Global fine-mode aerosol radiative effect, as constrained by comprehensive
observations

Chung, Chul E.; Chu, Jung-Eun; Lee, Yunha; van Noije, Twan; Jeoung, Hwayoung; Ha, Kyung-Ja; Marks, Marguerite

doi:https://doi.org/10.5194/acp-16-8071-2016

Articles | Volume 16, issue 13

https://doi.org/10.5194/acp-16-8071-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-16-8071-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 13

Research article

|

04 Jul 2016

Research article |

| 04 Jul 2016

Global fine-mode aerosol radiative effect, as constrained by comprehensive observations

Chul E. Chung, Jung-Eun Chu, Yunha Lee, Twan van Noije, Hwayoung Jeoung, Kyung-Ja Ha, and Marguerite Marks

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Final revised paper (published on 04 Jul 2016)
Preprint (discussion started on 22 Jan 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of Chung et al. (2016)', Anonymous Referee #1, 08 Feb 2016
- AC1: '(−0.28 ~ +0.05) Wm-2 is for total (anthropogenic + natural) carbonaceous, sulfate and nitrate aerosols.', Chul Chung, 09 Feb 2016
  - RC3: 'Your estimate is not inconsistent with IPCC AR5', Anonymous Referee #1, 04 Mar 2016
    - AC3: 'Our net forcing estimate is still less negative than the consensus.', Chul Chung, 04 Mar 2016
RC2: 'Review of "Global direct aerosol radiative forcing, as constrained by comprehensive observations"', Anonymous Referee #2, 01 Mar 2016
- AC2: 'direct radiative effect vs. direct radiative forcing', Chul Chung, 02 Mar 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Chul Chung on behalf of the Authors (31 Mar 2016) Author's response

ED: Referee Nomination & Report Request started (09 Apr 2016) by Ari Laaksonen

RR by Anonymous Referee #1 (18 Apr 2016)

RR by Anonymous Referee #2 (28 Apr 2016)

Suggestions for revision or reasons for rejection

The authors have made efforts to improve the manuscript with extra text and shifting away from conclusions about the anthropogenic aerosol forcing; however, there are still some clarity issues. I see that the other reviewer also had trouble reading the paper. The additions unfortunately do not do much to improve this. In many places the language is opaque and additional statements like "overestimate or underestimate" and "higher or lower" do not allow concrete conclusions to be easily drawn. I've tried to cover some of the clarity issues below but this list is by no means exhaustive.

I'm still not fully convinced by the explanation for the large positive radiative effect observed over the Sahara (and Africa in general) after dust and sea-salt have been removed (Fig 6a). These are almost equal in magnitude to the DRE over the regions that are far more affected by biomass burning. It is also at odds with other estimates e.g. Bellouin et al. (2013; Fig. 4) and Heald et al. (2014; Fig 2). I realize that these are at least partially model-based, but the discrepancy in the TOA DRE is very large and needs to be addressed. The strong positive radiative effect at TOA shown in this paper suggests very little scattering impact of organic aerosol in the biomass burning aerosol. Other studies suggest the OA and BC contributions cancel to produce forcings closer to zero (e.g. Myhre et al., 2013) The results of this paper may be correct but they need contrasting with other studies and the reasons for differences explaining. It is worrying because the conclusion of the paper is that the radiative effect of the anthropogenic aerosol is less negative than previously thought. If there is a general overestimation of positive DRE then that might invalidate the conclusion. I don't think this is clearly explained and may not be adequately covered by the sensitivity tests.

The 5th assessment total aerosol radiative effect and forcing estimate is not entirely from models, as is somewhat implied in the final paragraphs of this paper. I'm sure the authors realize it does include partially-observational evaluation (e.g. Bellouin et al., 2013; Su et al., 2013) as well as the AEROCOM study that is used for individual components (Myhre et al. 2013), but this is not clear in the text. In the IPCC report, the authors do point out that both model and observations give similar results (see Chapter 7.5 and Chapter 8.3.4.2 Radiation Forcing of the Aerosol–Radiation Interaction by Component). The range on the direct aerosol effect given is -0.85 to +0.15 W/m2, encompassing the result of this study. Although this study has a narrower uncertainty I still find it hard to accept that this is a legitimate reduction rather than possibly not accounting for certain uncertainties (for example my point about the strong positive fine-mode radiative effect that is surprising and potentially presents a positive bias in the direct aerosol effect).

After reading it several times I feel that while the premise of the paper is interesting and the work has merit, I'm finding it difficult to recommend for publication in the current form because (1) I'm not fully convinced that the range in the uncertainty estimate encompasses the real uncertainty in this estimate, (2) the conclusions drawn about the less negative aerosol DRE are not robust as a result of the uncertainty, and (3) other readers are likely to struggle to understand the details of the paper considering both the reviewers here did. Furthermore, criticisms of the methodology in Lee and Chung (ACP, 2013), that is regularly cited in this work, were made by reviewers in the ACP discussions and do not seem to be fully addressed. I only bring this up because similar criticisms were raised during the initial review of this paper, so it may be worth the authors taking the time to better justify their methods and the uncertainties involved so that readers are convinced this methodology is robust.

I hope the authors understand that the criticisms here are made in order to improve the manuscript and to ensure that the conclusions are justified so that readers are convinced by the results.

Below are some minor comments.

pg2 ln 52 "simulated aerosol" or "modeled aerosol"
pg2 ln52 the uncertainty isn't really "addressed", I would change that to "explored"
pg2 ln79, multiple references to Lee and Chung (2013), here I think you should reference O'Neill et al. (2003) as that is the source of the AERONET SDA.
pg3 ln89, the Chin et al. reference is in a strange location, please alter.
pg4 ln 104, I dont think the "but a clear explanation..." is necessary here.
pg4 ln155-117, I still think that these would benefit to be written in an equation editor, with symbols for the AOD (either "AOD" ot tau) and subscript used for the species types.
pg8 ln241, "SPRINTARS"
pg9 ln286-289, this is very confusing and does not really add any information unfortunately.
pg10 ln309, the uncertainty range of -0.85 to +0.15 W/m2 should be quoted here for clarity
pg10 ln316, "< -0.11 W/m2" means more negative, not closer to zero. I don't think this is what the authors actually mean.
pg10 ln
Figure 6 - year/period needs adding to the caption

References

Myhre, G., Samset, B. H., Schulz, M., Balkanski, Y., Bauer, S., Berntsen, T. K., Bian, H., Bellouin, N., Chin, M., Diehl, T., Easter, R. C., Feichter, J., Ghan, S. J., Hauglustaine, D., Iversen, T., Kinne, S., Kirkevåg, A., Lamarque, J.-F., Lin, G., Liu, X., Lund, M. T., Luo, G., Ma, X., van Noije, T., Penner, J. E., Rasch, P. J., Ruiz, A., Seland, Ø., Skeie, R. B., Stier, P., Takemura, T., Tsigaridis, K., Wang, P., Wang, Z., Xu, L., Yu, H., Yu, F., Yoon, J.-H., Zhang, K., Zhang, H. and Zhou, C.: Radiative forcing of the direct aerosol effect from AeroCom Phase II simulations, Atmos. Chem. Phys., 13(4), 1853–1877, doi:10.5194/acp-13-1853-2013, 2013.

Bellouin, N., Quaas, J., Morcrette, J.-J. and Boucher, O.: Estimates of aerosol radiative forcing from the MACC re-analysis, Atmos. Chem. Phys., 13(4), 2045–2062, doi:10.5194/acp-13-2045-2013, 2013.

Heald, C. L., Ridley, D. A., Kroll, J. H., Barrett, S. R. H., Cady-Pereira, K. E., Alvarado, M. J. and Holmes, C. D.: Contrasting the direct radiative effect and direct radiative forcing of aerosols, Atmos. Chem. Phys., 14(11), 5513–5527, doi:10.5194/acp-14-5513-2014, 2014.

Su, W., Loeb, N. G., Schuster, G. L., Chin, M. and Rose, F. G.: Global all-sky shortwave direct radiative forcing of anthropogenic aerosols from combined satellite observations and GOCART simulations, J. Geophys. Res. Atmos., 118(2), 655–669, doi:10.1029/2012JD018294, 2013.

Lee, K. and Chung, C. E.: Observationally-constrained estimates of global fine-mode AOD, Atmos. Chem. Phys., 13(5), 2907–2921, doi:10.5194/acp-13-2907-2013, 2013.

O’Neill, N. T., Eck, T. F., Smirnov, A., Holben, B. N., and Thulasiraman, S.: Spectral discrimination of coarse and fine mode optical depth, J. Geophys. Res., 108, 4559, doi:10.1029/2002JD002975, 2003.

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ED: Reconsider after major revisions (10 May 2016) by Ari Laaksonen

AR by Chul Chung on behalf of the Authors (20 May 2016) Author's response Manuscript

ED: Publish as is (13 Jun 2016) by Ari Laaksonen

AR by Chul Chung on behalf of the Authors (14 Jun 2016)

Short summary

Currently, the magnitude of aerosol direct forcing is estimated to range from −0.85 W m⁻² to +0.15 W m⁻². The uncertainty in estimated aerosol direct forcing is largely due to uncertainties in global aerosol simulation models. We processed a comprehensive suite of observations and developed creative uses of observations to constrain aerosol simulations. The net results are that (i) we reduced the forcing uncertainty and (ii) we showed that the forcing must be less negative than the consensus.