Impacts of global change on tropospheric ozone and mercury

Global change including changes in anthropogenic emissions, climate, land use and land cover have been imposing significant perturbations to atmospheric chemistry and air quality. In this work, we use a global 3-D chemical transport model (GEOS-Chem) to investigate the impacts of global change on two important air pollutants that have caused local, regional and global concerns: ozone and mercury. Ozone is a criteria air pollutant in the surface air. Tropospheric ozone is also a major greenhouse gas and has important implications for tropospheric chemistry. Mercury pollution poses risks to humans and wildlife, especially when it is converted to methylmercury in the aquatic system and bioaccumulate in the food chain. The stratospheric ozone level is predicted to recover towards its pre-1980 levels with the implementation of the Montreal Protocol and its amendments and adjustments. Our global model simulations show that the expected stratospheric ozone recovery would significantly decreases the photolysis rates for tropospheric ozone with the surface O3 photolysis rates being reduced by up to 22%. The photolysis rates for tropospheric NO2 show much weaker sensitivity to the changes in stratospheric ozone. In addition, the stratospheric ozone recovery causes strong seasonal variation and general increases in surface ozone, particularly over some ocean areas where surface ozone could increase by up to 5%. The lifetime of tropospheric ozone is augmented by stratospheric ozone recovery, which in turn enhances the intercontinental transport of ozone. We examine the impacts of changes in climate and land use and land cover on atmospheric mercury by coupling the GEOS-Chem model with a general circulation model (GISS GCM3) and a global dynamic vegetation model (LPJ). The land use and land cover change causes an increase in the annual mean Hg(0) dry deposition flux over the majority of the continental regions as a result of increasing leaf area index. Climate change drives the surface Hg(0) concentration to increase globally primarily due to suppressed tropospheric mercury oxidation and increased in-cloud mercury reduction and subsequent increase in Hg(0) dry deposition flux. Furthermore, the change in future precipitation greatly affects the mercury wet deposition flux with increases occurring over most continental regions and decreases over most of the mid-latitude and tropic oceans. Both changes in climate and land use and land cover would potentially drive more gross mercury deposition towards the terrestrial system and less to the ocean system. The growing concerns of elevated methyl mercury contamination have made it important to improve the understanding of the source-receptor relationship of mercury deposition, especially in the context of global change. We investigate the source-receptor relationship of atmospheric mercury deposition among several regions induced by changes in anthropogenic emissions, climate, land use and land cover, using the GEOS-Chem model driven by GISS ModelE2 meteorology. We examine the relative contribution to the total deposition in receptor regions from various sources (e.g. domestic vs foreign anthropogenic emission sources). Changes in 2050 land use and cover, and climate have more impacts on the directional changes of atmospheric mercury while changes in 2050 anthropogenic emissions show more uniformed distributions of changes in atmospheric mercury flux. In the Great Lakes region, the average gross deposition flux attributed from total anthropogenic emissions in the Great Lakes region show little change in 2050 climate but with increase in the lower part of the Great Lakes region near the point sources and decrease in the upper part. Changes in 2050 anthropogenic emissions under B1 scenario and land use and land cover drive the average gross deposition flux to decrease by 2 and 1.5 ug/m2/yr relevant to present-day, respectively