Changing optical properties of Black Carbon and Brown Carbon aerosols during long-range transport from the Indo-Gangetic Plain to the equatorial Indian Ocean

Budhavant, Krishnakant; Manoj, Mohanan Remani; Gaita, Samuel Mwaniki; Holmstrand, Henry; Salam, Abdus; Muslim, Ahmed; Satheesh, Sreedharan K.; Gustafsson, Orjan

doi:https://doi.org/10.5194/egusphere-2024-104

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https://doi.org/10.5194/egusphere-2024-104

Preprints

17 Jan 2024

| 17 Jan 2024

Changing optical properties of Black Carbon and Brown Carbon aerosols during long-range transport from the Indo-Gangetic Plain to the equatorial Indian Ocean

Krishnakant Budhavant, Mohanan Remani Manoj, Samuel Mwaniki Gaita, Henry Holmstrand, Abdus Salam, Ahmed Muslim, Sreedharan K. Satheesh, and Orjan Gustafsson

Abstract. Atmospheric aerosols strongly influence the global climate by their light absorption (e.g., black carbon, BC, brown carbon, BrC) and scattering (e.g., sulfate) properties. This study presents simultaneous measurements of ambient aerosol light absorption properties and chemical composition from three large-footprint South Asian receptor sites during the South Asian Pollution Experiment (SAPOEX) in December-March 2018. The BC mass absorption cross-section (BC-MAC₆₇₈) values increased from 3.5 ± 1.3 at the Bhola Climate Observatory-Bangladesh (i.e., located at exit outflow of Indo-Gangetic Plain) to 6.4 ± 1.3 at the two regional receptor observatories at Maldives Climate Observatory-Hanimaadhoo (MCOH) and Maldives Climate Observatory-Gan (MCOG). This likely reflects a coating-enhancement effect due to ageing of the aerosols during long-range transport. At the same time, the BrC-MAC₃₆₅ decreased by a factor of three from the IGP exit to the equatorial Indian Ocean, likely due to photochemical bleaching of organic chromophores. The high chlorine-to-sodium ratio at the near-source-region BCOB suggests a significant contribution of chorine from anthropogenic activities. This particulate Cl^- has the potential to convert into Cl-radicals that can affect the oxidation capacity of the polluted air. Moreover, Cl^- is shown to be near-fully consumed during the long-range transport. The results of this synoptic study over the large South Asian scale have significance for understanding the ageing effect of the optical and chemical properties of aerosols as the pollution from the Indo-Gangetic Plain disperses over regional scales.

Received: 12 Jan 2024 – Discussion started: 17 Jan 2024

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24 Oct 2024

Changing optical properties of black carbon and brown carbon aerosols during long-range transport from the Indo-Gangetic Plain to the equatorial Indian Ocean

Krishnakant Budhavant, Mohanan Remani Manoj, Hari Ram Chandrika Rajendran Nair, Samuel Mwaniki Gaita, Henry Holmstrand, Abdus Salam, Ahmed Muslim, Sreedharan Krishnakumari Satheesh, and Örjan Gustafsson

Atmos. Chem. Phys., 24, 11911–11925, https://doi.org/10.5194/acp-24-11911-2024,https://doi.org/10.5194/acp-24-11911-2024, 2024

Short summary

Krishnakant Budhavant, Mohanan Remani Manoj, Samuel Mwaniki Gaita, Henry Holmstrand, Abdus Salam, Ahmed Muslim, Sreedharan K. Satheesh, and Orjan Gustafsson

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-104', Anonymous Referee #1, 23 Feb 2024

Budhavant et al. reports a work depicting the extent of the long-range transport of pollutants from the Indo-Gangetic Plain to Maldives via Bangladesh, and the effects of atmospheric processes happened during the transport on the properties of light absorbing species (BC and BrC). This is an interesting study with the aerosol sampling done over strategically located sites. However, there are several other studies on the similar topic using the sampling at the same locations (Bosch et al., 2014; Dasari et al., 2019; Nair et al., 2023, etc.). Author shall explicitly mention the novelty of the current study over the reported studies. There are some interesting results but their explanations are not satisfactory, and sometimes there are conflicting arguments while explaining the observed data. Multiple parameters (MAC678, MAC365, OC/BC, Cl/Na, [NH4], [SO4], AAE) indicate that a better data churning is required. Attribution of all observations to long-range transport and processes is not convincing. The data shall be segregated with respect to wind back trajectories and different categories shall be discussed differently with more converging inferences. There is a sufficient room for modifications before the manuscript can be considered for publication in ACP.

Major Comments:
L74: Why the water-insoluble BrC is ignored? There are several studies showing water-insoluble BrC is a considerable part of BrC. A comparison of water-soluble BrC alone with BC gives an incomplete and biased picture of BrC effects.

L189-193: If this logic is true, why OC didn't come from those region? What processes can remove only OC but not SO42- and NH4+ coming from same region?

L204-208: Discussion on Cl/Na ratio is weak and too hypothetical. The seawater ratio of Cl/Na is 1.8 (on mass basis). A dotted line with ratio 1.8 can be added to the Figure 4. At BCOB site, Cl/Na ratio is varying from 0 to 11. It shall be explained that what sources or processes are adding or removing particulate Cl from the samples. There shall be convincing supporting evidences.

L217-221: The inherent assumption in this statement is that all BC at both the receptor sites is transported from South Asia, which is not concurred by wind trajectories.

L225-227: This is a strong but hypothetical statement. It has to be proven with other supportive measurements. Why the difference in MAC678 can't be due to different sources of BC over these sites? In fact, this inference may be more logical because OC/EC ratios are also different at those sites.

L248-250: As per the reasons given for MAC678, shouldn't be MCOG have lower MAC365 compare to MCOH?

L282-284: This is quite illogical and not convincing. There are several inferences which contradicts each other.

Table 1: How come SO4 and NH4 conc are similar or even higher than BCOB. It is counter intuitive and warrants satisfactory justification. Concentrations of other species like OC, WSOC, NO3, K, etc. look as per expectations.
Increase in BC MAC678 is attributed to coating during transport from IGP to MCOH and MCOG. However, back trajectories analysis shows that during a considerable time period, winds were not from the IGP. How to justify higher MAC678 in those samples?

Fig. 1: 10-days air mass back trajectories are used in this paper, which is in contrast to most of the studies which are using 5 or 7 day back trajectories. The reason of using 10 day back trajectory shall be explained.

Fig.2: This data shall also be plotted in different ways. Samples with similar BC fractions are showing quite different MAC678. It shall be explained.

Fig. 3: As per this plot, WIOC is the dominant fraction of OC. A significant part of this WIOC could be BrC, which is not measured, reflecting limitation of this work. Further, in many samples with low WSOC fraction, MAC365 is quite high. Possible reasons shall be discussed.

Fig. 4: In many of the samples collected over MCOH and MCOG, OC/BC looks close to 1, which is not normal. How is it inferred?
It would be better to plot WSOC/OC rather than WSOC/TC because WSOC/OC ratio can be better interpreted.

Fig.5: As the major focus of this paper is on BC and BrC, it would be appropriate to calculate RF for BC and BrC, and their contribution to total aerosols RF.

Fig.S2-S4: Why 10 days and not 5 or 7 days? Why at 50 m only? It would be better to add a few higher altitudes relevant for long-range transport.

Fig,S5: AOD data appears to follow expected trends, unlike chemical data.

Fig. S6: At MCOH and MCOG, MAC678 looks independent of x-axis and inferences are not supported by this data.

Table S1: SO4, NH4, nss-Ca, nss-Mg are not correlated with any species, why? Where is Na and Cl? Major ions data (absolute concentrations) shall also be given here.

Table S2: Why do NO3, Ca, and Mg are not correlating with any other species?

Table S3: Why does K not correlating with any other species?

Minor Comments:
L53-56: add appropriate references from this region
L120: Eq 3: How was the babs measured? Why at 365 nm only?
L255-257: There are numerous recent studies and MAC365 shall be compared with the recent studies.
L262-264: mention the wavelength range used for AAE calculation for the better clarity for readers.

Citation: https://doi.org/10.5194/egusphere-2024-104-RC1
- AC1: 'Reply on RC1', Krishnakant Budhavant, 04 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-104/egusphere-2024-104-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-104-AC1
RC2:
'Comment on egusphere-2024-104', Anonymous Referee #2, 27 Mar 2024

The Budhavant manuscript describes the results of SAPOEX campaign at three sites in South Asia. There is potential with this dataset, but the authors need to do more to connect across the results in each section (e.g. back trajectory analysis, aerosol composition, optical properties, radiative forcing). For example, the absolute concentrations of aerosol composition were only presented as campaign averages, which made it difficult to compare to the aerosol radiative forcing which was presented and discussed as an average and by month. There was often an over-simplification of the results, for example, the assertion that the MCOH site received transported air masses from the BCOB site: this did not seem to be always the case so it added confusion when discussing composition and aging. Overall the manuscript needs more refining of focus and connection among the different sections. Even in the introduction, the discussion of source and processing impacts on BC and BrC was over-simplified and lacking in precision.
detailed comments:
Ln 25: can this be better linked in the abstract to the aerosol optical properties?
Ln 46: this is an awkward phrase here. please edit
Ln 47: is this referring specifically to this region? this is undoubtedly true for BC, but WSOC may have other sources? e.g. biogenic and SOA?
Ln 47 – 56: this paragraph is difficult to follow as written. the authors need to clarify their purpose here. There is some confusion as they are trying to simultaneously discuss BC and WS-BrC. It doesn't really work and needs editing for clarity.
Ln 49: This sentence is awkward and the logical transition here is unclear
Ln 65: edit for clarity
Ln 88: What is the particle size here? TSP? PM2.5
Ln 111: are there any concerns about the high loading on these filters? typically filter-based photometers limit the filter loading to that which corresponds to a 50% transmission. the filters collected for this offline analysis would not have their loading limited by light transmission. i understand that this correction is intended to address the filter loading, but these correction schemes were originally designed for online instruments which have a filter advancement/change at a set transmission threshold. can this concern be addressed?
Ln 180-182: its not clear how these facts are relevant here. please remove or expand the discussion to make this more clear.
Ln 191-195: what about biogenic SO4? do you have a constraint on the possible marine contribution that goes beyond sea salt? Additionally, this rationale of sources from central and east India is a bit confusing as the BTs indicate that air masses predominantly leave the Indian subcontinent near BCOB or from west India before traveling to MCOH. If the aerosol composition from the west side of India is markedly different (e.g. higher SO4 fraction) than the IGP and BCOB, than the aging discussion of MCOH representing aged BCOB aerosol needs to be more refined.
Ln 208: Is the EC supposed to be BC?
Ln 217-223: I'm still stuck on the SO4 discussion. the provenance of the SO4 seems very relevant in determining if these 3 sites do represent different ages of the same air mass. as briefly mentioned in this section, increased SO4 would also seem to be very relevant for the coatings question. however some of the previous discussion of the loss of water soluble fraction during aging and transport (WSOC) seems to conflict with this. certainly the increase in SO4 at MCOH, absolute concentration as well as an extreme increase in relative contribution, is very relevant for coating of BC and internal mixture of BC and SO4 aerosol. i would like to see more discussion of this potentially conflicting observations between wsoc and so4, and using the three sites as steps in the aging process of one air mass. If the rationale for higher SO4 at MCOH is a shift in geographic source region, than the discussion of aging of two different aerosol systems needs to be included.
Ln 265-267: are these differences in AAE signficant? the std deviation is relatively high.
Ln 282-284: can the authors discuss why the atmospheric forcing was higher at MCOH while the surface concentrations for summed species was lower? it is also a bit difficult to interpret the relationship between the aerosol radiative forcing and the rest of the surface aerosol discussion as the time scales are not well aligned for aerosol concentrations and the ARF. overall, i'd like to see better connections among the sections of the discussion.
Table 1: this AAE is calculated off a very narrow range in wavelength. is 400 nm the longest wavelength measured here? What are the potential shortcomings of reporting AAE for such a narrow range in wavelength? Also, it is difficult to assess the trends in the ambient concentrations when only the averages for the entire period are reported. It is fine to present the ratios in the figures, but useful to also be able to see changes in the absolute concentrations as well.

Citation: https://doi.org/10.5194/egusphere-2024-104-RC2
- AC2: 'Reply on RC2', Krishnakant Budhavant, 04 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-104/egusphere-2024-104-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-104-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-104', Anonymous Referee #1, 23 Feb 2024

Budhavant et al. reports a work depicting the extent of the long-range transport of pollutants from the Indo-Gangetic Plain to Maldives via Bangladesh, and the effects of atmospheric processes happened during the transport on the properties of light absorbing species (BC and BrC). This is an interesting study with the aerosol sampling done over strategically located sites. However, there are several other studies on the similar topic using the sampling at the same locations (Bosch et al., 2014; Dasari et al., 2019; Nair et al., 2023, etc.). Author shall explicitly mention the novelty of the current study over the reported studies. There are some interesting results but their explanations are not satisfactory, and sometimes there are conflicting arguments while explaining the observed data. Multiple parameters (MAC678, MAC365, OC/BC, Cl/Na, [NH4], [SO4], AAE) indicate that a better data churning is required. Attribution of all observations to long-range transport and processes is not convincing. The data shall be segregated with respect to wind back trajectories and different categories shall be discussed differently with more converging inferences. There is a sufficient room for modifications before the manuscript can be considered for publication in ACP.

Major Comments:
L74: Why the water-insoluble BrC is ignored? There are several studies showing water-insoluble BrC is a considerable part of BrC. A comparison of water-soluble BrC alone with BC gives an incomplete and biased picture of BrC effects.

L189-193: If this logic is true, why OC didn't come from those region? What processes can remove only OC but not SO42- and NH4+ coming from same region?

L204-208: Discussion on Cl/Na ratio is weak and too hypothetical. The seawater ratio of Cl/Na is 1.8 (on mass basis). A dotted line with ratio 1.8 can be added to the Figure 4. At BCOB site, Cl/Na ratio is varying from 0 to 11. It shall be explained that what sources or processes are adding or removing particulate Cl from the samples. There shall be convincing supporting evidences.

L217-221: The inherent assumption in this statement is that all BC at both the receptor sites is transported from South Asia, which is not concurred by wind trajectories.

L225-227: This is a strong but hypothetical statement. It has to be proven with other supportive measurements. Why the difference in MAC678 can't be due to different sources of BC over these sites? In fact, this inference may be more logical because OC/EC ratios are also different at those sites.

L248-250: As per the reasons given for MAC678, shouldn't be MCOG have lower MAC365 compare to MCOH?

L282-284: This is quite illogical and not convincing. There are several inferences which contradicts each other.

Table 1: How come SO4 and NH4 conc are similar or even higher than BCOB. It is counter intuitive and warrants satisfactory justification. Concentrations of other species like OC, WSOC, NO3, K, etc. look as per expectations.
Increase in BC MAC678 is attributed to coating during transport from IGP to MCOH and MCOG. However, back trajectories analysis shows that during a considerable time period, winds were not from the IGP. How to justify higher MAC678 in those samples?

Fig. 1: 10-days air mass back trajectories are used in this paper, which is in contrast to most of the studies which are using 5 or 7 day back trajectories. The reason of using 10 day back trajectory shall be explained.

Fig.2: This data shall also be plotted in different ways. Samples with similar BC fractions are showing quite different MAC678. It shall be explained.

Fig. 3: As per this plot, WIOC is the dominant fraction of OC. A significant part of this WIOC could be BrC, which is not measured, reflecting limitation of this work. Further, in many samples with low WSOC fraction, MAC365 is quite high. Possible reasons shall be discussed.

Fig. 4: In many of the samples collected over MCOH and MCOG, OC/BC looks close to 1, which is not normal. How is it inferred?
It would be better to plot WSOC/OC rather than WSOC/TC because WSOC/OC ratio can be better interpreted.

Fig.5: As the major focus of this paper is on BC and BrC, it would be appropriate to calculate RF for BC and BrC, and their contribution to total aerosols RF.

Fig.S2-S4: Why 10 days and not 5 or 7 days? Why at 50 m only? It would be better to add a few higher altitudes relevant for long-range transport.

Fig,S5: AOD data appears to follow expected trends, unlike chemical data.

Fig. S6: At MCOH and MCOG, MAC678 looks independent of x-axis and inferences are not supported by this data.

Table S1: SO4, NH4, nss-Ca, nss-Mg are not correlated with any species, why? Where is Na and Cl? Major ions data (absolute concentrations) shall also be given here.

Table S2: Why do NO3, Ca, and Mg are not correlating with any other species?

Table S3: Why does K not correlating with any other species?

Minor Comments:
L53-56: add appropriate references from this region
L120: Eq 3: How was the babs measured? Why at 365 nm only?
L255-257: There are numerous recent studies and MAC365 shall be compared with the recent studies.
L262-264: mention the wavelength range used for AAE calculation for the better clarity for readers.

Citation: https://doi.org/10.5194/egusphere-2024-104-RC1
- AC1: 'Reply on RC1', Krishnakant Budhavant, 04 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-104/egusphere-2024-104-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-104-AC1
RC2:
'Comment on egusphere-2024-104', Anonymous Referee #2, 27 Mar 2024

The Budhavant manuscript describes the results of SAPOEX campaign at three sites in South Asia. There is potential with this dataset, but the authors need to do more to connect across the results in each section (e.g. back trajectory analysis, aerosol composition, optical properties, radiative forcing). For example, the absolute concentrations of aerosol composition were only presented as campaign averages, which made it difficult to compare to the aerosol radiative forcing which was presented and discussed as an average and by month. There was often an over-simplification of the results, for example, the assertion that the MCOH site received transported air masses from the BCOB site: this did not seem to be always the case so it added confusion when discussing composition and aging. Overall the manuscript needs more refining of focus and connection among the different sections. Even in the introduction, the discussion of source and processing impacts on BC and BrC was over-simplified and lacking in precision.
detailed comments:
Ln 25: can this be better linked in the abstract to the aerosol optical properties?
Ln 46: this is an awkward phrase here. please edit
Ln 47: is this referring specifically to this region? this is undoubtedly true for BC, but WSOC may have other sources? e.g. biogenic and SOA?
Ln 47 – 56: this paragraph is difficult to follow as written. the authors need to clarify their purpose here. There is some confusion as they are trying to simultaneously discuss BC and WS-BrC. It doesn't really work and needs editing for clarity.
Ln 49: This sentence is awkward and the logical transition here is unclear
Ln 65: edit for clarity
Ln 88: What is the particle size here? TSP? PM2.5
Ln 111: are there any concerns about the high loading on these filters? typically filter-based photometers limit the filter loading to that which corresponds to a 50% transmission. the filters collected for this offline analysis would not have their loading limited by light transmission. i understand that this correction is intended to address the filter loading, but these correction schemes were originally designed for online instruments which have a filter advancement/change at a set transmission threshold. can this concern be addressed?
Ln 180-182: its not clear how these facts are relevant here. please remove or expand the discussion to make this more clear.
Ln 191-195: what about biogenic SO4? do you have a constraint on the possible marine contribution that goes beyond sea salt? Additionally, this rationale of sources from central and east India is a bit confusing as the BTs indicate that air masses predominantly leave the Indian subcontinent near BCOB or from west India before traveling to MCOH. If the aerosol composition from the west side of India is markedly different (e.g. higher SO4 fraction) than the IGP and BCOB, than the aging discussion of MCOH representing aged BCOB aerosol needs to be more refined.
Ln 208: Is the EC supposed to be BC?
Ln 217-223: I'm still stuck on the SO4 discussion. the provenance of the SO4 seems very relevant in determining if these 3 sites do represent different ages of the same air mass. as briefly mentioned in this section, increased SO4 would also seem to be very relevant for the coatings question. however some of the previous discussion of the loss of water soluble fraction during aging and transport (WSOC) seems to conflict with this. certainly the increase in SO4 at MCOH, absolute concentration as well as an extreme increase in relative contribution, is very relevant for coating of BC and internal mixture of BC and SO4 aerosol. i would like to see more discussion of this potentially conflicting observations between wsoc and so4, and using the three sites as steps in the aging process of one air mass. If the rationale for higher SO4 at MCOH is a shift in geographic source region, than the discussion of aging of two different aerosol systems needs to be included.
Ln 265-267: are these differences in AAE signficant? the std deviation is relatively high.
Ln 282-284: can the authors discuss why the atmospheric forcing was higher at MCOH while the surface concentrations for summed species was lower? it is also a bit difficult to interpret the relationship between the aerosol radiative forcing and the rest of the surface aerosol discussion as the time scales are not well aligned for aerosol concentrations and the ARF. overall, i'd like to see better connections among the sections of the discussion.
Table 1: this AAE is calculated off a very narrow range in wavelength. is 400 nm the longest wavelength measured here? What are the potential shortcomings of reporting AAE for such a narrow range in wavelength? Also, it is difficult to assess the trends in the ambient concentrations when only the averages for the entire period are reported. It is fine to present the ratios in the figures, but useful to also be able to see changes in the absolute concentrations as well.

Citation: https://doi.org/10.5194/egusphere-2024-104-RC2
- AC2: 'Reply on RC2', Krishnakant Budhavant, 04 Apr 2024
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-104/egusphere-2024-104-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-104-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Krishnakant Budhavant on behalf of the Authors (20 Apr 2024) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (05 May 2024) by Manvendra Krishna Dubey

RR by Anonymous Referee #1 (05 Jun 2024)

RR by Anonymous Referee #2 (26 Jul 2024)

ED: Publish as is (18 Aug 2024) by Manvendra Krishna Dubey

AR by Krishnakant Budhavant on behalf of the Authors (24 Aug 2024)

Journal article(s) based on this preprint

24 Oct 2024

Changing optical properties of black carbon and brown carbon aerosols during long-range transport from the Indo-Gangetic Plain to the equatorial Indian Ocean

Atmos. Chem. Phys., 24, 11911–11925, https://doi.org/10.5194/acp-24-11911-2024,https://doi.org/10.5194/acp-24-11911-2024, 2024

Short summary

Krishnakant Budhavant, Mohanan Remani Manoj, Samuel Mwaniki Gaita, Henry Holmstrand, Abdus Salam, Ahmed Muslim, Sreedharan K. Satheesh, and Orjan Gustafsson

Supplement

https://doi.org/10.5194/egusphere-2024-104-supplement

Krishnakant Budhavant, Mohanan Remani Manoj, Samuel Mwaniki Gaita, Henry Holmstrand, Abdus Salam, Ahmed Muslim, Sreedharan K. Satheesh, and Orjan Gustafsson

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The South Asian Pollution Experiment-2018 utilized access to 3 strategically located atmospheric receptor observatories. These observational constraints revealed opposite trends during long-range transport in BC-MAC and BrC-MAC. Models estimating the climate effects of particularly BC aerosols may have underestimated the ambient BC-MAC over distant and extensive receptor areas, which could contribute to the discrepancy between aerosol absorption predicted by models constrained by observations.


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Changing optical properties of Black Carbon and Brown Carbon aerosols during long-range transport from the Indo-Gangetic Plain to the equatorial Indian Ocean

Journal article(s) based on this preprint

Interactive discussion

Interactive discussion

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Journal article(s) based on this preprint

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