This paper makes a significant improvement to the calculation of mean age in the lowermost stratosphere from in situ trace gas measurements by quantifying the effect of extratropical entry into this region.  This study builds on previous work that has shown the lowermost stratosphere to be composed of a mixture of air masses from different source regions with a large seasonal dependence.  This mixture can have an important impact on mean age estimates, different in each hemisphere, and the interhemispheric contrast in mean ages that are revealed with this method are quite interesting.  The techniques and methods used are well described and the uncertainties are clearly shown.  The paper is laid out well and easy to read.  Really excellent work!  I have no significant suggestions for improvement, and only a very few minor comments listed below.  I suggest publication in the present form.

Specific comments:

Line 192:  add ‘of’ after ‘instead’

Table 2 could go in the supplement.

Line 246:  ‘datasets were processed in three steps.’

Line 250:  I don’t see this age correction formula in the Leedham Elvidge et al. paper.  How was this derived?

Lines 313, 385, 390, 420:  ‘extend’ should be ‘extent’

This manuscript reports on a new method to calculate the mean age in the lowermost stratosphere that explicitly takes into account the contributions of air masses that cross the tropopause into the stratosphere via the extratropics (not tropics).  Physically, this is well motivated as studies have long shown evidence of extratropical stratosphere-troposphere exchange occurring through isentropic exchange over subtropical and middle latitudes.  Despite this, standard methods to calculate the mean age still assume a tropical entry point.  To this end, the authors propose a so-called “exTR-TR” method which they apply to aircraft data to obtain new observational constraints on the mean age in the lowermost stratosphere.  The method does not contribute much physical understanding, given that it consists mainly of developing an empirical parameterization of origin fractions computed in a previous study and then using these, in combination with measurements, to obtain new observational age estimates.  In that respect, this manuscript should be really be considered an extension/follow-up to that work.  Nonetheless, the study is overall very clear, well motivated and generally well-written.  I have a few minor comments, however, that should be addressed before I can recommend publication. 

Minor Comment 1, Line 86:  It is not clear to me why G(x,t’) in equation (2) does not depend on the source region x_i.  That is, G should also be conditionally dependent on x_i, such that G(x,t’) should be rewritten as G(x,t’|x_i).  The authors here have assumed that the transport operators propagating tracer concentrations for all regions i are the same, but I can envision several cases where this would not be true.  For example, air propagating into the stratosphere at high latitudes will have no clear path into the stratosphere, as opposed to air straddling the midlatitude tropopause, where isentropic surfaces provide a clear pathway for stratosphere-troposphere exchange.  The authors need to provide their rationale here.   

Minor Comment 2, Section 2.2.2:  I am curious about the calculation of t_xi.  The procedure outlined in steps (i)-(iii) essentially sounds like a description of how to calculate the SF6-age, which previous studies have used to calculate the tropospheric mean age (albeit using an SF6 surface boundary condition that only averages stations over northern midlatitudes).  The details of the regions considered may be slightly different, but the procedure is basically the same.  So why not reference this literature?  In particular, the authors should review these studies:

Waugh, Darryn W., A. M. Crotwell, E. J. Dlugokencky, G. S. Dutton, J. W. Elkins, B. D. Hall, E. J. Hintsa et al. "Tropospheric SF6: Age of air from the Northern Hemisphere midlatitude surface." Journal of Geophysical Research: Atmospheres 118, no. 19 (2013): 11-429.

Orbe, Clara, Darryn W. Waugh, Stephen Montzka, Edward J. Dlugokencky, Susan Strahan, Stephen D. Steenrod, Sarah Strode et al. "Tropospheric Age‐of‐Air: Influence of SF6 Emissions on Recent Surface Trends and Model Biases." Journal of Geophysical Research: Atmospheres 126, no. 19 (2021): e2021JD035451.

Technical Comments:

Line 83: The concept of “origin fraction” referred to here certainly precedes the Hauck et al. (2020) study and the authors should properly reference the literature. For example, see these studies:

Orbe, Clara, Mark Holzer, Lorenzo M. Polvani, and Darryn Waugh. "Air‐mass origin as a diagnostic of tropospheric transport." Journal of Geophysical Research: Atmospheres 118, no. 3 (2013): 1459-1470.

Orbe, Clara, Darryn W. Waugh, and Paul A. Newman. "Air‐mass origin in the tropical lower stratosphere: The influence of Asian boundary layer air." Geophysical Research Letters 42, no. 10 (2015): 4240-4248.

Wagenhäuser et al. derive stratospheric mean age of air in the lowermost stratosphere from observational data, using a novel method that considers the addition of extra-tropical input into the stratosphere. This is an important contribution to this field of study as previous work assumes that the tropical tropopause is the only entry point of air into the stratosphere when deriving mean age of air. Overall, the paper is clearly written and easy to read. The approach and techniques are very well described and the results and uncertainties clearly presented.