Methane

2007
Xu, S., P. Jaffe, and D. L. Mauzerall. “A Process-based Model for Methane Emission from Flooded Rice Paddy Systems.” Ecological Modeling 205 (2007): 475-491.Abstract
Methane is the second most important greenhouse gas after carbon dioxide. Rice paddy soils release approximately 15–20% of total methane emitted to the atmosphere. A processbased methane emission model was developed for rice paddy systems that highlights plant mediated methane transport. Sequential utilization of alternative electron acceptors such as oxygen, nitrate, Mn(IV), Fe(III) and sulfate in flooded soils is included and permits examination of the effects of fertilizer application and field drainage on methane emissions. Acetate and hydrogen, two representative electron donors produced from the biologically mediated decomposition of solid organic matter, are assumed to be the substrates driving the electron transfer processes. Effects of temperature on reaction kinetics and diffusion processes are based on empirical relationships observed in the laboratory and field. Other processes considered include the exudation of organic carbon and radial release of oxygen from roots, the infiltration flow induced by plant transpiration, the growth dynamics of rice plants, the vertical distribution of soil organic carbon and root biomass, dieback of roots, and loss of gaseous species through ebullition. The performance of the model is evaluated using methane flux data collected in Chongqing and Sichuan, China. Model simulations reveal that although hybrid rice cultivars are several times more efficient in mediating methane transport than traditional tall cultivars at seedling stage, the development of methane transport capacity over the growing season leads to a relatively small difference in total seasonal methane flux (∼15%) among fields planted with tall and hybrid cultivars. Application of nitrate fertilizer at a rate of 64 kg N/ha (about 50% of total nitrogen applied at the Chongqing site) could reduce methane emission by 7%. By converting both iron and manganese to oxidized forms, pre-season drainage is found to be able to reduce methane emissions by 8–10%. A 1-week drainage of a rice field during the growing season could further reduce the methane emission by 22–23% and might be a very promising methane-emission mitigation technique, since such drainage practices can also conserve water and improve rice yields. This model will be implemented on a national scale to establish national methane emission inventories and to evaluate the feasibility and cost-effectiveness of various mitigation options that could vary from site to site.
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2006
West, J.J., A. F. Fiore, L. W. Horowitz, and D. L. Mauzerall. “Mitigating Ozone Pollution with Methane Emission Controls: Global Health Benefits.” Proceedings of the National Academy of Science 103, no. 11 (2006).Abstract
Methane (CH4) contributes to the growing global background concentration of tropospheric ozone (O3), an air pollutant associated with premature mortality. Methane and ozone are also important greenhouse gases. Reducing methane emissions therefore decreases surface ozone everywhere while slowing climate warming, but although methane mitigation has been considered to address climate change, it has not for air quality. Here we show that global decreases in surface ozone concentrations, due to methane mitigation, result in substantial and widespread decreases in premature human mortality. Reducing global anthropogenic methane emissions by 20% beginning in 2010 would decrease the average daily maximum 8-h surface ozone by 1 part per billion by volume globally. By using epidemiologic ozonemortality relationships, this ozone reduction is estimated to prevent 30,000 premature all-cause mortalities globally in 2030, and 370,000 between 2010 and 2030. If only cardiovascular and respiratory mortalities are considered, 17,000 global mortalities can be avoided in 2030. The marginal cost-effectiveness of this 20% methane reduction is estimated to be $420,000 per avoided mortality. If avoided mortalities are valued at $1 million each, the benefit is $240 per tonne of CH4 ($12 per tonne of CO2 equivalent), which exceeds the marginal cost of the methane reduction. These estimated air pollution ancillary benefits of climate-motivated methane emission reductions are comparable with those estimated previously for CO2. Methane mitigation offers a unique opportunity to improve air quality globally and can be a cost-effective component of international ozone management, bringing multiple benefits for air quality, public health, agriculture, climate, and energy.
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