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GLOBAL RESEARCH
Inter-continental Transport of Air Pollutants
Research Team: Denise Mauzerall, Junfeng Liu (doctoral student), Larry Horowitz (GFDL scientist), and collaborators (see references)
As Asia industrializes there is increasing concern in the United States over the impact that long-range transport of Asian emissions will have on air quality in the US. Important questions are whether the effect will be felt primarily by rising background concentrations of air pollution in the northern hemisphere or by occasional rapidly transported pollution plumes, and how large the seasonal and inter-annual variability in transport will be.
We first characterized the seasonal cycle of ozone production over Asia and export to the Pacific Ocean [Mauzerall et al., 2000]. We found that although ozone production was largest in summer, export was largest in spring. We next developed the global chemical transport Model of Ozone and Related Tracers, version 2 (MOZART-2) [Horowitz et al., 2003]. We designed it to accept a variety of meteorological inputs to facilitate examination of processes controlling inter-annual variability in inter-continental transport of air pollution.
We analyzed the effect of meteorology on the magnitude of seasonal and inter-annual variability in trans-Pacific transport of pollutants by using MOZART-2 with assimilated meteorology (real winds) for 1991-2001 and uniformly emitted continental tracers with prescribed first-order decay rates. We found that the average length of time required for transport of continental tracers from East Asia to the US in spring is approximately 3-weeks, significantly longer than the 1-2 week transport time that scientists have observed in rapidly transported pollution plumes [Liu and Mauzerall, 2005]. In addition, because no existing atmospheric indices were able to explain the inter-annual variability in trans-Pacific transport potentials (defined as transport across 130º W in the eastern Pacific), we developed a new index we call the Eastern Pacific Index (EPI). The EPI successfully reproduces most of the variability in influx of East Asian tracers to the US between 1991-2001 [Liu, Mauzerall and Horowitz, 2005].
Although the atmospheric science community has been researching the export of pollution from Asia to the Pacific and the import of Asian pollution into the United States, there has been little examination of the influence influx of Asian pollutants have on human health and welfare in the United States. We are now updating the emission inventory used in MOZART-2 to more accurately represent present global emissions from anthropogenic sources and biomass burning in order to simulate inter-continental transport of carbon monoxide, ozone and aerosols (including black carbon, organic carbon and sulfate) from 1991 to 2004. By tagging emissions from Asia and other continents, our intention is to estimate the health impacts that occur in the US, Europe and Asia resulting from pollutants that are transported to these regions from other continents.
As a teaching tool and for public outreach, I have built animations of MOZART-2 computer simulations to show global transport of emissions from fossil fuel and biomass burning (forest and savanna fires) from a variety of continents in both 1990 and projected for 2020. They are available on my animations page. These animations clearly show that air pollution can no longer be considered just a local or regional issue but is in fact increasingly becoming a hemispheric and global problem.
Linkages between Air Pollutant and Climate Change Science and Policy
Net Radiative Forcing Due to Changes in Regional Emissions of Ozone Precursors
Research Team: Denise Mauzerall, Vaishali Naik (postdoc), Larry Horowitz, Dan Schwarzkopf, V. Ramaswamy (GFDL scientists), Michael Oppenheimer (WWS professor)
Air pollutants impact human health and welfare; they can also influence climate change. From a policy perspective it would be advantageous to be able to credit countries for reducing their emission of air pollutants that contribute to climate warming and thus to secure both local and global benefits. However, determining the impact that emissions of short-lived air pollutants have on climate is challenging for several reasons. First, when and where they are emitted influences the impact they have on climate. Second, they interact in the atmosphere resulting in increases in the concentrations of certain gases and decreases in the concentrations of others with the net change determining the climate effect.
We have used MOZART-2 and a global radiation model to quantify the variable effect on climate resulting from emissions of tropospheric ozone (NOx, carbon monoxide and non-methane hydrocarbons) precursors. We found that changes in radiative forcing depend strongly on the geographical location of the reduction, with reductions in the tropics having the largest effect. Our key finding is that simultaneous reductions of CO, NMHCs, and NOx reduce radiative forcing resulting from emission of tropospheric O3 precursors while reductions in NOx emissions alone do not.
With NOx emission reductions alone, there is no net climate benefit because the cooling effect of decreasing ozone is offset by the warming effect of increasing methane [Naik, Mauzerall, et al., 2005]. This is a significant finding as preliminary policy discussions considered providing climate credit for NOx emission reductions alone. However, we show that carbon monoxide and hydrocarbons must be reduced as well in order to obtain a climate benefit.
We are now in the process of evaluating the effect of regional biomass burning (forest and savanna fires) on global tropospheric ozone distributions and resulting radiative forcing from ozone and methane. Biomass burning emissions contain more hydrocarbons than typical anthropogenic emissions. Hence, we expect a larger climate benefit to result from reductions in biomass burning than from reductions in industrial emissions of the same magnitude. We will next explore scientifically supportable policy mechanisms by which it may be possible to include emission reductions for a basket of ozone precursors in a climate agreement.
Mitigating ozone pollution with methane emission controls: Global health benefits
Research Team: Denise Mauzerall, Jason West (post-doc); Arlene Fiore, Larry Horowitz (GFDL scientists)
Methane is the second most important greenhouse gas after carbon dioxide. It also contributes to the production of tropospheric ozone. Ozone exposure leads to an increase in premature mortality rates and morbidity. Reducing methane emissions both decreases climate warming as well as reduces surface ozone concentrations everywhere. We found that reducing annual global anthropogenic methane emissions by 20% starting in 2010 would decrease average concentrations of surface ozone by ~1 ppbv globally. In a novel analysis, we estimated this ozone reduction would prevent ~30,000 premature all-cause mortalities globally in 2030, and ~370,000 between 2010 and 2030 [West, Fiore, Horowitz, and Mauzerall, submitted]. If avoided mortalities are valued at $1 million each, a ~$240 per ton CH4 ($12 per ton CO2 equivalent) benefit is realized which exceeds the cost of methane reduction.
Although uncertainties exist in applying epidemiological concentration-response functions from developed to less developed countries and our valuation of avoided mortalities is at best approximate, our work makes the valuable, policy relevant, finding that cost-effective methane emission reductions not only have benefits for mitigating climate change but also reduce global ozone concentrations. These reductions result in global air pollution control benefits and associated global reductions in premature mortalities from decreased ozone exposure.
Mitigation of Methane Emissions from Rice Paddies
Research Team: Denise Mauzerall, Shangping Xu (doctoral student/post doc), Peter Jaffe (environmental engineering professor)
Rice paddy systems emit approximately 15-20% of methane released to the atmosphere. In order to evaluate possible strategies for reducing these emissions, we developed a process-based methane emission model for rice paddy systems that highlights plant mediated methane transport and permits examination of the effects of fertilizer application and field drainage on methane emissions [Xu, Jaffe, Mauzerall, submitted]. We plan to use the model on a national scale in China to evaluate the feasibility and cost-effectiveness of various methane mitigation options for rice paddy systems.
Effective Policy Implementation: Sustainable Development
Research Team: Denise Mauzerall and Tom Hale (undergraduate)
To be successful in introducing new science based policy, it is valuable to also critically examine existing policy infrastructure. We therefore examined the efficacy of partnerships between governments, industry and non-governmental organizations in furthering the goals of sustainable development , including projects on energy, climate change and air pollution mitigation. Partnerships, also called Type II agreements, were launched at World Summit on Sustainable Development with the intention of catalyzing nongovernmental participation in and additional funding of sustainable development projects around the world. We found, however, that little partnership financing is coming from new sources; most is coming from governments and less than 1% from the private sector.
Guided by empirical findings from the partnerships to date, we proposed the following to make the partnership program more effective: (a) establishing a learning network through which information can be broadly exchanged; (b) increasing the transparency of partnerships; (c) increasing private sector and small stakeholder participation; (d) establishing an institutional home to support partnerships; and (e) ensuring that the partnerships are consistent with multilateral priorities [Hale and Mauzerall, 2004]. Since publication there is evidence that several of our recommendations have been implemented - in particular increasing transparency of partnerships and the establishment of a web-based learning network.
PUBLICATIONS:
Hale, T. N. and Mauzerall, D. L., “Thinking Globally and Acting Locally: Can the Johannesburg Partnerships Coordinate Action on Sustainable Development?” Journal of Environment and Development, September 2004. [full text (pdf)]
Horowitz, L. W., Walters, S., Mauzerall, D. L., Emmons, L. K., Rasch, P. J., Granier, C., Tie, X., Lamarque, J.-F., Schultz, M. G., Tyndall, G. S., Orlando, J. J., and Brasseur, G. P., “A Global Simulation of Tropospheric Ozone and Related Tracers: Description and Evaluation of MOZART, Version 2,” J. of Geophys. Res., 108 (D24), 4784, doi:10.1029/2002JD002853, 2003. [full text (pdf)]
Liu, J., Mauzerall, D. L., Horowitz, L.W., “Analysis of Seasonal and Interannual Variability in Transpacific Transport,” J. Geophys. Res., 110, D04302, doi: 10.1029/2004JD005207, 2005. [full text (pdf)]
Liu, J., and Mauzerall, D. L., “Estimating the Average Time for Inter-continental Transport of Air Pollutants,” Geophys. Res. Lett., 32, L11814, doi:10.1029/2005GL022619, 2005. [full text (pdf)]
Mauzerall, D. L., Narita, D., Akimoto H., Horowitz L., Walters S., Hauglustaine D., Brasseur, B., “Seasonal Characteristics of Tropospheric Ozone Production and Mixing Ratios Over East Asia: A Global Three-dimensional Chemical Transport Model Analysis,” J. Geophys. Res., 105, pp. 17895-17910, 2000. [Abstract] [full text (pdf)]
Naik, V., Mauzerall, D. L., Horowitz, L. W., Schwarzkopf, D., Ramaswamy, V., and Oppenheimer, M., “Net Radiative Forcing Due to Changes in Regional Emissions to Tropospheric Ozone Precursors,” J. Geophys. Res., Vol. 110, D24306, doi:10.1029/2005JD005908, December 2005. [full text (pdf)]
Xu, S., Jaffe, P., Mauzerall, D. L., “A Process-based Model for Methane Emission from Flooded Rice Paddy Systems,” Ecological Modeling, submitted 2005. [full text (pdf)]