A complementary method to estimate air pollutant emissions is in using a "top-down" approach. With this method, emissions are back calculated from pollutant measurements acquired across an entire airshed. This is typically done with a remote sensing instrument -in orbit or on an aircraft . Analyses have been conducted for global megacities and power plants using the Tropospheric Monitoring Instrument (TROPOMI) and a complementary satellite instrument, the Ozone Monitoring Instrument (OMI). The emission rates are inferred by analyzing the concentration maps over a large region and incorporating the lifetime (chemical and dispersion lifetime) of the pollutant to back-calculate the emission rate at the source.