Aerosol optical depth regime over megacities and possible links with their population changes

More than 50% of the global population is hosted in cities of the world. The United Nations estimate that the number of inhabitants in the cities will be increased by about 2.5 billion people by 2050. Worldwide, the most pronounced urban population growth (up to 90%) is projected   for Asia and Africa. Such a population increase is expected to also increase the emissions of fine aerosols originating from anthropogenic activities. The World Health Organization estimates that the exposure to fine aerosol particles is responsible for 4.2 million premature deaths on an annual basis. In some countries, strategic national measures (i.e., U.S. and European Clean Air Acts, China's Air Pollution Prevention and Control plan) are applied for the mitigation of aerosol emissions. Such initiatives have been designed to tackle the urgent needs for combating air quality degradation, which in turn has subsequent impacts on human health.

In this study, high resolution satellite-based data of aerosol optical depth (AOD) from the MODerate resolution Imaging Spectroradiometer onboard the Aqua satellite (MODIS-Aqua), in addition to ground-based sun photometric aerosol measurements and population data for 81 megacities (cities with more than 10 million inhabitants) are analyzed. Aspects that are addressed deal with the correlation of AOD variability and population growth and the effect of regional emissions in the AOD vs population links. As an example, India and China show contradictory AOD trends, being continuously increasing for India and declining for China, despite the recorded population growth in both countries. In addition, high spatial resolution data identify intra-city correlations among aerosols and population growth. In this case ground-based observations are also used aiming to understand possible spatial AOD inhomogeneities within a few kilometers in megacities and in the neighboring areas. Moreover, in the context of this work, long-term AOD retrievals obtained by the MAIAC (Multi-Angle Implementation of Atmospheric Correction) will be used for better data availability and for the potential to use additional auxiliary data to investigate the effects on several aspects. Finally, as a number of megacities are affected by natural aerosols (e.g. desert dust) we have tried to eliminate such effects as they are not linked with population growth, by using a fine spatial resolution dust optical depth product derived by the MIDAS (ModIs Dust AeroSol) dataset.

Acknowledgements

This study was funded by the COST (European Cooperation in Science and Technology) funding agency for research and innovation network; COST Action HARMONIA (International network for harmonization of atmospheric aerosol retrievals from ground-based photometers), CA21119.