Preface to a special issue "Megacity Air Pollution Studies (MAPS)"

As several common factors and drivers cause air pollution problems in urban areas, it is widely thought that the air quality trends in different cities exhibit similarities in certain aspects correlated with urban development activity and air pollution mitigation practices.

However, unique characteristics in the emission, chemistry, and dispersion of air pollutants that are specific to each urban area also exist, which often reflect differing approaches to air pollution control policy. Based on this reasoning, several comprehensive studies in big megacities, mostly in North America and Europe, have been implemented extensively over the last two decades. However, the air quality is often much poorer in the megacities of developing countries, and extensive research is still needed to tackle the urgent goals of understanding the increasing complexity of air quality and identifying appropriate mitigation measures.