the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jan 2026
Measuring molecular singlet oxygen (1O2*) from atmospheric photosensitizers: Intercomparison of techniques, irradiation setups, data analysis and protocol recommendations
Abstract. Molecular singlet oxygen (1O2*) is the first excited state of molecular oxygen O2 and can be formed through indirect photochemistry during irradiation of chromophoric organic matter. Once formed in the particle and droplet phases in the atmosphere, 1O2* can be a competitive oxidant in the photochemical processing of organic matter. Yet, as more researchers study the atmospheric photochemistry of 1O2*, it is useful to establish protocols by evaluating and comparing experimental setups across laboratories. Here, we present 1O2* measurements from four photosensitizing molecules in four photoreactor setups at three research institutions, including two xenon lamps of different strengths and two multi-bulb UVA + UVB broadband systems. The production of 1O2* was investigated from perinaphthenone, lignin, and juglone, which are photosensitizers with atmospherically relevant light absorbing moieties, as well as from Rose Bengal, a standard photosensitizer. Two chemical actinometers, 2-nitrobenzaldehyde and p-nitroanisole/pyridine, were used to quantify photon fluxes and calculate rates of light absorbance for photosensitizers for each photoreactor. We compared two commonly used 1O2* quantification methods, chemical probe method using furfuryl alcohol, as well as direct 1O2* phosphorescence detection at 1270 nm. Rates of light absorbance across experimental setups for each photosensitizer ranged between 0.2 and 62 × 10-5 molphotons L-1 s-1, while 1O2* steady-state concentrations ranged between 0.01 and 129 × 10-11 M. Despite order of magnitude differences in rate of light absorbance and 1O2* steady state concentrations, normalizing to 1O2* quantum yields showed good inter-laboratory agreement but only for perinaphthenone (94 ± 9 % – 112 ± 17 %) and for Rose Bengal (67 ± 15 % – 87 % ± 5 %). 1O2* quantum yields for lignin and juglone increased with decreasing irradiation wavelength, consistent with a wavelength-dependence. Finally, we make five recommendations to improve the accuracy and reproducibility of 1O2* measurements. These include considering wavelength-dependent quantum yields, avoiding suppression of 1O2*, controlling and reporting photoreactor temperature, considering light scattering from nanoparticles, and conducting control experiments. These recommendations will help standardize 1O2* measurements in studying photochemical processing of atmospheric aerosols and droplets.
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Status: open (until 10 Mar 2026)
- RC1: 'Comment on egusphere-2025-6292', Anonymous Referee #1, 26 Feb 2026 reply
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RC2: 'Comment on egusphere-2025-6292', Anonymous Referee #2, 01 Mar 2026
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Review for egusphere-2025-6292
This work investigates key aspects of photochemical experiments such as repeatability and reliability of measurements across multiple light sources and actinometry methods for one of the most critical photochemical oxidants, singlet oxygen. Quantum yields for singlet oxygen were measured using both chemical probe experiments and phosphorescence measurements. Results suggest that reaction conditions and the selection of the light source can have strong impacts on observed, and hence predicted, levels in atmospheric scenarios such as wildfire aerosol emissions. This work is thorough and makes clear the importance of inter-lab comparisons across multiple approaches. The considerations for singlet oxygen assessment are valuable. However, the presentation quality is frequently problematic, particularly if this work is to be generally helpful to researchers with a wide range of backgrounds. At times poor wording and detailed lines of logic that omit key points may give rise to misapplication of the important points this manuscript seeks to demonstrate. I recommend this manuscript for publication, but not without such numerous corrections, listed below, as to amount to major revisions, whether in effort or importance. This work seems to be focused on getting the details right, which is absolutely commendable, yet the quality of presentation can work against this.
General Comments
Some of the applicability to a wider range of systems seems limited, in terms of the recommendations.
How can the suggestion “less than 1% of photosensitizers” be applied to more complex systems when the sample itself includes photosensitizer species? The production rate of singlet oxygen would also seem to be an important metric. Perhaps the authors can also suggest ways to diagnose whether quenching of singlet oxygen is a problem in more complex samples. Much more could be said in the final recommendations as to the utility of relative yield measurements in complex systems.
Can more be said about the universality of furfuryl alcohol as a chemical probe and what other chemical probes would be useful in some applications? Are there limitations, perhaps chemical compatibility to consider?
Sunlight itself is not a single, constant light source, as it varies with solar zenith angle. Using a solar simulator to reproduce sunlight is certainly a good approach, but it is still a single light source that does not apply to all solar conditions.
More Specific Comments
Equation 1
While numerically the values may work out, this equation is very, very poorly expressed. The reference (Kaur 2019b) does this much more clearly, and the authors would do best to follow that example exactly.
Absorbance does not have units.
It seems this should be absorption coefficient.
The summation over the wavelengths approximates the integration over wavelength, meaning that the 1/nm units are canceled via multiplication by dλ . Add a Δλ in units of wavelength.
The goal of this paper is very clear validation, and these seemingly small shortcuts may quickly lead to poor application of the principles this manuscript is working to support.
Equation 3
This equation is also problematic.
The final equivalence should be kobs*t, not just kobs.
The slope of the log plot is the observed rate constant.
Equation 5
Absorbance does not have units, again, this should have absorption coefficient, not abs.
The screening factor should be more clearly defined, in particular its general purpose, which would include the reduction of light available to the photosensitizer. It would seem in the spirit of this manuscript to assume the audience is not strictly photochemists, but rather atmospheric chemists more broadly (or others) who need to consider the wider impacts of UV irradiation on their experiments.
Equation 8
All terms, especially the rate constants, must be clearly defined
Table 2
The table should include a header row indicating “light absorbance equivalent hours,” this information should not be solely in the caption.
Table 3 and Section 3.3.6
The apparent low sensitivity of the phosphorescence method is noted, were higher concentrations of photosensitizer and/or oxygen saturation attempted?
The authors should also further address, or at last specifically state for the reader, that 4-nitroanisole was measured to have a much higher yield than juglone using chemical probe measurements, yet phosphorescence measurements of 4-nitroanisole were apparently below detection limits while juglone produced singlet oxygen above phosphorescence detection limits.
Line 28
“potent but oxidant”. Please complete the sentence.
55
Lack of reproducibility is vague here. Please revise to make this clearer.
95
UBC Setup : “…20 mg/L of lignin were used to make up” It would be clearer if you state something more
like “20 mg/L of lignin were the concentrations”. The same applies to the other setups.
185
What quencher?
221
The use of parentheses to indicate the symbol for the relative quantum yield is confusing here.
Please instead use a set of commas to indicate this use of an appositive.
364
The potential for screening of 2-NBA at the excitation wavelength due to the overlapping absorption of the product during actinometry should be mentioned. Also, it should be made clear that both actinometers should be utilized at low optical depth.
426
It should be made clearer that “optimized” screening factors indicates reduction sample absorbance. Furthermore, the Ircelyon screening factor is 0.68, far from a value of 1, while this is a value that is reasonable to correct for, this deviation from 1 should be noted here.
448
It seems this is the “deactivation lifetime”. Please clarify.
470-480
Normalizing production yield to the rate of absorption is simply the definition of quantum yield. What is the purpose of this sentence? Is there something else implied here? The second paragraph simply states that the observed quantum yield with lignin and juglone was not consistent with the other photosensitizers. Indeed, the most likely source is some sort of wavelength dependence. These paragraphs could be condensed and edited to greatly increase clarity.
480 and elsewhere
It is more descriptive to describe wavelengths as “short” and “long”, rather than low and high. Low and high are better adjectives for frequency.
493
Combinations of several narrowband excitations allow for the adjustments between changing light source wavelength profiles, while at the same time requiring many more experimental trials. As noted above a solar simulator set to one solar flux profile does not cover all atmospheric conditions either.
Section 3.3.7
The first paragraph is poorly worded to the point of confusion. The tedious explanation of how to determine the oxygen concentration in solution is not necessary, stating that the dissolved oxygen concentration was determined the atmospheric partial pressure and the Henry’s law constant is sufficient, and adjustments to altitude can be noted.
The second paragraph tells us that the discrepancy between chemical and phosphorescence detection discussed in in section 3.3.6 is now due to reaction with furfuryl alcohol. I understand the presentation of results in 3.3.6 and discussion in 3.3.7. Perhaps somehow these sections can be combined in a succinct way to make a clearer presentation.
523
Why wasn’t an equation noted? Instead “this calculation” is simply stated without clearly indicating what calculation was done. This is insufficient.
524-525
If an equation was noted or referenced, it be immediately clear to any reader that lower oxygen concentration leads to a lower quantum yield for any photosensitizer. This statement suggests that something special is occurring here, when this is exactly as expected. Do the authors suggest that there is a unique reason why lower oxygen concentration leads to lower quantum yields?
535
This sentence does not really define photochemical “action spectra”. Typically, an action spectrum would refer to a chemical response as a function of wavelength, such as an OH photolysis yield. The rate of absorption doesn’t need a new definition or seem to quite fit the term “action spectrum.”
600
It should be noted that the solar spectrum is not constant in either intensity or wavelength dependence across solar zenith angles.
Citation: https://doi.org/10.5194/egusphere-2025-6292-RC2
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