Technical note: Northern midlatitude baseline ozone – long-term changes and the COVID-19 impact

A nonlinear 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 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.