Space-based sensors help detect otherwise invisible changes, peering either sideways or down through the atmosphere to build up three-dimensional views of its chemical composition, sensitive to a few parts per billion.
The Copernicus Sentinel-5P mission is dedicated to monitor the atmosphere and its TROPOMI sensor is capable of measuring atmospheric constituents.
Satellite measurements show that nitrogen dioxide in the lower atmosphere over parts of Europe and the US has fallen over the past decade. More than 15 years of atmospheric observations have revealed trends in air quality.
The loss of the Envisat satellite is affecting services by Europe's Global Monitoring for Environment and Security programme. Efforts are being coordinated with other space agencies to fill some of the gaps, but the situation adds further urgency to launch the Sentinel missions.
