert tracers for LB ozone

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Summary

Introduction

Studies based on chemical transport models (CTMs) have shown that air quality in the US can be considerably influenced by pollutants beyond the US boundaries, such as through intercontinental transport and through stratosphereto-troposphere exchange Nopmongcol et al, 2016; Langford et al, 2017; Lin et al, 2017; Hogrefe et al, 2018). Similar findings have been reported based on routine observations and field campaign measurements (e.g., Cooper et al, 2012; Gratz et al, 2015; Langford et al, 2015), especially at rural and elevated locations in the western US.

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