The GCAS instrument was installed on the NASA G-V aircraft during the Tracking Aerosol Convection. ExpeRiment -Air Quality (TRACER-AQ) field campaign in Houston, Texas during September 2021. The GCAS instrument employs charge-coupled device array detectors to observe backscattered light. These data can be used to calculate column densities of gases, such as NO2, below the aircraft using differential optical absorption spectroscopy (DOAS) .