the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Sep 2026
Particle-associated benzotriazole ultraviolet stabilizers exhibit atmospheric persistence against •OH heterogeneous oxidation
Abstract. Benzotriazole ultraviolet stabilizers (BT-UVs) are ubiquitous in airborne particles due to continuous emissions from widespread usage in plastic and rubber, with several congeners recognized as Persistent Organic Pollutants (POPs) under the Stockholm Convention. While •OH-initiated heterogeneous oxidation is the primary atmospheric degradation pathway for particle-associated organics, the degradation kinetics and fate of particulate BT-UVs remain poorly constrained. Herein, an oxidation flow reactor (OFR) was utilized to systematically investigate the •OH-initiated heterogeneous oxidation processes of eight commonly used BT-UVs. The results revealed that the intrinsic second-order rate constants ranged from (4.12 ± 0.30) × 10–13 to (13.2 ± 1.17) × 10–13 cm3 molecules–1 s–1, corresponding to atmospheric lifetimes exceeding one week and significant potential for long-range environmental transport (LRET). Furthermore, elevated ambient humidity enhanced their degradation rates by approximately 30%, whereas coexisting soluble iron exhibited a marginal effect. The oxidation proceeded mainly through alkyl-group and phenyl-ring reactions, yielding 30 tentatively identified transformation products (TPs), 24 of which were subsequently confirmed in ambient PM2.5. These findings quantitatively confirm the high persistence of particulate BT-UVs and highlight the necessity of evaluating the environmental risks and human inhalation hazards of their secondary transformation products.
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Status: open (until 10 Nov 2026)
- RC1: 'Comment on egusphere-2026-5464', Anonymous Referee #1, 07 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5464', Anonymous Referee #2, 18 Oct 2026
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The manuscript entitled “Particle-associated benzotriazole ultraviolet stabilizers exhibit atmospheric persistence against •OH heterogeneous oxidation” by Gao et al. is concerned with the atmospheric fate of benzotriazole ultraviolet stabilizers (BT-UVs) in airborne particles. An oxidation flow reactor (OFR) was utilized to investigate the OH-initiated heterogeneous oxidation of several BT-UVs. The derived reaction kinetics suggest atmospheric lifetimes exceeding one week, thereby allowing for long-range environmental transport. This study also examined the impact of elevated humidity and presence of soluble iron on the chemical degradation kinetics. While increased humidity increased the chemical reactions, the presence of soluble iron displayed only a minor effect on the kinetics. Furthermore, the reaction products were examined showing that oxidation proceeded mainly through alkyl-group and phenyl-ring reactions. Lastly, some of the identified reactions products were also observed in ambient PM2.5 indicating the persistence of particulate BT-UVs in the environment.
This is a comprehensive study of the degradation kinetics of particulate BT-UVs by OH reaction and corresponding reaction products. As such, the topic fits nicely in the scope of Atmospheric Chemistry and Physics.
Reading this manuscript, it is clear that the authors put great effort into this study. However, I feel that the derived heterogeneous kinetics could be significantly flawed and this would impact most results and interpretations, expect possibly for the reaction product derivations. The reason for this assessment is based on several studies on heterogeneous chemical kinetics from about 20 years ago. I will try to provide a short explanation but encourage the authors to study those papers.
Typically, oxidation flow reactors are not suited or recommended to perform detailed heterogeneous chemical kinetics studies between gases and particles, i.e., measuring the reactive uptake coefficient or first order or second order loss rates. OFR’s advantages lie in simulating atmospheric VOC oxidation and SOA formation (Peng and Jimenez, 2020). One can look at the IUPAC’s published heterogeneous chemical kinetic evaluations. The listed methods to derive heterogeneous chemical kinetics do not include oxidation flow reactors as far as I know.
When using a flow reactor system, for accurate determination of the transport of and reaction with a particle or substrate, the reactant, here OH, has to be uniformly / homogeneously mixed in the (laminar) flow. As soon as there are non-specified reactant concentration profiles, other than diffusional transport to the “reactive” surface, significant biases in derived chemical kinetics can occur. In an OFR, where, e.g., the UV light is mounted on top or in one specific area, an OH gradient will establish in radial and axial direction. I am not going into more details here but recommend four studies by Geoffrey Smith’s and Neil Donahue’s groups who use relative rate approaches to derive heterogeneous kinetics under such difficult flow reactor or chamber conditions. I briefly outline these here:
The studies by Hearn and Smith (2006) and Hearn et al. (2007) showcase a mixed-phase relative rates approach to quantitatively determine the degradation of bis(2-ethylhexyl) sebacate (BES) and dioctyl sebacate (DOS) particles by Cl and OH radicals. The Cl radicals were produced along the center of the flow reactor by a laser serving as UV source. This is similar to this study having a UV light source for radical generation while having an improved geometric configuration. To circumvent the uncertainty in radical concentration in the flow tube and the reaction time to which the particles are exposed to, the authors use a gas-phase reference compound with a known rate constant (need to be accurate). The degradation of the reference compound is measured concurrently with the degradation in the particulate phase. To quote them “In essence, the gas-phase reference compound was used as a ‘‘chemical clock’’ with which to measure the integrated exposure of the particles to the gas-phase reactant making it is unnecessary to know the absolute concentrations and reaction times.”
Similarly, (Donahue et al., 2005) makes the case to use a relative rate approach for oxidation of multicomponent organic particles to determine heterogeneous kinetics. (Robinson et al., 2006) applies this relative rate approach.
In short, when using a flow reactor system to study reactive uptake kinetics, the flow characteristics have to be known (laminar, parabolic profile, Reynolds numbers, etc.) and the reactant has to be uniformly distributed. Then reactive uptake by particles or substrates can be analytically derived by various established methods. Otherwise, a relative rates approach could allow for derivation of uptake kinetics but is of course more cumbersome and depends on choice/suitability of reference material and studied system. This is the reason why one typically does not find heterogeneous kinetics studies based on the application of an OFR.
For these reasons, I am very doubtful of the presented kinetics data. How far the reported data are off from the actual values, is difficult to tell. The derived uptake coefficients range between 0.07 to 0.13 which are typical values for OH reacting with organic particles (see, e.g., Bertram, Wilson, Knopf, Kroll groups). One might expect these values to represent lower values since measured particle composition included particles that likely have experienced a lesser degree of oxidation. Though, one cannot be entirely sure. The OH concentration is estimated by reaction with SO2 averaged over the OFR and intrinsic OH profiles. The reaction rate is likely to have an uncertainty of a factor of 2 (Atkinson et al., 2004)? The derived averaged OH concentration likely cannot alleviate the negligence of accounting for the OH profile since we deal with aerosol particles with different surface areas while OH concentration was derived from gas-phase kinetics. Hence, the call by Smith and Donahue groups to use a relative rates approach. My “feeling” would be that the actual values are not far off from reported ones, i.e., not orders of magnitudes, but one cannot just adjust the error bar.
I am not sure what to recommend with regard to publication of this manuscript. Additional experiments, using the relative rates approach might shed light on the kinetics (also measuring the surface area dependency of observed kinetics is crucial when applying polydisperse aerosol). However, this could not be used to correct all the data. One would need to repeat all the kinetics experiments. One may focus on the product study only combined with the ambient measurements.
Minor comments:
Line 213: Correct reference – “Davidovits et al.”. Also, if corrections were required, then the uptake was limited sufficiently. Please correct.
Line 242: Like water, OH, a small molecule, would diffuse rapidly through an organic particle, even if it is glassy at room temperature (Koop et al., 2011). So, this statement is incorrect. However, OH is highly reactive and even in liquid particles will most likely not penetrate further than 1 nm (Lee and Wilson, 2016). Hence, it will not mix throughout the particle as implied.
Line 251-253: This statement needs references. Typically larger organic molecules are more surface active, contrary to statement. Again, OH does not diffuse deep into the particle but reacts close to surface. The reactive uptake coefficient would allow interpretation for a possible surface coating effect (see Thornton and Bertram studies that examine the effect of surface coating on uptake kinetics). Then the gamma value would be lower (or one might see a trend to lower values), but it is not. This might point to issues of kinetics measurements described above?
Line 280: I do not think that OH will diffuse throughout the particle. OH reaction occurs close to surface and, thus, not yet oxidized organic molecules might diffuse to surface, leading to the overall greater oxidation and uptake.
References
Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J., and Troe, J.: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of Ox, HOx, NOx and SOx species, Atmos. Chem. Phys., 4, 1461-1738, 2004.
Donahue, N. M., Robinson, A. L., Huff Hartz, K. E., Sage, A. M., and Weitkamp, E. A.: Competitive oxidation in atmospheric aerosols: The case for relative kinetics, Geophys. Res. Lett., 32, 5, 10.1029/2005gl022893, 2005.
Hearn, J. D. and Smith, G. D.: A mixed-phase relative rates technique for measuring aerosol reaction kinetics, Geophys. Res. Lett., 33, 10.1029/2006gl026963, 2006.
Hearn, J. D., Renbaum, L. H., Wang, X., and Smith, G. D.: Kinetics and products from reaction of Cl radicals with dioctyl sebacate (DOS) particles in O2: a model for radical-initiated oxidation of organic aerosols, Phys. Chem. Chem. Phys., 9, 4803-4813, 10.1039/b707523e, 2007.
Koop, T., Bookhold, J., Shiraiwa, M., and Pöschl, U.: Glass transition and phase state of organic compounds: dependency on molecular properties and implications for secondary organic aerosols in the atmosphere, Phys. Chem. Chem. Phys., 13, 19238-19255, 10.1039/c1cp22617g, 2011.
Lee, L. and Wilson, K.: The Reactive-Diffusive Length of OH and Ozone in Model Organic Aerosols, J. Phys. Chem. A, 120, 6800-6812, 10.1021/acs.jpca.6b05285, 2016.
Peng, Z. and Jimenez, J. L.: Radical chemistry in oxidation flow reactors for atmospheric chemistry research, Chemical Society Reviews, 49, 2570-2616, 10.1039/c9cs00766k, 2020.
Robinson, A. L., Donahue, N. M., and Rogge, W. F.: Photochemical oxidation and changes in molecular composition of organic aerosol in the regional context, J. Geophys. Res., 111, D03302, 10.1029/2005jd006265, 2006.
Citation: https://doi.org/10.5194/egusphere-2026-5464-RC2
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This study investigates the OH-initiated heterogeneous oxidation of eight particle-associated benzotriazole ultraviolet stabilizers (BT-UVs) and evaluates their degradation kinetics, atmospheric persistence, and long-range transport potential. In particular, UV-328 has been listed as a Persistent Organic Pollutant (POP) under the Stockholm Convention; therefore, understanding the atmospheric behavior and fate of these compounds is of significant importance. The study is well conceived and conducted, the data are sufficient to support the conclusions, and the results are meaningful and meet the research objectives. However, there are still some issues that need to be considered and discussed. The comments are listed as follows.
(1) Line 23: The use of “confirmed” in the Abstract to describe the screening results of suspected TPs in PM is inappropriate. In Section 3.4, these suspected transformation products were classified as “tentatively identified,” whereas in the subsequent PM2.5 samples they were only “detected” and “semi-quantified,” not “confirmed”. Accordingly, the terminology should be made consistent throughout the manuscript.
(2) Line 156: For the Fuks and Sutugin correction, the values, sources, and uncertainties of the key parameters (e.g.,mean speed and mean free path of OH) should be provided.
(3) Line 202-206: The statement that photolysis and ozonolysis losses of BT-UVs were both below 5% and that evaporative loss was below 0.2% requires further supporting data and calculation details to demonstrate that these processes did not significantly affect the derived kexp values.
(4) Line 215: The order of the lower and upper limits in the reported γexp range is inconsistent and should be checked. The authors should also ensure that this range is consistent with the later statement.
(5) Line 222-226: Although the authors compare the kexp values of BT-UVs with those of LCMs, OPFRs, and terbutylazine, these studies may differ in particle phase state, OH exposure, and reaction time. It is recommended that the authors provide a comparison table summarizing the differences in experimental conditions and clarify the applicable scope and limitations of such comparisons.
(6) Line 237-241: The applicability of the AOPWIN SAR method to the molecular fragments and substituents of BT-UVs should be considered, and the main uncertainties in using gas-phase predictions to assess particle-phase persistence should be clarified.
(7) Line 304: In Fig. 5, TP384, TP368, and related products appear to contain isomers sharing the same designation or products from different pathways with similar labels. The authors should clearly distinguish the different isomers and clarify the assignment of each product to its corresponding pathway.
(8) Line 337-338: The “highest PNEC value” was conservatively adopted for TPs with multiple possible isomers to avoid underestimating risk. Since a higher PNEC generally corresponds to lower toxicity, adopting the highest PNEC may lead to an underestimation, rather than a conservative estimation, of toxicity risk. The authors should clarify the logic of this strategy and specify what positive and negative TVC values indicate in terms of relative toxicity.
(9) Line 303: The manuscript contains numerous grammatical errors that require careful checking and correction to improve readability. For example: “UV-328 and UV-234 was chosen” should be “were chosen”; “short-chain fragment” should be “short-chain fragments”; “carboxylic acids products” should be “carboxylic acid products”.