Simulations and projections of major air pollutants over the United States and uncertainty analyses, effects of natural change and human activities

Changes in global climate and pollutant emissions are very likely to continue in the coming decades driven by the human-related activities and natural fluctuations in the Earth climate system. These potential changes would have very important consequences on regional air quality over the contiguous United States due to their effects on atmospheric chemical and physical processes. To understand these effects, the present studies use the global climate chemistry model, CAM-Chem version 3, to systematically assess potential changes in major air pollutants including surface ozone, particulate matter and mercury from the present (1998???2002) to the 2050 (2048???2052). The projections of future air quality consider changes in global climate, precursor emissions from anthropogenic and biogenic sources, and pollutant transport. Moreover, to evaluate the projection uncertainties resulting from different plausible trends of climate and emissions as a result of unknown human-related activities and climate variations, three IPCC SRES scenarios, A1FI, A1B and B1, are considered and compared to evaluate the resulting uncertainty in projecting future pollutant concentrations. To achieve a better understanding on the effect of mineral dust emissions on changes in future air quality especially the PM concentrations, a physical dust aerosol module is developed and incorporated into the CAM-Chem model. A mercury module is developed for the CAM-Chem model to simulate the atmospheric cycle of mercury and its consequences on the toxicity of U.S. air quality. For the study of ozone air quality, we focus on the risk of high ozone episodes and the relative contributions from changes in local anthropogenic emissions (LE) versus changes in intercontinental transport (ICT) on 2050 U.S. surface ozone air quality. It is found that the projected changes in air temperature, precipitation, lighting, planetary boundary layer height and cyclone activities tend to intensify the associated extreme weather conditions that foster the risk of high ozone pollution episodes over many parts of the world. As a result of both changes in climate and emissions, the frequency of hazardous ozone days in the summertime is derived to be 6-73 days for North America. Through analysis of the contributions from changes in domestic and international emissions, we find that projected changes in the U.S. domestic anthropogenic emissions and the changes in long range transport under the A1FI scenario have a comparable contribution in affecting western U.S. ozone air quality in 2050. However, under the A1B and B1 scenarios, contributions from changes in domestic emissions are more significant than that from changes in long range transport. The results indicate that for the United States, pollution control is a domestic issue under low global emission situations, while it becomes an international issue when fossil fuel use is rapidly increasing throughout the world. For the study of future surface particulate matter (PM) air quality, a physical dust aerosol module is developed and incorporated into the CAM-Chem to better simulate the dust emission over the globe and improve the simulations of PM air quality. This dust module incorporates the wind erosion physics of Shao [2008] and various other improvements necessary for a global modeling. These improvements include the parameterizations of frozen soil effect and snow cover effect on dust emission. The module is tested in reproducing the present dust pollution, through the comparison with available observations. Then, the surface PM concentration under the 2050 climate is projected to increase the levels of particulate matter in 2050 by 0-10 ??g/m3 for PM2.5 and 0-14 ??g/m3 for PM10 in the eastern and western U.S., but decrease PM2.5 by 0-6 ??g/m3 and 10-0 ??g/m3 for PM10 in the central United States. When considering only the reduced anthropogenic emissions relevant to aerosols, there will be overall decreases in PM2.5 concentrations around 2-18 ??g/m3 and PM10 concentrations around 2-22 ??g/m3. Mineral dust and organic compounds are projected to increase in the 2050 U.S. PM composition, but secondary inorganic aerosols will decrease. For the study of atmospheric mercury, a global mercury scheme is developed and incorporated into the CAM-Chem model. The scheme represents the emission, transport, transformation and deposition of atmospheric mercury (Hg) in three forms: elemental mercury (Hg0), reactive mercury (HgII), and particulate mercury (HgP). It is tested by using the NCEP/DOE AMIP II reanalysis meteorology data. The result shows the model can simulate the spatial and temporal distribution of atmospheric mercury concentrations. Then, the future mercury air quality is projected to 2050 under each IPCC climate and emission change scenario. We find that the U.S. total gaseous mercury (TGM) concentration in 2050 may have a 2.1-4.0 ??g/m2???s increase on the eastern U.S. and around 1.4-3.0 ??g/m2???s increase on the western U.S. under the A1FI scenario. The corresponding changes in wet deposition are around 10-14 and 2-4 ??g/m2, respectively. The increase of HgII in emissions tends to enhance the wet deposition