Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods

Shetty, Nishit J.; Pandey, Apoorva; Baker, Stephen; Hao, Wei Min; Chakrabarty, Rajan K.

doi:https://doi.org/10.5194/acp-19-8817-2019

Articles | Volume 19, issue 13

https://doi.org/10.5194/acp-19-8817-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-19-8817-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 13

Research article

|

11 Jul 2019

Research article |

| 11 Jul 2019

Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods

Nishit J. Shetty, Apoorva Pandey, Stephen Baker, Wei Min Hao, and Rajan K. Chakrabarty

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Final revised paper (published on 11 Jul 2019)
Supplement to the final revised paper
Preprint (discussion started on 21 Nov 2018)
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 Shetty et al.', Anonymous Referee #1, 21 Dec 2018
RC2: 'Interactive comment on “Measuring Light Absorption by Organic Aerosols: Correction Factors for Solvent Extraction-Based Photometry Techniques” by Nishit Shetty et al.', Anonymous Referee #2, 22 Dec 2018
RC3: 'Review', Anonymous Referee #3, 24 Dec 2018
RC4: 'Review of “Measuring Light Absorption by Organic Aerosols: Correction Factors for Solvent Extraction-Based Photometry Techniques” by Nishit Shetty et al.', Anonymous Referee #4, 03 Jan 2019
AC1: 'Authors' comments', Rajan Chakrabarty, 30 Mar 2019

Peer-review completion

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

AR by Rajan Chakrabarty on behalf of the Authors (30 Mar 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Apr 2019) by Sergey A. Nizkorodov

RR by Anonymous Referee #2 (17 Apr 2019)

RR by Anonymous Referee #5 (23 May 2019)

Suggestions for revision or reasons for rejection

1) The trends in Figure 2 and Figure 5 seem to be unrealistic, or at least inconsistent with the argument (Line 358-365) that extraction efficiency increases with increasing OC/TC (and thus, SSA). Specifically, in panel (b), how do the authors explain the sharp drop in babs,OA/babs,sol to values close to zero at high OC/TC and SSA? One would expect babs,OA/babs,sol to approach a constant value (due only to particle size effects) with increasing extraction efficiency, so one should expect an asymptotic behavior of babs,OA/babs,sol at large OC/TC and SSA values, opposite to what’s observed. It seems to me that the observed trends could be partially an artifact of the way babs,OA is calculated as the difference between the measured babs,tot and a calculated babs,BC (section 2.2.2). In any case, a discussion of the meaning of the trends observed at large SSA values should be added to section 3.1.

2) Along the same lines, the data, especially the part of the space that has the sharp change (i.e. SSA > 0.9), should be further explored. For example, the data in Figure 2 could be plotted with the points color-coded by fuel type and with different scales on the y-axis to better zoom in the changes at SSA > 0.9 to see if the variability that is not explained by SSA (i.e. where the changes are very steep) could be explained by fuel type. The same applies to OC/TC.

3) Line 275 – 281: the choice of SSA = 0.7 and 0.825 as a cutoff between low uncertainty and high uncertainty ranges seems arbitrary. What constitutes high uncertainty? Also, in Figure 2, similar size error bars (especially in panel b) seems to exist on both sides of the cutoff lines.

4) The purpose and value of the Mie calculations in section 3.4 are not clear. It reads: previous studies have used scaling factors of 2. This study found a scaling factor of 2. In conclusion, a scaling factor of 2 should not be used because of reasons stated in the previous sections. Then why do the Mie calculations to begin with? The cautionary statement about the non-universality of the scaling factor of 2 should be inserted in section 3.1.
The presentation of the results could be reformulated in a more informative way. For the 3 experiments reported in section 3.2, the authors can compare babs,OA, babs,sol, and babs,Mie and use this comparison to determine the fractional contribution of particle size and extraction efficiency to the discrepancy.

5) Table 4: It would be more informative to present the data in Table 4 as scatter plot of AAE_OA vs AAE_sol, color coded with fuel type and with a 1:1 line.

6) Table 5: with geometric mean diameter of 397 nm, the size distribution is expected to extend beyond the SMPS measurement window. It is not clear how this issue was addressed.

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ED: Publish subject to minor revisions (review by editor) (28 May 2019) by Sergey A. Nizkorodov

Dear authors. As reviewers pointed out, the revised version has improved considerably. Please address the remaining points raised by reviewers #1 and #2 of the revised manuscript. In addition, I have a few editorial review notes below, which may also need to be addressed before the final acceptance.

Line 157: a space is needed between l and min

Line 170: there is another important artifact of changing the chemical composition by sonochemistry, as described in [Mutzel, A.; Rodigast, M.; Iinuma, Y.; Böge, O.; Herrmann, H., An improved method for the quantification of SOA bound peroxides. Atmos. Environ. 2013, 67, (0), 365-369.] You might want to cite it.

Line 173: impurities -> suspended particles (because filtration does not remove soluble impurities)

Line 182: this effectively sets the absorption coefficient at 700 nm to zero. Please discuss in the paper whether it was actually the case for your samples (in many cases absorbance at 700 nm is indeed negligible). It might be more precise to subtract an average absorbance over a wavelength interval where you expect no absorption, such as 700-750 nm as opposed to subtracting a single value, which is prone to more uncertainty. I do not suggest you should redo all of your calculations, just a suggestion for the future.

Line 189: please fix grammar in this sentence

Line 235: I think conversion of WSOC into mass concentration of dissolved organics requires an OM/OC ratio. What was the assumed value?

Table 4: there are too many significant digits reported in AAE values. There is no reason to report uncertainties with more than 1-2 significant digits. The entry 13.74+/-2.27 would be more faithfully represented as 14+/-2 or 2 13.7+/-2.3. Similar considerations apply to numbers reported in a few other tables as well as text.

Several references have an incomplete list of authors, for example, Akagi et al. (2011), Arnott et al. (2003). I think your reference manager software is cutting the list. ACP lists all authors to the best of my knowledge, so it needs to be fixed.

Also, fix page numbers in references Arnott et al. (2003), Bahadur et al. (2012), Kirchstetter et al. (2004), Saleh et al. (2015), Sun et la. (2007), Zhang et al. (2008). I am guessing your reference library is missing these page numbers and formatting the references incorrectly. It is a common problem form AGU journals.

Panteliadis et l. (2015) reference is missing the journal

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AR by Rajan Chakrabarty on behalf of the Authors (07 Jun 2019) Author's response Manuscript

ED: Publish as is (07 Jun 2019) by Sergey A. Nizkorodov

AR by Rajan Chakrabarty on behalf of the Authors (17 Jun 2019) Manuscript

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Short summary

We investigated biases in particle-phase absorption coefficients for organic aerosol from bulk-phase absorbance measurements of solvent extracts in the visible spectrum. These biases were systematically studied as a function of organic-to-total carbon mass ratios and aerosol single scattering albedo. A linear correlation between SSA and OC / TC ratios was observed. Differences in the absorption Ångström exponents from bulk- and particle-phase measurements were also investigated.