UBC. The photoreactor used at UBC was a Rayonet RPR-200 (The Southern NE Ultraviolet Co.) equipped with 16 removable bulbs and a rotating sample carousel (Fig. a). The photoreactor was temperature controlled using the vented gas from a 15 L liquid nitrogen dewar (Cryofab, Inc. CLPB-15-GF). Temperature was monitored using a thermocouple probe (Thermosense BTM-4208SD 12 Channels Temperature Recorder) at each time-point. Experimental solutions (5 mL) were transferred into borosilicate glass test tubes and set into a rotating carousel. Empty slots on the carousel were filled with borosilicate glass vials containing 5 mL of MilliQ water in order to ensure homogeneous light distribution. Experimental solutions contained 20 µM furfuryl alcohol, 1 mM of isopropanol and 10 µM of either perinaphthenone, Rose Bengal, juglone, or 20 mg L⁻¹ of lignin. At 6–10 designated time points during illumination, 100 µL aliquots were removed for furfuryl alcohol quantification. Two UBC photoreactor setups were used for the present study. One setup used 12 UVA centred bulbs (Southern NE Ultraviolet Co., RPR-3500A), herein referred to as UBC UVA. The other setup used 8 UVA centred bulbs and 8 UVB centred bulbs (Zoo Med 26396 Reptisun 15 W 10.0 T5-Ho UVB Fluorescent Lamp, 12 in.), herein referred to as UBC UVA+UVB broadband.

UCD. At UCD, tropospheric sunlight was simulated with a 1000 W xenon arc lamp with three downstream optical filters: a water filter, an AM1.0 air mass filter (AM1D-3L, Sciencetech), and a 295 nm long-pass filter (20CGA-295, Thorlabs) (Fig. b). The temperature of the illumination chamber was controlled with a water bath set to 20 °C. Illuminations were either performed in 1 mL GE 021 quartz tubes (5 mm inner diameter) or in 5 mL rectangular quartz cuvettes (1 cm pathlength; Starna Cells). Experimental solutions contained 20 µM furfuryl alcohol, 1 mM of isopropanol and 10 µM of either perinaphthenone, Rose Bengal, juglone, or 20 mg L⁻¹ of lignin. At five designated time points during each illumination experiment, 130 µL aliquots were removed for furfuryl alcohol quantification. For the experiments performed in 1 mL tubes, the entire solution was illuminated. The solutions did not need to be stirred during illumination but the solutions were shaken vigorously prior to aliquot removal. For the experiments performed in cuvettes, the light beam only illuminated a subset of the solution and therefore the solutions were stirred during illumination. The volume removed during the cuvette experiments did not exceed 15 % of the initial illuminated volume.

Ircelyon. The set-up applied in Ircelyon consisted of a glass photoreactor equipped with a water jacket to allow for temperature control (Fig. c). Experimental solutions contained 20 µM furfuryl alcohol, 1 mM of isopropanol and 10 µM of either perinaphthenone, Rose Bengal, or 30 µM juglone, or 40 mg L⁻¹ of lignin. 20 mL of experimental solution were transferred to the photoreactor for irradiation. All experiments were performed at 293 K (20 °C). The solution was stirred by a magnetic stirrer. A quartz lid was placed on top of the reactor to avoid exchange with the air surrounding the set-up. The light source applied was a xenon lamp (LOT LSE140/160.25C, 150 W), which had an infrared (IR) filter, a mirror bending the light 90°, followed by a Pyrex filter to avoid light with wavelengths below 280 nm.

UBC. High performance liquid chromatography (HPLC, Agilent 1260, Agilent Technologies) equipped with a photodiode array detector was used to quantify the decay of the photooxidant probes. The analytical method was conducted with a reverse phase C18 column (Agilent, 5 µm, 4.6×150 mm) and an eluent gradient of acetonitrile (ACN) and MilliQ water for furfuryl alcohol. For the furfuryl alcohol quantification, a flow rate of 1 mL min⁻¹ was used. The gradient was 6 min at 20 / 80 (water / ACN), and 2 min at 90 / 10 (water / ACN) for a total run duration of 8 min. Furfuryl alcohol was monitored at 219 nm and typically observed at 3.32 min . For the monitoring of 2-nitrobenzaldehyde, a flow rate of 0.5 mL min⁻¹ was applied to an isocratic gradient of 60 / 40 (water / ACN), and maintained for 20 min. 2-nitrobenzaldehyde was monitored at 254 nm and was typically observed at 9.6 min. For p-nitroanisole, the gradient was 6 min at 50 / 50 acetonitrile/acetate buffer (pH = 6). The p-nitroanisole peak was typically quantified at 316 nm.

UCD. Furfuryl alcohol was monitored using high performance liquid chromatography (HPLC: Shimadzu LC-20AB pump, Thermo Scientific Accucore XL C18 column (50 × 3 mm, 4 µm bead), and Shimadzu-M20A Photodiode Array detector. A flow rate of 0.45 mL min⁻¹ and an eluent gradient with 2 min of 10 / 90 (acetonitrile / water), 3.5 min of 40 / 60 (acetonitrile / water), and the remaining 12.5 min with 10 / 90 (acetonitrile / water) for a total run time of 18 min was used. Furfuryl alcohol was quantified at 210 nm and eluted at 3.0 min, and the remainder of the run was to ensure all photosensitizers were flushed from the column.

Ircelyon. Samples were extracted at varying time intervals, depending on the experiment, and set aside for UHPLC/UV analysis. The method applied for UHPLC/UV analysis had an ACQUITY UPLC HSS T3 by WATERS column (100 mm × 2.1 mm, 1.8 µm) and is also described in . With a flow of 0.3 mL min⁻¹ the method started using two solvents: H2O with 0.1 % formic acid (solvent A), and acetonitrile (ACN) with 0.1 % formic acid (solvent B). Initially, 1 % solvent B (and 99 % solvent A) was applied for 2 min. In the next 11 min, the gradient gradually shifted from 1 % to 100 % solvent B. After this, solvent B was kept at 100 % for the following 2 min, which was followed by a change to 1 % solvent B (over 0.1 min), which was kept until the end of the sequence (total runtime of 22 min). This allowed for an equilibrium of the column before the next sample was injected. Furfuryl alcohol, 2-nitrobenzaldehyde and p-nitroanisole were analyzed at 218, 254, and 320 nm, respectively.

All absorbance spectra of experimental solutions was measured from 200–800 nm and contained furfuryl alcohol, isopropanol, the photosensitizer. Samples were measured in 1 cm pathlength quartz cuvettes, and were corrected for baseline and for the absorbance of a blank furfuryl alcohol + isopropanol solution to isolate the absorbance of the photosensitizer. At UBC, absorbance spectra were recorded using a double beam UV/Visible spectrometer (Carry 5000, Varian). At UCD, absorbance spectrum of each sample solution were measured with a Shimadzu UV-2501PC spectrophotometer. At Ircelyon, solutions were measured using an Agilent Cary 60 spectrophotometer.

An Ocean Optics FLAME-T-UV-VIS spectrophotometer equipped with a QP600-1-XSR fiber optic cable and a CC-3-UV-S cosine receptor was used to measure the irradiance spectrum of the photoreactor setups at UBC. Due to the rotation of the carousel sample holder, the spectrophotometer probe was positioned in one of the sample slots and rotated for measurement. One rotation of the sample carousel took 10 s. To not oversaturate the spectrophotometer detector and to obtain the entire photoreactor light output the spectrophotometer integration time was set to 0.25 s and averaged over 40 scans. At UCD, a TIDAS S 300 VIS/NIR 3011 (MMS 300–1100 nm) spectrophotometer was used to measure the irradiance spectrum of the solar simulator. The Ircelyon laboratory used an Avantes AVASPEC-HSC1024 x 58TEC-EVO spectrophotometer, equipped with an optical fiber (FC-UV/IR-400-1-PR, Avantes) that has a cosine corrector, was employed to measure the irradiance spectrum of the lamp.

The rate of light absorbance (Rabs, mol_photons L⁻¹ s⁻¹) of the photosensitizer was calculated by: 1Rabs=∑λIλ,0⋅αλ⋅Δλ⋅2.303⋅103 where αλ is light absorption coefficient of the sample (cm⁻¹, baseline corrected), Iλ,0 is the spectral irradiance of the light source (mol_photons cm⁻² s⁻¹ nm⁻¹), Δλ is the interval between adjacent wavelengths, and the values 2.303 and 103 are conversions for base and units, respectively . Following the recommendations of , UV-vis spectra were corrected by averaging and subtracting the absorbance of a sample from 700–800 nm (or the region of noise determined in logarithmic absorbance space of the UV-Vis spectrum) from the entire absorbance spectra. Additionally, negative absorbance values were screened and set to be 0 to avoid an artificial decrease in the rate of light absorbance. A baseline spectrum was also taken with the solvent (water) as well as isopropanol and furfuryl alcohol.

1O2* was detected using furfuryl alcohol as a chemical probe, which has a well constrained rate constant with 1O2* of 1×108+2.1×106 (T [°C] - 22) M⁻¹ s⁻¹ and has been used for several decades for 1O2* quantification . The decay of the furfuryl alcohol probe was followed with (U)HPLC/UV. The loss of the 1O2* probe, furfuryl alcohol, can be expressed as: 2-d[FFA]dt=kFFA+1O2*×[FFA]×[1O2*]ss+kFFA+3C*×[FFA]×[3C*]ss+kFFA+⚫OH×[FFA]×[⚫OH]ss+jFFA

The reaction of furfuryl alcohol with hydroxyl radicals (⚫OH) was set to be 0 due to the use of isopropanol as a quencher , and the direct photodegradation (jFFA) was also negligible for the irradiance times used in this study, evidenced by the direct photodegradation control experiments (Fig. S1 in the Supplement). The reaction of furfuryl alcohol with 3C* is also negligible, evidenced by deoxygenation control experiments (Fig. S2). Therefore, the loss of furfuryl alcohol can then be expressed only as a function of its reaction with 1O2*. 3ln⁡[FFA]t[FFA]0=kFFA+1O2*×[1O2*]ss×t=kobs,FFA×t

Through this decay, the pseudo-first-order rate constant, kobs,FFA, was obtained (Figs. S2–S7). kobs,FFA was then corrected for light screening of the sample: 4kobs,corr=kobs,FFAsf

Internal light screening due to light absorption is the reduction of light intensity within a sample as photons are absorbed before they can reach the entire irradiated volume. The light screening of the sample depends on the light absorbance of the sample, the path length of the light through the sample, and the irradiance from the light source. The light screening factor (sf) is described by Eq. (). 5sf=∑(1-10-αλ⋅l)Iλ,0∑(2.3⋅αλ⋅l⋅Iλ,0) where l is the path length of the sample. As described in Eq. (), the screening factor was used to obtain the corrected rate constant for decay of furfuryl alcohol, kobs,corr. kobs,corr was used to calculate the steady-state 1O2* concentration by applying the second-order rate constant between furfuryl alcohol and 1O2*, k1O2*,FFA, according to Eq. (). 6[1O2*]ss=kobs,corrk1O2*,FFA

The quantum yield expresses the efficiency of 1O2* production, i.e., it is the fraction of absorbed photons that lead to 1O2* production (Eq. ). 7Φ1O2*=#of1O2*moleculesformed#ofphotonsabsorbed The quantum yield is particularly powerful for comparing 1O2* production across different experimental setups . For example, strong irradiation sources would produce larger absolute quantities of 1O2* that are difficult to directly compare to weaker irradiation sources.

Considering the photophysical and chemical processes in the system, the 1O2* quantum yield can be expressed in Eq. () : 8Φ1O2*=ΦISCkO2[O2]kdT+kO2[O2]fΔ where ΦISC is the fraction of excited singlet photosensitizer molecules that undergo intersystem crossing to the excited triplet state, kdT is the deactivation of triplets, kO2 is the second-order rate constant for the physical quenching of 3C* with O₂, kO2[O2]kdT+kO2[O2] is the fraction of 3C* that is quenched by O2, and fΔ is the fraction of the quenching that leads to the formation of 1O2*.

In practice, two methods can be used to calculate Φ1O2*. The direct method (Φ1O2*dir), uses rate of light absorbance as well as a rate constant to account for the deactivation of 1O2* in the solvent (Eq. ). 9Φ1O2*dir=kobs,corr⋅kdk1O2*,FFA⋅Rabs where kd is the deactivation of 1O2* by water, 2.76×105 s⁻¹ . The direct quantum yield depends on the rate of light absorbance, Rabs, which were determined using spectral irradiances determined using actinometry of either 2-nitrobenzaldehyde or p-nitroanisole/pyridine.

In contrast, the relative quantum yield of 1O2*, Φ1O2*r, does not require photon flux to be quantitatively measured, and only normalizes rate of light absorbance of a compound of interest to a reference compound. In this study, perinapthenone (PN) was applied as reference compound. Perinaphthenone as a reference photosensitizer allows the normalization of the production of photooxidants and of the rate of absorbance for each experiment, due to possible slight changes in irradiation and/or experimental setup, since it has a well characterized 1O2* quantum yield . Additionally, the triplet state of perinaphthenone does not react with furfuryl alcohol under typical experimental conditions (concentrations in the µM range), meaning that furfuryl alcohol loss reflects only reaction with singlet oxygen. This greatly simplifies [1O2*]_SS and Φ1O2* calculations by enabling relative rate comparisons between singlet oxygen production and furfuryl alcohol consumption . Finally, perinaphthenone possesses no acid-base functionality. Therefore, its triplet reactivity is not expected to exhibit a pH dependence, further supporting its use as a robust reference photosensitizer. 10Φ1O2*r=kobs,corrkobs,PN,corrRabs,PNRabsΦPN

In Eq. (), kobs,PN,corr is the observed decay of furfuryl alcohol due to 1O2* produced by perinaphthenone that has been corrected for light screening, and Rabs,PN is the rate of light absorbance from the perinaphthenone sample.

Control experiments were performed to isolate the reaction of furfuryl alcohol + 1O2* and eliminate any other sources of furfuryl alcohol degradation. A blank control containing MilliQ water and 20 µM of furfuryl alcohol was irradiated to ensure the absence of probe decay due to impurities in the solvent (MilliQ water) or direct photodegradation of the chemical probe due to impurities. Based on the results of this control experiment, jFFA in Eq. () was set to zero.

A dark control solution was prepared for each set of photosensitizer experiments by adding a glass vial of sample solution covered with aluminum foil to protect the solution from irradiation. The dark control sample was conducted to account for any potential reactions between the photosensitizer and the probe in the absence of light.

In order to investigate potential reactions of the triplet state of the photosensitizer with furfuryl alcohol, deoxygenation experiments were conducted. The borosilicate tube was sealed with a septum and bubbled with N2 for 15 min to evacuate O2 dissolved in solution and present in the headspace. Throughout the irradiation process, the time points were taken by adding a N2 flow through the system, to maintain an inert environment. O2 deoxygenation experiments were only conducted at the UBC laboratory. Since the same photosensitizer compounds were used, any triplet state reactivity with furfuryl alcohol observed, or lack thereof, is expected to be reproducible across all laboratories. Based on the results of the deoxygenation control experiments, we set the value of kFFA+3C* to zero in Eq. ().

Intercomparing photoreactor light sources is essential for evaluating the reproducibility and the atmospheric relevance of 1O2* measurements across different laboratory setups, because light absorption by chromophoric organic matter is the first step in the formation of 1O2*. We compared xenon lamps as solar simulator systems at UCD and at Ircelyon , as well as two configurations of 16 bulbs of broad-band UV lights in a commercial Rayonet photoreactor at UBC (Fig. , Table ).

These choice of light source has advantages and disadvantages to consider. Xenon lamps produce a broad spectrum most similar to the solar spectrum, including the photochemically active UV range (280–400 nm). Xenon lamp photoreactors provide a better mimic of natural sunlight, but may be less suited for mechanistic, wavelength-specific experiments. In contrast, UV-centred bulbs enable selective irradiation in higher-energy UV regions, but fail to replicate the full spectral profile of solar irradiance (Sect. , Fig. b).

Although this intercomparison focuses on these four systems, many other photoreactors are also used in the community. Examples reported in recent atmospheric photochemistry studies include multiple 300 W xenon lamps , custom LED reactors equipped with discrete monochromatic LEDs , and additional photoreactors with different power xenon-lamp sources. In Sect. , we quantify each photoreactors light source compared to natural sunlight, and in Sect.  we assess the potential wavelength dependence of 1O2* formation. Together, these comparisons provide the foundation for interpreting inter-laboratory differences in 1O2* production, while recognizing the broader diversity of photoreactor setups used across the community.

The number of samples that can be irradiated simultaneously depends on the light setup (Fig. , Table , column 4). If the setup has one point source, such as a xenon lamp, then only one sample can be irradiated at at time (Figs. b, c and S6). On the other hand, if the setup has multiple bulbs arranged in an array, like a Rayonet or an incubator, then dozens of samples can be irradiated simultaneously. These multi-bulb setups can have a carousel, like the setup at UBC (Fig. S4), or have rotating lights (Fig. a). Multiple lights provide the advantage of increasing the experimental throughput. Additionally, chemical actinometry methods that were exposed to the exact same irradiation conditions were expected to provide the most accurate photon flux measurements, which is only possible when multiple samples can be irradiated simultaneously.

All three photoreactors can control sample temperature, which is important for chemical kinetic experiments, including for the reaction of 1O2* with its furfuryl alcohol probe. Indeed, calculates a +2 % increase in rate constant per °C increase in temperature. The Rayonet temperature at UBC is measured using a thermal couple probe and is regulated using a perforated copper coil inside the photoreactor where liquid nitrogen flows through a needle valve (Fig. S4). However, this setup does not benefit from the same accuracy or temperature range that the water-cooled temperature regulators provide in the Ircelyon and UCD photoreactors (20–34 °C for the liquid nitrogen cooling system vs. 0–50 °C for water bath cooling systems, Table ). Operational temperatures used for intercomparison experiments in this work were 22 °C at UBC and 20 °C at Ircelyon and UCD (Table , column 3).

Chemical actinometry allows for the quantification of the photon flux from an irradiation source, necessary to intercompare photochemical results across samples, setups and studies. Chemical actinometers are compounds with well characterized direct photolysis chemistry and with known quantum yields and absorption cross sections. For our intercomparison, we used two chemical actinometers: 2-nitrobenzaldehyde and p-nitroanisole/pyridine . Molar absorption coefficients for both compounds define the wavelength ranges over which they can be used as chemical actinometers, i.e., their validity ranges (Fig. S10). The validity ranges for the p-nitroanisole/pyridine system and for 2-nitrobenzaldehyde are 300–400 nm , and 280–400 nm , respectively. For these actinometers to be effective, these ranges should overlap with the emission spectra of the photoreactor, which is the case for our intercomparison (Sect. ). Additionally, both chemical actinometers should be used at low optical depth, and potential light screening by all chromophores, including reaction products, should be considered.

The p-nitroanisole (PNA) and pyridine (Pyr) actinometery pair has the advantage that the time decay can be adjusted from min to h (depending on the strength of the light source) by varying the pyridine concentration (i.e ΦPNA=0.29[Pyr]+2.9×10-4). For example, if the sample's furfuryl alcohol decay takes 12 h, the concentration of Pyr can be tuned to match the timeline of the probe kinetics. In contrast, the quantum yield of the 2-nitrobenzaldehyde system is fixed (Φ2NB=0.41±0.02) and cannot be tuned to match the timescale of 1O2* probe decay experiments. For these reasons, when an experiment may be subject to varying photon flux over the irradiation period (for example, in natural sunlight experiments when the angle of the sunlight changes throughout the day), the p-nitroanisole/pyridine actinometer is preferred due to its tunable quantum yield. However, in the context of lab-based photoreactors, we found that 2-nitrobenzaldehyde was operationally simpler to use. If photon flux is expected to remain stable throughout a 1O2* probe experiment, the 2-nitrobenzaldehyde actinometer may be the more suitable option.

To demonstrate that these two chemical actinometers work interchangeably, we compared them across setups. As expected, they provided nearly identical photon fluxes as a function of wavelength for the same photoreactor (Fig. S11). Consequently, the rate of light absorbance of each photosensitizer in different photoreactor configurations did not differ by more than 11 % when using both chemical actinometers (Table S5). Nevertheless, we recommend conducting illumination experiments with both 2-nitrobenzaldehyde and p-nitroanisole/pyridine chemical actinometers at the start of a study, since the reproducibility of the photon fluxes determined by two independent actinometers provides confidence that the photon flux is accurately calculated. When the agreement is established, subsequent experiments may rely on a single actinometer.

To compare the absolute and normalized photon fluxes from each setup, we measured their spectrally resolved irradiance profiles using a spectrophotometer. Absolute irradiance was calculated by combining the irradiance profiles with 2-nitrobenzaldehyde actinometry, following Eqs. ()–(). As a standard reference, we integrated the photon flux over 200–800 nm, noting that wavelengths below 290 nm contribute negligibly to tropospheric irradiance whereas wavelengths above approximately 600 nm are unlikely to initiate photochemistry. The UCD photoreactor, with a 1000 W xenon light source, had the highest absolute photon flux, with an integrated irradiance in the 200–800 nm wavelength range of 4686 W m⁻² (Fig. a). The Ircelyon photoreactor, with a 150 W xenon lamp, had a lower integrated irradiance at 171 W m⁻². The UBC photoreactor had an integrated irradiance of 103 W m⁻² for the UVA configuration, and 163 W m⁻² for the UVA+UVB broadband configuration. The Tropospheric Ultraviolet and Visible (TUV) radiation modelled sunlight for the 2025 summer solstice in Vancouver, BC, was 774 W m⁻² over 200–800 nm. This modeled sunlight value is higher than those of the UBC and Ircelyon photoreactors, but lower than the UCD photoreactor, hence the need to quantify the approximate sunlight equivalents for each photoreactor (Sect. ).

Calculating absolute irradiance required combining the spectrophotometer measurements of relative photon fluxes as a function of wavelength with chemical actinometry. UCD found significant variability in the relative photon fluxes for measurements made on the same day but with different optical probe positions in the sample chamber (see Sect. S12). We believe that this variability was due to internal reflections within the UCD illumination chamber. To determine the most correct relative photon fluxes, we used experiments to determine the 1O2* quantum yield from perinaphthenone to constrain the 548 nm / 347 nm intensity ratio, which we used as a marker of the photon fluxes at long and short wavelengths (Sect. S12 and Fig. S21 in the Supplement). The combination of actinometry (to get the short-wavelength region) and a reference photosensitizer (to characterize the long-wavelength region) allowed us to constrain the UCD photon flux by identifying the influence of internal reflections on the spectral shape. By testing if experiments with two actinometers yielded equivalent photon fluxes, and by ensuring that experiments with reference photosensitizers yielded published values, this tested the photon flux across a wide range of wavelengths. This highlights the utility of actinometry and model photosensitizers as robust tools to constrain the photon flux in an experimental illumination system.

The spectral range of wavelengths and their peak intensity are also important considerations for interpreting photochemical results. The Ircelyon photoreactor has light that extends to 285 nm, a lower threshold than the simulated sunlight from the TUV model (Fig. b). The UBC photoreactor configurations had UVA bulbs (348–405 nm), and UVA+UVB bulbs (300–405 nm), with some additional irradiance at longer wavelengths due to fluorescence from the UVB bulbs. The UBC spectral profiles differed from the broad distribution of natural sunlight, with implications for extrapolating to the real atmosphere (Fig. b). Normalized photon fluxes showed similar spectral shapes for the xenon lamp setups at UCD and Ircelyon (Fig. b), suggesting consistency in the emission characteristics despite differences in absolute intensity. Based on differences in spectral range of the different light sources, we also explored the possible quantum yield dependence on wavelength in Sect. .

To estimate the equivalent hours of sunlight experiences by the samples in each photoreactor, we compared rates of light absorbance by photosensitizers in each four photoreactor configurations to conditions under natural sunlight. The conversion factor, calculated using Eq. (), represents the number of hours of solar exposure at 5 km altitude over Vancouver, Canada (a representative mid-tropospheric location), required to match 1 h of irradiation in a given photoreactor.

Equivalent hours of sunlight were determined for perinaphthenone, lignin, and juglone, based on rates of light absorbance from 2-nitrobenzaldehyde actinometry (Table ). Consistent with our observed photon fluxes (Sect. ), the UCD photoreactor had the highest effective light intensity with conversion factors of 5.0 to 7.1 (Table ). In contrast, the Ircelyon photoreactor yielded the lowest values, with less than 0.3 h of natural sunlight for each hour in the photoreactor, consistent with a weaker irradiance. The UBC photoreactor setups had intermediate conversion factors, ranging from 0.23 to 0.72. These trends reflect differences in both total irradiance and spectral overlap with the absorbance features of the photosensitizers, highlighting the need to characterize and report spectral irradiance, light intensity, and action spectra of light absorbance for each illumination system.

Furfuryl alcohol is widely used as a chemical probe for 1O2* due to its well characterized and selective reactivity . Recent work has expanded the family of furan-based probes. For example, reported singlet oxygen reaction kinetics for 17 furan derivatives, highlighting that alternative probes may be selected for specific experimental constraints. For example, less volatile probes such as 2-methylfuran-3,4-dicarboxylic acid may be advantageous in open systems where volatility is a concern, although this was not an issue for the capped solutions used here. To the best of our knowledge, chemical compatibility has not been systematically explored as a limitation of FFA as a probe for 1O2*.

To accurately quantify 1O2* production, furfuryl alcohol must display pseudo-first order kinetics as a function of irradiation time (Figs. S2–S7). This linearity is necessary, as a deviation indicates that 1O2* is no longer under steady-state conditions and other sources and sinks of 1O2* are likely contributing to furfuryl alcohol's decay. Furthermore, deviations from pseudo-first-order kinetics can also be observed for longer irradiation times and weak light sources. To account for these deviations, we have previously suggested removing furfuryl alcohol time points resulting in a change in slope greater than 25 % .

When using a chemical probe to determine 1O2*, all possible sinks of the probe need to be considered and accounted for. These sinks include photolysis, light screening, reactivity with other oxidants such as ⚫OH, and reactivity with other molecules and excited state photosensitizers (see Sect. ). For example, a dark control will confirm the lack of reactivity between the photosensitizer and the probe in the absence of light (Fig. S1). Additionally, correcting observed decays of the probe for light screening according to Eq. () is necessary to account for any heterogeneity of chromophores in the sample and particularly due to concentrated and coloured samples. In our case, the screening factors were optimized to be close to 1, by reducing sample absorbance, to specifically avoid screening (Tables S1–S4). However, in the Ircelyon photoreactor, the 3.5 cm irradiation pathlength through the sample produced screening factors that deviated from 1. Furthermore, to account for ⚫OH reactivity with furfuryl alcohol, we recommend adding an ⚫OH quencher such as isopropanol to the solution . Alternatively, ⚫OH can be quantified explicitly with a probe such as benzoic acid or terephthalic acid . found that ⚫OH accounted for up to 32 % of the observed furfuryl alcohol loss under 311 nm irradiation, but only up to 2 % under 365 nm irradiation, highlighting the importance of quenching ⚫OH or accounting for probe loss from ⚫OH in the presence of UVB light.

Yellow impurities in furfuryl alcohol can also influence the reaction kinetics and the absorbance of the sample in the UVA region (Fig. S12). These impurities can contribute to the direct photodegradation of furfuryl alcohol by 3.1 %, as shown in Fig. S13 . To limit this issue, stock solutions should be stored at 4 °C, and in dark environments . Overall, using a probe such as furfuryl alcohol is an indirect method of quantification but does lend itself readily to high throughput experiments of atmospheric sample extracts.

Removing oxygen by bubbling with N₂ from sample solutions is an important control for verifying that the 3C* of the photosensitizer did not react with furfuryl alcohol at a competitive rate, which is the case for the the photosensitizing molecules used in our photoreactor intercomparison (Figs. S2, S3). However, we found that excited states of atmospherically relevant nitroanisoles , can react with furfuryl alcohol in the absence of O₂ leading to a 1O2* false positive (Fig. S14). Nitrogen-containing compounds are ubiquitous in organic aerosols and have been found in wildfire smoke and PM_2.5 filters , highlighting the importance of conducting this control experiment for the atmospheric context.

Furthermore, previous studies have used the kinetic solvent isotope effect as a diagnostic tool to assess the decay of furfuryl alcohol from oxidants other than 1O2* . The kinetic solvent isotope effect relies on the difference in the deactivation lifetime of 1O2* in H2O (3.5 µs; ) and in D2O (67 µs; ). Although not conducted in our intercomparison study, kinetic solvent isotope effect experiments can be a useful diagnostic tool to quantify the furfuryl alcohol from oxidants other than 1O2*.

There are two approaches to calculate a quantum yield. There is an absolute method, Φ1O2*dir, which relies on chemical actinometry and is measured for instance by 2-nitrobenzaldehyde or p-nitroanisole/pyridine according to Eq. () and used to generate Fig. c. In addition, there is a relative method, Φ1O2*r, which uses a reference photosensitizer such as perinaphthenone along with Eq. (). This method requires that a solution of perinaphthenone be run side by side with the samples in a multi-sample holder and captures changes in photon flux throughout the irradiation experiment.

For this intercomparison study, we ran both sets of experiments to evaluate the differences between the absolute and relative methods. Values between the two methods (Φ1O2*dir vs. Φ1O2*r) were consistently within 15 % of each other for all sensitizers, and we thereby suggest 15 % as a reasonable metric for acceptable agreement between quantification methods (Tables S1–S4). Agreement between these methods is expected and supports the stability of the lights sources in all four experimental setups (Table S5). Chemical actinometry has the advantage of providing both the photon flux and the corresponding quantum yield (Eq. ), while the relative method only provides quantum yields (Eq. ). Applying both approaches at the onset of a study can provide a consistency check to identify systematic biases in the light distribution and actinometry based quantification within the photoreactor. Nevertheless, previous aqueous environmental studies of 1O2* have favoured the use of actinometry over reference photosensitizer to quantify 1O2* quantum yields (24 studies to 4 studies, reported by ).

Rates of light absorbance and 1O2* steady-state concentrations differed by several orders of magnitude across the different laboratories (Fig. ). These differences reflect variation in photon flux among the light sources, with higher intensity sources producing higher absorbance rates and correspondingly higher 1O2* concentrations (Fig. a, b). Despite these order of magnitude differences in Rabs and [1O2*]_SS, the apparent quantum yield, Φ1O2*, for Rose Bengal and for perinaphthenone were consistent and reproducible across photoreactors and aligned with literature values (Fig. c), indicating that these sensitizers singlet oxygen generation efficiency exhibit minimal dependence on the irradiation conditions explored here.

In contrast, lignin and juglone exhibited deviations in quantum yields across photoreactors. Specifically, xenon lamp systems (UCD and Ircelyon) yielded lower Φ1O2* values, whereas the UV bulb system (UBC) produced higher values. For juglone, this discrepancy was particularly pronounced, as the highest photon flux (UCD; Fig. 4a) corresponded to the lowest measured quantum yield (Fig. 4c, left). These results suggest that, unlike Rose Bengal and perinaphthenone, the apparent quantum yields of lignin and juglone are influenced by the spectral distribution of the light source, consistent with a wavelength dependent mechanism.

1O2* quantum yields have been observed to decrease as a function of wavelength in surface water samples and in juglone aqueous solutions . In this study, we observed similar trends for two samples specifically: lignin and juglone. Indeed, shorter wavelengths led to an increase in Φ1O2* (Figs. , S20). This wavelength dependency may be due to higher energy photons leading to higher singlet to triplet intersystem crossing rates, kISC. This increase in intersystem crossing rate has been observed for a range of aromatic molecules relevant to the atmosphere . While a quantitative relationship between photon energy and increasing Φ1O2* does not yet exist, there is clear evidence for wavelength-dependent quantum yields of photosensitizing molecules within chromophoric dissolved organic matter,, and we now extend this observation to atmospherically relevant molecules like lignin and juglone. For inter-comparing Φ1O2*, it is thus necessary to report the irradiance spectrum and if possible use different light sources for the interpretation of Φ1O2* values. This wavelength sensitivity may introduce a measurement-dependent bias when different light sources are used and should therefore be treated as a methodological constraint.

Due to wavelength dependencies, results obtained using narrow or single wavelength irradiation can be difficult to reliably extrapolate to the broader solar spectrum. Accordingly, using sunlight-mimicking irradiation sources, or combinations of several narrow band excitations are best recommended. In fact, recommended that, to obtain more accurate measurements, researchers should: (1) select a chemical actinometer that absorbs light in the same spectral region as the compound of interest, and (2) minimize inner filter effects by employing optically transparent solutions.

We conducted phosphorescence measurements of 1O2* as a direct spectroscopic technique to further intercompare reported Φ1O2* values measured using a chemical probe. All four sensitizers, as well as nitrophenol and nitroanisoles as additional sensitizers relevant for BrC, were excited at 355 nm and the relaxation of 1O2* to the ground state was detected at 1270 nm . The Φ1O2* values between the two methods only matched well for the reference sensitizer Rose Bengal (Table ).

Interestingly, lignin, 2-nitroanisole, 4-nitroanisole and 4-nitrophenol had 1O2* quantum yields of 0 in the phosphorescence set up, but measurable 1O2* quantum yields using the chemical probe method (Table ). We hypothesize that this discrepancy is due to the detection limit of the photomultiplier tube in the phosphorescence set up. To further test this hypothesis, we used a range of concentrations of the sensitizers (absorbance 0.1–0.5), yet 1O2* signals remained undetectable. We did not test the role of oxygen saturation to maintain comparability with the ambient-air conditions of the chemical probe experiments.The differences observed among the photosensitizers support the conclusion that the direct phosphorescence method, despite its high specificity, may lack sufficient sensitivity to detect 1O2* emission for photosensitizers with low 1O2* quantum yields.

For juglone, a phosphorescence signal was detected, and the 1O2* quantum yield obtained was three times higher than with the chemical probe method (Table ). This discrepancy is hypothesized to be due to an observed wavelength dependence on 1O2* quantum yield from the single wavelength laser used (355 nm) in phosphorescence experiments (Sect. ).

The relatively high quantum yield for 3-nitroanisole by both methods compared to 4- and 2-nitroanisole (Table ) may be explained by the unique meta position of the methoxy and the nitro substituents. Indeed, the meta position has been suggested to lead to an excited state through a low lying π,π* triplet state , consistent with a high 1O2* quantum yield . Furthermore, a quantum yield of 8.83 % was measured for 4-nitroanisole using the chemical probe method, however no corresponding signal was detected via direct phosphorescence. We hypothesize that this discrepancy can be attributed to the reactivity of the 3C* of the this nitroanisole with furfuryl alcohol observed in the absence of O₂ (Sect. , Fig. S14). The proposed mechanism is a substitution reaction involving the nitro group on nitroanisole and and the alcohol on furfuryl alcohol, proceeding under light with wavelengths greater than 300 nm in water . In these cases, furfuryl alcohol is not a suitable probe for 1O2* in the presence of nitro groups, which are particularly prevalent in BrC and the atmospheric chemistry context.

Phosphorescence measurements can be used to diagnose whether quenching of 1O2* is significant in complex samples . Although not conducted here due to the use of single molecule sensitizers, 1O2* can be generated using a known sensitizer at a wavelength outside the absorbance range of the sample matrix and the phosphorescence signal at 1270 nm compared in the presence and absence of the sample. A reduction in the signal when the sample is present indicates additional quenching of 1O2* and this approach is complementary to chemical probe methods. For example, in systems that do not absorb above 500 nm, Rose Bengal can be excited at 550 nm to generate 1O2* and evaluate quenching by the sample matrix.

Overall, the direct phosphorescence method and the indirect probe method remain complimentary. The phosphorescence technique does not suffer from the interferences outlined for the indirect probe method since it tracks 1O2* directly (Sect.  and ). However, it does not provide information on 1O2* reactivity (i.e. no information on kobs that chemical probe methods provide .

Discrepancies between phosphorescence and chemical probe methods may also be attributed to differences in dissolved O2 (Eq. ). To test this hypothesis, we estimated the partial pressure of O2 at the location of the measurement, used O2's Henry's Law constant to estimate the amount of dissolved O2 and then calculated the fraction of triplets quenched by O2 using the constant for the deactivation triplets, kdT from and calculating (kO2[O2]) (see additional details in the Supplement). The calculation of [O2] in solution led to calculated 3C* deactivation pathways yields, kO2[O2], that were 2.8 times larger than kdT in Vancouver, and 2.5 times larger in Calgary. In other words, a lower pO2 results in a lower Φ1O2*, according to Eq. (), consistent with the quantum yield for Rose Bengal that we reported in Table 3.

Photochemical action spectra of light absorbance represent rates of light absorbance as a function of wavelength, and provide insights into the specific wavelength regions that drive photochemical reactions. Rose Bengal exhibited a strong absorbance peak at 549 nm in water, which dominated its photochemical action spectra of light absorbance in all photoreactors except the UBC UVA setup, where no irradiance was present at this wavelength (Fig. a). Remarkably, despite this difference, Φ1O2*,RB remained comparable across all setups, indicating no wavelength dependence. Perinaphthenone action regions of light absorbance also differed, with xenon lamp photoreactors displaying broader regions (e.g 294–480 nm at Ircelyon) and narrower regions in the UV photoreactor setups (e.g 348–405 nm for UBC UVA) (Fig. b). The differences in action regions of light absorbance did not lead to an observed difference in Φ1O2*,PN, suggesting that the quantum yield is independent of wavelength, and effectively normalizes Rabs and 1O2* production.

The juglone action spectra of light absorbance peaked at longer wavelengths in xenon lamps (433 nm at UCD, 421 nm at Ircelyon), where lower Φ1O2*,Juglone values were observed (Fig. c). UBC photoreactors showed peaks at 365 nm, with the UVA+UVB broadband setup extending further into the UV (down to 304 nm) and producing the highest Φ1O2*,Juglone, consistent with increased excitation by higher energy photons (Fig. d). A similar trend was observed for lignin; increased contributions from high energy photons in the UBC UVA and UVA+UVB broadband systems led to higher Φ1O2*,Lignin compared to the UCD and Ircelyon xenon lamp setups. This general trend is consistent with previous studies for quantum yields of photodegradation .

An important consideration in the analysis of photochemical action regions of light absorbance is the potential for light scattering in absorbance measurements. Light scattering should be considered a source of uncertainty in UV–Vis absorbance measurements, even in filtered samples, as particles smaller than filter pore size (0.22 µm here) can still contribute to scattering and filtration alone will not necessarily eliminate this effect. Dissolved organic matter can form colloidal structures or aggregates smaller than the filter pore size, which remain in solution and can scatter light. The formation and optical properties of these colloids depend on the composition and source of the organic matter, and thus filtration does not guarantee the removal of all scattering effects (Sect. S8, Figs. S15, S16) . Scattering from nanoparticles is particularly challenging, and can artificially increase rates of light absorbance. We therefore recommend that future studies recognize and discuss the potential influence of light scattering when interpreting light absorbance measurements. In this work, we also conducted a sensitivity analysis of the light scattering from nanoparticles (Sect. S8). Particle concentrations below the detection limits of nanoparticle tracking analysis or dynamic light scattering can produce errors over 100 % in calculated absorbance rates (Fig. S17). These findings indicate that calculated rates of light absorbance, and consequently, 1O2* quantum yields, may be sensitive to scattering artifacts an introduce an additional source of uncertainty.