Review comments on “Evaluation and updates to the oxidized reactive nitrogen trace gas dry deposition parameterization from the GEOS-Chem CTM, including a pathway for ground surface NO2 hydrolysis” by Boys et al.

This study conducted extensive sensitivity tests on the dry deposition code of GEOS-Chem focusing on comparing modeled NO2 dry deposition velocity (Vd) with eddy covariance measurements collected at the Harvard Forest station. The method used in this study is scientifically sound. Results from these tests are properly interpreted and may help understand the causes of model-measurement biases and reduce model uncertainties. One addition from this study to the existing literature is the modification of the GEOS-Chem deposition scheme by including the effect of NO2 hydrolysis at surface so that nocturnal NO2 Vd is increased, which is worth to be published, although its general application may need further evaluation in future studies. It is also noted that results from some of the conducted tests are well-known and expected as there are many similar model intercomparison and/or sensitivity studies in literature.

This is a very long manuscript with too many unnecessary detailed descriptions. While I appreciate the big amount of the work the authors have done, I do feel that it can be substantially simplified because many materials presented here are available in literature and do not necessarily provide any new information. For example, the following places can be substantially simplified: (1) Discussions related to the uncertainties in Ra and Rb (section 3.1 and proceeding sections related to 3.1) have been published in several studies (Toyota et al., 2016, Atmos. Environ., 147, 409–422; Wu et al., 2018, Geoscientific Model Development, 14, 5093–5105). In fact, the same data set of Nguyen et al. (2015), as used here for testing Ra and Rb, was also used in Wu et al. (2018) for the same purpose. Results from these earlier studies are readily applicable here. (2) Details of all the measurement data and model formulas (Section 2) are available from literature; they can either be referred to the literature and/or moved to Supplement Information of this manuscript. Only keep the minimum information that is needed for the big picture. (3) Do you really need formulas and detailed descriptions of Vd models in Section 1? Should you focus more on NO2 hydrolysis effect on Vd, instead of describing Vd modes similar to a textbook? (4) You can simply state that you used a stand-alone version of GEOS-Chem Vd code in this study for sensitivity tests, why bother describing how you extract the code from GEOS-Chem CTM (in Abstract, Introduction, section 2.1)?

The modification of GEOS-Chem Vd code by including surface NO2 hydrolysis pathway increases NO2 Vd (probably because the original Wesely code predicts very low Vd for NO2). If it is not too much work, can the authors estimate how higher the increased NO2 Vd would be compared to other Vd models such as the Zhang et al. (2003) code that is referenced here? How big differences would such a modification of NO2 Vd make on predicted ambient concentration and deposition flux of NO2 on seasonal to annual basis?

Overall, this is a big effort and can be published after considering the above comments.

Review of “Evaluation and updates to the oxidized reactive nitrogen trace gas dry deposition parameterization from the GEOS-Chem CTM, including a pathway for ground surface NO2 hydrolysis”

Summary: In this manuscript the authors test the sensitivity of GEOS-Chem assumptions about gas-phase diffusivity, site-specific parameters that influence the R_a parameter in the Wesley gaseous dry deposition model, and non-stomatal influences on R_c. They use a dataset collected in 2000 from Harvard Forest to evaluate their non-stomatal treatment of R_c for NO₂ deposition. They then consider a set of various parameterizations that implement different treatments of parameters that affect R_a, R_b, and R_c for NO₂ in GEOS-Chem. Informed by the results of these updated parameterizations, the authors improve agreement between modeled and measured V_dep for highly soluble species measured from the study of Nguyen et al. 2015. Among other things, they also highlight the likely importance of heterogenous hydrolysis of NO₂ as a deposition mechanism.

I agree with the perspective provided by Reviewer 1 that the manuscript could benefit from condensing some of the sections that have a lot of detail and/or read like a textbook. I found the manuscript difficult to review because the amount of detail that was included about the analyses. However, this was a monumental effort, and the authors methods of analyses are scientifically sound and robust. I left some comments below that should not be necessary to address but could be considered in constructing future manuscripts.

Comments

(General) About halfway through the manuscript I had to remind myself, by re-reading the introduction, what motivated this study. For me, the question of why NO₂ and what are CTMs getting wrong about NO₂ deposition wasn’t really addressed until page 4. Echoing Reviewer 1 I felt like I had a textbook introduction to dry deposition of gases before the uncertainties of NO₂ deposition (likely non-stomatal deposition) were introduced. I don’t have specific recommendations for restructuring/condensing the introduction, but it could benefit the reader to be concise in stating the problem that updating the gaseous diffusivity and non-stomatal deposition treatment for NO2 in GEOS-Chem could improve measurement-model agreements.

(Table 2) There’s a missing “)” for the “base GC (Chapman–Enskog theory with constant mfp(b)” in the middle box of the table.

(Page 19 Line ~500) Detailing the historical observations and exact calculation methodology of V_chem and F_soil is an example of where some information could potentially be condensed. From Fig. 2 it looks like correcting for soil NO emission results in a relatively small increase in V_ex (maybe 5 % to 10 % at most?). I suggest even simplifying Fig. 2 (bottom panel) by removing the “observed” and just showing the soil NO corrected. In the caption a statement of “correcting for soil NO increased observed V_ex by no more than 10 %” or something like that.