Compound events (e.g., co-occurrence of two extreme weather events simultaneously) have the potential to aggravate the impacts of heat wave and stagnation on ozone through nonlinear effects. Using the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem), this study investigated the impact of compound extreme weather events on ozone enhancement in the contiguous U.S. The analyses are based on simulations for the historical (2001–2006) and future (2046–2055) periods previously performed, with emissions in the future following the A1B/B2 emission scenarios produced by the Technology Driver Model (TDM). While A1B reflects rapid economic growth, B2 emphasizes more on local environmental sustainability, hence lower anthropogenic emissions. With overall warming in the future, the frequency and intensity of compound extreme events increase more than that of heat waves. Thus compound events promote higher ozone concentration compared to the single extreme events of heat wave and stagnation in larger areas of the U.S. in the future relative to the present. However, averaged over the U.S., the amplification effect of compound events on ozone enhancement is diminished in the future. Sensitivity experiments to isolate the effect of climate change versus emission change showed that climate change under the future emission scenarios dominates the weakening amplification effect of compound events. Further analyses to examine the nonlinear effect of compound events (NLRE), defined as the difference between the enhanced effect on ozone during compound events and the additive effect of heat waves and stagnation relative to non-extreme, showed that regions prone to positive NLRE tend to have compound events marked by higher temperature and stronger stagnation than the single extreme events. Simulations showed reduced mean positive NLRE in the U.S. in the future, suggesting that future emission reduction may provide a previously unrecognized benefit in ozone pollution control by reducing the impacts of compound events on ozone.

复合事件（例如，同时发生两个极端天气事件）有可能通过非线性效应加剧热波和停滞对臭氧的影响。本研究使用天气研究与预报模型与化学（WRF / Chem）结合，研究了复合极端天气事件对美国连续海域臭氧增强的影响。该分析基于对历史（2001-2006年）和未来（ 2046–2055）期间执行，未来将按照技术驱动程序模型（TDM）产生的A1B / B2排放情景进行排放。尽管A1B反映了经济的快速增长，但B2则更加强调了当地环境的可持续性，因此降低了人为排放量。随着未来的整体变暖，复合极端事件的频率和强度比热浪增加更多。因此，与将来在美国大片地区相对于目前的热浪和停滞的单个极端事件相比，复合事件促进了更高的臭氧浓度。但是，按照美国的平均水平，复合事件对臭氧增强的放大作用将来会减弱。隔离气候变化对排放变化影响的敏感性实验表明，未来排放情景下的气候变化将主导复合事件的减弱放大作用。进一步分析以检查复合事件的非线性影响（NLRE），定义为复合事件对臭氧的增强作用与热波和停滞相对于非极端的相加作用之间的差异，表明易于发生NLRE的区域往往具有比单个事件更高温度和更强停滞特征的复合事件极端事件。模拟显示，未来美国的平均正NLRE降低，这表明未来的减排可能会通过减少复合事件对臭氧的影响而为臭氧污染控制带来以前无法认识的好处。