With the acute societal concerns about air quality, climate change and their effects on health and ecosystems, there is an increasing need for comprehensive, reliable and fast information services on the atmospheric environment. This is also of importance for a range of policy-relevant applications at different scales, from international treaty verification to urban planning for instance. Succeeding GEMS (Global and regional Earth-system Monitoring using Space and in-situ data) and MACC (Monitoring Atmospheric Composition and Climate), MACC-II* (MACC-Interim Implementation) is the third in a series of projects funded since 2005 through the European Union's Seventh Framework programme to build up the atmospheric service component of the Global Monitoring for Environment and Security (GMES) /Copernicus European programme. MACC-II combines the expertise of its 36 partner institutes from 13 European countries to bridge the gap between the meteorological and environmental communities engaged in research and operational service provision. Using the extensive experience of both communities, MACC-II provides information on atmospheric composition using satellite observations, ground-based observations, and state-of-the-art numerical models (http://www.copernicus-atmosphere.eu). MACC-II not only monitors atmospheric composition over time but also provides forecasts of air quality, dust storms, fire emissions and solar/UV radiation for a few days ahead both globally and in more detail for Europe. Furthermore, MACC-II supports studies of pollution events and possible responses to mitigate their effects, annual assessments of air quality, and the monitoring of greenhouse gases and their sources and sinks at the Earth's surface. This special issue focuses on the world-class research aspects that underpin the continuous development, evaluation and delivery of the GMES/Copernicus services for atmospheric composition.

*The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7 THEME [SPA.2011.1.5-02]) under grant agreement no. 283576.