There is growing evidence that human activities in an increasingly globalized, industrialized and interconnected world are influencing both air quality and climate ranging from urban and regional to continental and global scales. Rapid population growth and increased energy demand are the primary forces driving unprecedented environmental changes. Most growth occurs in urban regions and surrounding suburban areas, leading to the phenomenon of megacities.

This special issue is dedicated to the contributed papers presented at the special sessions on "Megacities: Air Quality and Climate Impacts from Local to Global Scales", which were convened at the EGU General Assembly in 2009 and 2010 and will include contributions from the EGU-2011 session.

In addition, this special issue will consider articles related to the topic, including key findings from the three large projects MILAGRO, MEGAPOLI, and CITYZEN, as well as other atmospheric science studies conducted in large urban centers around the world, highlighting the following areas:

• emissions;
• photochemistry;
• aerosol and trace gas measurements;
• meteorological measurements;
• dynamics and radiative effects;
• regional climate;
• Air quality modeling;
• satellite remote sensing of air quality in cities;
• implications of policy to reduce air pollution (air quality, health, climate change).