WP 5: Regional and global atmospheric composition

Overview and background

The overall objective of WP5 is to quantify the effects of megacities on air quality of the surrounding regions, and on the downwind atmospheric composition on regional to global scales. Over the past 10 years, it has been realized that air pollution emitted on one continent also influences the pollutant concentrations on downwind continents (e.g., Stohl, 2004). The simulations using chemistry transport models have shown a substantial influence also on surface air quality (e.g., Li et al., 2002). Pollution emitted in the highly populated areas on the east coasts of Asia and North America is typically lifted to the upper troposphere (Stohl, 2004), whereas emissions from Europe tend to remain in the lower troposphere. Lawrence et al. (2006) found such transport patterns to be typical for individual megacities, but also found major differences in the transport patterns for cities within the same general region. High concentration pollution plumes originating from the U.S. east coast conurbations can occasionally be transported over the Atlantic Ocean in the lower troposphere (Neuman et al., 2006). Satellite observations provide a connection from regional to global scales, due to their global coverage and continually improving horizontal resolution. These observations can also be used as evaluation data for the models, and in assessment of specific air pollution episodes (e.g., Leue et al., 2001, Velders et al., 2001, Eckhard et al., 2003, Stohl et al., 2003, Wenig et al., 2003).

Methodology and advancement beyond the state-of-the-art

We will use a multi-scale modelling approach for regional-to-global analyses. Advanced, dedicated remote-sensing assessments will be linked to ground-based and aircraft campaigns (WP 3). Two model ensembles, on regional and global scales, will be constructed, evaluated and applied. Activities in this WP include:

1) Improved satellite products regarding the dispersion of pollution from megacities:
Most currently available satellite products are based on algorithms with moderate resolutions and accuracy; these will be improved to increase their applicability for detailed studies of plumes from megacities. Retrievals from multiple instruments, especially for aerosols, will be used synergistically, combining complimentary pieces of information. The satellite data will be used to characterize megacity pollution levels and outflow plumes, as well as to evaluate the regional and models; in turn, the use of model output (e.g. profiles) to improve the retrieval algorithms will be explored.

2) Improved modelling systems:
Existing aerosol process models will be refined, new aerosol process modules will be developed, and these will be integrated with regional scale models. The application of such new modelling systems will result in new insights and a better understanding on the importance of various aerosol processes on atmospheric particulate matter concentrations on regional and continental scales. The project will also result in improved regional and global CTMs regarding especially model urbanisation, i.e., parameterization of factors such as turbulence inside megacities, the effects of surfaces, and energy exchanges.

3) Evaluation of regional and global atmospheric dispersion models against experimental data:
The complete regional CTM ensemble will be applied in the regions surrounding and within Paris for the field campaign period, and their performance will be evaluated against the available measurements, as well as other selected available measurement datasets. This is expected to result in a better understanding of the advantages and limitations of the models, and of the model ensemble.

4) Impact of megacities on regional and global atmospheric composition:
Carefully designed sensitivity studies with the models will characterize the regional and global atmospheric composition changes which result from the presence of current megacities, as well as the changes which would be expected for scenarios of moving a fraction of the population into surrounding regions. The influence of pollutants from other sources relative to the megacity emissions for the megacity AQ will be determined. Furthermore, the generic outflow of pollutants from megacities will be examined, both on a global basis comparing all megacities with each other, and with an additional focus on the transatlantic transport of pollution from US east coast urban regions like New York city to Europe. Finally, the key sensitivity simulations for the present conditions will also be repeated for conditions of future emissions and/or future climate conditions.

FP7 EC MEGAPOLI, 2008-2011