Interannual Variation of Tropospheric Ozone over the Tibetan Plateau in Summer and Its Influencing Factors

The Tibetan Plateau, located in the mid-latitude region of the Asian continent, is commonly referred to as the third pole and the water tower of Asia. The high-altitude terrain and distinct circulation systems contribute to the formation of an ozone valley in the atmosphere above the Plateau. The discovery of this ozone valley has garnered significant attention from the international scientific community regarding the ozone levels over the Tibetan Plateau. At the same time, in the context of global warming, the increase of surface ozone concentrations in various regions of the world has posed significant threats to both human health and ecological environment. However, due to limited observational data and satellite data of short time scales, past studies on the Tibetan Plateau's ozone primarily focused on total column ozone, the upper troposphere and lower stratosphere, or surface ozone, while fewer studies have examined the tropospheric ozone column over the region. Therefore, longer-term data are needed to investigate the interannual variability and influencing factors of the tropospheric ozone column over the Tibetan Plateau. Based on AIRS (atmospheric infrared sounder) satellite data from 2003 to 2022, the tropospheric ozone column over the Tibetan Plateau during summer seasons from 2003 to 2022, as well as its interannual variation characteristics are analyzed. Additionally, using ERA5 reanalysis data from 2003 to 2022 and surface station data from China's Ministry of Ecology and Environment from 2015 to 2022, the study employs composite and correlation analyses to explore the factors influencing the tropospheric ozone column over the region. Results show that the tropospheric ozone column over the Tibetan Plateau during the summer exhibits significant interannual variability, increasing at a rate of approximately 0.08 DU per year. The difference of total tropospheric ozone column in high and low years is not only directly related to the difference of vertical distribution of ozone in the upper and lower troposphere, but also related to the difference of dynamic and chemical processes in the upper and lower troposphere. When the total tropospheric ozone column over the Tibetan Plateau is elevated, the tropopause on the northern side of the Plateau is lower, and the subtropical westerly jet is weak and fragmented. The weak transmission barrier enhances stratospheric tropospheric exchange, which is beneficial to the downward transmission of stratospheric high-concentration ozone air, while the vertical circulation in the lower troposphere affects the ozone concentration in the whole troposphere by upward transmission of low-concentration ozone air in the lower troposphere. The tropospheric ozone column anomaly in the northern Plateau is primarily associated with tropopause folding, while the lower ozone concentration across the entire troposphere in the southwestern Plateau is linked to anomalies in the South Asian High. The elevated tropospheric ozone column over the central Plateau may be associated with unusually high levels of surface solar radiation and emissions from surface pollutants.