The atmosphere is a mix of gases and fine particles called aerosols. Some of these particles, like ash and dust, are large enough that they can be seen by the human eye, while others are microscopic. Regardless of their size, aerosols scatter and absorb light, which means they can be detected from space. Aerosol optical thickness data is shown here from the Suomi NPP VIIRS sensor. Areas with thick aerosol layers (i.e., a lot of light scattering/absorbing) are colored in deep orange, areas with low aerosol optical thickness are colored light yellow. This image shows the weekly average aerosol optical thickness over Europe from March 3 through March 9, 2014 as shown in NOAA View.
An unannotated version of this image can be found here.
Information about air quality in European cities can be found at CITEAIR, an operational service for European Cities.