Surface-level ozone (O₃) continues to be a significant health risk in the Greater Toronto Hamilton Area (GTHA) of Canada even though precursor emissions in the area have decreased significantly over the past two decades. In July 2015, Environment and Climate Change Canada (ECCC) led an intensive field study coincident with Toronto hosting the 2015 Pan American Games. During the field study, the daily 1-h maximum O₃ standard (80 ppbv) was exceeded twice at a measurement site in North Toronto, once on July 12 and again on July 28. In this study, ECCC’s 2.5-km configuration of the Global Environmental Multi-scale (GEM) meteorological model was combined with the Modelling Air-quality and CHemistry (MACH) on-line atmospheric chemistry model and the Town Energy Balance (TEB) urban surface parameterization to create a new urban air quality modelling system. In general, the model results showed that the nested 2.5-km grid-spaced urban air quality model performed better in statistical scores compared to the piloting 10-km grid-spaced GEM-MACH model without TEB. Model analyses were performed with GEM-MACH-TEB for the two exceedance periods. The local meteorology for both cases consisted of light winds with the highest O₃ predictions situated along lake-breeze fronts. For the July 28 case, O₃ production sensitivity analysis along the trajectory of the lake-breeze circulation showed that the region of most efficient O₃ production occurred in the updraft region of the lake-breeze front, as the precursors to O₃ formation underwent vertical mixing. In this updraft region, the ozone production switches from volatile organic compound (VOC)-sensitive to NOx-sensitive, and the local net O₃ production rate reaches a maximum. This transition in the chemical regime is a previously unidentified factor for why O₃ surface-level mixing ratios maximize along the lake-breeze front. For the July 12 case, differences between the model and observed Lake Ontario water temperature and the strength of lake-breeze opposing wind flow play a role in differences in the timing of the lake-breeze, which impacts the predicted location of the O₃ maximum north of Toronto.

中文翻译：

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2015年泛美运动会期间表面臭氧过量的化学分析

尽管在过去的二十年中，该地区的前驱物排放量已大大减少，但在加拿大大多伦多汉密尔顿地区（GTHA），地表臭氧（O ₃）仍然是重大的健康风险。2015年7月，加拿大环境与气候变化委员会（ECCC）进行了深入的现场研究，恰逢多伦多主办了2015年泛美运动会。在实地研究中，每天1小时最大O ₃7月12日和7月28日，北多伦多的一个测量点都超过了标准（80 ppbv）两次。在这项研究中，ECCC的2.5公里全球环境多尺度（GEM）气象模型配置与建模空气质量和化学（MACH）在线大气化学模型以及城镇能源平衡（TEB）城市表面参数化，以创建新的城市空气质量建模系统。总体而言，模型结果表明，与不带TEB的10公里网格间隔GEM-MACH试点模型相比，嵌套2.5公里网格间隔城市空气质量模型的统计得分更好。使用GEM-MACH-TEB对两个超期进行了模型分析。两种情况的当地气象由O ₃最高的小风组成沿湖风前沿的预测。对于7月28日的事件，沿湖风循环轨迹的O ₃产生敏感性分析表明，最有效的O ₃产生区域发生在湖风锋的上升区，因为形成了O ₃的前体。垂直混合。在该上升气流区，臭氧的产生从对挥发性有机化合物（VOC）敏感到对NOx敏感，本地净O ₃产生率达到最大值。化学过程中的这种转变是为什么O _3的一个先前未知的因素沿湖风前沿的地表混合比最大。对于7月12日的情况，模型和观测到的安大略湖水温之间的差异以及与微风相对的风的强度在微风时间的差异中起作用，这影响了O ₃最大值的预测位置多伦多北部。