The Impact of Meteorological Conditions and Emissions on Tropospheric Column Ozone Trends in Recent Years

Based on OMI/MLS data (2005-2020) and Community Earth System Model (CESM2) simulated results (2001-2020), annual variation trends of tropospheric column ozone (TCO) in the recent two decades are explored, and the separate impacts of meteorological conditions and emissions on TCO are quantified. The stratospheric ozone tracer (O3S) is used to quantify the contribution of stratospheric ozone to the trend of TCO. The evaluation shows that the simulated results capture the spatial-temporal distributions and the trends of tropospheric column ozone well. Over the East Asia and Southeast Asia regions, TCO is increasing, with a rate of similar to 0.2 DU/yr, which is primarily attributed to the emission changes in ozone precursors, nitrogen oxide (NOx) and volatile organic chemicals (VOCs). But the changes in meteorological conditions weaken the increase in TCO, even leading to a decrease in East Asia in spring and summer. TCO is decreasing in the middle and high latitudes of the southern hemisphere, which is mainly attributed to the changes in meteorological conditions. The increasing rates are the highest in autumn, especially over North America, East Asia, Europe and South of East Asia, with rate values of 0.20, 0.31, 0.17, and 0.32 DU/yr, respectively. Over the equatorial region, the contribution of stratospheric ozone to TCO is below 10 DU, and shows a weak positive trend of similar to 0.2 DU/yr. In the latitude of similar to 30 degrees N/S, the stratospheric contribution is high, similar to 25 DU, and is affected by the sinking branch of the Brewer-Dobson circulation and stratosphere-troposphere exchange in the vicinity of tropical jet stream. The stratospheric contribution to TCO in the north of 30 degrees N is significantly decreasing (similar to 0.6 DU/yr) under the influence of meteorological conditions. Changes in emissions weaken the decrease in stratospheric contributions in the north of 30 degrees N and enhance the increase in 30 degrees S-30 degrees N significantly. The trends of stratospheric contributions on TCO partly explain the trends of TCO which are mostly affected by the change in emissions. To control the increasing TCO, actions to reduce emissions are urgently needed.