|The study by Lampert et al. describes an approach by which air samples can be obtained from different altitudes reaching far into the atmospheric boundary layer (ABL) using an unmanned aerial system. The authors have added important information and I really appreciate the RANS modelling concerning rotor downwash. However, the authors have stopped short with fully exploiting the information provided, and turning them into valuable information for others in the discussion. For my understanding, this requires further elaboration. My most important points are as follows: |
1. Still lack of precision in formulations, quite often I was confused at first and had to read several times to understand the meaning. For instance, the term origin sometimes refers to spatial allocation, and sometimes to the production mechanism. I suggest to use spatial origin and source process respectively.
2. Determination of origin at different heights above ground is mentioned as one of the two most important aspects of the study this i) not the same aims as described in the introduction and ii) the abstract never gets back to a conclusion on this issue. The implications of aim 2 given in the introduction remain unclear. What new methods need to be developed?
3. The RANS simulations are really interesting. Thank you for adding. However, it remains unclear to me how the calculation of the spatial allocation of the sampled air arises. Please clarify as I am not sure if I understood correctly: The authors have taken the largest relative air speed to calculate the travel distance during the 1.3 seconds it takes to fill the glass vessels. To my understanding, the sample air inlet was above the rotors, and I don’t see these relative speeds above the rotors. This is because air from a spherical half-space above the rotor is washed down by the rotor. The calculation would make sense if air exclusively straight above the rotors would be washed down, but the figure does not support this notion. It seems like the highest relative speed above the rotors is in the order of 12-15ms-1. Wouldn’t it be more accurate to take the highest speed above the rotors to estimate the maximum travel distance? In addition it is not clear to me if only the downward pointing velocities are shown. Further, this figure suggests that having a sideways pointing tube as air inlet that reaches 25cm beyond the rotor would be a good option for further uas based systems, or a tube that reaches 50 cm above the rotors. Also the discussion if hovering would be the best option for taking a sample is not discussed at all. This is a pity and deserves a section in the discussion.
4. Figure 1 doesn’t tell anything
See some more detailed comments below.
P1L2: “quadrocopter-borne air sampling and of methane isotopic analyses, is applied to determine the origin of methane”: I am not sure what the authors mean exactly with origin. Methane isotopic composition can give hints on the process that had produced the methane, but does not tell anything about the location. It could have been produced anywhere in the landscape where the respective production process had taken place. In addition, I don’t see where the authors get back to the origin of methane in the abstract. There should be a reference to the origin if this is mentioned at the foremost place in the abstract.
P2L5: one reference can be removed
P2L19: Transformation of the isotope ratio to the delta scale _13C‰ results in values that allow to distinguish the source: Also the isotope ratios allow this process identification. The conversion to delta scale is just more intelligible for us, but is no other information. Just stick to : Isotopic composition allows to distinguish different source categroies.
P2L24: please change from predominantly to “an increasing share of ”
P3L36: I suggest adding that this is typically happens during night.
P5L15: please change from content to volume
P8L8: heated away is colloquial, please rephrase
Materials and Methods
Table 1: the data shown is not isotope ratio but delta value.
P10L15: vertical mixing is possible, but hampered.
L18: please change from dilusion to dilution. If the assumption is that air from above the inversion mixes with air below as the inversion climbs up and is dissolved, I suggest calling it mixing. Dilution alone, in the sence of mixing with for instance ch4 free air would not alter the isotopic composition. What you observed is the temporal variability of CH4 isotopic composition which is caused by transport and mixing.
L20: dilusion again. Is this a real technical term that I am not aware of?
P12L5: in agreement with atmospheric stability: what does this mean? Maybe caused by atmospheric stability?
P12L16: isn’t this bullet point the same as above?