The observed influence of local anthropogenic pollution on northern Alaskan cloud properties

Maahn, Maximilian; de Boer, Gijs; Creamean, Jessie M.; Feingold, Graham; McFarquhar, Greg M.; Wu, Wei; Mei, Fan

doi:https://doi.org/10.5194/acp-17-14709-2017

Articles | Volume 17, issue 23

https://doi.org/10.5194/acp-17-14709-2017

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

https://doi.org/10.5194/acp-17-14709-2017

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

Articles | Volume 17, issue 23

Research article

|

11 Dec 2017

Research article |

| 11 Dec 2017

The observed influence of local anthropogenic pollution on northern Alaskan cloud properties

Maximilian Maahn, Gijs de Boer, Jessie M. Creamean, Graham Feingold, Greg M. McFarquhar, Wei Wu, and Fan Mei

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Final revised paper (published on 11 Dec 2017)
Preprint (discussion started on 16 May 2017)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'review', Anonymous Referee #2, 19 May 2017
RC2: 'Review', Anonymous Referee #3, 27 Jun 2017
AC1: 'Response to reviewers', Maximilian Maahn, 23 Aug 2017

Peer-review completion

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

AR by Maximilian Maahn on behalf of the Authors (23 Aug 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Sep 2017) by Xiaohong Liu

RR by Anonymous Referee #2 (11 Sep 2017)

Suggestions for revision or reasons for rejection

The authors spent a great deal of effort addressing the reviewer suggestions, which is much appreciated, and the paper is substantially improved. The logic of the paper and the figures are now much clearer, and I am happy to recommend it for publication, after a few minor comments detailed below are addressed.

Specific comments:
p.16, l.20: “Because enhanced rBC and CN concentrations are expected to be good indicators of anthropogenic activity, they are used to isolate clouds impacted by anthropogenic emissions.”
I agree that rBC is a good indicator, but I am not so sure about CN. In this paper, CN is defined as the number count of aerosols between 3 or 10 nm up to 3 microns in diameter (p.5, l.35). Many local non-combustion derived aerosols pop up in the lower diameter range between 3-60 nm (e.g., Tunved et al. 2013). These very small background particles can be so numerous that they can dominate the CN count (e.g., Zamora et al., 2016). That could be one explanation of the large discrepancy between Figures 8 and 10 in the aerosol distributions relative to reff, why Figure 12 and Figure 8 are so similar, and why there are more brown points in Figure 13b than in Figures 13a and 13c.

p.17 l.4: “We conclude that CN and rBC particles, which were used to classify local clouds, have the potential to grow to accumulation mode particles measured by the PCASP.”
Again, I feel that CN is inappropriate to use for this purpose – PCASP concentrations would likely be better. Also, from the author’s definition of CN, the CN data should already include the data in the PCASP ranges. This is confusing to me.

P.19, l.7: “When applying the linear regression to the data sets corresponding to the two sites separately, the obtained ACI values differ (Table 1), with OLI having a lower ACI value (0.12±0.05) than NSA (0.20±0.07). Given the small sample size (24 and 16 cases for OLI and NSA) which was caused by the PCASP data being quality-flagged for some cases, it is not possible to determine whether this is caused by a difference in nucleation efficiency between aerosols at the two sites or a random effect.”
How were the errors in ACI value calculated? Is the difference in ACI values between the two sites significant? Given the error ranges listed here, and the high spread of data shown in Figure 14, it looks likely to me that they are likely not significantly different, in which case discussion of the differences is not appropriate.

P20, l.5: For the following reasons, I feel that the following new text is too speculative, and suggest removing it:
“The lower R2 value for OLI (0.24) in comparison to NSA (0.40) could indicate that the assumption that PCASP particle concentrations represent a good approximation for CCN concentrations is partly violated at OLI.”
As mentioned, I’d like to know if the ACI values actually were significantly different.
“This could result from those particles being less aged and consequently less coated by sulphates and organics in comparison to those observed around NSA.”
These data are not provided.
"In addition, some data points lie above the 1:1 line which might indicate that particles smaller than the PCASP size range (i.e. < 100 nm) are acting as CCN (Leaitch et al., 2016)."
What figure is being referred to here - Figure 14?
"Further, the assumption that the below-cloud aerosol properties govern the cloud microphysical properties might not be true for all clouds depending on sub-cloud vertical mixing. Therefore, we identified all clouds where the above-cloud PCASP concentration is larger than below-cloud (red dots in Fig 14), and indeed half of these clouds are above the 1:1 line. When using the above-cloud concentration for these clouds, only two of these clouds are above the 1:1 line. However, there are still 11 more clouds above the 1:1 line. Since these clouds generally feature PCASP concentrations < 50 cm-3, the fact that they are above the 1:1 line could be related to increasing sampling errors for small concentrations. They may also confirm the finding of Leaitch et al. (2016) that aerosols below 100 nm can act as CCN for thin clouds.”
This last argument is based on a very low sample number. Especially due to this low sample size, detailed description of the instrument/sampling error is key to giving any credence to the hypothesis. To “confirm” the Leaitch et al findings, the authors would need to show a lot more information on this error than what is provided here. Without that information, I believe the above discussion is much too speculative.

Technical suggestions
p.2, l.3: Because of these observations…
p.5, l.5: Suggest: In the following work
p.6, l.15: I still disagree with the phrasing here, because while these tracers are emitted by fires, they are also affected by other processes. BC and CO are only good tracers of smoke for in situ data if the other processes affecting the BC/CO concentrations are unimportant at a specific place and time. I suggest rephrasing to something like, “Therefore, we manually inspected the vertical profiles of rBC and CO, which together can be used to trace biomass burning in otherwise clean environments (Warneke et al., 2009, 2010).” (Note: if you make this change, the reference is probably not appropriate anymore, since I believe Warneke et al. used a model to identify smoke-related BC in non-clean environments)
For the same reason, on p.6, l.17, I suggest you change to: “For each spiral obtained at the two sites, elevated layers with CO >= 0.1 ppmv or rBC >= 20 ng kg-1 were flagged as potentially corresponding to forest fires.”

p. 6, l.18-19: “Local emissions, on the other hand, are expected to be found in a layer connected to the surface” – how sure are you about that? Do you mean “Local emissions, on the other hand, are expected to be concentrated in the layer…”?

p.8, l. 29: e.g.?
p.10, l.24: what does the question mark mean?

Figure 1: it would be helpful to state in the caption how fire source was identified

References

Tunved, P., Ström, J., and Krejci, R.: Arctic aerosol life cycle: link- ing aerosol size distributions observed between 2000 and 2010 with air mass transport and precipitation at Zeppelin station, Ny-Ålesund, Svalbard, Atmos. Chem. Phys., 13, 3643–3660, doi:10.5194/acp-13-3643-2013, 2013.

Zamora, L. M., Kahn, R. A., Cubison, M. J., Diskin, G. S., Jimenez, J. L., Kondo, Y., McFarquhar, G. M., Nenes, A., Thornhill, K. L., Wisthaler, A., Zelenyuk, A., and Ziemba, L. D.: Aircraft- measured indirect cloud effects from biomass burning smoke in the Arctic and subarctic, Atmos. Chem. Phys., 16, 715–738, https://doi.org/10.5194/acp-16-715-2016, 2016.

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RR by Anonymous Referee #3 (26 Sep 2017)

Suggestions for revision or reasons for rejection

The paper is much improved and worthy of publication in ACP subject to a few minor comments.

1. Abstract, line 4 – “which are governed IN PART by the availability of…”

2. Page 1, line 21 to Page 2, line 4 – I would like to see you add to this discussion that natural emissions during summer also can result in higher number concentrations of aerosol particles that serve as CCN, and these can have significant influences on cloud albedo in the summertime Arctic (Croft et al., Nat. Commun. 7, 13444, doi: 10.1038/ncomms13444, 2016). Aside - In the Arctic and in the context of the indirect effect, clean must be defined in terms of the numbers of CCN that can influence cloud properties as opposed to mass concentrations of particles. Anthropogenic and BB sources will increase mass but not always the numbers of CCN; although BB seems to be more important for CCN numbers over North Alaska, it appears to be less so at higher latitudes. Rather than enhancing any AIE, the introduction of an anthropogenic or BB aerosol may just replace the effects of the natural aerosol on clouds by shifting the dominate mode of the particle size distribution from Aitken to accumulation.

3. The internal pump employed in the CPC 3025 by TSI is often not strong enough to control the in-flight flow in an aircraft where pressure changes (either due to changes in airspeed, altitude or bypass flows used to draw the aerosol across the CPC intake) may under- or overpressure the inlet. Was that a problem?

4. Page 12, line 6 - The mass discussion seems consistent with the tail in your PCASP number distributions in Fig. 3.

5. Regarding Fig 14 and the discussion on page 18-19 – It is not uncommon for the lower activation diameter to be somewhat lower than the lower limit of the PCASP, even without activating particles as small as in Leaitch et al. (2016). That may happen because some of the OLI particle size distributions are skewed towards slightly smaller sizes compared with NSA, as suggested in Figures 10 and 11 and implied by your first conclusion. It could also happen because some updrafts in the NSA region were slightly stronger than over OLI. Is there any evidence to suggest that some of the NSA clouds may have had slightly higher updrafts? Related, Section 2 should include a short discussion of the nature of the clouds sampled.

6. Please change conclusion 4 to “Given the …. which might be impacted by LOCAL anthropogenic emissions.”

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ED: Reconsider after minor revisions (Editor review) (12 Oct 2017) by Xiaohong Liu

AR by Maximilian Maahn on behalf of the Authors (19 Oct 2017) Author's response Manuscript

ED: Publish as is (06 Nov 2017) by Xiaohong Liu

AR by Maximilian Maahn on behalf of the Authors (06 Nov 2017)

Short summary

Liquid-containing clouds are a key component of the Arctic climate system and their radiative properties depend strongly on cloud drop sizes. Here, we investigate how cloud drop sizes are modified in the presence of local emissions from industrial facilities at the North Slope of Alaska using aircraft in situ observations. We show that near local anthropogenic sources, the concentrations of black carbon and condensation nuclei are enhanced and cloud drop sizes are reduced.