Projects

PRE-MEX: Modeling Air Quality in Mexico City and the Surrounding Region

Summary

We are requesting support from the Opportunity Fund ($111.6K for FY-03) to model the air quality in Mexico City and the surrounding region.Anthropogenic and natural emission inventories will be compiled and used in a nested grid chemistry-transport model (HANK/MM5). Chemical and meteorological characteristics of the urban outflow, and their interactions with the surrounding ecosystems, will be analyzed.

The proposed study is aimed at helping the development of a large strategic initiative, the Megacities Impacts on Regional and Global Environments (MIRAGE) program, which is currently being discussed within NCAR and with the scientific community. These discussions have identified the Mexico City region as the leading candidate for a major community-wide field campaign. Pre-campaign modeling is essential both to assist in the planning of this field campaign, and more broadly to begin exploring the major scientific uncertainties related to export of pollutants from megacities.

Background

The world will face unprecedented growth in the coming decades, much of it in urban areas. Pollution exported from large cities will likely have increasingly adverse effects on human health, agriculture and natural ecosystems, and visibility on regional scales, and may ultimately have global consequences. For example, even at today's relatively moderate levels, pollution-related tropospheric ozone and aerosols are known to be significant contributors to the atmospheric “greenhouse” radiation budget. While the importance of urban pollution to regional and global environments is widely acknowledged, it remains poorly quantified because of the complexity of physical and chemical processes that must be understood over a broad span of spatial and temporal scales.

The Megacity Impacts on Regional and Global Environments (MIRAGE) program is a new activity currently being developed by scientists at NCAR and involving the wider community, based on the recognized need for an integrated, multidisciplinary approach to this complex environmental issue. Its broad goals are to help provide a scientifically sound basis for answering some of the most significant questions of concern to the future of the atmosphere. These are:

How will future urban growth affect atmospheric composition on regional and global scales? Conversely, how will improvements in urban air quality benefit the larger geographic scales?

What will be the impacts of such composition changes on regional and global climate, specifically on atmospheric oxidation capacity and on the direct and indirect aerosol radiative budgets?

To what extent can better urban design and planning reduce these impacts?

Toward these goals, MIRAGE discussions have identified the need for a comprehensive research program focusing on the interactions between gas phase photochemistry, physical and chemical evolution of aerosols, local and regional meteorology, and atmospheric radiation budgets, as well as the development of socio-economic models to improve understanding and prediction of emission characteristics of different megacities. Specifically, the discussions have identified the need for at least one large measurement program to examine the chemistry and physics of exported urban air. Currently, some 20 cities have populations in excess of 10 million people, and 2/3 of these are located in the tropics. While no one megacity can be considered fully representative, selection criteria also included the severity of air pollution, the extent of local and regional meteorological and air quality data, and accessibility. Based on these criteria, Mexico City was determined to be the most suitable candidate for an intensive field campaign.

Work Plan

A vigorous modeling activity will be required at all stages of the Mexico City field campaign. Well in advance of the campaign, meteorological and chemistry-transport models, with both climatologic and representative episodic conditions, are required to design the overall flight plan strategy. During the campaign, forecast models will be used to assist with specific flight planning. Post-campaign, the observational data base will be used to improve and evaluate detailed models of processes (e.g. hydrocarbon oxidation, aerosol formation and evolution, gas-aerosol interactions), to test high (or variable) resolution regional chemistry-transport models, and to assess large-scale impacts using global models.

The proposed modeling studies will provide (i) a more quantitative framework for the planning of the Mexico City field campaign, (ii) an initial scientific thrust towards the interpretation of the observations, and (iii) an early exploration of major scientific uncertainties related export of pollutants from megacities. Specifically, we propose the following tasks:

Compile a gridded inventory of anthropogenic and biogenic emissions for Mexico City and its surroundings. Allocate emissions to the HANK chemical mechanism, based on reactivity.

Perform preliminary simulations of the urban-regional linkage with the MM5 and HANK (off-line chemistry) models. Nested grid simulations at different resolutions will be performed to examine the non-linearity of the chemistry (3x3, 9x9, 27x27 km). Simulations will be made for different seasons. (Future simulations may be done with on-line chemistry using WRF-chem, when this model becomes available).

Evaluate consistency with urban concentration measurements. Identify key needs for additional emissions measurements.

Examine chemical outflow in relation to field experiment planning, esp. time scales for composition changes, prevailing local meteorological conditions for different seasons, role of convective clouds.

Compare with global model simulations (MOZART) that use low resolution urban emissions, and evaluate consequences for regional scale chemistry.

Expected Benefits

The proposed work will benefit directly the planning of a field campaign in the Mexico City region. It will provide preliminary estimates of the locations and amounts of chemicals to be measured, and will be used in selecting the most suitable time of year for the field campaigns and specific flight plans. (A similar pre-campaign modeling effort was critical to the success of the ACD-led TOPSE field campaign in Spring 2000.)

More broadly, the proposed work will enhance the scientific understanding of the non-linear chemistry inherent in highly polluted air masses during the transition from urban to regional scales. Additionally, the work will help elucidate the role of convective clouds in the rainout and ventilation of pollutants over urban areas. Such understanding will support the development of sub-grid scale parameterizations of urban areas for use in larger scale models (now very poorly represented in regional and global models, e.g. WRF-chem, CCM/MOZART). Of particular importance is the effect of urban pollutants on the regional and global levels of ozone, aerosols, and hydroxyl radicals (which determine the lifetime of many greenhouse gases such as methane, and control the formation of sulfate aerosols).

  Finally, the proposed work will likely benefit the evolution of the MIRAGE program into a comprehensive new Strategic Initiative, by highlighting areas of major uncertainties and helping to identify the needed experimental and modeling resources. For example, residual sub-grid scale issues may ultimately require even more detailed models (e.g. LES or DNS); similarly, uncertainties in current (and future!) emissions may need to be addressed by improved socio-economic models. The benefits of such an integrated approach will extend well beyond the Mexico City case study, to a world increasingly dotted with burgeoning megacities.

Budget Justification

We are requesting incremental funding to support:

Xuexi Tie (Scientist III) 50% - Compile known emissions, estimates; gridding, coupling to HANK; run MOZART. Analysis and interpretation of model runs.

Christine Wiedinmyer (Project Scientist I) 20% - Assist with biogenic emissions inventory.

ACD co-sponsorship will consist of:

Sasha Madronich (Sr. Scientist) 20% - Oversight of all aspects; coordinate with Mexico agencies for emissions inventories and concentration measurements. Analysis and interpretation of model runs.

Peter Hess (Scientist III) 20% - Oversight of HANK model modifications, runs, analysis and interpretation.

Alex Guenther (Scientist III) 10% - Assist with biogenic emissions inventory.