Abstract. A non-linear change in baseline ozone concentrations at northern midlatitudes has been quantified over preceding decades. During the past few years several studies, using linear trend analyses, report relatively small trends over selected time periods – results inconsistent with the earlier developed picture. We show that reported COVID-19 related ozone changes in the background troposphere based on the linear analysis are significantly larger than those derived considering recent long-term decreases in background ozone, which the linear trend analyses do not quantify. We further point out that the extensive loss of lower stratospheric ozone in the unprecedented 2020 springtime Arctic stratospheric ozone depletion event likely reduced the natural source to the troposphere rendering the background anomalously low that year. Consideration of these two issues indicates that the COVID-19 restrictions had a much smaller impact on background tropospheric ozone in 2020 than previously reported. A consensus understanding of baseline ozone changes and their causes is important for formulating policies to improve ozone air quality; cooperative, international emission control efforts aimed at continuing or even accelerating the ongoing decrease in in hemisphere-wide background ozone concentrations may be the most effective approach to further reducing urban and rural ozone in the more developed northern midlatitude countries, as well as improving ozone air quality in all countries within these latitudes. Analysis of baseline ozone measurements over several years following the COVID-19 impact is expected to provide a firm basis for resolving the inconsistencies between the two views of long-term northern midlatitude ozone changes and better quantifying the COVID-19 impact.

中文翻译：

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北部中纬度基线臭氧：长期变化和 COVID-19 影响

摘要。在过去的几十年中，已经量化了北部中纬度地区基线臭氧浓度的非线性变化。在过去几年中，一些使用线性趋势分析的研究报告了选定时间段内相对较小的趋势——结果与早期发展的图景不一致。我们表明，基于线性分析报告的背景对流层中与 COVID-19 相关的臭氧变化明显大于考虑到背景臭氧最近长期减少而得出的那些，线性趋势分析没有量化。我们进一步指出，在 2020 年春季史无前例的北极平流层臭氧消耗事件中，低层平流层臭氧的大量损失可能会减少对流层的自然来源，从而导致当年的背景异常低。对这两个问题的考虑表明，COVID-19 限制对 2020 年背景对流层臭氧的影响比之前报道的要小得多。对基线臭氧变化及其原因的共识理解对于制定改善臭氧空气质量的政策很重要；旨在继续甚至加速整个半球背景臭氧浓度持续下降的合作性国际排放控制努力可能是进一步减少较发达的北部中纬度国家的城市和农村臭氧以及改善臭氧空气的最有效方法这些纬度内所有国家的质量。