Transport of regional pollutants through a remote trans-Himalayan valley in Nepal

Dhungel, Shradda; Kathayat, Bhogendra; Mahata, Khadak; Panday, Arnico

doi:https://doi.org/10.5194/acp-18-1203-2018

Articles | Volume 18, issue 2

https://doi.org/10.5194/acp-18-1203-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Atmospheric pollution in the Himalayan foothills: The SusKat-ABC...

https://doi.org/10.5194/acp-18-1203-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 18, issue 2

Research article

|

30 Jan 2018

Research article |

| 30 Jan 2018

Transport of regional pollutants through a remote trans-Himalayan valley in Nepal

Shradda Dhungel, Bhogendra Kathayat, Khadak Mahata, and Arnico Panday

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Final revised paper (published on 30 Jan 2018)
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Preprint (discussion started on 08 Nov 2016)
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Interactive discussion

Status: closed

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

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RC1: 'Review of the paper “Transport of regional pollutants through a remote trans-Himalayan valley in Nepal “ by Dhungel et al., 2016.', Anonymous Referee #1, 06 Dec 2016
RC2: 'Comments on acp-2016-824', Anonymous Referee #2, 10 Dec 2016
AC1: 'Author Comment', Shradda Dhungel, 01 Mar 2017

Peer-review completion

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

AR by Shradda Dhungel on behalf of the Authors (01 Mar 2017)

ED: Referee Nomination & Report Request started (13 Mar 2017) by Elizabeth Stone

RR by Anonymous Referee #1 (26 Mar 2017)

Suggestions for revision or reasons for rejection

Review of the paper “Transport of regional pollutants through a remote trans-Himalayan valley in Nepal” by Dhungel et al.

This is my second review of this paper. In respect to the first submission, the authors added some new elaborations/analyses and accepted some of my previous indications. The paper is improved from the original submission (as an instance the authors improved the motivation of the study and the manuscript organization) but more work is still necessary before it can be considered suitable for publications by ACP. Indeed, the paper still suffers of major deficiencies (both technical and scientific). It still appears somewhat confused and still difficult to follow. Besides the presentation of O3 and BC measurements at a new Himalayan location, no new information in respect to previous similar works have been provided (the main outcome of this paper is that the valley represent a “channel” for pollution to be transported at high altitudes). However, as I reported in my previous review, in the Himalaya region a need for measurements and scientific evidences still is needed, and the observations from KGV can provide useful information for (i) confirm the role of Himalaya valley as “preferred” way for the anthropogenic pollution to be transported up to high Himalaya, (ii) support the hypothesis that pollution from IGP and Himalaya foothills can be transported across mountain ridge to Tibetan Plateau or vented to the free troposphere, (iii) assess the contribution of open fires (or other emissions) to the occurrence of the ABC in South Asia/Himalayas region.

From a technical point of view, the paper need a strong language revisions and the quality of some figures must be improved.

For these reasons, I cannot recommend publication but reconsideration after major revisions.

MAJOR POINTS
In respect to my previous review you mentioned that “winter” was not considered in your paper because is “not relevant”. Please, better argument this point: January and February are not winter months?

Line 273: as already reported in my previous review, this is not true. The seasonal cycles ARE DIFFERENT between IGP and Himalaya foothills and Himalayas. Your BC peak is in pre-monsoon. While over the plains or foothills the peak is during winter (see also Putero et al., ACP, 2015 same special issue). The seasonal cycles are different not similar.

Line 347-366: this discussion is out of the scope of the paper. Please remove.

Line 430: to provide a more robust assessment about the degree of mixing within the valley you should calculate the gradient of potential temperature between the two AWS...
“Because….estimate”. On which basis can you affirm that BC can be vented above 800 m agl. Provide references.

Section 3.4: you should better discuss the different relationship between BC and O3 for the different class of events, not only limiting to describe the observed behavior but also speculating about the reasons leading to the different co-variabilities. As an instance, why during the Pattern A no O3 enhancement is observed? On the other side for Pattern B the O3 is highest when BC is low. A mixed situations occur for pattern C. Please comment and provide explanations. Figure 8: please in the caption provide explanation for the dotted lines and blue/orange dots.

The Conclusion Section looks like a simple summary/highlights of the evidences reported long the paper.

Once again: you should discuss the fact that only 2.5 years of observations are available. Thus you are not able to discuss and assess interannual variability of frequency and magnitude of observed pollution transport.

TECHNICAL/SPECIFIC/LANGUAGE POINTS
Abstract-line 28: please revise this sentence (“particulary during preceeding the monsoon” replace with “pre-monsoon”)

Line 182, please replace “climatologically important pollutants” with “essential climate variables (ECVs)”

Line 192: I’m sorry but the valley orientation description is still obscure to me. What do you mean with “The valley is oriented southeasterly to northwesterly”? Maybe “The valley is oriented from SE to NW”? The same for the following sentences…

Figure 2: please for each site provide the start-end date of the available observations.
Move Table 1 Supplementary to the main manuscript (section 2.1).

Line 208: Again, more details about QA/QC should be reported. How the O3 sampling is executed? Do you use anti-particulate filters at the inlet lines (which type of filters?).By which frequency the instrument is calibrated? Against which standard? How the inlet line is made? Did you calculate the measurement uncertainty?

Line 259: is not suppressed but reduced

Line 283: replace “seasonable” with “seasonal”

Line 296: please add formula to better explain you calculations. I’m still convinced that showing concentration (for BC) and mixing ratio (for O3) will provide more useful info for the audience.

Line 319: use subscript for “3”

Line 319. “non-combustion sources”: please explain more explicitly

Line 329: something is missing in this sentence. “… are at minimum in the KGC, thus supporting the contribution of regional/long-range transport”.

Line 334: “in each seasons”. Do you mean the single months showed by Figure 5? In the case, please, replace “pre-monsoon” with “Apr. 2013” and so on…The quality of Figure 2 is poor. Especially for post-monsoon is really difficult to read the wind roses. Please, zoom by decreasing the upper range of radius…

Line 392: Figure 4 appears after Figure 5 and 6 (section 3.2, 3.3). Please check and re-number figures where appropriate. Once again. Figure 4 is not useful for the paper. Each feature that you describe here can be (more easily) find looking at Figure 5 and 6. I suggest to move Figure 4 in the supplementary materials just to provide an overview of data.

Line 366: replace “one hour” with “1 hour”

Line 403: this is not true looking at Figure 5 where only 1 peak can be seen in late afternoon.

Line 406: replace “relatively calmer” with “weaker”

Line 407: BC diurnal behavior at NCOP(reported by Marinoni et al., ACP, 2010 and not by Bonasoni et al., 2010) is not bimodal but characterized by a single peak in late afternoon/evening.

Line 418: “the distribution of BC concentrations…”..maybe “ the differences of BC concentrations…”?

Figure 7: I would say that DV fluxes must be negative…

Line 437: from Supplementary figure 1, wind speed at JSM_2 is more than 5 m/s higher than at JSM_1 (which is about 30%). Hardly you can define them “similar”… Please uniform station codes with Figure 2.

Line 440: please provide formula for this calculation. Which is the usefulness of calculating this values of mass exported per day? Flux is defined as “kg/m2·s”: “kg /day” is not a flux! Please revise.

Line 570: so, which is the outcome of this comparison? Table 1: Are std.dev or 95th confidence levels reported?

Figure 7: what the whiskers and bars represent?

Supplementary Table 2: no measurement units are reported. Please adopt a nomenclature compliant with Figure 7

Supplementary Table 3 . this should be moved to the main manuscript! For each episode provide information about O3: when is it above the 90th percentile? When “cloud cover” is reported, did you mean that source identification was not possible? Please explain the manuscript text…

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RR by Anonymous Referee #3 (02 May 2017)

Suggestions for revision or reasons for rejection

The present manuscript explores air quality data from a remote
valley location in the Himalayas and tries to suggest that deep
Himalayan valleys may act as important export routes for pollution
from the Indo-Gangetic Plain (IGP) to the Tibetan Plateau (TP). The
authors have addressed several comments given for the previous
version of the manuscript and added some additional analysis.
However, they fail to dissolve many of the major remarks made
previously and the presented results still do not indisputably
support the conclusions drawn by the authors. In addition, the
technical quality of the manuscript has not improved and does not
meet ACP standards (language, presentation of results). Therefore,
I would suggest to reject the manuscript in its current for and
encourage the authors to take the extensive comments made by
reviewer 1 more seriously and carefully rework the manuscript for a
novel submission.

Specifically, I have three major concerns with the present study
that would need to be addressed more adequately before the
manuscript could be considered for publication.

Local influences

The local environment of the site and how local emissions may
influence the present results and conclusions needs way more
attention. A quick look on google Earth revealed that the town of
Jomsom is not that small at all and that there is even a small
airfield. A map (or googleEarth image) to clarify the site location
relative to local pollution sources would be helpful to exclude any
bias of the measurements due to local contamination. It would be
possible to use the local wind observations to eliminate phases
when local emissions directly influence the measurements. As the
authors themselves state, the observed morning peak in BC is most
likely due to local emissions from biofuel burning. The same might
be true for the evening peak, but is never considered. Next to the
local emissions also emissions further down the valley should be
carefully looked at before concluding that all the observed BC is
advected from the IGP rather than from the valley itself.

Flux calculation

In the revised manuscript the authors present an estimate of the BC
flux through the valley. Although it is never stated how this flux
was calculated, I assume the authors multiplied the local wind
speed and concentration measurements to derive a flux. This is
really oversimplifying the transport situation in a complex alpine
valley! At least vertical profiles of wind speed and concentrations
would be needed to derive a robust flux estimate, two-dimensional
transects would be even better. Especially the nighttime flux at
the near-surface site cannot be seen as representative for the
whole valley atmosphere, since the surface layer often decouples
from the flow above and considerable drainage flows may exist above. The
idea to present a flux estimation to corroborate their conclusion
of net pollution export through the valley is honourable but the
present database simply does not allow for a robust estimate of a
flux.

Trajectories

The presented analysis of trajectories and fire hot spots seems a
bit arbitrary. The description of the back-trajectories is
completely insufficient, both in terms of how they were calculated
as well as how they are interpreted. The authors need to explain
where and when the trajectories were initialized and why.
Apparently the underlying meteorological data used for their
calculation was insufficient to include the up-valley flow. So what
was the meteorology used? The authors need to argue why it would be
sufficient to include the transport from the source regions to the
valley mouth in any detail if model resolution is insufficient to
cover the valley flow. The connection with the sources is too
qualitative and vague to be used as evidence. The shown MODIS fire
pixels can only be seen as an indication that there were some fires
in the region, but they don't tell us anything about the absolute
emission strength nor the relative emission strength during the
whole observation period. More quantitative products of fire
emissions are freely available and should be looked at (e.g., GFED,
CAMS GFAS).

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ED: Reconsider after major revisions (04 May 2017) by Elizabeth Stone

AR by Shradda Dhungel on behalf of the Authors (21 Aug 2017)

ED: Publish subject to technical corrections (07 Sep 2017) by Elizabeth Stone

Additional technical corrections are needed prior to publication and are listed below by line number:
Lines 306, 311, and 313: Clarify what is meant by “changes” and “variabilities.” As stated these are too vague and it is unclear what specific measures are being discussed. Perhaps “relative standard deviation” is the correct term?
Line 756: The directions “East-South/North East” is confusing. Please break this up into parts to omit the slash.
45: This comparison suggested that
213: iron, fabrics, etc.
215: 4 km to the west of Bode.
220: located approximately 4.5
234: that results in high sensitivity. (Omit precision here, as this is not affected by the path length.)
236: interest to reduce drift.
238: precisions for CO2…
244: data every 5 seconds and… corrected data for CO2
247: field measurements.
254: simultaneous measurements…
258: The Horiba…
265: A statistically…
271: deemed unnecessary.
272: Besides being highly…
292: from entering into…
293: an AWS….
299: 3.5 discuss the interrelation…
306: mixing ratio were higher…
327: spell out “ca.”
344: Valley, the measurement…
348: (small-medium) and biomass…
356: The small differences…
357: the smaller number…
373: than in the pre-monsoon….
383: all seasons…
387: CH4 emissions…
390: brick kilns
396: suppresses emissions.
398: and, where relevant, reduction…
400: during high intensity
409: related to little or no rainfall
412: relatively low during…
426: increased by…
430, 432: East-Northeast (E-NE)
447: local sources…
462: emissions form the main…
464: higher values of all…
487: farm fires…
492: were coincident
498: diurnal patterns
506: decayed
509: may explain…
533: verb is needed to complete the clause following “and”
546: from the plains to the mountains
548: Although the CO…
565: Omit “Measurement in”
583: change in the mixing later
596: vegetation fires
771: seasons and wind directions suggested
Table 6 caption: and CO2 and their lower and upper bounds (LB and UB).
Table 5 caption: change “gram of gas” to “mass of gas”
Table 5 caption: “, except for the transport sector”
Table 1: use capitalization consistently in the headings.
Figure 9 caption: in all seasons except…

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ED: Referee Nomination & Report Request started (12 Sep 2017) by Elizabeth Stone

RR by Anonymous Referee #1 (03 Oct 2017)

RR by Anonymous Referee #4 (27 Oct 2017)

Suggestions for revision or reasons for rejection

This is my first time to review this MS. I totally agree with comments from those previous reviewers. I find that numbers of problems still existed in the new version of the MS. I suggest the MS a major revision and the following are some more suggestions to the authors.

1. P3 Line 3 reference of (Kopacz et al., 2011) should be replaced by (Zhang et al., 2015) because the results of the former overlooked the potential contribution of local Tibet, which is out of date.
2. P3 Line 31 “no pollution data from Jomsom have been previously reported previously”. The authors overlooked one just published work last year that has done 14C study of BC at Jomsom, which showed the influence of BC from South Asia to the Himalaya and the local biomass burning sourced BC to valleys of the Himalaya (Li et al., 2016). In addition, please add (Chen et al., 2017) and (Luthi et al., 2015), which are also two related article of this study.
3. I am sorry to say that the figures and tables are not typical for both ACP and other scientific journals, which have been pointed out by other reviewers. However, the authors make little changes and still have lots of problems. For instance, you should at least add longitude and latitude to the figure 1 so that the readers know where your research site located. You also need to point out where is South Asia, where is the Tibetan Plateau at this figure.
4. Figure 2 looked a little strange and is useless. Instead, the words that point out four sites should be moved to right photo of figure 1. The table of Figure 2 should be moved to supporting information.
5. The expression of tables is not typical, which should be “Three line table”. Please change the format of tables.
6. Finally and honestly, the English of the article is not fluent, please improve it by expert English editor.

References:
Chen, P., Li, C., Kang, S., Rupakheti, M., Panday, A.K., Yan, F., Li, Q., Zhang, Q., Guo, J. and Ji, Z. (2017) Characteristics of Particulate-Phase Polycyclic Aromatic Hydrocarbons (PAHs) in the Atmosphere over the Central Himalayas. Aerosol & Air Quality Research x, 1-13.
Li, C., Bosch, C., Kang, S., Andersson, A., Chen, P., Zhang, Q., Cong, Z., Chen, B., Qin, D. and Gustafsson, Ö. (2016) Sources of black carbon to the Himalayan–Tibetan Plateau glaciers. Nature Communications 7, 12574.
Luthi, Z.L., Skerlak, B., Kim, S.W., Lauer, A., Mues, A., Rupakheti, M. and Kang, S. (2015) Atmospheric brown clouds reach the Tibetan Plateau by crossing the Himalayas. Atmospheric Chemistry and Physics 15, 6007-6021.
Zhang, R., Wang, H., Qian, Y., Rasch, P.J., Easter, R.C., Ma, P.L., Singh, B., Huang, J. and Fu, Q. (2015) Quantifying sources, transport, deposition, and radiative forcing of black carbon over the Himalayas and Tibetan Plateau. Atmospheric Chemistry and Physics 15, 6205-6223.

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RR by Anonymous Referee #3 (29 Oct 2017)

Suggestions for revision or reasons for rejection

This is my third review of the manuscript. In comparison to my
previous review the authors have partly addressed my three main
points of concern although not necessarily in as much detail as I
had hoped for. However, they have removed some more speculative
parts of the discussion of the pollution transport towards the
valley, so that this part is not hindering publication anymore.
Nevertheless, I still have some further comments concerning the
last additions and I make some additional recommendations
concerning the presentation of figures and tables, which in my view
are still not up to ACP standards. Once these issues are addressed
publication in ACP could be considered.

Local influences

The authors have replied to my concerns about local pollution
episodes by adding a sentence on the use of the local air strip
which suggests that this will only have minor influences. However,
the authors did not follow the suggestion of further screening
their data based on local wind direction and location of sources,
but argue that the vertical distance to the town is large enough to
minimise the influence. Without further analysis and data this
remains hard to judge, but there seems to be sufficient mention of
local sources now to create the required cautiousness when
interpreting the results.

Flux calculation

The authors added a whole section explaining the calculation of net
upvalley BC fluxes and also explain the limitations of their
approach. One of the main assumptions of the approach is that of
constant depth of the upvalley or downvalley flows which is clear
oversimplification of the flow especially during nighttime. The
authors should add another sentence on the nighttime flow. It
should be mentioned that their assumption is most likely an
overestimation of the nighttime flow depth and, hence, nighttime
downvalley flux. Therefore, their net upvalley flux is most likely
an underestimation and at least the conclusion that there is net
upvalles mass flux seems to be supported by the approach, whereas
the absolute numbers should definitely taken with caution. I would
also suggest a rewrite of the equation of the daily net mass
transport. Instead of using 1/6 in front of each term, it would be
more logical to just a multiplication with Delta t at the end of
the equation. This would immediately make clear that we are looking
at an integration over time. The value of Delta t can then be given
in the text.

Trajectories

The previously criticised section on transport towards the valley
mouth discussed by the analysis of air mass trajectories was
removed from the manuscript. Therefore, my suggests for improving
this kind of analysis were not followed, however, the manuscript
does not draw any vague conclusions on this point anymore.

Presentation of figures and tables

Some of the figures and tables still need improvement before
publication in ACP.

Figure 2 should be split into a figure and a table! I think I
mentioned this before in an earlier review ...

Table 1 looks more like a figure than a table. Please check if this
agrees with ACP guidelines for tables!

Figure 3: Maybe wind-roses are just not the best way to present the
wind statistics in the case of such a clear bidirectional flow.
Maybe a pdf of wind speed for 3 different wind sectors would be
much clearer. The figure right now consists mostly of white space
and the interesting information makes up only a very small fraction
of the plot and is hard to see.

Figure 7: In general the figure quality could be improved by not
squeezing everything together in the vertical. Make the figure less
wide, but add some vertical space between individual panels. I
would also suggest to use a different y-axis scale for the monsoon
period. Like it is presented now one cannot even tell the magnitude
of the fluxes.

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ED: Publish subject to minor revisions (review by editor) (07 Nov 2017) by Elizabeth Stone

AR by Shradda Dhungel on behalf of the Authors (18 Nov 2017) Author's response Manuscript

ED: Publish subject to technical corrections (20 Dec 2017) by Elizabeth Stone

AR by Shradda Dhungel on behalf of the Authors (26 Dec 2017) Author's response Manuscript

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Short summary

We analyze seasonal and diurnal concentrations of black carbon (BC), ozone, and associated meteorological conditions within a remote trans-Himalayan valley in western Nepal. We observe elevated BC concentrations during non-monsoon seasons, frequent and persistent episodes of higher-than-average concentrations, and net up-valley fluxes throughout the year. The findings provide direct observational evidence of trans-Himalayan valleys serving as vital pollutant transport pathways.