Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing

Martinsson, Bengt G.; Friberg, Johan; Sandvik, Oscar S.; Hermann, Markus; van Velthoven, Peter F. J.; Zahn, Andreas

doi:https://doi.org/10.5194/acp-17-10937-2017

Articles | Volume 17, issue 18

https://doi.org/10.5194/acp-17-10937-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-17-10937-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 18

Research article

|

15 Sep 2017

Research article |

| 15 Sep 2017

Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing

Bengt G. Martinsson, Johan Friberg, Oscar S. Sandvik, Markus Hermann, Peter F. J. van Velthoven, and Andreas Zahn

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Final revised paper (published on 15 Sep 2017)
Preprint (discussion started on 23 Jan 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review of Martinsson', Anonymous Referee #1, 24 Feb 2017
- AC1: 'Reply to referees', B. G. Martinsson, 08 May 2017
RC2: 'Referee comment on Martinsson et al', Anonymous Referee #2, 22 Mar 2017
- AC2: 'Reply to referees', B. G. Martinsson, 08 May 2017
RC3: 'Referee Report', Anonymous Referee #3, 28 Mar 2017
- AC3: 'Reply to referees', B. G. Martinsson, 08 May 2017

Peer-review completion

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

AR by B. G. Martinsson on behalf of the Authors (30 May 2017) Manuscript

ED: Referee Nomination & Report Request started (04 Jun 2017) by Kostas Tsigaridis

RR by Anonymous Referee #1 (20 Jun 2017)

RR by Anonymous Referee #2 (14 Jul 2017)

Suggestions for revision or reasons for rejection

This manuscript reports on measurements of aerosol sulfur in aerosol samples collected from the IAGOS-CARIBIC platform over a 16 year period. Analysis focuses on a new regression technique that the authors suggest can be used to infer both the gradient of sulfur concentration and the integrated burden in the lowermost stratosphere (LMS), starting at the dynamical tropopause and extending to 3 km above it. It is also suggested that this analysis provides an estimate of the relative contribution of stratospheric sulfate mixed downward and tropospheric sources on the sulfur concentration in the upper troposphere (UT), and how these contributions vary seasonally. Compared to the original version, changes in the data processing now produce more pronounced seasonality in the estimated contribution of S derived from tropospheric sources to the S burden in the UT. In particular, the analysis now suggests a quite strong peak which authors attempt to link to downward transport from the ATAL.

While the approach has changed in response to comments on first version, and some sections describing the data analysis are now more clear, I still find myself not very convinced. Step 1, doing forced linear regression (FLR) of S versus distance above the tropopause is relatively straight forward, except for one key point. The data “were grouped with respect to concentration levels of the different years, resulting in 4 to 5 groups of data for each season” (lines 179-180). The discussion about this step seems to justify at least 2 groups (background and influenced by volcanoes), and perhaps the volcanic group could be (was?) further divided into strong and weak influence. It seems that the authors recognized that there had to be more than 2 groups in each season to do step 2 regressions, but how these were created is not explained. Some objective method of creating these groups, perhaps based on mean, median, max measured S concentrations, or maybe based on time since eruptions were known to have perturbed the LMS burden (based on previous work by this group using largely the same data set) would seem essential, especially for anyone attempting to replicate the analysis or apply the technique to some future similar data set. As it reads now, it almost seems that the approach was to sort by year based on timing of volcanos, but then move years (or individual samples?) that didn’t work into another group until things looked better

The next step in the analysis is now basically clear conceptually: for each season the intercept and slope from 4 or 5 FLR in step 1 were regressed against each other, using weighted regression with the weighting based on estimated uncertainty in the FLR fits. The explanation of this weighting is not completely clear to me, but seems to generate uncertainty only in the FLR intercept (termed offset) when transferred to Fig 3 which shows results of the weighted regressions by season. Surely, the slopes from step 1 were also uncertain. It would seem that uncertainty in both slope and intercept from the FLR would propogate into uncertainty in the estimated slope and intercept from the weighted regression. Based on the small number of data points and the magnitude of the error bars in all 12 panels of Fig 3, I find the small error bars in Fig 5 very surprising (seems that error bars in 5 a should come directly from uncertainty in the slope from fits in Fig 3, while those in 5 b come from uncertainty in the intercepts in Fig 3). In particular, the authors need to carefully consider how to propagate uncertainty from both initial measurement, uncertainty in Z, and uncertainty in slope and intercept of the FLR to the intercept values for the weighted regressions in Fig 3 and clearly explain how they do this, since they assert that the derived values represent the concentration of S in the UT that is derived from tropospheric sources and use these numbers for most of the remaining analyses in the paper. Lines 244 – 249 address how they attempted to account for the uncertainty from FLR, but not in very clear manner.

If the seasonality shown in Figures 5 and 7 is real (signal truly larger than uncertainty) that is an interesting finding. Going to CALIOP data to try to explain the fall peak in tropospheric influence on S in UT is also a good idea, but I am not sure how much support the progressive descent of the ATAL in the top row of Fig 8 provides to the athors interpretation of CARIBIC UT/LS S. How much does the ATAL spread zonally over just a few months? Would the upper level anticyclone associated with the Asian monsoon not keep the enhanced aerosol more or less over Tibet? Did many (any) of the CARIBIC flight pass through or near the 60-120E longitude band shown in Fig 8?

In summary, I am still not convinced that the data analysis is robust enough to believe that from now forward every CARIBIC S measurement can be converted into an estimate of the S column 3 km above the tropopause (Fig 6), or that the source of S in the UT changes from almost completely stratospheric in FM to about 90% tropospheric in ON (Fig. 7). Authors need to use and describe an objective means to bin their data, and more clearly discuss how confident they are about the uncertainty in their derived estimates of “particulate sulfur concentration of tropospheric origin” (also known as the intercepts of the weighted regressions).

Minor editorial comments
Line 82 what is meant by “crust and particle-bound water”?

Lines 147-149 reword this to make it clear what is meant by “not temperature decrease in relation to surface of the earth or the degree of cloud processing” and how either of these is related to distance below the PV tropopause.

Lines 178-194 This section is where the data binning and how 4 or 5 groups were objectively defined needs to be explained.

Line 223 and equation 2, seems that year (y) should be group (g). If you really did the regression for 12 seasons and 16 years the number would be >> 52

Line 284 Fig 4--→Fig 4a

Line 289 Fig 4-→Fig 4b

Line 415-417 This sentence is describing the seasonality of “particulate sulfur concentration of tropospheric origin” not the concentration of S in UT. Figure 7 shows nearly no seasonal change in UT S concentration in background year, and a spring peak in the volcanic year.

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RR by Anonymous Referee #4 (02 Aug 2017)

Suggestions for revision or reasons for rejection

Review of Paper: Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing by Martinsson et al. 2017, submitted to Atmospheric Chemistry and Physics

Martinsson et al. studied sources regions of upper tropospheric sulfur using long-term IAGOS-CARIBIC data. The authors show that the relative contributions (either from transport below or mixed down from the stratosphere) in UT sulfur are dependent on seasons, volcanic activities. The paper has gone through the first round of review with substantial changes based on previous reviewers especially reviewer #1. In general the paper is in good shape, but some concerns (especially on tropospheric contributions) need to be addressed before publication.

a. In the abstract, please give more definition on UT and LMS, i.e. LMS is ~3 km above dynamic tropopause; while UT is how many km below?

b. Line 25 “We find that tropospheric sources dominate during summer andthe fall as a result of downward transport of the Asian tropopause aerosol layer (ATAL) formed in the Asian monsoon”
Line 576 “The ATAL is transported downwards, and affects the extratropical tropopause region in August to December.”
These arguments are unclear to me, what does “downwards transport” mean? A recent paper “efficient transport of tropospheric aerosol into the stratosphere via the asian summer monsoon anticyclone” shows that ATAL is mainly lifted above locally to the stratosphere and further transport to high latitudes. See their Figure 4.

c. The same paper above argues that in summer/fall seasons, convections transport tracer gases (SO2) and aerosol to UT. Do you think it may be more likely a reason for higher tropospheric contribution of UT sulfur rather than downward transport of ATAL?

d. “SO2 measurements by the satellite-based instrument MIPAS indicate a UT seasonal variation in the NH midlatitudes with low concentrations in December to March and the highest concentrations in June to September (Höpfner et al., 2015).”
In my impression MIPAS SO2 product is of large uncertainties, and may not be meaningful in UT. Shown in Rollins et al. (2017, GRL), MIPAS overestimates the SO2 concentration in TP by a factor of ~3 or so.

e. I found the discussions on SO2, CO, OH are difficult to understand. Seems authors claim that direct transport of SO2 and sulfate aerosol from lower troposphere is not going to explain the seasonal variation on tropospheric contribution to UT sulfur, because they will be washed out or cloud-processed. And the authors suggest the concentrations of OH and SO2 are responsible. Well, are we certain that new TP, SO2 oxidation is OH-limited? Any know seasonal cycle on SO2 emissions can explain the seasonal variation? I think some surface SO2 can survive in UT especially in deep convection.

Referee Report: PDF

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ED: Reconsider after minor revisions (Editor review) (02 Aug 2017) by Kostas Tsigaridis

AR by B. G. Martinsson on behalf of the Authors (11 Aug 2017)

ED: Publish as is (15 Aug 2017) by Kostas Tsigaridis

AR by B. G. Martinsson on behalf of the Authors (16 Aug 2017)

Short summary

We find that the aerosol of the lowermost stratosphere has a considerable climate forcing. The upper tropospheric (UT) particulate sulfur is strongly influenced by stratospheric sources the first half of the year, whereas tropospheric sources dominate in fall; 50 % of the UT particulate sulfur (S) was found to be stratospheric at background condition, and 70 % under moderate influence from volcanism. The Asian monsoon is found to be an important tropospheric source of S in the NH extratropical UT.