Status: this preprint is currently under review for the journal ACP.
Light Absorption by Brown Carbon over the South-East Atlantic Ocean
Lu Zhang1,Michal Segal-Rozenhaimer1,2,Haochi Che1,Caroline Dang3,4,Arthur J. Sedlacek III5,Ernie R. Lewis5,Amie Dobracki6,Jenny P. S. Wong7,Paola Formenti8,Steven G. Howell9,and Athanasios Nenes10,11Lu Zhang et al.Lu Zhang1,Michal Segal-Rozenhaimer1,2,Haochi Che1,Caroline Dang3,4,Arthur J. Sedlacek III5,Ernie R. Lewis5,Amie Dobracki6,Jenny P. S. Wong7,Paola Formenti8,Steven G. Howell9,and Athanasios Nenes10,11
1Department of Geophysics, Tel Aviv University, Tel Aviv, Israel
2Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, California, USA
3NASA Ames Research Center, Moffett Field, California, USA
4Universities Space Research Association, Columbia, Maryland, USA
5Brookhaven National Laboratory, Upton, New York, USA
6Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA
7Mount Allison University, New Brunswick, CA
8Université de Paris and Univ Paris Est Creteil, CNRS, LISA, Paris, France
9University of Hawaii at Manoa, Department of Oceanography, Honolulu, USA
10Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, Switzerland
11Center for Studies of Air Quality and Climate Change, Institute of Chemical Engineering Sciences, Foundation for 20 Research and Technology Hellas, Greece
1Department of Geophysics, Tel Aviv University, Tel Aviv, Israel
2Bay Area Environmental Research Institute, NASA Ames Research Center, Moffett Field, California, USA
3NASA Ames Research Center, Moffett Field, California, USA
4Universities Space Research Association, Columbia, Maryland, USA
5Brookhaven National Laboratory, Upton, New York, USA
6Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA
7Mount Allison University, New Brunswick, CA
8Université de Paris and Univ Paris Est Creteil, CNRS, LISA, Paris, France
9University of Hawaii at Manoa, Department of Oceanography, Honolulu, USA
10Laboratory of Atmospheric Processes and their Impacts, School of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, Switzerland
11Center for Studies of Air Quality and Climate Change, Institute of Chemical Engineering Sciences, Foundation for 20 Research and Technology Hellas, Greece
Received: 01 Dec 2021 – Discussion started: 28 Jan 2022
Abstract. Biomass burning emissions often contain brown carbon (BrC), which represents a large family of light-absorbing organics that is chemically complex and therefore difficult to estimate their absorption of incoming solar radiation, resulting in large uncertainties in the estimation of the global direct radiative effect of aerosols. Here we investigate the contribution of BrC to the total light absorption of biomass burning aerosols over the South-East Atlantic Ocean with different optical models utilizing a suite of airborne measurements from the ORACLES 2018 campaign by introducing an effective refractive index of black carbon (BC), meBC = neBC+ikeBC, that accounts for all possible absorbing components at 660 nm wavelength to facilitate the attribution of absorption at shorter wavelengths.
Most values of the imaginary part of the refractive index, keBC, were larger than those commonly used for BC from biomass burning emissions, suggesting contributions from absorbers beyond BC at 660 nm. The TEM-EDX single particle analysis further suggests that these long-wavelength absorbers might include iron oxides, as iron is found to be present only when large values of keBC are derived. Using this effective BC refractive index, we find that the contribution of BrC to the total absorption at 470 nm (RBrC,470) ranges from ~5–15 %, with the organic aerosol mass absorption coefficient (MACOA,470) at this wavelength ranging from 0.25 ± 0.34 m2 g−1 to 0.43 ± 0.12 m2 g−1. The core-shell model yielded much higher estimates of MACOA,470 and RBrC,470 than homogeneous mixing models, underscoring the importance of model treatment. Another key finding was that estimates of the BrC contribution at 470 nm from the commonly used AAE (absorption Ångström exponent) attribution method (< 5 %) are much lower than the BrC contribution estimates (RBrC,470) using our new methodology that accounts for contributions from both BrC and non-carbonaceous, long-wavelength absorbers, such as magnetite. Thus, it is recommended that application of any optical properties-based attribution method use absorption coefficients at the longest possible wavelength to minimize the influence of BrC at the long wavelength and to account for potential contributions from other absorbing materials.
This study used comprehensive aircraft measurements to investigate the aged biomass burning plumes from south African transported to the Atlantic Ocean. The authors used optical closure between iterated refractive indices measured scattering-absorption at λ=660nm, to derive a proposed “ effective refractive index of BC ” in order to account for all BC absorption at multiple wavelengths. This study had a valuable dataset and could potentially contribute to the understanding on the aerosol absorption at this climatically important region. However I have a few concerns about the methods of this study, before it can be considered for publication.
Major:
1) My main concern is about the nil absorption of brown carbon at 660nm. There are many studies stating OA could be absorbing at relatively long visible wavelength, particularly for less-volatile organics from biomass burning (Saleh et al. ES&T letters, 2018). These OA has a large molecular weight and more functionalized (hereby lower volatility) and may survive after transport, when more volatile species were evaporated with remaining more absorbing component to be transported to a longer distance, as evidenced by a recent field study (Liu et al., ACP 2021).
So it is very possible that the long visible absorption (e.g. 660nm) in your results contained some BrC. I would suggest it may not be necessary to really assume a nil absorption of BrC but using a combined positive k value for OA to feed the optical closure. See a study to derive the kOA by assuming some absorptivity for both externally and internally mixed OA in BC-containing particles (Liu et al., 2021 ES&T). This may lead to a more solid conclusion.
2) Figure 4 is crucial and needs large improvement. Are these from all the straight-level runs from all flights? Error bars for each dot are required. It said only 6 dots had potential additional absorbing component, if so, why there were some “magnetite” but some without, these need to be discussed.
There are still no solid evidences that the magnetite did exist and contributed to the absorption at long visible. I may suggest having a plot like “additional absorption besides BC” vs magnetite fraction, the former is from Fig. 4 and the latter is from Fig. 5. Additionally, the origin of magnetite is speculated but not evidenced, where did they come from? This needs exterior support.
3) The optical closure of the partial dataset is performed on absorption-only and others are absorption-scattering. My concern is this may induce some discrepancies, especially when presenting both datasets on the same table. Why not using consistent absorption-only approach, at least they are consistent.
Others:
There should be more details discussing about the determination of bulk mass ratio of coating over rBC core from the SP2 measurement.
The title itself only includes brown carbon, though the authors also largely discussed about the possible absorption by iron.
It would be useful to briefly mention what effective refractive index of BC is in the abstract.
Conclusions should include some discussions about evolution.
Page 2, line 3, what is the commonly used AAE? To what extent lower?
The last sentence in the abstract needs rewriting, it is too long and a bit confusing.
Page 8, line 9, no need italic font.
Page 10, line 5, MAC is mass absorption cross section, not coefficient.
Equation (9) why using sigma, but only one wavelength is used.
References
Liu, Q., et al.: Reduced volatility of aerosols from surface emissions to the top of the planetary boundary layer, Atmos. Chem. Phys., 21, 14749–14760, https://doi.org/10.5194/acp-21-14749-2021, 2021.
Liu D. et al.: Evolution of Aerosol Optical Properties from Wood Smoke in Real Atmosphere Influenced by Burning Phase and Solar Radiation, ES&T, 55(9), 5677–5688, 2021.
Saleh, R., Z. Cheng, and K. Atwi (2018), The Brown–Black Continuum of Light-Absorbing Combustion Aerosols, Environmental Science & Technology Letters, doi: 10.1021/acs.estlett.8b00305.
Widespread biomass burning (BB) events occur annually in Africa, which contribute about one-third of global BB emissions. These emissions contain a large family of light-absorbing organics, known as brown carbon (BrC), whose absorption of incident solar radiation is difficult to estimate, leading to large uncertainties in the global radiative forcing estimation. This study quantified the BrC absorption of aged BB particles and highlighted the importance of absorbing iron in the meanwhile.
Widespread biomass burning (BB) events occur annually in Africa, which contribute about...
