Atmospheric Chemistry and Transport Modeling

A primary tool used in our model-based research on atmospheric transport and chemistry is the GEOS-Chem chemical transport model developed by an international community led by Harvard, and a nested-grid version of the model focused on East Asia developed by WANG Yuxuan as a student and researcher of the Harvard-China Project (Tsinghua University Center for Earth System Science and University of Houston) and Project Chair Michael B. McELROY.

Validated by agreement of modeled concentrations with measurements made by ground stations and offshore aircraft, the nested-grid GEOS-Chem model differentiates air transport mechanisms for individual sub-regions of China on a finer scale than previously possible and captures critical seasonal effects of meteorology—such as cold fronts in winter and monsoonal patterns in summer—on regional and urban air quality. This helps researchers differentiate natural meteorological and chemical effects from policy effects on air quality.
 
Using the model, researchers have studied the transport and secondary chemistry of diverse air pollutants and the effectiveness of control policies on concentrations. Applied in inverse mode—in which atmospheric concentrations observed by ground stations, aircraft, and satellites are use to derive optimized emissions—the model also provides independent checks on inventories of nitrogen oxides (NOX), carbon dioxide (CO2), carbon monoxide (CO), and other pollutants and greenhouse gases. 

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The China Project's atmospheric research is committed to building observationally validated, fundamental research on the physical and chemical dimensions of China’s atmospheric environment, from urban to global scales. In addition to the model-based research described here, it includes field observations described here and bottom-up emissions research described here. It is also a core component of a Project-wide interdisciplinary framework now being applied to evaluation of national GHG and pollution control policies, described here.

Acknowledgment: Some of the material summarized here is based on work supported by the National Science Foundation under Grants No. ATM-1019134 or ATM-0635548 (indicated by acknowledgments in the papers themselves). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF).

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To evaluate the effectiveness of national air pollution control policies, the emissions of SO2, NOX, CO and CO2 in China are estimated using bottom-up methods for the most recent 15-year period (2000–2014). Vertical column densities (VCDs) from satellite observations are used to test the temporal and spatial patterns of emissions and to explore the ambient levels of gaseous pollutants across the country. The inter-annual trends in emissions and VCDs match well except for SO2. Such comparison is improved with an optimistic assumption in emission estimation that the emission standards for given industrial sources issued after 2010 have been fully enforced. Underestimation of emission abatement and enhanced atmospheric oxidization likely contribute to the discrepancy between SO2 emissions and VCDs. As suggested by VCDs and emissions estimated under the assumption of full implementation of emission standards, the control of SO2 in the 12th Five-Year Plan period (12th FYP, 2011–2015) is estimated to be more effective than that in the 11th FYP period (2006–2010), attributed to improved use of flue gas desulfurization in the power sector and implementation of new emission standards in key industrial sources. The opposite was true for CO, as energy efficiency improved more significantly from 2005 to 2010 due to closures of small industrial plants. Iron & steel production is estimated to have had particularly strong influence on temporal and spatial patterns of CO. In contrast to fast growth before 2011 driven by increased coal consumption and limited controls, NOX emissions decreased from 2011 to 2014 due to the penetration of selective catalytic/non-catalytic reduction systems in the power sector. This led to reduced NO2 VCDs, particularly in relatively highly polluted areas such as the eastern China and Pearl River Delta regions. In developed areas, transportation is playing an increasingly important role in air pollution, as suggested by the increased ratio of NO2 to SO2 VCDs. For air quality in mega cities, the inter-annual trends in emissions and VCDs indicate that surrounding areas are more influential in NO2 level for Beijing than those for Shanghai.

Long Wang, Shuxiao Wang, Lei Zheng, Yuxuan Wang, Yanxu Zheng, Chris P Nielsen, Michael B. McElroy, and Jiming Hao. 2014. “Source apportionment of atmospheric mercury pollution in China using the GEOS-Chem model.” Environmental Pollution, July, 190: 166-175. Publisher's Version Abstract

China is the largest atmospheric mercury (Hg) emitter in the world. Its Hg emissions and environmental impacts need to be evaluated. In this study, China's Hg emission inventory is updated to 2007 and applied in the GEOS-Chem model to simulate the Hg concentrations and depositions in China. Results indicate that simulations agree well with observed background Hg concentrations. The anthropogenic sources contributed 35–50% of THg concentration and 50–70% of total deposition in polluted regions. Sensitivity analysis was performed to assess the impacts of mercury emissions from power plants, non-ferrous metal smelters and cement plants. It is found that power plants are the most important emission sources in the North China, the Yangtze River Delta (YRD) and the Pearl River Delta (PRD) while the contribution of non-ferrous metal smelters is most significant in the Southwest China. The impacts of cement plants are significant in the YRD, PRD and Central China.

Xuan Wang, Yuxuan Wang, Jiming Hao, Yutaka Kondo, Martin Irwin, J. William Munger, and Yongjing Zhao. 2013. “Top-down estimate of China's black carbon emissions using surface observations: Sensitivity to observation representativeness and transport model error.” Journal of Geophysical Research, 11, 118: 5781-5795. Publisher's Version Abstract

This study examines the sensitivity of “top-down” quantification of Chinese black carbon (BC) emissions to the temporal resolution of surface observations and to the transport model error associated with the grid resolution and wet deposition. At two rural sites (Miyun in North China Plain and Chongming in Yangtze River Delta), the model-inferred emission bias based on hourly BC observations can differ by up to 41% from that based on monthly mean observations. This difference relates to the intrinsic inability of the grid-based model in simulating high pollution plumes, which often exert a larger influence on the arithmetic mean of observations at monthly time steps. Adopting the variation of BC to carbon monoxide correlation slope with precipitation as a suitable measure to evaluate the model's wet deposition, we found that wet removal of BC in the model was too weak, due in part to the model's underestimation of large precipitation events. After filtering out the observations during high pollution plumes and large precipitation events for which the transport model error should not be translated into the emission error, the inferred emission bias changed from −11% (without filtering) to −2% (with filtering) at the Miyun site, and from −22% to +1% at the Chongming site. Using surface BC observations from three more rural sites (located in Northeast, Central, and Central South China, respectively) as constraints, our top-down estimate of total BC emissions over China was 1.80 ± 0.65 Tg/yr in 2006, 0.5% lower than the bottom-up inventory of Zhang et al. (2009) but with smaller uncertainty.

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