The group of Birner has developed new techniques to analyse with high precision helium in atmospheric samples in oredr to resolve a long-standing question: the potentail rise of crustal 4He in the atmosphere since the beginning of large-scale hydrocarbon exploitation, i.e the 20th century. Results have been published elesewhere but here authors present an improved system to measure atmospheric variability of helium and associated gases (CO2) in urban area air samples to check possibly short and long-term variability and its association to natural (i.e., gas transport and degassing from oceanic breeze) and anthropogenic sources (urban traffic and other releated sources). The paper is well organized and first results interesting, I suggest some improvement to clarify a few technical points and scientific ones.

Now to explain the improvements done on their system to analyse with precision variations in He, the previous system should be explained a little bit better (lines 35-40). Sentences as "The observed changes in He/M are then converted to equivalent changes in helium to nitrogen ratio (He/N2) using separate observations of CO2, O2, and Ar" are not really clear for people unaware of teh previously developed system and this papsr should be read as a "stand-alone". The previous sentence means that you measure variations in CO2, O2, Ar and then use constant atmospheric ratios with N2 to extrapolate N2 data? It is unclear to me, likely worst for a non-specialist.

Everywhere the analytical system is indicated as MS (Mass spectrometer). At lines 79 is indicated as a GV Isoprime 100 magnetic sector mass spectrometer, which is unusual for measuring He, while in previous papers of Birner a MAT253 was used, also unusual for measuring helium. Can clearly state from beginning which MS was used in the older system and which one is now used in the improved system (possibly adding it into Figure 1’ caption)? Helium is historically measured by quadrupole MS or noble-gas-MS (NGMS). Can you briefly explain the advantages and disadvantages of using an Isoprime compare other magnetic sector gas-source MS usually used by noble gas geochemists?

At lines 207-210 the diurnal variations in He and CO2 are presented. During day marine breeze would bring natural He and CO2 from the ocean, while during night, reverse air circulation would bring anthropogenically enriched He+CO2 mixtures. I suppose that the mechanism is diffusive degassing of dissolved He and CO2 at ASSW conditions. Can the CO2/He ratios sufficiently precise to support this mechanism? Or eventually can you slightly develop this point air-seawater noble gas exchange has been the focus of several papers (Seltzer et al., 2019; Hood et al., 1997) but not all community is aware about.

Lines 284-296. This last part of the future applications is less clear. I don’t understand the interest of measuring neon isotopes or concentrations in the atmosphere except the fact that atmospheric Ne should be constant and thus help, if I well understand, separated atmospheric N2 variability by some geological source. Why not Kr, is mostly constant in the Universe and has no problems of double charges which could hamper your measurements in dynamic mode. About H2 would be more interesting, but authors do not relay expand this part. I would suggest stopping at line 284 their future application considerations.

Evaluation of « Short Term variability of atmospheric Helium…”by Birner et al.

Variations in the abundance of atmospheric helium have the potential to trace the anthropogenic cycle of CO2. Radiogenic helium-4, generated as an alpha particle by the decays of U and Th in the crust, is readily released to the atmosphere together with carbon during exploitation of natural gases. The flux rate of C is well constrained from mining statistics but the potential sinks of CO2 are less known, and using an independent tracer of carbon release le 4He can help to better understand the carbon cycle over the last decades. The Scripps group has developed a novel analytical method to measure variations in the amount of atmospheric helium by measuring at high precision the 4He/N2 ratio in air sampled at different periods during the Anthropocene. The rationale is that, assuming that N2 is constant, secular increase of the ratio should be the result of release of hydrocarbon-related 4He from the crust. The group has validated the method by reporting an evolution curve of 4He/N2 since 1974 that fits well the expected flux of 4He released by hydrocarbon exploitation. This remarkable achievement, which constitutes a world premiere, required processing large quantities of air and long duration measurements. It was not possible to investigate short-term variations over short periods of time like weeks or months that could have a potential interest for tracing local sources ort air mass movements.  

To circumvent this problem, the group has improved the analytical method by concentrating the amount of 4He with a cryogenic trap and by automatizing the whole process including regeneration of the trapping mechanism. This contribution describes in detail the new system and reports validation tests in terms of analytical precision, reproducibility, and short term variation of 4He concentration in air daily sampled on the Pacific coast.

This is a well written technological paper that opens new areas of research in atmospheric chemistry and climate change monitoring. The paper is well suited for publication in Atmospheric Measurement Techniques and I recommend its acceptance after minor corrections.

The overall rationale of the method is somewhat difficult to follow in the first part as it is based on a diagram (Figure 1) where the reader has to guess the significance of the acronyms, and the essence of the method becomes clear only in the second part of the paper. I suggest the authors to add a paragraph explaining clearly the method and the use of the different devices displayed in Fig. 1. The conversion of He/M to He to N2 needs to be better explained. It is not clear to me why the direct measurement of the 4He/N2 ratio (4/28 peak ratio) cannot be done. I suspect that there are some mass spectrometric issues (e.g., magnet hysteresis) that should be explained. The use of two mass spectrometers, if I understand well, is also unclear. Maybe the authors could add a flow chart diagram of the method. What are the use of the different calibration tanks and what are their composition?

• There are a few typos, e.g. “reference” in Fig. 1
• Explain meg in abstract
• Why not using the mu notation for 1E-6 like “solid” isotopists ?
• Is it possible to have C3+ and 04+ contributing to mass 4 ?
• What are the compositions of reference gas, and what is the composition of the standard ?