Investigation of the Influence of Atmospheric Scattering on Photolysis Rates Using the Cloud-J Module

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is well written and worthy of publication. This reviewer only has two minor comments for authors considering, 1) the current version of Cloud-J 8.0 builds on the old version. The authors present details on the Cloud-J. However, the weaknesses of the old version (CLOID-J 7.3) are missed, particular for those updates in the new version should be presented; 2) why can new approaches effectively improve the performance in predicting photolysis rates?    

 

 

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Reviewer 2 Report

Comments and Suggestions for Authors

This paper discusses the calculation of photolysis rates in the atmosphere using radiative transfer models. Rate constants calculated using the (presumably) more accurate LibRadtrans method are compared with the more approximate Cloud-J model, and the photolysis rates agree within ±20% below ~60 km, and at higher altitudes except for the photolysis of O2, which the Cloud-J model is apparently not designed to handle. Effects of albedo, Raleigh scattering, clouds and aerosols are also shown for representative photolysis reactions. These effects can be significant depending on the photolysis reaction and its action spectrum.

Photolysis reactions are important in atmospheric chemistry models at all altitudes, and many modelers do not seem to appreciate the issues and uncertainties in calculating photolysis rates even for reactions whose action spectra are well characterized. Therefore, this is an important subject and appropriate for this journal. However, paper needs to give somewhat more information about the radiative transfer models discussed and the terms they employ to improve its utility to modelers and atmospheric chemists who may not be familiar with the details and literature of radiative transfer. In addition, there are areas of the presentation that should be improved. This is discussed below.

The introduction section gives an adequate background concerning photolysis process in the atmosphere and why they are important, though it should also mention that photolyses are important for some organic compounds and affect the chemistry of polluted atmospheres. The introduction also includes a discussion of radiative transfer models, but this discussion needs to give more information about what the integral radiative transfer equation is and why it is difficult to solve. The references can be cited for details, but not all readers who are (or should be) interested in issues related to calculation of photolysis rates will be sufficiently familiar with the terms and methods used to understand the significance of this work. Therefore, at least more of a summary of the terms and methods relevant to this work would be useful. What's the difference between narrow-band and wide-band methods? Briefly explain the approximations involved in the simplified approaches used in this work.

The references cited in the introduction should include a reference giving some general discussion of atmospheric chemistry in the lower atmosphere such as "Chemistry of the Lower Atmosphere" by Findlayson-Pitts and Pitts or similar. Currently the citations in the introduction are restricted to references on radiation, ozone depletion, and a photolysis data evaluation.

The LibRadtran and "uvspec" software is used as the reference method when assessing the adequacy of the presumably more approximate Cloud-J method that this paper is evaluating. Is the LibRadian software the most accurate available method to calculate photolysis rates? The text seems to imply that but I couldn't find where that was stated clearly.

The caption of Figure 1a should state that it is showing the deep space spectrum, not ground level. It might be interesting to see a ground level spectrum (for typical conditions) on this figure as well, since this should clearly show the attenuation of intensity at the shortest wavelengths.

Figure 1 should actually have action spectra (absorption x quantum yields) rather than absorption cross sections, since that is what determines photolysis rates. The caption of Figure 1b states that it is showing absorption cross sections, but it is actually showing action spectra for the two O3 photolysis reactions. I believe that quantum yields are assumed to be unity at all wavelengths for HNO3 and probably Cl2O2 (I didn't check), but they fall off at higher wavelengths for NO2. Is the curve on Figure 1a for NO2 the absorption cross section or action spectrum? In this regard, the paragraph starting at line 159 should be discussing action spectra rather than absorption.

On Lines 215-217 seems to imply that LibRadtran model uses spectral resolutions that are 500 times courser in the 130-175 and 205-305 nm ranges, and 1000 times courser in the 350-700 nm range than it does in the 121-130 and 175-205 nm ranges. Is this correct? If so, it needs some discussion.

Table 1 needs a caption or footnote stating what the percentages given after the wavelength intervals mean.

It might be illustrative for Section 3.1 to also show plots the solar fluxes vs. wavelength at the different altitudes for the two models. This would clearly indicate why and how the photolysis rates differ with altitude and model. I also think that the data on Table 2 might be more effective as a figure giving plots of relative contributions as a function of wavelength than as a table.

The same colors should be used for the various reactions on Figure 2a and 2b. For example, HNO3 is green on 2b but red on 2a.

I found Figure 3 difficult to understand. Would just showing the solar spectra in the two wavelength regions or their differences at different altitudes be sufficient to illustrate the point being made?

Figures 4-6 should show 0 on the x axis. As it is, the effects of changing the parameters are exaggerated and their relative effects on the different photolysis rates shown on the three plots would be more clearly seen. As it is, the figures are somewhat misleading.

The caption on Figure 7 should state what the green dotted lines are. I could get this from the text, but it should also be stated on the caption.

Plots of differences of solar spectra at different parameters or models would be good complement to Figure 4-8. It's still useful to show differences in photolysis rates, but differences in actinic fluxes are the fundamental reason for the differences photolysis rates.

On lines 599-600 they summarize the three values for the ATM0 parameter, but do not indicate what is meant by "refractive", "geometric" or "spherical" models and how they differ. They state that their calculations use a "refractive" model, but from their discussion it looks like they are comparing the flat model with a spherical model.

Portions of this paper are difficult to read because many paragraphs tend to run on and are  excessively long. The long paragraphs should be broken up for easier reading.

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript introduces a new version of cloud J algrothim to calculate the photolysis rate. The calculation is strongly depend on the software, which make this manuscript like a software evaluation. Does it really have enough scientific property?

What is changed from v7.3 to v 8.0?

p3, eq. 5 has a little problem, eq. 13 shall be revised into concentration formulas?

Caption of Figure 3  does not express clearly.

Conclusion part is a little long. Also it needs to write more like a science paper, not a software evaluation.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Accept as it is.