This paper analyzes OCS exchange in pine canopies using inversion of an intermediate-scale model that simulates vertical gradients of temperature, humidity, and wind speed, along with trace gas concentrations in the canopy, coupled to a convective boundary layer and the overlying troposphere. The main focus is on OCS exchange and on the parameter LRU, which relates the deposition velocities of CO2 and OCS to their local concentrations. The inversions indicate a vertical gradient of LRU within the canopy, which is qualitatively consistent with previous studies showing that LRU responds to PAR and VPD. This modeling framework is unique and potentially valuable for interpreting eddy covariance observations of OCS exchange.

However, the emphasis on the absolute values of LRU produced by the model appears overstated. Robust evaluation of these values requires careful consideration of (i) the observational constraints used in the inversion and (ii) the realism of the model parameterizations. While the study uses a widely applied parameterization for OCS exchange, the descriptions of CO2 uptake and stomatal conductance—both critical for determining LRU—are unfamiliar, non-standard, and not well explained in the main text. Beyond the list of equations in the supplementary material, there is no clear description of the response characteristics of this parameterization or how it compares with more established approaches. A direct comparison with the parameterization used by Kooijmans et al. at this site would be particularly informative.

The reported LRU values fall within a reasonable range, but it is not clear that they represent independent estimates directly comparable to those in the literature. The most reliable way to determine LRU remains direct gas-exchange measurements of CO2 and OCS fluxes and concentrations (e.g., Stimler et al. 2011; Kooijmans et al. 2019). In this study, the [OCS] and [CO2] measurements appear sparse and, at times, show gradients with the opposite sign to what would be expected. This raises doubts about whether there is sufficient information to constrain LRU directly from the observed concentrations and fluxes. Instead, it seems likely that the inversion primarily fits stomatal conductance and photosynthesis to the vertical profiles of temperature, VPD, CO2, and PAR, with LRU then emerging as an implicit  consequence of applying the chosen OCS parameterization. In that sense, the regression that they propose linking LRU to VPD and PAR may mainly reflect the built-in response behavior of the parameterization rather than the physiological behavior of the leaves themselves.

The study nevertheless provides a useful illustration of how vertical gradients in light, CO2, and humidity can generate vertical structure in LRU, and it demonstrates an interesting modeling capability to quantify the gradients in [OCS] betwee the bulk atmosphere and the leaf surface that confound estimation of GPP from the OCS flux and LRU. From this perspective, the work is valuable. However, it should not be presented as an alternative to direct gas-exchange measurements for determining LRU, and the manuscript should be revised to clarify this distinction. The Kooijmans et al. paper cited above provides code and data that could be used to calibrate, test, or possibly replace the current parameterization, and the manuscript would benefit from more extensive explanation of the parameterizations and inversion framework in the main text. Finally, the comments regarding the failure of the Lai et al. model to reproduce the study’s results are not currently supported by data and should either be removed or substantiated with appropriate analysis.

The authors present their work developing a coupled modelling framework simulating the exchange of CO₂, OCS and H₂O between surface processes and the lower atmosphere – including throughout the canopy. Parameterisations in the coupled model are adjustable and are done so by assimilating observations using an inverse modelling framework. The aim of this work is to improve representation of the ratio of exchange between CO₂ and OCS, the leaf relative uptake (LRU), for needleleaf boreal forest biomes. The methodology used here is robust and is exceptionally important in the field of OCS and the carbon cycle in general. The comparison across multiple sites and variations within-canopy are of particular note and interest for the OCS and carbon cycle community.

The strength of this work is the improved understanding of LRU variations within the canopy and on the development of a simple and well represented regression model. More work is required if the goal is to make model parameterisations transferrable between biomes (in further publications, not this one). While the performance of the physical and regression models at Mieming are slightly overplayed, it is worth noting that results do not necessarily have to be sold as ‘good’ or ‘better’ to be interesting and valuable. A little more in-depth discussion on the cause of the discrepancy between the physical model and Lai24, and the performance against measurements would enhance this already well-written publication.

Please see the supplementary document for more in-depth and specific comments. Note they are written up chronologically, not ranked by importance or significance.