The manuscript describes an instrument sensitive to radon and thoron in concentrations frequently found in near-surface air above continents. Characterisation and calibration of the instrument were thorough, though only for radon and not for thoron (lines 114-115). Hence, the manuscript title requires modification. The instrument can help to make sure that radon concentrations measured at different stations within a monitoring network are real and unaffected by differences between instruments' performance, their individual calibration, or differences in data processing.

There is little I can add to the earlier discussion between Scott Chambers and the Authors. The main additional issue I would like to raise is the long-term stability of the instrument's calibration. Is the instrument assumed to maintain its current calibration throughout its lifetime, even when travelling frequently and extensively? Or, is regular re-calibration foreseen? If so, how often? Lines 496 to 499 hint at a calibration unit for "...very short calibration or recalibration [...] under field conditions..." The next sentence states this issue "...will be the object of a future paper."  There are two further papers announced, one on a field intercomparison with an ANSTO detector (lines 499 to 501) and another one on the "full calibration procedures" (last line in AC2). Is it really necessary to distribute the outcome of this enterprise among four (!) papers? From my point of view, the mobile calibration unit definitively has to be included in the present manuscript, as should be more details about the air dryer and the thoron delay volume that will go with the instrument once it will be 'on the road' as a travelling standard instrument. Please add these items also to Figures 1 and A2. In addition, please show in the schematic diagram of Fig. 1 the position of the air pump.

At the end of AC1 you state that "... air inside the sphere is at atmospheric pressure because it is an open circuit." This presumption cannot be correct. If air pressure inside the sphere would be exactly the same as outside, there would be no air flowing through the sphere. Yet, the sphere is continuously flushed with 2 L/min. Upstream of the sphere a filter, downstream a flow meter (Fig. 1). Both restrict flow in addition to the tubing connecting the sphere with the outside. It may not be necessary to continuously monitor pressure inside the sphere, but I would suggest to determine once the pressure difference between inside and outside the sphere and add the offset to the pressure reading from the atmospheric station. Even if the correction is small, it should be included because not doing so introduces a perhaps small but systematic error.

3.) Perhaps tell the reader already in line 139 that the detection volume is 20 L.

4.) Line 445: The humidity was < 150 ppm, so why Eq. 2 and not Eq. 13?