This manuscript describes methods by which to interpret isotope and humidity data from drone-mounted permeable gas sampling bags. The conceptual model appears to be generally useful for inferring atmospheric conditions based on gas composition inside sampling bags. However, the conceptual derivation of the model should be more robustly described, improving precision of communication but especially considering all relevant effects such as temperature and pressure. There is also a lot of improvement needed in precision of notation and terminology, such as the definition of diffusion, silent substitution of delta notation for isotopic ratio in a key equation, and others that I have noted below in the detailed comments. Finally, the actual sampling techniques are not described, so the example drone flight profile is difficult to interpret. 

L90 differential diffusion does cause fractionation, but they are not synonymous.  

L90 there are two relevant gradients causing fractionation: one in concentration of water (causing mass flux), and one in isotopic composition (resulting in no net mass flux). The latter can cause fractionation even if humidity is the same inside and outside of the bag. Neglecting the second gradient may be justified if it is small, but it should not be ignored completely by the theoretical derivation. 

L125 I have several comments about Eq 1:  

(1) it neglects effects of pressure and temperature differences across the bag membrane. (2) wouldn't it be more general to formulate this equation in terms of partial pressure of water vapor instead of mass concentrations? That would partially resolve #1 

(2) L124 It's not flux toward the bag, but into the bag, correct? 

(3) defining k in g/kg adds a potentally confusing dimensionality to k, so it is not dimensionless as might be assumed and so that F takes on the units of g/m2/s instead of the si base units kg/m2/s that readers might assume without reading carefully. Maybe there is a good reason for the choice; please tell us. 

L133 and L141 the definitions of alpha and lambda are both crucial equations. Assigning them equation numbers would make them easier to find. 

L139  the mass balance assumptions are not clear. How can M be constant if there is water flux into the bag? Does this mean we must assume that all vapor transport into the bag is balanced by an equal mass of non-water vapor transport out of the bag? Or maybe this theory only works if dM/dq is very small? What are the limits of this assumption? 

L141 this definition of lambda can be loosely defined as a diffusion coefficient, but it is probably better termed (non-dimensional) conductance. It defines the rate of net mass flux in response to a gradient in concentration, but neglects gradients in pressure, temperature, and isotopic composition. A standard definition of a diffusion coefficient would have dimensions cm2/s. Further confusion arises in sec 3.2, where lambda is called "permeability" L218. Please choose consistent terms. 

L167 it would be helpful to be extra clear here that the alphas being obtained are those due to fractionation due to mass flow through bags. 

L181-183 there is a lack of clarity here in notation. Delta 18O and delta 2H do not appear in eq 11. Substituting delta notation for ratio notation has no consequences for the alpha in Eq 11 because the standard ratio cancels out, but it would be kinder to readers to justify the use of delta notation either by deriving Eq 11 in terms of deltas or to explain here that ratios of R and ratios of deltas are equivalent. 

L202 flow rate is not measured in psi.  

L209 "can” or "did"? And what is a parallel sample? 

L210 I'm not following—which bias is this? I am guessing this is in the laser spec, but it would be nice to be clear. 

L210 this paragraph is difficult to follow because it appears to use jargon specific to the piece of equipment used (but not fully specified—was it a Picarro A0101?). 

Table 2.1 is not needed 

L228 alpha_delta is an unfortunate choice in nomenclature. The standard variable is alpha, which can be made more specific by listing the isotopes involved (eg alpha H2/H1), but it adds only confusion to add "_delta" because the delta notation has nothing to do with the isotope fractionation factor. Alpha is defined in terms of isotopic ratios (i.e., not in terms of delta). 

L232 injected how? Liquid? This is ~10^-2 ml, correct? L255 And 10^-3 ml in experiment 3? I'm surprised this was easier than using lab air and later adjusting the humidity or isotopic composition of a testing chamber. 

L259 "can” or "did"? 

L266-268 the logic of how the sampling system works is important. Full details might not be appropriate here, but a citation to them would be nice. At minimum I would expect an outline of how it works, given that understanding the results depends on understanding the methods. 

L269 of course the bags do not deflate because of mass loss, but because of increased pressure outside the bag, and the pressing danger would therefore seem to be preventing ingress of new air, not egress of sample. Do the one-way valves protect against this? 

L280 the vapor is not measured in situ. Samples are removed from their locations and measured elsewhere. 

L287 this sentence illustrates what I mean in my comment L266: readers cannot appreciate the sampling environment in any useful detail, so there is no way to fully understand why, for example, "it is difficult to experimentally estimate λ for different altitudes". 

L291 longer sampling time where? To collect the air or to analyze the samples?  

In general, section 3.4.1 seems to all collapse to "mass was estimated proportional to pressure at the sampling altitude and pumping time". The overly detailed presentation makes the logic seem more complicated than it is. 

L306 comment 1: what does "diffusion model correction process" mean? Parameterization? Correction of model structure? 

L306 comment 2: I don't think lambda_surface and lambda_alt are proper variables. Lambda is a property of a bag that should not depend on altitude. Its apparent dependence on altitude in this work is due to errors estimating masses. Therefore, unless I am missing something, the better variable to report here as a source of error is the estimate of air mass. 

L306 comment 3: mismatches between model and data are not sources of error, they are themselves the error. 

Sec 3.4.2 the uncertainty section is difficult to follow and needs a revision for conciseness and clarity. The section includes too much information (e.g., how mean parameter estimates were obtained—i.e., nothing to do with uncertainty), is not well organized, and is also not always specific when it needs to be. An example of this last point is the ¼ estimate for pumping time. Is this really uncertain to that degree? It seems more likely (lacking actual experimental details), that pumping time is well known and the real variable is mass captured. 

L376 this temperature (and pressure) dependence should be recognized in the theoretical development. 

Fig 3 it would improve the accessibility if the caption told us which experiment these data come from 

Fig 4a misspelling of Environment. Also, Environment should be defined in the main legend, not in each panel 

Fig 4 I don't understand why the isotopic equilibration models cross over each other at long time. What equation exactly generates the solid lines model fits? 

L398 should specify HD16O—or omit it, since the sentence is about 18O 

Fig 5 delete "(a, b) (a-b)" at the beginning of the caption  

Fig 5 is difficult to follow. What is "real value"--it looks like initial isotopic composition inside the bag, but why does it not change with time and why is it measured at different times compared to the colored dots? What equation exactly generates the solid lines model fits? 

For figures 4 and 5, plotting the humidity inside the bags over time would help a lot in illustrating the processes and ensuring the models are describing the processes correctly.  

For figures 5 and 6, I appreciate the idea to diagram the processes, but I found the diagrams unhelpful because they illustrate only the magnitudes of fluxes and ignore the crucial differences in humidity inside the bag. 

L405 referring to Fig 4 as the first scenario and Fig 5 as the second and third scenarios is confusing, because they are not so labeled in the figures and difficult to keep straight in the text. 

L431 again it would be helpful to be explicit about which equation is "the model" 

L441 ok but (1) there are other ways to flag for unrealistic results, so focusing on this one seems odd; and (2) leaving in those six data points would presumably not have much effect on the results, so this detail seems distracting. 

Minor global comment: the word "value” is redundant almost everywhere. 

L456 "model corrections” can never affect d18O in the bag. I think this is saying that applying corrections for vapor pressure differential and fractionation by the bag changes the estimate of the atmospheric d18O. L454-459 why say all this twice? The correction process is the same for 2H and 18O, and the d response follows. This description makes it sound more complicated than that. 

Fig 10 it is not clear what "Picarro” means here. Is this the measurement of ambient vapor at the surface at the time of sampling aloft? Or is this bag samples vs. satellite-inferred estimates? 

L551 the methods in this manuscript have nothing to do with laser spectrometry; the samples could as well be measured by other methods.