The Atmospheric Survey Aircraft (ASA) is a modified Cessna 421 operated by the Met Office in association with Alto Aerospace and based at Bournemouth, UK. Its original mission was to be on standby to monitor volcanic ash in UK airspace. With funding from the UK Strategic Priorities Fund (SPF) it was modified with additional instrumentation to make regular air quality measurements with the primary objective to measure pollution in the lower atmosphere. You can read more about where it flies and the data it collects here.