Characteristics and sources of submicron aerosols above the urban canopy (260 m) in Beijing, China, during the 2014 APEC summit

Chen, C.; Sun, Y. L.; Xu, W. Q.; Du, W.; Zhou, L. B.; Han, T. T.; Wang, Q. Q.; Fu, P. Q.; Wang, Z. F.; Gao, Z. Q.; Zhang, Q.; Worsnop, D. R.

doi:https://doi.org/10.5194/acp-15-12879-2015

Articles | Volume 15, issue 22

https://doi.org/10.5194/acp-15-12879-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-15-12879-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 22

Research article

|

19 Nov 2015

Research article |

| 19 Nov 2015

Characteristics and sources of submicron aerosols above the urban canopy (260 m) in Beijing, China, during the 2014 APEC summit

C. Chen, Y. L. Sun, W. Q. Xu, W. Du, L. B. Zhou, T. T. Han, Q. Q. Wang, P. Q. Fu, Z. F. Wang, Z. Q. Gao, Q. Zhang, and D. R. Worsnop

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Status: closed

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

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RC C7278: 'Comments to Characteristics and sources of submicron aerosols, C. Chen et al.', Anonymous Referee #1, 26 Sep 2015
RC C7511: 'Review', Anonymous Referee #2, 02 Oct 2015
AC C8925: 'Response to the two reviewers' comments', Yele Sun, 04 Nov 2015

Peer-review completion

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

AR by Yele Sun on behalf of the Authors (04 Nov 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 Nov 2015) by Jianzhong Ma

RR by Anonymous Referee #2 (05 Nov 2015)

Suggestions for revision or reasons for rejection

I thank the authors for considering my comments. However, I still find myself in disagreement with the conclusion regarding aerosol acidity. The stated intent of section 3.2.3 is to evaluate acidity because of its relevance to health impacts. However, as conceded by the authors, any observation of an apparently acidic aerosol is at odds with the observation of ammonium nitrate being present. To summarise the points raised by myself and the authors, the three possibilities of reconciling these observations are:

1. Inaccurate calibration data
2. A high metal cation content
3. Nitrate partitioning into dilute aqueous droplets at high humidities

Personally, I still find possibility 1 to be highly plausible. While the instrument was cross-calibrated with a HRAMS, this sill only makes it as accurate as the HRAMS calibration, which having read the corresponding paper, I have the exact same concerns about. Possibility 2 could also be plausible because the ACSM does not quantify potassium, although if this was the case, I would expect the deviations from perceived neutrality to correlate with a marker for biofuel or coal burning (this could be done by colouring figure R5 with an appropriate marker, e.g. m60).

Possibility 3, on the other hand, I do not find at all plausible in view of figure R5. As far as I can tell, there is no dependency between relative humidity and apparent acidity except for the fact that the RH tends to be low when loadings are low, but this is probably an aspect of the meteorology and not a causal relationship. The apparent agreement noted at low concentrations is within the scatter of the points, which can easily be explained by the low signal-to-noise ratio of the ammonium measurement or possibly the influence of organic acids, which aren't included in the calculation. While there are points with a low NH4meas/NH4pred on figure R5b that correspond to RHs approaching 100%, there are also points on the exact same fit line that also correspond to RHs as low as 50%. At these relative humidities, the water content of particles is very low and this causes nitrate to partition to nitric acid in the event of an inbalance (this can be shown using e-AIM, http://www.aim.env.uea.ac.uk/aim/model3/model3b.php). In short, If possibility 3 were to be occurring, I would expect the behaviour to vary between 50% and 100%, but this simply isn't the case, according to figure R5b.

To return to the original point, if it is either possibility 1 or 2 that is occurring, then the logical conclusion would be that the ACSM (as operated here) is not a reliable measure of particle acidity. In this context, the NH4meas vs NH4pred is only really useful as a technical instrument diagnostic and as such, I still don't see the conclusion that the particles are acidic is supportable.

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ED: Reconsider after minor revisions (Editor review) (07 Nov 2015) by Jianzhong Ma

Dear Dr. Sun,

One referee still has a serious concern on your conclusion regarding the aerosol acidity. Please consider the comments from this referee carefully and make necessary revisions and/or explanations.

From my point of view, the method you applied to predict ammonium and the aerosol acidity might not be completely applicable for the real atmosphere in Beijing. First, all the cations and anions that are likely to be present in a considerable amount in the aerosol salt solution should be included for an ion balance. For example, potassium (as suggested by the referee), calcium, carbonate (or hydrogen carbonate) and organic ions. Second, other ions like carbonate and organic anions tend to make solution basic and neutralize the acidity induced by ammonium cations. You may have to consider these before drawing a conclusion, not mentioned to the data calibration issue.

In addition, your arguments in some places seem to be arbitrary with inconsistent expressions.
In the last part of Sect. 3.2.1 (Lines 392-400 of version 3), the lower sulfate contribution during APEC has been attributed to the lower RH. In Sect. 3.2.3 (Lines 468-471), however, the reduction of SO2 is also considered as on one reason for the reduction of sulfate. Moreover, what do you mean by lower RH here, the lower aerosol water content or the lower water vapor amount? The two cases would mean different pathways for the oxidation of SO2.

You have stated that the emission controls during APEC did not significantly affect the regional aerosol bulk composition (do you mean its fraction?) (Line 393-394), but say that you detected a slightly higher reduction of sulfate than other inorganic species (Line 469-470). This seems to be in self-conflict.

Please do not hesitate to contact me if there is something unclear for you.

Sincerely,

Jianzhong Ma

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AR by Yele Sun on behalf of the Authors (10 Nov 2015) Author's response Manuscript

ED: Publish as is (11 Nov 2015) by Jianzhong Ma

AR by Yele Sun on behalf of the Authors (11 Nov 2015)

Short summary

A comprehensive characterization of submicron aerosol composition and sources at 260m in urban Beijing during APEC 2014 is presented. Aerosol species were shown to decrease substantially by 40–80% during APEC, whereas the bulk composition was relatively similar to the result of synergetic controls of secondary precursors. Our results elucidated that the good air quality during APEC was the combined result of emission controls and meteorological effects, with the former playing the dominant role.