Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

Bryant, Daniel J.; Dixon, William J.; Hopkins, James R.; Dunmore, Rachel E.; Pereira, Kelly L.; Shaw, Marvin; Squires, Freya A.; Bannan, Thomas J.; Mehra, Archit; Worrall, Stephen D.; Bacak, Asan; Coe, Hugh; Percival, Carl J.; Whalley, Lisa K.; Heard, Dwayne E.; Slater, Eloise J.; Ouyang, Bin; Cui, Tianqu; Surratt, Jason D.; Liu, Di; Shi, Zongbo; Harrison, Roy; Sun, Yele; Xu, Weiqi; Lewis, Alastair C.; Lee, James D.; Rickard, Andrew R.; Hamilton, Jacqueline F.

doi:https://doi.org/10.5194/acp-20-7531-2020

Articles | Volume 20, issue 12

Atmos. Chem. Phys., 20, 7531–7552, 2020

https://doi.org/10.5194/acp-20-7531-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Atmos. Chem. Phys., 20, 7531–7552, 2020

https://doi.org/10.5194/acp-20-7531-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 12

Research article

30 Jun 2020

Research article | 30 Jun 2020

Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

Daniel J. Bryant et al.

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Final revised paper (published on 30 Jun 2020)
Supplement to the final revised paper
Preprint (discussion started on 23 Oct 2019)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Review of Bryant et al. 2019', Anonymous Referee #1, 09 Dec 2019
RC2: 'Review comments', Anonymous Referee #2, 11 Dec 2019
RC3: 'Review', Anonymous Referee #3, 17 Dec 2019
AC1: 'Author reply to reviewers comments.', Daniel Bryant, 28 Feb 2020

Peer-review completion

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

AR by Daniel Bryant on behalf of the Authors (28 Feb 2020) Author's response

ED: Referee Nomination & Report Request started (05 Mar 2020) by Alex Lee

RR by Anonymous Referee #1 (25 Mar 2020)

Suggestions for revision or reasons for rejection

Major comments:
The authors significantly improved the description of the LC-MS measurements and the corresponding discussion. However, there remain some major weaknesses and wrong conclusions. To some degree, these are based on poor method development for the analysis approach (and maybe inexperience) with the Q Exactive mass spectrometer.

a) The authors did a good job in comparing the results from HILIC and RPLC, however, I was wondering why only the peak areas of 2-MT-OS were correlated. What about the 2-MG-OS?

b) I disagree with the conclusion that “there is no evidence of ion source induced artifacts” (L527). As the authors acknowledge, there is still some co-elution of sulfate and nitrate ions with detected OS species. Therefore, there is still a chance of adduct formation between inorganic ions and organic compounds. The “test for artefact formation” conducted by the authors is not convincing, as it is just proving that sulfate and 2-methylglyceric acid / 2-methyltetrol do not produce artifacts. This cannot be generalized for all organic compounds! (By the way, I totally agree with the authors’ conclusion that all previous direct infusion studies suffer from in-source adduct formation. Therefore, I’m actually very happy to see that people are now more and more using chromatographic separation before ionization. Nonetheless, if the separation is not done appropriately, we will not get away from ion source artifacts.)

c) The selected settings of the Q Exactive mass spectrometer are really far from appropriate for a quantitative analysis approach. With the settings applied here (i.e., full scan at R=70,000 combined with ddMS2 Top 10), the authors obtain ~1 full scan spectrum per second (i.e, full scan + 10 x ddMS2 = 256 ms + 10 x 64 ms = 896 ms). As a rule of thumb, for accurate quantification, about 12 data points are needed to get a good peak quality in the extracted ion chromatograms. Therefore, the chromatographic peak width needed here would have been roughly 11–12 seconds, which is a quite broad peak in typical UHPLC applications and only reached with quite concentrated samples. In particular, since all the OS peaks elute quite early, I would be surprised to see that the authors obtained more than ~5–6 data points for each of the peaks here. In principle, it is possible to increase data quality by measuring replicates. However, the authors decided to measure each sample only once. Therefore, I think the concentrations reported here are connected to very large uncertainties (which could have been avoided easily).

d) Regarding the uncertainties mentioned above, it is still unclear to me how the authors estimate an uncertainty of 60% for the measured OS concentrations.

Minor / Technical comments:
1) There are several mentions of “corplot” and “PollutionRose plot” in the manuscript, which might be disturbing to readers less familiar with R. I would recommend replacing these R-specific names with more general terms (e.g., corplot = correlation plot).

2) Figure 5: Some of the label names capitalized and some not.

3) L901: Ultra-performance liquid chromatography (UPLC) is actually specific for components from Waters Corporation. The more general term is ultra-high performance liquid chromatography (UHPLC).

4) Figure S3: I suppose the x-axis label should read 2-MG-OS.

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RR by Anonymous Referee #4 (29 Mar 2020)

Suggestions for revision or reasons for rejection

The authors presented measurement results of isoprene-derived secondary organic aerosol (iSOA) tracers in PM2.5 in urban Beijing by off-line analysis. Particulate organosulfates (OSs) and nitrooxy-organosulfates (NOSs) were quantified by ultra-performance liquid chromatography mass spectrometry (UPLC-MS). Results showed that OS formation depended on both photochemistry and sulfate availability, and that the high-NOx conditions in Beijing favored formation of those iSOA tracers compared to other regions. The tracers determined can be up to 3% of the measured oxygenated organic aerosol (by aerosol mass spectrometry, AMS) when NOx conc. The authors concluded that iSOA formation in urban Beijing is strongly controlled by anthropogenic emissions and results in extensive conversion to OS products from heterogeneous reactions.

The study was well designed and the data analysis rigorous. The authors also addressed the comments of the previous reviewers fairly well and made substantial improvements to the manuscript based on the review comments. Given the combination of off-line detailed chemical characterization and some on-line data in this study, as well as limited reports in SOA from biogenic volatile organic compounds (VOCs) in urban Beijing, this study is of interest to the atmospheric chemistry community in general. I recommend publication after the authors address a few comments below.

Major:

Since AMS data is available in this campaign (L984), could the authors look into the tracer of IEPOX-SOA (i.e., m/z 82 or C_5H_6O^+) to see what is the relationship between the measured tracers and that ion tracer in AMS? Presumably NOx conditions may also affect the channels of IEPOX or OS formation if the chemistry is similar in other regions with SOA from isoprene.

Minor:

1. The authors used “ultra-pressure liquid chromatography” in the Abstract, but “ultra-performance liquid chromatography” in L901. Please be consistent.

2. Figure 7. Please use [O3] × [SO_4^2-] in the title of y axis. And better to use SO_4^2- for sulfate as there is no such thing as SO_4.

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ED: Reconsider after major revisions (30 Mar 2020) by Alex Lee

AR by Daniel Bryant on behalf of the Authors (02 Apr 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (01 May 2020) by Alex Lee

RR by Anonymous Referee #1 (15 May 2020)

ED: Publish as is (15 May 2020) by Alex Lee

Short summary

Using the chemical composition of offline filter samples, we report that a large share of oxidized organic aerosol in Beijing during summer is due to isoprene secondary organic aerosol (iSOA). iSOA organosulfates showed a strong correlation with the product of ozone and particulate sulfate. This highlights the role of both photochemistry and the availability of particulate sulfate in heterogeneous reactions and further demonstrates that iSOA formation is controlled by anthropogenic emissions.