Methodology:

The ground-based microwave radiometer of the British Antarctic Survey (BAS) measures spectra in the region of the rotational transitions centered at 250.796 GHz (nitric oxide, NO), 249.79 GHz and 249.96 GHz (ozone, O3) and 230.538 GHz (carbon monoxide, CO). A three-way mechanical chopper system selects the microwave signal either from the main reflector pointing to the sky at 30 deg elevation angle, a 60 K cold load or an ambient-temperature calibration load mounted on the chopper wheel, and directs it into the main receiver. The sky signal is then calibrated using the Dicke switching technique. One calibration cycle takes 26 s with 8 s integration time on each target. However, in order to achieve a high enough signal to noise ratio for the profile retrieval, several measured spectra are integrated. To produce this data set 80 spectra from consecutive calibration cycles were averaged, which under good observing conditions results in a measurement noise of less than 0.15 K and an integration time of less than 30 minutes. Spectra with measurement noise higher than 0.25 K, due to e.g. high tropospheric opacity, were not considered for profile retrieval. During the winters 2008 and 2009 (492 measurement days) the radiometer acquired a total of 1974 integrated spectra over 394 days that are suited for profile retrieval.

The inversions of the CO spectra were performed using the optimal estimation method implemented in the Qpack software package (v2.0.0). The forward model is provided by the Atmospheric Raditative Transfer Simulator (ARTS), a modular program simulating atmospheric radiative transfer.