the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Dec 2024
Reactive oxygen species build-up in photochemically aged iron-and copper-doped secondary organic aerosol proxy
Abstract. The toxicity of particulate matter (PM) is highly related to the concentration of particle-bound reactive oxygen species (ROS). Chemical properties, including metal dissolution and the sources of PM, influence ROS production and its oxidative potential. Here, the photochemical aging of a secondary organic aerosol proxy (citric acid, CA) with metal complexes (iron-citrate, FeIIICit) is assessed toward the production of particle-bound ROS with an online instrument (OPROSI). We studied the photochemically induced redox chemistry in iron/copper-citrate particles experimentally mimicked with an aerosol flow tube (AFT) in which UV-aging was probed. Different atmospheric conditions were tested, influencing the physicochemical properties of the particles. We found that UV-aged CA aerosol containing 10 mole% FeIIICit generated ROS concentrations on the order of 0.1 nmol H2O2 eq μg−1, indicating the photochemically driven formation of peroxides. An increase in relative humidity (RH) leads to only a slight but overall lower concentration of ROS, possibly due to a loss of volatile HO2 and H2O2 in the gas phase in the less viscous particles. The RH effect is enhanced in nitrogen sheath flow, but in air and compared to the FeIIICit/CA particles, the iron/copper-citrate samples show a uniformly decreased ROS level. Interestingly, in the high humid nitrogen experiment with copper, we found a much more pronounced decline of the ROS concentration down to 2×10−2 nmol H2O2 eq μg−1 compared to all other irradiation experiments. We suggest that copper may suppress radical redox reactions and therefore consume ROS in an anoxic regime.
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RC1: 'Comment on ar-2024-36', Anonymous Referee #1, 28 Jan 2025
The manuscript, “Reactive Oxygen Species Build-up in Photochemically Aged
Iron-and Copper-doped Secondary Organic Aerosol Proxy” by Kilchhofer et al., conducted a study to measure photochemically induced reactive oxygen species (ROS) production on SOA proxy with metal complexes.
The manuscript overall is well-written and easy to follow. However, the manuscript could be improved by providing a more thorough explanation of the methods and results, as well as addressing a few limitations before it is ready for final publication.
Thanks for the opportunity to review this interesting manuscript.
Major comments:
- The authors provide a clear explanation for their choice of Cu and Fe particles to evaluate ROS production in the atmosphere. However, there is no justification for selecting citric acid (CA) as a surrogate for SOA. Can CA effectively represent SOA? The authors should address this point in the introduction.
- As mentioned by the authors in the introduction, the rationale behind using the DCFH assay to measure ROS in SOA particles is explained. Since the DCFH assay measures only specific ROS in SOA particles, the authors should provide more details on the potential limitations of OPROSI in measuring ROS from aerosols that contain multiple types of ROS.
Minor comments:
- The authors define the ROS concentration (ROSDCFH) unit as nM H2O2 L-1 air and the mass-normalized ROS concentration (Cnorm) unit as nM H2O2 eq. μg-1. However, in the results and discussion sections, as well as in some figures, these two abbreviations are used interchangeably. For instance, in Figure 5, the y-axis unit is labeled as ROSDCFH (nmol H2O2 eq. μg-1). According to the authors' definition, the unit for ROSDCFH should be volume-normalized (nM H2O2 eq. L-1 air). Additionally, nmol and nM are distinct units. Please review and clarify this inconsistency.
- Line 231 to 233: Figure 7 indicate about ROSDCFH level of FeCit/CA and FeCit/CuHCit/CA under air and N2 conditions, with 25% and 75% RH. The authors explain “ROSDCFH in FeCit/CuHCit/CA were about 0.05 nmol H2O2eq ug-1 lower compared to FeCit/CA.”. However, the observed difference in ROSDCFH level between FeCit/CA and FeCit/CuHCit/CA under air conditions appears to be smaller than the stated 0.05 nmol H2O2eq ug-1. Please review this discrepancy and clarify.
- Line 242: “The findings back previous efforts to model FeIII reoxidation in photochemically aged~.” can be reworded into “These findings support previous efforts to ~.”
Citation: https://doi.org/10.5194/ar-2024-36-RC1 -
RC2: 'Comment on ar-2024-36', Anonymous Referee #2, 20 Feb 2025
This study investigated the reactive oxygen species (ROS) produced through photochemical aging of SOA proxy (citric acid) with metals and further examines key factors influencing ROS generation, including RH, oxygen availability, and the presence of copper. The manuscript is well-written and logically sound. However, I have several concerns that need to be addressed before it can be considered for publication.
- One main limitation is that the manuscript does not clearly articulate the study's novelty. What new scientific insights does this work provide? At present, it appears to be an experimental validation of previous simulation studies.
- The experimental design is too simple. For example, only one molar ratio of FeIIICit:CA, CuIIHCit:CA, FeIIICit:CuIIHCit:CA was investigated. While the selected ratio is reasonable and mimics real atmospheric conditions, the actual atmospheric FeIIICit:CA ratio likely varies spatially. The authors should assess the effect of varying this ratio on ROS production to enhance the study’s significance.
- The rationale for using citric acid as an SOA proxy is not clearly justified. Is citric acid an oxidation product of any volatile organic compounds in the atmosphere? How abundant is it? The authors should clarify these points.
Other comments:
- Line 18: Please clarify why especially India and China. Is this due to their high PM pollution levels or large populations?
- Line 28: The statement regarding oxidative potential (OP) should be revised. OP was not first defined by Bate et al. (2019).
- Here and there (e.g., the captions of Figures 1 and 2), you wrote CuIIIHCit, please correct to CuIIHCit.
- Lines 138-140, according to Table 2, Experiments 1 to 4 are background ROS tests? And Experiments 5 and 6 designed to confirm that CuIIHCit did not autonomously produce ROS?
- Line 141, it states that two mole ratios of FeIIICit:CA (1:10 and 1:100) were selected, but only one ratio is present in Table 2. Also, the reported ROS results are derived from only one ratio. This inconsistency should be addressed.
- Line 179, replace “oxidative potential” to ROSDCFH for consistency.
- Line 222, the authors should perform a statistical analysis (e.g., t-test) and provide p values to quantify the significance of differences observed under various experimental conditions.
- Line 238: typo, change “oygen” to “oxygen”
- Line 279: what assays can be recommended to detect the whole range of particle-bound ROS? The authors should discuss potential methodologies.
Citation: https://doi.org/10.5194/ar-2024-36-RC2
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