the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2024
Copper accelerates photochemically induced radical chemistry of iron-containing SOA
Abstract. Photochemical aging in secondary organic aerosol (SOA) particles alters their chemical composition and affects their adverse health effects. However, there is limited mechanistic insight on the role of transition metals in photochemical SOA aging and the evolution of the oxidative potential through their effect on radical chemistry. Here, we investigated the influence of copper (Cu) on the photochemical aging of iron (Fe) containing SOA in single particles using scanning transmission X-ray microscope measurements and chemical box modeling. The SOA proxy included citric acid (CA), iron(III) citrate (FeIII (Cit)) and copper(II) citrate (CuII (HCit)), which were exposed to UV light (λ = 365 nm) in a humidified environmental cell. We modeled known catalytic radical destruction mechanisms resulting from cross-redox reactions between copper and iron. Simulating anoxic FeIII (Cit)/CuII (HCit)/CA aging experiments showed a lower initial iron(III) reduction compared to FeIII (Cit)/CA particles, indicating a reduced iron(II) quantum yield than the photolysis of the FeIII (Cit) alone. We hypothesize that this effect may be due to copper replacing an iron center in a polynuclear complex. At higher relative humidity (RH) up to 60 %, a lower iron(II) quantum yield could not account for our observations of iron reoxidation in the dark. Instead, reoxidation appears to be highly sensitive to a potential copper(II)-induced reoxidation reaction. We provide a comprehensive discussion and evaluation of the poorly understood role of copper in modifying redox and radical chemistry, which is relevant for reactions involving transition metals mixed with SOA in the atmosphere.
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RC1: 'Comment on egusphere-2024-3226', Anonymous Referee #1, 19 Nov 2024
Kilchhofer et al. report interesting data sets to examine the effect of the co-existence of Fe and Cu on the photochemistry of citrate-containing particles. Based on the experiments, the present study found a synergy between iron and copper in photoreactions. However, the text requires minor revisions, as addressed below:
The main text needs to mention Figure 1. It would be easier to follow if the authors had added a pathway number to Figure 1 and referred to it along with the text (lines 52-64).
Line 102: What is “it”?
Line 125: What is the size to select here?
What is the rationale for assuming the constant acidity of pH = 2?
Figure 5 shows two identical images (5a and b only), which need revision.
Lines 273-275 conclude that the observations are due to a reduced quantum yield for iron(III) and faster reoxidation in the presence of copper. As noted in the text, particle viscosity also affects the beta. It needs to be confirmed that the particle viscosity is the same among different particle compositions at a given RH.
How was the particle viscosity change at different RHs represented in the model calculation?
Does Figure 9 show the beta averaged over all particles deposited on the substrate? The authors should show the position-dependent beta values to strengthen the discussions about the O2 diffusion-limited process.
Line 286: Please specify what is “These results”. So far, I don’t see which results support the acceleration and the reduced quantum yield by copper and how it works.
The section starting with line 292 seems more like a literature review about the quantum yields, but the connection to the present study needs to be discussed sufficiently. Furthermore, how is the discussion useful for the effect of copper on the reduced quantum yields?
Citation: https://doi.org/10.5194/egusphere-2024-3226-RC1 -
RC2: 'Comment on egusphere-2024-3226', Anonymous Referee #2, 30 Dec 2024
This is an interesting and well-executed study investigating the role of Cu in the photochemical aging of Fe-containing secondary organic aerosols. The authors present compelling experimental data showing how Cu impacts Fe(III) reduction during photochemical aging. Additionally, the study demonstrates that the results are significantly influenced by relative humidity (RH). While the findings are valuable, several aspects require further clarification and revision before I can recommend the paper for publication.
Major Comments
1. Reason for Rapid Fe Reoxidation
Figures 10, 12, and 13 indicate that the observed Fe reoxidation occurs much faster than predicted by the model at RH = 60%. The authors attribute this discrepancy to the RH dependence of Fe(II) quantum yield. However, an alternative explanation could be the underestimation of internal H₂O₂ or OH production in the model, especially considering Cu’s role. Cu reacts with HO₂ (or O₂⁻) far more efficiently than Fe in the aqueous phase, leading to increased H₂O₂ production (see for example Mao et al., 2017). Elevated H₂O₂ or OH levels could accelerate Fe(II) oxidation to Fe(III), explaining both the rapid reoxidation and the reduced Fe(III) reduction observed in the Fe(III)(Cit)/Cu(II)(HCit)/CA system compared to the Fe(III)(Cit)/CA system.
As authors state in the text, O2 is consumed by abundant organic radicals. Organic radicals formed during photochemical aging (wavelength 360–380 nm) likely produce significant amounts of HO₂/RO₂ radicals, contributing to additional H₂O₂/OH production that the current model may not account for.
Additionally, the reaction rate constant for R34 appears to be incorrect. Table 5 lists it as 1x10^(-9) /M/S, but it should be 8x10^(-9) /M/S. A lower rate constant in the model would underestimate O₂⁻ production, leading to underestimated H₂O₂ levels. The authors should explore the possibility of underestimated oxidant levels in the model instead of relying solely on RH-dependent quantum yield for Fe(II), which lacks a clear scientific basis.
2. Reactive Uptake and Diffuso-Reactive Length of O₂
The study discusses the effective reactive uptake coefficient of O₂ but applies different values to fit measurements, highlighting diffusion and reaction limitations. A more rigorous approach would involve calculating the diffuso-reactive length of O₂ in the aerosols to examine whether diffusion or reaction limits O₂ uptake. Providing this calculation would add depth to the discussion.
3. Role of Gas-Phase HO₂ and H₂O₂
While O₂ plays a critical role in the multiphase chemistry, the contribution of gas-phase HO₂ and H₂O₂ might be worth consideration. As Mao et al. (2013) indicate, gas-phase HO₂ and H₂O₂ influxes are significantly higher than their internal production rates in aerosols. The authors should address whether gas-phase HO₂ and H₂O₂ could affect ROS levels in the aerosol phase for their experimental setup.
4. Assumptions Regarding R38 and R39
The assumptions for R38 and R39 are counterintuitive. Combining R35 and R39 suggests that the Cu(I) + Fe(III) redox reaction would not proceed, contrary to laboratory measurements. Alternative mechanisms, such as underpredicted H₂O₂/OH/HO₂ production, could explain the high Fe(III) fractions observed. The authors should revisit these assumptions.
5. Aerosol pH
The authors assume a constant aerosol pH of 2 in their model, but pH can vary significantly with RH due to changes in liquid water content. Many reactions in this study, such as Cu2+ + HO₂ and Cu2+ + O₂⁻, are highly pH-sensitive, as pH determines the partitioning between HO₂ and O₂⁻. How pH changes with RH and its impact on key processes should be discussed. Additionally, the validity of the assumption that aerosol pH remains constant at 2 needs to be evaluated.
Technical Comments
1. Beta Averaging
How was beta calculated for Figures 10, 12, and 13? Was it averaged across measurements from the surface to the aerosol core? As shown in Figures 6/7/8, the beta values vary with distance from surface.
2. Figure 7 Caption
The caption for Figure 7 appears incomplete.
Overall, this study provides valuable insights into the role of Cu in the photochemical aging of Fe-containing aerosols. Addressing the comments above will significantly strengthen the manuscript.
References
Mao, J., Fan, S., Jacob, D. J., and Travis, K. R.: Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols, Atmospheric Chem. Phys., 13, 509–519, https://doi.org/10.5194/acp-13-509-2013, 2013.
Mao, J., Fan, S., and Horowitz, L. W.: Soluble Fe in Aerosols Sustained by Gaseous HO2 Uptake, Environ. Sci. Technol. Lett., 4, 98–104, https://doi.org/10.1021/acs.estlett.7b00017, 2017.
Citation: https://doi.org/10.5194/egusphere-2024-3226-RC2
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