An update on variable and changing global tropospheric ozone trends

Tropospheric ozone is a greenhouse gas and pollutant detrimental to human health and crop and ecosystem productivity. However, it is difficult to observe and quantify on the global scale, due to its acute spatial variability, resulting from its variable lifetime and its range of sources (injection from the stratosphere, or photochemical production from natural and anthropogenic precursor gases) and sinks (surface deposition and chemical destruction). To improve our understanding of ozones global distribution and trends, the International Global Atmospheric Chemistry Project (IGAC) initiated the Tropospheric Ozone Assessment Report (TOAR) in 2014. TOAR built the worlds largest database of surface ozone observations and generated ozone exposure metrics at thousands of measurement sites around the world. Several publications used the TOAR surface database, in conjunction with free tropospheric observations and satellite data, to quantify and summarize global surface ozone trends. However, these studies only addressed trends through 2014 or 2016, and are now out of date. Even the most recent assessment of tropospheric ozone trends, just published in the IPCC Sixth Assessment Report, is already out of date because the assessed studies did not include recent data collected during the COVID-19 economic downturn. This presentation will review the latest observed global ozone trends as assessed by IPCC (spanning the 1970s through 2018), emphasizing: 1) the broad range of positive and negative trends in the boundary layer of the Northern Hemisphere extra tropics; 2) strong positive trends in the tropical lower troposphere; 3) weak positive trends in the free troposphere of the Northern Hemisphere extra tropics; 4) stronger trends throughout the tropical free troposphere; 5) limited evidence for ozone increases across the Southern Hemisphere. New studies covering the ozone response during the COVID-19 pandemic will also be reviewed, including original analysis of distinct negative ozone anomalies during 2020 at WMO GAW background sites and in the Northern Hemisphere free troposphere.