Tropospheric Ozone Interacts With Climate Change:Mechanisms And Model Development

Tropospheric ozone is one of the key species in atmospheric chemistry,and strongly interacts with climate.Human activities and climate change have led to significant changes in tropospheric ozone over the past 30 years.However,current research on long-term changes in tropospheric ozone in many regions is limited by the sparcity of observations,and are mostly focused on exploring the effects of anthropogenic emissions,while research on the contribution of climate change is rather limited.The aim of this study is to use the latest observations and numerical models to quantify the interannual variabilities and long-term trends of tropospheric ozone,to explore the mechanism of tropospheric ozone-climate change interactions,and to develop new climate-chemistry coupled numerical models.Meteorological conditions significantly influence regional ozone air quality by affecting the its natural sources and photochemistry.China is one of the fastest developing countries since the 1980s,and the rapid increase in anthropogenic pollutant emissions has also led to serious ozone pollution.This study shows that the current level of urban ozone pollution in China is significantly higher than that of developed countries such as Europe,US,Japan and South Korea,and ozone level has shown a significant upward trend since 2013.GEOS-Chem simulations show that the background sources have the largest impact on China’s ground-level ozone levels in 2016-2017（80%-90%）,with natural sources accounting for 72-80%.Biogenic emissions,lightning and soil NO_x emissions,stratospheric influences are all important natural sources of ozone,and their contributions to surface ozone are largely affected by meteorological conditions such as temperature and vertical transport.The surface ozone levels in summer 2017 increased compared with 2016 in many cities in eastern China,mainly due to the hotter and dryer weather conditions in 2017,which increased the ozone contribution of soil and biogenic emissions by 1-2 ppbv,respectively,and accelerated the thermal decomposition of PAN and decreased dry deposition rate of ozone.More stringent control measures on anthropogenic emissions should be taken to alleviate ground-level ozone pollution in China expecially under adverse meteorological conditions（such as high temperatures）.Research on ozone over India is further limited due to the lack of observations.This study used satellite observations and GEOS-Chem simulations to explore the effects of the South Asian summer monsoon（SASM）on the seasonal and interannual variations of ozone in India from 1990 to 2010.Both satellite observations and model simulations indicate that the ozone levels in the Indian lower troposphere（surface to 600 h Pa）peak before the arrival of SASM,driven by the large ozone photochemical production induced by the dry and hot weather conditions.Ozone levels significantly reduced during the SASM prevailing seasons,due to the significant decline of ozone photochemical production and storng ozone upward transport induced by the SASM.The interannual variation of the Indian lower tropospheric ozone levels are significantly affected by the intensity of the SASM.Ozone levels are 3.4 ppbv higher during the weak SASM years compared to those of the strong SASM years,driven by the higher photochemical production of ozone（0.4 Tg）and weaker upward ozone transport（0.2Tg）.GEOS-Chem simulations show that the Indian lower tropospheric ozone levels significantly increase at the rate of 0.19±0.07 ppbv year^-1from 1990 to 2010,which are mainly driven by the increase in anthropogenic emissions(0.18 ppbv year^-1).Climate change influences long-term trends of surface ozone via affecting its natural sources such wildfire emisssions.Wildfires are an important natural source of ozone in the United States Intermountain West（USIW region）.We developed a new method based on Lagrangian simulation and statistical models（FLEXPART-SMLR）to quantify the effects of wildfires on ozone in the USIW region during the summer of 1989-2010.We found that the FLEXPART-SMLR method not only avoids the over-estimation of ozone near the source regions as presented in the many Eulerian model,but also better captures the ozone enhancement in wildfire plumes downwind.Based on the new model,we estimate that the contributions of wildfires to surface ozone in this region are 0.5-1.3 ppbv on average,but can reach 10-20 ppbv in some cases.We found that the strong interannual correlation between ozone concentration and wildfire ozone enhancements during 1989-2010（r>0.80）in the region is mainly due to their common correlations with meteorological parameters such as relative humidity and temperature.At the meantime,wildfires have important impacts on the ozone air quality in the region.The number of days with ozone>70ppbv in the USIW region would be reduced by about 30%in the absence of wildfires.Climate change also influence the interannual variability of tropospheric ozone via affecting the large-scale atmospheric circulations.We present that tropospheric ozone in the Southern Hemisphere（SH）have been slightly but significantly increasing since 1990s from a number of ground,ozonesondes,and satellite observations.The GEOS-Chem simulation shows that,climate change,particularly changes in atmospheric circulation,dominated the increases in tropospheric ozone in the SH in 1990-2010,while increases in anthropogenic emissions and global methane contributed much less.We propose that the poleward expansion of the SH Hadley circulation and the induced changes in the atmospheric circulation,dominate the tropospheric ozone enhancement in the extratropical SH,by increasing the ozone transport from the stratosphere to the troposphere and the ozone photochemical production.The study not only explains for the first time the driving factors of the increasing trend of SH tropospheric ozone,but also reveals a potential climate-chemical positive feedback process.Climate-chemsitry coupled models are indispensable tools for quantifying the interactions between climate change and atmospheric chemistry.The study also developed a new global climate-chemistry coupled model BCC-GEOS-Chem,and evaluated its performance with a focus on tropospheric ozone.The BCC-GEOS-Chem includes an interactive atmospheric general circulation model and the land surface model,and uses the GEOS-Chem as the atmospheric chemistry module.The model includes advanced tropospheric chemistry mechanisms,on-line dry and wet deposition algorithms,and also considers a number of natural emissions process.The simulation of 2012-2014 show that the model can well capture the vertical gradient and seasonal variation of tropospheric ozone,with the calculated tropospheric ozone and OH burden of 336 Tg and 1.16×10⁶molecule cm^-3,respectively.The development of BCC-GEOS-Chem marks an important step for China’s climate system model to move towards the earth system model,and also provides a good scientific tool for further research on climate-chemistry interactions.In summary,by quantifying and analyzing the interannual and long-term trends of tropospheric ozone in China,India,USIW,and the SH,this study explores the effects of climate change on tropospheric ozone through modulating the natural emissions,chemical and deposition processes,and transport.The study also proposes a new feedback mechanism for tropospheric ozone on climate change,and develops a new climate-chemistry coupled numerical model.The results of the study show that we need further studies on the interactions between tropospheric ozone and climate change,for better understandings of tropospheric ozone changes,ozone pollution control,and climate change.