the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 May 2025
Field Observations Reveal Substantially Higher Scattering Refractive Index in Secondary Versus Primary Organic Aerosols
Abstract. Aerosol-radiation interactions play a crucial role in air pollution and climate change with scattering being the dominant process. The complex refractive index of organic aerosols is essential for accurately simulating these interactions, with the scattering capability is predominantly determined by the real part of the refractive index (mr). Prevailing models often assume a constant mr for organic aerosols (e.g., 1.53 or 1.45) at different wavelengths or claim that mr of primary organic aerosols (POA) is substantially higher than that of secondary organic aerosols (SOA) (e.g., 1.63 for POA and 1.43 for SOA), largely due to a lack of direct measurements. This study employs direct measurements from the DMA-SP2 system to demonstrate a strong diameter dependence of dry state mr at 1064 nm, closely associated with primary aerosol emissions and secondary aerosol formation. Source apportionment of aerosol size distributions reveals that the mr of SOA is substantially higher than that of POA. Optical closure calculations, based on extensive dry state observations of aerosol scattering at 525 nm, size distributions, and chemical compositions, confirm this finding. These results challenge existing model assumptions. In addition, further analysis reveals mr of SOA increases with oxidation level, which is contrary to results of most laboratory studies on evolution of mr of SOA, which is likely associated with multiphase SOA formation. Our analysis recommends mr values at 525 nm of 1.37 for POA and 1.59 for SOA. These findings underscore that current modeling practices may introduce substantial inaccuracies in estimating the radiative effects of organic aerosols.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1056 KB) - Metadata XML
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Supplement
(1639 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
This study provides direct observational evidence that secondary organic aerosol formation enhances the aerosol scattering refractive index and has substantially higher real refractive indices than primary organic aerosols in humid southern China, challenging current model assumptions and offering recommended values that might improve the accuracy of aerosol radiative effect simulations.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-1410', Anonymous Referee #1, 05 Jun 2025
- AC1: 'Reply on RC1', Ye Kuang, 02 Jul 2025
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RC2: 'Comment on egusphere-2025-1410', Anonymous Referee #2, 27 Jun 2025
While most researchers nowadays focus on light absorption by brown carbon when trying to address the climate effect of aerosols, Shen et al. focused on the overlooked aerosols scattering, which is simplified in a lot of studies, that may induce large uncertainty in radiative forcing estimation. The topic fits well with the scope of ACP. The manuscript is generally well written and adequately organized. I have a few comments before it can be accepted for further consideration.
- In the abstract and lines 471-474. The authors found an increasing trend of real refractive index with O/C or aging and stated this finding conflicts with most lab results. For example, He et al. (2018) observed a declining trend of real refractive index when OA aged from LO-OOA to MO-OOA for photooxidation of b-pinene and p-xylene. However, He et al (DOI: 10.1021/acs.est.1c07328) and Li et al. (DOI: 10.5194/acp-19-139-2019) also observed increasing refractive index with aging for naphthalene+NOx and biomass burning aerosols. Lab experiments did not conclude on this aspect, as it seems to depend on what kind of aerosol was studied. I would say such of statement exaggerates the significance of the finding in this study.
- The results obtained from this study are from a single observation site near the emission source. How could the results and conclusions from this unique location be applied to a larger scale or different locations with potentially different emission sources and experiencing different atmospheric processes?
- I am not clear how one could use PMF to do source apportionment for PNSD. The authors also mentioned in their manuscript that LOOA and MOOA are generally not externally mixed and are likely to be prone to internal mixing (lines 463-465). But in the discussion of lines 440-446, the authors stated that POA and SOA tend to be optically independent at the single-particle level. The authors did observe SOA at the observation site. According to our knowledge of new particle formation and condensation growth, SOA and POA would not be completely externally mixed. Condensation of secondary products on existing particles (e.g., POA) would change the size of the particles, but does not change the number. How could PMF deal with this situation?
- Figure 1b, for me, it looks like the mr1064,400/mr1064,235 changed suddenly at mr1064,400=1.53. What is the mechanism behind this?
- Line 386-387, would the correlation coefficient of R=0.25 and R=-0.24 be significantly different?
- The authors used the empirical method proposed by Li et al. (2023). I would say this might not be so relevant for this paper. Li’s method was developed based on refractive index data for pure compounds without N element. However, in urban locations, aerosols already have nitrogen-containing species that affect the refractive index of the aerosols. The explanations in lines 474-478 are not well supported, as the aerosols themselves are different. It is not clear how much the formation pathway matters.
Citation: https://doi.org/10.5194/egusphere-2025-1410-RC2 - AC2: 'Reply on RC2', Ye Kuang, 02 Jul 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-1410', Anonymous Referee #1, 05 Jun 2025
- AC1: 'Reply on RC1', Ye Kuang, 02 Jul 2025
-
RC2: 'Comment on egusphere-2025-1410', Anonymous Referee #2, 27 Jun 2025
While most researchers nowadays focus on light absorption by brown carbon when trying to address the climate effect of aerosols, Shen et al. focused on the overlooked aerosols scattering, which is simplified in a lot of studies, that may induce large uncertainty in radiative forcing estimation. The topic fits well with the scope of ACP. The manuscript is generally well written and adequately organized. I have a few comments before it can be accepted for further consideration.
- In the abstract and lines 471-474. The authors found an increasing trend of real refractive index with O/C or aging and stated this finding conflicts with most lab results. For example, He et al. (2018) observed a declining trend of real refractive index when OA aged from LO-OOA to MO-OOA for photooxidation of b-pinene and p-xylene. However, He et al (DOI: 10.1021/acs.est.1c07328) and Li et al. (DOI: 10.5194/acp-19-139-2019) also observed increasing refractive index with aging for naphthalene+NOx and biomass burning aerosols. Lab experiments did not conclude on this aspect, as it seems to depend on what kind of aerosol was studied. I would say such of statement exaggerates the significance of the finding in this study.
- The results obtained from this study are from a single observation site near the emission source. How could the results and conclusions from this unique location be applied to a larger scale or different locations with potentially different emission sources and experiencing different atmospheric processes?
- I am not clear how one could use PMF to do source apportionment for PNSD. The authors also mentioned in their manuscript that LOOA and MOOA are generally not externally mixed and are likely to be prone to internal mixing (lines 463-465). But in the discussion of lines 440-446, the authors stated that POA and SOA tend to be optically independent at the single-particle level. The authors did observe SOA at the observation site. According to our knowledge of new particle formation and condensation growth, SOA and POA would not be completely externally mixed. Condensation of secondary products on existing particles (e.g., POA) would change the size of the particles, but does not change the number. How could PMF deal with this situation?
- Figure 1b, for me, it looks like the mr1064,400/mr1064,235 changed suddenly at mr1064,400=1.53. What is the mechanism behind this?
- Line 386-387, would the correlation coefficient of R=0.25 and R=-0.24 be significantly different?
- The authors used the empirical method proposed by Li et al. (2023). I would say this might not be so relevant for this paper. Li’s method was developed based on refractive index data for pure compounds without N element. However, in urban locations, aerosols already have nitrogen-containing species that affect the refractive index of the aerosols. The explanations in lines 474-478 are not well supported, as the aerosols themselves are different. It is not clear how much the formation pathway matters.
Citation: https://doi.org/10.5194/egusphere-2025-1410-RC2 - AC2: 'Reply on RC2', Ye Kuang, 02 Jul 2025
Peer review completion
Journal article(s) based on this preprint
This study provides direct observational evidence that secondary organic aerosol formation enhances the aerosol scattering refractive index and has substantially higher real refractive indices than primary organic aerosols in humid southern China, challenging current model assumptions and offering recommended values that might improve the accuracy of aerosol radiative effect simulations.
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Junlin Shen
Li Liu
Fengling Yuan
Biao Luo
Hongqing Qiao
Miaomiao Zhai
Gang Zhao
Hanbing Xu
Tao Deng
Xuejiao Deng
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(1056 KB) - Metadata XML
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Supplement
(1639 KB) - BibTeX
- EndNote
- Final revised paper
my review is attached.