The Met Office Atmospheric Survey Aircraft (MOASA) is a twin-engine Cessna 421 mounted with science instrumentation for the measurement of aerosol (PM2.5, PM10) and gaseous pollutants, specifically nitrogen dioxide, ozone, and sulphur dioxide. MOASA is also equipped with an aerosol backscatter LIDAR; that can point upwards to detect boundary layer height.

The aircraft was originally commissioned as a civil contingency capability but usage was extended to include clean air survey flights for the SPF Clean Air programme.