A case study of surface ozone source contributions in the Seoul metropolitan area using the adjoint of CMAQ

To quantitatively investigate the transboundary behaviors and source attributions of ozone (O3) and its precursor species over East Asia, we utilize the adjoint technique in the CMAQ modeling system (the CMAQ adjoint). Our focus is on the Seoul Metropolitan Area (SMA) in South Korea, which is the receptor region of this study. We examine the contributions of both local and transported emissions to an O3 exceedance episode observed on June 3, 2019, estimating up to four days in advance. By using the CMAQ adjoint, we can determine the sensitivity of O3 remaining in the SMA to changes in O3 precursor emissions (emissions-based sensitivity) and concentrations (concentrations-based sensitivity) along the long-range transport pathways and emission source regions overseas. These include Beijing-Tianjin-Hebei (BTH), Shandong, Yangtze River Delta (YRD), and Central China. CMAQ adjoint-derived source attributions suggest that overseas precursor emissions and O3 contributed significantly to the O3 exceedance event in SMA. The emissions-based sensitivities revealed that precursor emissions originating from Shandong, YRD, Central China, and BTH contributed 11.42 ppb, 4.28 ppb, 1.24 ppb, 0.9 ppb, respectively, to the O3 exceedance episode observed in the SMA. Meanwhile, Korean emissions contributed 31.1 ppb. Concentrations-based sensitivities indicated that 19.3 ppb of contributions originated in regions beyond eastern China and directly affected the O3 level in the SMA in the form of background O3. In addition to capturing the transboundary movements of air parcels between the source and receptor regions, we performed HYSPLIT backward trajectory analyses. The results align with the trajectories of O3 and its precursors that we obtained from the adjoint method. This study represents a unique effort in employing the adjoint technique to examine the impacts of regional O3 on South Korea, utilizing a combination of emissions-based and concentrations-based sensitivities.
Implications: This research brings to light the critical role of both local and regional precursor emissions in contributing to an ozone (O3) exceedance event in the Seoul Metropolitan Area (SMA), South Korea. Utilizing the CMAQ adjoint technique, a novel approach in the context of South Korea’s O3 investigations, we were able to delineate the quantitative contributions of different regions, both within South Korea and from overseas areas such as Beijing, Shandong, Shanghai, and Central China. Importantly, the results underscore the substantial influence of transboundary pollutant transport, emphasizing the need for international collaboration in addressing air quality issues. As metropolitan areas around the globe grapple with similar challenges, the methodology and insights from this study offer a potent tool and framework for regions seeking to understand and mitigate the impacts of O3 on human health and the environment. By integrating different sensitivity types, coupled with HYSPLIT backward trajectory analyses, this research equips policymakers with comprehensive data to design targeted interventions, emphasizing the significance of collaborative efforts in tackling regional air pollution challenges. However, it’s important to note the limitation of this study, which is a case study conducted over a short time period. This constraint may impact the generalizability of the findings and suggests a need for further research to validate and expand upon these results.

为定量探究东亚地区臭氧(O3)及其前体物的跨界传输行为与来源贡献，本研究采用公共多尺度空气质量模型(CMAQ)系统中的伴随技术(CMAQ伴随模型)。本研究以韩国首尔都会区(SMA)为受体区域，针对2019年6月3日观测到的臭氧超标事件，分析了本地排放与输送排放对该事件的贡献，并提前至多四天进行了模拟估算。借助CMAQ伴随模型，我们可以明确首尔都会区留存臭氧对长距离传输路径及海外排放源区的臭氧前体物排放(排放基敏感性)与浓度(浓度基敏感性)变化的敏感性，这些源区包括京津冀(BTH)、山东、长江三角洲(YRD)以及华中地区。基于CMAQ伴随模型的来源归因结果显示，海外前体物排放与臭氧对首尔都会区的臭氧超标事件具有显著贡献。排放基敏感性结果显示，源自山东、长三角、华中及京津冀的前体物排放分别为首尔都会区观测到的臭氧超标事件贡献了11.42 ppb、4.28 ppb、1.24 ppb与0.9 ppb；与此同时，韩国本地排放贡献了31.1 ppb。浓度基敏感性结果则表明，19.3 ppb的贡献源自中国东部以外区域，并以背景臭氧的形式直接影响了首尔都会区的臭氧浓度水平。除了追踪源区与受体区域之间气团的跨界传输路径外，我们还开展了混合单粒子拉格朗日综合轨迹(HYSPLIT)后向轨迹分析，其结果与我们通过伴随方法得到的臭氧及其前体物的传输轨迹一致。本研究结合排放基与浓度基敏感性分析，采用伴随技术探究区域臭氧对韩国的影响，具有独特的研究价值。

研究启示：本研究揭示了本地与区域前体物排放对韩国首尔都会区臭氧超标事件的关键驱动作用。本研究采用CMAQ伴随技术——这在韩国臭氧相关研究中属于创新方法——定量明确了韩国境内及北京、山东、上海、华中地区等海外源区的贡献份额。尤为重要的是，研究结果凸显了跨界污染物传输的显著影响，强调了开展国际协作以应对空气质量问题的必要性。全球诸多都会区均面临类似的臭氧污染挑战，本研究的方法与结论可为各地区理解并减缓臭氧对人类健康与生态环境的影响提供有力工具与分析框架。本研究结合多种敏感性分析方法与HYSPLIT后向轨迹分析，为政策制定者提供了制定精准干预措施的全面数据支撑，进一步凸显了协同应对区域空气污染挑战的重要意义。但需注意本研究存在一定局限：仅针对短时段开展了个例研究，这一限制可能影响研究结果的外推性，因此需开展进一步研究以验证并拓展该结论。