Attributing differences in the fate of lateral boundary ozone in AQMEII3 models to physical process representations

Abstract

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.

Multisensor and Multimodel Monitoring and Investigation of a Wintertime Air Pollution Event Ahead of a Cold Front Over Eastern China

“…A multimodel approach instead has been shown more beneficial to investigate these air pollution issues, which can constitute an envelope of realizations of the scenario that adequately represents our understanding of the processes involved (Colette et al., 2011, 2012, 2017; Fiore et al., 2009; Kim et al., 2019; Sokhi et al., 2008; Tai et al., 2012). These multimodel investigations (e.g., Liu et al., 2018; Ma et al., 2019; Solazzo et al., 2017) use a variety of air quality models, including the Community Multiscale Air Quality (CMAQ) Modeling System (Binkowski & Roselle, 2003; Byun & Schere, 2006), and Comprehensive Air quality Model with extensions (CAMx), which are commonly used by the U.S. Environmental Protection Agency (EPA) and other planning organizations for formulating emission control strategies. For air quality simulations in China, as other regions in the world, the primary uncertainties stem from emission inputs (Bouarar et al., 2019; Chen, Liu, Ban, et al., 2019; Kong et al., 2020; Zhao et al., 2019), as well as physical/chemical schemes (Hu et al., 2008; Li, Wang, et al., 2019).…”

The GFDL Variable‐Resolution Global Chemistry‐Climate Model for Research at the Nexus of US Climate and Air Quality Extremes

“…High‐resolution regional models such as the Weather Research and Forecasting model with Chemistry (WRF‐Chem) and the Community Multiscale Air Quality Modeling System (CMAQ) (US EPA, 2022) provide an alternate approach, but they require prescribed atmospheric boundaries from other models. There are substantial issues with imposing global model boundary conditions in a regional model, due to inconsistencies in the physical and chemical schemes and vertical resolution among models, and the neglect of climate feedbacks from regional environments to the global system (e.g., Gao et al., 2013; Hogrefe et al., 2018; Lin et al., 2009, 2010; Liu et al., 2018; Pfister et al., 2014). Furthermore, regional air quality models typically rely on prescribed vegetation characteristics (e.g., Foroutan et al., 2017), limiting their ability to study the impacts of future climate change on vegetation dynamics and feedbacks to air quality.…”

Assessment of the sensitivity of model responses to urban emission changes in support of emission reduction strategies

“…Long-term modelling exercises such as EURODELTA (Bessagnet et al 2016;Ciarelli et al 2019;Colette et al 2017;Mircea et al 2019;Thunis et al 2010;Vivanco et al 2017), AQMEII (Im et al 2018;Liu et al 2018;Solazzo et al 2012Solazzo et al , 2013 and CityDelta (Cuvelier et al 2007;Thunis et al 2007;Vautard et al 2007) were designed to evaluate and intercompare model responses to emission changes. With the exception of Citydelta, these exercises mostly focused on continental and regional model responses.…”