Ozone Stratosphere-Troposphere Exchange in the Past, Present, and Future [electronic resource]

다국어 초록 (Multilingual Abstract)

As a key component of the Earth system, stratospheric ozone protects life on Earth from hazardous ultraviolet radiation and has a crucial impact on tropospheric chemistry. Stratosphere-troposphere exchange (STE) of ozone represents a significant source term in the tropospheric ozone budget and can impact surface ozone concentrations, tropospheric oxidation capacity, and methane lifetime. The stratospheric ozone and ozone STE in the past, present, and future climates are investigated in this dissertation.In Chapter 2, we estimate the STEs of air masses and ozone concentrations averaged over 2007 to 2010 using the Modern Era Retrospective analysis for Research and Application, version 2 (MERRA2) and ERA5 reanalyses, and observations. The extratropical downward ozone fluxes are 528-543 Tg year-1 from these reanalyses and observations, consistent with previous studies. Previous studies, however, did not consider tropical upward ozone flux. Here we show that the tropical upward ozone flux is 185 Tg year-1, compensating about 35% of the extratropical downward ozone fluxes, and should not be neglected. After considering the tropical upward ozone flux, the global ozone STE is 347±12 Tg year-1, which can be used as the contribution of ozone STE to the tropospheric ozone budget. Cloud radiative effects on the STE of air mass and ozone are also investigated. At 380 K, cloud radiative effects enhance downward fluxes in the extratropics from both reanalyses and observation, but reduce and enhance upward fluxes in the tropics from reanalyses and observation, respectively. The discrepancy in the tropics is related to the tropical tropopause layer thin cirrus that is missing in the reanalyses. It is found the cloud radiative effects enhance the global ozone STE by about 25%.In Chapter 3, STE of air mass and ozone in ERA5 and MERRA2 reanalyses from 1980-2022 are investigated. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to tropical tropopause as the upper boundary of lowermost stratosphere. The seasonal cycle, annual-mean climatology, and monthly anomalies of air mass and ozone STEs are studied. The annual-mean ozone STEs over the NH extratropics, SH extratropics, tropics, extratropics, and globe in ERA5 are -342, -239, 201, -581, and -380 Tg year-1, respectively, versus -305, -224, 168, -529, -361 Tg year-1 from MERRA2. The annual-mean global ozone STE difference between ERA5 and MERRA2 is dominated by diabatic heating difference, partly compensated by ozone concentration difference. There are about 40% (-40%) differences between ERA5 and MERRA2 in global ozone STEs in boreal summer (autumn), mainly due to the difference in seasonal breathing of the lowermost stratosphere ozone mass between reanalyses. For the global ozone STE monthly anomalies, ERA5 and MERRA2 can only explain each other's variance by 30%. Multiple linear regression analysis shows that El Nino-Southern Oscillation, Quasi-Biennial Oscillation, Brewer-Dobson circulation (BDC), solar cycle, and volcanic aerosols can only explain the variance in global ozone STE monthly anomalies by 0.9-1.8, 2.5-3.9, 0.1-1.0, 0.2-0.3, 1.7-4.1%, respectively, with a residual variance larger than 90%. Furthermore, the volcanic aerosol impacts on ozone STEs from ERA5 and MERRA2 have opposite signs and thus are inconclusive. Cautions are therefore needed when using ERA5 and MERRA2 to investigate the STE seasonal cycle and interannual variability.Chapter 4 investigates the changes in the stratospheric ozone in the Last Glacial Maximum (LGM) as compared with preindustrial (PI) climate, using the Whole Atmosphere Community Climate Model 6 (WACCM6). It is shown that, compared with PI times, LGM modeled stratospheric temperatures are increased by up to 8 K, leading to faster ozone destruction rates for gas phase reactions, especially via the Chapman mechanism. On the other hand, stratospheric hydroxyl radical (OH) and nitrogen oxides (NOx) concentrations are decreased by 10-20%, which decreases catalytic ozone destruction, thereby decreasing ozone loss rates. The net effect of these two compensating mechanisms in the upper stratosphere (above 15 hPa) is a vertically-integrated 1-3 Dobson Unit (DU) decrease during the LGM. In the lower stratosphere (tropopause to 15 hPa), changes in the stratospheric overturning circulation and resulting transport dominate changes in ozone. Consistent with a weakening of the residual circulation in the LGM, lower stratospheric ozone is increased by 2-5 DU in the tropics and decreased by 5-10 DU in the extratropics, but the latter is partly compensated by ozone increases due to a lower tropopause. It is found that tropospheric ozone is decreased by about 5 DU in the LGM versus PI. Combined changes in stratospheric and tropospheric ozone lead to a decrease in total ozone column everywhere except over the northeast North America, equatorial Indian and west Pacific Oceans, and East Antarctica. Surface ultraviolet radiation in the LGM versus PI is increased over the Northern Hemisphere mid- and high-latitudes, especially over the ice caps, and over the Southern Hemisphere near 60° S.In Chapter 5, changes in the air mass and ozone STEs in the Last Glacial Maximum (LGM) as compared with preindustrial (PI) climate are studied using WACCM6. We use dynamic isentropic surfaces that are determined by fitting to the tropical tropopauses as the upper boundary of the lowermost stratosphere in a mass budget approach, a method particularly suitable for estimating air mass and ozone STEs across different climates. Relative to the PI, the magnitude of ozone STE in the LGM is decreased by 14-19%, 18-24%, 18-23%, 16-21%, 15-21% over the NH extratropics, SH extratropics, the tropics, the extratropics, and the globe, respectively. The extratropical and global decreases are mainly caused by decreased ozone in the extratropical lower stratosphere associated with a weakening of BDC, while changes in air mass fluxes play a minor role because the effects of weakening BDC and increased isentropic density partly cancel each other. Analysis of the modelled tropospheric ozone budget indicates that the ozone STE in the LGM is 28% of the tropospheric ozone production rate, as compared to about 9% in the modern climate (year 2000) and 19% in the PI.In Chapter 6, we investigate changes in STE of air masses and ozone concentrations from 1960 to 2099 using multiple model simulations from the Chemistry Climate Model Initiative (CCMI) under the climate change scenario RCP6.0. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to the tropical tropopause as the upper boundary of lowermost stratosphere. The multi-model mean (MMM) trends of air mass STEs are all small over all regions, which are within 0.3 (0.1) % decade-1 for 1960-2000 (2000-2099). The MMM trends of ozone STE for 1960-2000 are 0.3, -2.7, 3.4, -0.9, and -2.7% decade-1 over the NH extratropics, SH extratropics, tropics, extratropics, and globe, respectively. The corresponding ozone STE trends for 2000-2099 are 3.0, 4.3, 0.8, 3.5, and 4.7% decade-1. Changes in ozone STEs are dominated by ozone concentration changes, driven by climate-induced changes and ozone-depleting substance (ODS) changes. For 1960-2000, small changes in ozone STEs in the NH extratropics are due to a cancellation between effects of climate-induced changes and ODS increases, while the ODS effect dominates in the SH extratropics, leading to a large ozone STE magnitude decrease. Increased ozone transport from tropical troposphere to stratosphere for 1960-2000 is due to increased tropospheric ozone. A decreased global ozone STE magnitude for 1960-2000 was largely caused by ODS-induced ozone loss that is partly compensated by climate-induced ozone changes. For 2000-2099, about two-thirds of global ozone STE magnitude increases are caused by ozone increases in the extratropical lower stratosphere due to climate-induced changes. The remaining one-third is caused by ozone recovery due to the phaseout of ODS.