the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Nov 2023
Prominent role of organics in aerosol liquid water content over the south-eastern Atlantic during biomass burning season
Abstract. The interaction between atmospheric aerosols and moisture is crucial for aerosol properties and their climate effects. In this study, thanks to the rich measurements of aerosol properties during the 2016 and 2018 ORACLES campaigns, we investigate the aerosol liquid water content (ALWC) over the south-eastern Atlantic Ocean during the biomass burning (BB) season, as well as the seldom-reported ALWC associated with organic aerosols (OA) (ALWCOA) (OA). ALWCOA is determined using the OA hygroscopicity parameter κOA, derived from in-situ hygroscopicity measurements, particle number size distribution, and chemical composition. The ALWC can be determined either with the overall hygroscopic parameter κf(RH) or from the sum of ALWCOA and the ALWC simulated from ISORROPIA-II, a thermodynamic equilibrium model for inorganic aerosol. The ALWC from both methods is highly correlated at all RHs with an R2 of 0.99. The ALWC increases with aerosol loading and ambient relative humidity (RH). Due to the lower RH and higher aerosol loading in the 2016 campaign, the ALWC for both campaigns are generally consistent. ALWCOA accounts for 38±16 % of the total ALWC during both campaigns. Notably, the contribution of ALWCOA is greater than commonly reported in the literature, highlighting the significance of OA in ALWC and therefore the aerosol direct radiative forcing in this climatically significant region. The strong correlation between κOA and ALWCOA/ALWC, as indicated by an R2 value of 0.72, underscores the importance of a good estimation of κOA in the ALWC estimation. Additionally, the significant difference between ALWCOA values calculated using real-time κOA and those calculated with the campaign mean κOA, highlights the limitation of using a constant κOA value, a practice commonly adopted in climate models.
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RC1: 'Comment on egusphere-2023-2319', Anonymous Referee #1, 19 Dec 2023
The manuscript by Zhang et al. (Prominent role of organics in aerosol liquid water content over the south-eastern Atlantic during biomass burning season) reported hygroscopicity/liquid water content of aerosol particles at the eastern Atlantic ocean measured during the ORACLES aircraft campaign. The campaign was conducted during biomass the biomass burning season in Africa. Humidity-controlled nephelometers were employed for estimating water contents. The data were combined with the AMS measurement results and ISORROPIA thermodynamic models for retrieving contributions of organic aerosol on water contents. The topic is within the scope of the interests of readers of the journal. Such an aircraft observation in the region is relatively rare. The research community will be benefitted by the publication of this manuscript. However, the reviewer has some concerns, as detailed below.
Major comments
Section 2.1 Aircraft Instrumentation and Data Analysis
The section should provide further details about the measurement. The authors cited Titos et al. (2016) and Zieger et al. (2013) as references for the technique. However, neither of them is for aircraft observations/research groups of the authors. The reviewer believes that the authors developed the humidification setup by themselves, and tested it. All the technical details of the setup should be described in the manuscript, at least as a part of SI. Calibration results of the setup also need to be provided for convincing the readers about the data quality. Both the precision and accuracy of RH should be provided, as they are the key parameters for measuring hygroscopic growth. For comparing the nephelometer data with the AMS results, it is important to make sure that the two instruments measured particles in the same size range. Some information about the inlet design and size-cut of aerosol particles will need to be added for addressing the concern. The authors mentioned that PNSD was measured using the UHSAS and APS. Did the data from these instruments agree reasonably well with the AMS results? The comparison would help checking if chemical species that are unmeasurable by the AMS occupied a substantial fraction of aerosol particles.
Figure 3
The reviewer does not understand what the color scales for LV-OOA, SV-OOA, BBOA, and HOA mean in the figure. Please clarify it.
L239-240
The reviewer’s understanding on the concepts of LV-OOA and SV-OOA is that these categorizations were developed following factor analysis of the aerosol mass spectra. Did the authors conduct factor analysis?
L241
Is this statement for LV-OOA, SV-OOA, or oxidized BBOA?
L243
Why does volatility matter for the present study? Please clarify.
L247
Kappa_OA of 0.00 is extremely low, suggesting that OA was practically water-insoluble. Is it an accurate number, or is it due to the measurement uncertainties? If the low value is not due to the measurement uncertainty, is the idea supported by the AMS mass spectra observed at the same time (e.g., low O:C or f44)?
L252
What kind of uncertainties are available in the calculation? Please clarify. Is there a possibility that the issue is induced by measurement uncertainties?
L254-L259
Has such a phenomenon been observed for sub-micrometer particles (sub-micrometer particles should be the major focus of the present study, considering that the AMS is used for measuring chemical composition).
L265
It is better to show the corresponding figure, rather than only showing the correlation coefficient.
L267
Why is there a limitation in relating kappa_OA and OA oxidation level? What would be the thermodynamic reason behind it?
L275
The paragraph which starts from L275 was unclear to the reviewer, as various types of kappa appeared without clear definitions. As a result, what the authors are trying to mean by Figure 4 was also unclear to the reviewer. Please update the corresponding descriptions.
L281
What kind of mixing state was assumed? Could it really happen under the ZSR approximation?
L288-289
The description about the phase state of aerosol particles is highly speculative. Do the authors have any data to support the statement?
Figure 5
The sums of values in the PDF are larger than unity. How was the data normalized?
L311
Would it be possible to use the AMS or size distribution data here for representing mass loading?
L316-322
It is better to show the corresponding figures, rather than only showing the correlation coefficients.
Figure 6
The reviewer was confused to see this figure, as the reviewer thought that RH of the nephelometer was fixed during the observation. Is it an estimation that employed the thermodynamic model and kappa_OA? If so, do the authors have any evidence to support the idea that kappa_OA does not change with RH?
L350
What does ‘hydrated’ mean here? Does it imply formation of hydrates, or formation of aqueous phase following hygroscopic growth?
L361-364
Jin et al. (2020) and Gao et al. (2015) observed totally different types of air masses than the present study. Is there any reason why the authors selected these two references from the numerous literature about hygroscopicity?
Minor comments
L61 thermal equilibrium
Thermal equilibrium or thermodynamic equilibrium?
L139
The values of the densities that were used for the study should be provided in the manuscript, rather than referring a previous manuscript by the authors.
L240 LV-OOA/SV-OOA
These abbreviations are not defined in the manuscript.
L279
Figure 4a does not exist.
L309
What does ‘38% of the RH’ mean? RH = 38%?
L344
What does ‘the compound state’ mean?
Citation: https://doi.org/10.5194/egusphere-2023-2319-RC1 -
RC2: 'Comment on egusphere-2023-2319', Anonymous Referee #2, 28 Jan 2024
This study investigated the contribution of organics to ALWC during the 2016 and 2018 ORACLES campaigns which was conducted over the south-eastern Atlantic Ocean during the biomass burning season. Although the observation data is valuable, I don't think this study is worth publishing as an independent manuscript, as many of the main findings is already implied in the authors' another study (Zhang et al., 2023, https://doi.org/10.5194/egusphere-2023-2199). Trying to split one study into too many pieces heavily diluted the creativity of this study, as detailed below.
(1) In the abstract, the authors highlighted three major conclusions. The first one, that there's high contribution of ALWC from organics during the biomass burning, is worth reporting but not surprising. However, the other two points, that the hygroscopicity of OA are positively correlated with ALWC_OA/ALWC and that hygroscopicity of OA can vary much, are not surprising at all. Actually, they're well recognized fact and should be applied to explain some other observed phenomenon.
(2) I was expecting that more pattern analysis of OA hygroscopicity during the campaign would be provided, but then realized that those are the main part of Zhang et al. (2023), and Fig. 2 in this study actually expressed the same idea as the Fig. 5 in Zhang et al. (2023). Similarly, major findings in section 3.2 is already somehow implied and discussed in Zhang et al. (2023).
(3) The Fig. 1 in this study is the same as in Zhang et al. (2023), which needs to be clarified.
(4) The comparison as shown in Fig. 4 is kind of circular reasoning. As the kappa_OA is derived from the kappa_f(RH) and kappa_INORGANICS, it was not surprising at all that the ALWC_f(RH) agrees well with the ALWC_ISOR+OA. In fact, the good agreement would only prove that the kappa of inorganic species is well represented in ISORROPIA -- which has already been proved during the validation of ISORROPIA.Overall, I suggest merging this study into other companying studies, otherwise the major points and unique opinions of this study needs to be reframed and highlighted more clearly.
Citation: https://doi.org/10.5194/egusphere-2023-2319-RC2
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