Particle water and pH in the eastern Mediterranean: source variability and  implications for nutrient availability

Bougiatioti, Aikaterini; Nikolaou, Panayiota; Stavroulas, Iasonas; Kouvarakis, Giorgos; Weber, Rodney; Nenes, Athanasios; Kanakidou, Maria; Mihalopoulos, Nikolaos

doi:https://doi.org/10.5194/acp-16-4579-2016

Articles | Volume 16, issue 7

https://doi.org/10.5194/acp-16-4579-2016

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

https://doi.org/10.5194/acp-16-4579-2016

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

Articles | Volume 16, issue 7

Research article

|

13 Apr 2016

Research article |

| 13 Apr 2016

Particle water and pH in the eastern Mediterranean: source variability and implications for nutrient availability

Aikaterini Bougiatioti, Panayiota Nikolaou, Iasonas Stavroulas, Giorgos Kouvarakis, Rodney Weber, Athanasios Nenes, Maria Kanakidou, and Nikolaos Mihalopoulos

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Interactive discussion

Status: closed

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

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RC C9108: 'Referee Comment', Anonymous Referee #3, 10 Nov 2015
- AC C11841: 'Answer reviewer 3', Nikolaos Mihalopoulos, 21 Jan 2016
RC C9979: 'Review of Nikolaou et al.', Anonymous Referee #1, 01 Dec 2015
- AC C11842: 'Answer reviewer 1', Nikolaos Mihalopoulos, 21 Jan 2016

Peer-review completion

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

AR by Nikolaos Mihalopoulos on behalf of the Authors (09 Feb 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (14 Feb 2016) by Frank Dentener

RR by Anonymous Referee #1 (19 Feb 2016)

RR by Anonymous Referee #3 (29 Feb 2016)

Suggestions for revision or reasons for rejection

General Comments

The authors have satisfactory addressed most of my comments on the original version of this manuscript. However, I encourage them to consider additional revision to address the three issues detailed below. After suitable revision to address these issues, I recommend that the revised manuscript be published.

For completeness, the following points include my comment on the original version of the manuscript, the authors’ response, the authors’ action, and my follow-up comment regarding their response and action.

Specific Comments

Original Reviewer Comment: Page 29,526, last 3 lines. The relative importance of different pathways for SO2 oxidation is strongly pH dependent. The solubility of SO2 in aerosol solutions at pH less than 3 is quite low and, consequently, in the presence acidic aerosol in this pH range, most SO2 is oxidized in the gas phase [e.g., Keene et al., 1999, 251 J. Aerosol Sci.] not via “heterogeneous” pathways as suggested by the authors. The text should be clarified.

Authors’ Response: The statement was within the context of cloudy atmospheres and the presence of coarse mode dust particles. For clarification, the sentence now reads “…and the subsequent acidification through heterogeneous oxidation of the SO2 on deliquescent dust particles within the plume, Meskhidze et al. (2003) concluded that…”.

Authors’ Action: See page 18, lines 713-715 of the current file.

Follow-up Reviewer Comment: Results presented by the authors in Table 1 suggest that the supermicron-diameter aerosol size fractions were acidic, which is consistent with expectations based on kinetics. Alkalinity associated with freshly produced particles is typically titrated within seconds to a few minutes by multiple pathways of which condensation and subsequent in situ oxidation of SO2 in only one. Condensation of acids from the gas phase (including HNO3, HCl, H2SO4, HCOOH, and CH3COOH among others) also titrates alkalinity associated with, and thereby acidifies, freshly produced crustal dust as well as marine aerosol. Following alkalinity titration, pH drops sharply and the SO2 pathway becomes insignificant [e.g., Chameides and Stelson, 1992, JGR]. Measurements in many regions demonstrates that H2SO4 is generally not the primary source of acidity in supermicron-diameter aerosol size fractions. The authors present no evidence to support the speculation that the in situ oxidation of SO2 is the only or even most important process by which larger particles are acidified. I encourage them to drop the unsubstantiated suggestion that the in situ oxidation of SO2 is the only driver of acidification. The primary point in this section of the manuscript relates to aerosol pH and associated bioavailably of Fe, not to the processes by which aerosols are acidified.

Original Reviewer Comment: Page 29,537, line 2, Table 1, and elsewhere. Presumably, the reported “average…pH” and associated 508 standard deviation correspond to the average and standard deviation for aqueous concentrations of H+ expressed as pH. If so, the text should be clarified. If not, both the results and the corresponding text should be revised accordingly. It is inappropriate to directly average lognormally distributed values such as pH.

Authors’ Response: We kindly disagree here. Of course averaging of a non-linear metric, such as pH, would result in a biased value if we are considering mixing of samples into one volume that is allowed to react. Here, however, we are talking about temporal variability in aerosol properties; each time instant is independent from each other so averaging is acceptable.

Follow-up Reviewer Comment: pH is a mathematical expression of H+ concentration, which has physical meaning that does not change based on the application. Direct averages of pH values yield H+ concentrations that are biased relative to average H+ concentrations expressed as pH. It is evident that direct averages of pH values have no physical meaning. It appears that the authors’ wish to report a population statistic for the central tendency of the distribution of solution acidities rather than the average for that distribution of solution acidities. As such, I suggest that they report the median pH, which does have physical meaning. Other properties of the aerosol are reported in the paper as average concentrations (e.g., Fig. 4). The pH value corresponding to the average H+ concentration is the only unbiased statistic relevant for direct comparison to and interpretation in the context of averages for other aerosol properties.

Original Reviewer Comment: Page 29,539, line 1. “nss” is not defined, the method used to calculate nss-K is not described, no nss-K data are reported, and nss-K is not an acid or a base and, thus, has no direct influence on the acidity or alkalinity of the aerosol.

Authors’ Response: ‘Nss’ (Non-sea-salt) is now defined in the revised manuscript for clarity. Note however that the statement on nss-K is provided as an additional plausible reason found in the literature (Zhang et al. 2015 reference) why biomass burning aerosol exhibits higher pH values. It does not concern data from Finokalia station. No nss-K data are reported in our study, therefore no method to calculate nss-K is described. Non-sea-salt potassium (nss-K) is higher in air masses influenced by biomass burning (e.g. Zhang et al. 2015), and being an ionic species would partake in the pH calculations.

Authors’ Action: See page 30, lines 1086-1088 of the current file.

Follow-up Reviewer Comment: Nss K+ has virtually no direct effect on solution pH. For example, adding nss K+ to an aerosol solution in the form of KBr or K2SO4 and at a concentration relevant to particles from biomass burning would have essentially no influence on either the actual or the calculated acidity of that solution. In contrast to the authors’ suggestion in the manuscript, simple correlations between nss K+ and acidity in aerosols produced by biomass burning do not demonstrate a significant causal influence of nss K+ in “rendering aerosol more alkaline.” To suggest otherwise is misleading. The text should be clarified. In addition, since all pHs reported in the manuscript correspond to acidic solutions, it would seem more appropriate to characterize biomass-burning impacted aerosol as “less acidic” rather than “more alkaline.”

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ED: Reconsider after minor revisions (Editor review) (01 Mar 2016) by Frank Dentener

AR by Aikaterini Bougiatioti on behalf of the Authors (09 Mar 2016) Author's response

ED: Publish as is (24 Mar 2016) by Frank Dentener

AR by Aikaterini Bougiatioti on behalf of the Authors (24 Mar 2016)

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Short summary

Atmospheric aerosols and relevant parameters were measured in the eastern Mediterranean during summer and fall 2012. Submicron aerosol water can contribute up to 33 % of total mass, and 27.5 % of this can be associated with organics. Using these data, the pH of the submicron aerosols was calculated to be highly acidic, varying from 0.5 to 2.8 and independently of air masses origin. Such pH values could increase nutrient availability and thus sea water productivity of the Mediterranean Sea.