The interactive effect between ozone, sulfate aerosol, East Asia summer monsoon under present and future climate

Abstract

Abstract

Changing climate and air quality are strongly connected each other. For example, ozone and aerosol concentrations are strongly influenced by wind and temperature changes. Those air pollutants also play an important role in climate as a short-lived climate forcer. However, the understanding of the interaction between the two is still very low. This dissertation is to address the uncertainties of the interactive effects between climate change and air pollutants focusing on three objectives: (1) The effect of aerosol on the East Asian summer monsoon, (2) Future ozone and oxidants change under the RCP scenarios, and (3) Relationship changes between the East Asian summer monsoon and ozone in surface air in the present and future climate. I first examine the effect of anthropogenic aerosol forcing on the East Asian summer monsoon (EASM) using a general circulation model. One control and two sensitivity model experiments were conducted in order to diagnose the separate roles played by sea surface temperature (SST) variations and anthropogenic sulfate aerosol forcing changes in East Asia. I find that the SST variation has been a major driver for the observed weakening of the EASM, whereas the effect of the anthropogenic aerosol forcing has been opposite and has slightly intensified the EASM over the recent decades. The reinforcement of the EASM results from radiative cooling by the sulfate aerosol forcing, which decelerates the jet stream around the jets exit region. Subsequently, the secondary circulation induced by such a change in the jet stream leads to the increase in precipitation around 18-23°N. This result indicates that the increase in anthropogenic emissions over East Asia may play a role in compensating for the weakening of the EASM caused by the SST forcing. I investigate the ozone air quality changes in 2050 caused by global changes in climate and anthropogenic emissions of ozone precursors using a global chemical transport model driven by meteorological fields from a general circulation model. My model results show that annual mean concentrations of surface ozone will be lower in 2050 relative to 2000 by -3.3, -3.7, and -4.2 ppbv under RCP6.0, RCP4.5, and RCP2.6, respectively. In contrast, the RCP8.5 projection results in a slight increase of 2.1 ppbv caused by a methane increase. The ozone reductions are driven primarily by decreases in NOx emission, which dominate the climate penalty on ozone driven by temperature increases. I also estimate the effect of 21st century climate change on ozone air quality, assuming no changes in anthropogenic emissions of ozone precursors in the future. Temperature increase is found to result in ozone increases of up to 2.2 ppbv over land. Ozone over the oceans, however, is largely reduced with specific humidity increase, particularly in Northern Hemisphere, where the ozone concentration decreases by 0.8 ppbv. I also examine the effect of the EASM change on surface ozone concentrations over East Asia using the GEOS-Chem, which is driven by meteorological fields from the Community Earth System Model (CESM). I conduct model simulations using the RCP8.5 scenario to estimate the effects of the EASM on ozone 2000 and 2050. My model results show that ozone concentrations are positively correlated with the EASM in Central China. On the other hand, an opposite relationship is found in downwind regions including Eastern China, Korea, and Japan owing to a cyclonic circulation associated with the EASM. However, the relationship between the ozone change and the EASM becomes opposite in 2050 compared to that of 2000. The 2000-2050 change in the relationship between the ozone and the EASM is mainly due to a EASM domain shift under the warming climate in 2050, indicating the conventional EASM index based on the present climate condition cannot be applied to the future climate. Therefore, a modified EASM index is applied to examine the relationship between the two. I find, however, a weaker correlation between ozone and the EASM change in the downwind region in 2050, which is associated with the weakening of cyclonic circulation associated with the EASM over East Asia in 2050. These results indicate that the ozone change owing to the inter-annual variation of the EASM may change under the global warming climate.