Technical Note: Effect of varying the λ = 185 and 254 nm photon flux ratio on radical generation in oxidation flow reactors

Oxidation flow reactors (OFRs) complement environmental smog chambers as a portable, low-cost technique for exposing atmospheric compounds to oxidants such as ozone (O₃), nitrate (NO₃) radicals, and hydroxyl (OH) radicals. OH is most commonly generated in OFRs via photolysis of externally added O₃ at λ=254 nm (OFR254) or combined photolysis of O₂ and H₂O at λ=185 nm plus photolysis of O₃ at λ=254 nm (OFR185) using low-pressure mercury (Hg) lamps. Whereas OFR254 radical generation is influenced by [O₃], [H₂O], and photon flux at λ=254 nm (I₂₅₄), OFR185 radical generation is influenced by [O₂], [H₂O], I₁₈₅, and I₂₅₄. Because the ratio of photon fluxes, I₁₈₅:I₂₅₄, is OFR-specific, OFR185 performance varies between different systems even when constant [H₂O] and I₂₅₄ are maintained. Thus, calibrations and models developed for one OFR185 system may not be applicable to another. To investigate these issues, we conducted a series of experiments in which I₁₈₅:I₂₅₄ emitted by Hg lamps installed in an OFR was systematically varied by fusing multiple segments of lamp quartz together that either transmitted or blocked λ=185 nm radiation. Integrated OH exposure (OH_exp) values achieved for each lamp type were obtained using the tracer decay method as a function of UV intensity, humidity, residence time, and external OH reactivity (OHR_ext). Following previous related studies, a photochemical box model was used to develop a generalized OH_exp estimation equation as a function of [H₂O], [O₃], and OHR_ext that is applicable for $I_{\mathrm{185}}:I_{\mathrm{254}}\approx \mathrm{0.001}$ to 0.1.

1 Introduction

Hydroxyl (OH) radicals govern the concentrations of most atmospheric organic compounds, including those that lead to secondary organic aerosol (SOA) formation. For decades, environmental chambers and oxidation flow reactors (OFRs) have been used to simulate atmospheric aging processes through the controlled exposure of trace gases and aerosols to OH radicals. Environmental chamber studies are typically conducted over experimental timescales and equivalent atmospheric exposure times of hours up to 1 or 2 d. OFRs with residence times on the order of minutes achieve multiple days of equivalent atmospheric OH exposure (OH_exp), typically through the following reactions:

$$\begin{array}{}\text{(R1)}& {\displaystyle }& {\displaystyle }{\mathrm{O}}_{\mathrm{3}}+h{\mathit{\nu }}_{\mathrm{254}}\to {\mathrm{O}}_{\mathrm{2}}+\mathrm{O}{(}^{\mathrm{1}}\mathrm{D})\text{(R2)}& {\displaystyle }& {\displaystyle }\mathrm{O}{(}^{\mathrm{1}}\mathrm{D})+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{2}\mathrm{OH}\text{(R3)}& {\displaystyle }& {\displaystyle }\mathrm{O}{(}^{\mathrm{1}}\mathrm{D})+{\mathrm{N}}_{\mathrm{2}}\mathrm{O}\to \mathrm{2}\mathrm{NO}.\end{array}$$

This method is referred to as OFR254 and relies on addition of externally generated O₃ at the OFR inlet. In some cases, OFRs have additionally employed the secondary λ=185 nm emission line present in low-pressure mercury (Hg) lamps to generate radicals from the following reactions in addition to those listed above that are employed in OFR254:

$$\begin{array}{}\text{(R4)}& {\displaystyle }& {\displaystyle }{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+h{\mathit{\nu }}_{\mathrm{185}}\to \mathrm{H}+\mathrm{OH}\text{(R5)}& {\displaystyle }& {\displaystyle }\mathrm{H}+{\mathrm{O}}_{\mathrm{2}}\to {\mathrm{HO}}_{\mathrm{2}}\text{(R6)}& {\displaystyle }& {\displaystyle }{\mathrm{O}}_{\mathrm{2}}+h{\mathit{\nu }}_{\mathrm{185}}\to \mathrm{2}\mathrm{O}{(}^{\mathrm{3}}\mathrm{P})\text{(R7)}& {\displaystyle }& {\displaystyle }\mathrm{O}{(}^{\mathrm{3}}\mathrm{P})+{\mathrm{O}}_{\mathrm{2}}\to {\mathrm{O}}_{\mathrm{3}}\text{(R8)}& {\displaystyle }& {\displaystyle }{\mathrm{N}}_{\mathrm{2}}\mathrm{O}+h{\mathit{\nu }}_{\mathrm{185}}\to {\mathrm{N}}_{\mathrm{2}}+\mathrm{O}{(}^{\mathrm{1}}\mathrm{D}).\end{array}$$

This method is referred to as OFR185. Recent modeling studies suggest that OFR185 is less affected by experimental artifacts than OFR254 such as SOA photolysis and unwanted reactions with non-OH oxidants (Peng et al., 2016, 2018, 2019). Additionally, OFR185 is often more practical than OFR254 for application in field studies because O₂ and H₂O that are already present in ambient air are photolyzed to generate O₃, OH, and HO₂, whereas OFR254 requires addition of compressed air or O₂ for external O₃ generation and additional inlet plumbing to inject it at the OFR inlet. However, because the λ=185 nm photon flux (I₁₈₅) is influenced by OFR-specific design considerations that are mainly related to the Hg lamps being used, concentrations of O₃, HO${}_x=\mathrm{OH}+{\mathrm{HO}}_{\mathrm{2}}$, and ${\mathrm{NO}}_x=\mathrm{NO}+{\mathrm{NO}}_{\mathrm{2}}$ generated using OFR185 are potentially variable between different systems even when constant [H₂O], [N₂O], and I₂₅₄ are established. Thus, calibrations and models developed for one OFR185 system may not be applicable to another, making it more difficult to evaluate results or plan experiments. To investigate these issues, we designed a series of experiments in which I₁₈₅:I₂₅₄ was systematically varied over a wide range using multiple novel Hg lamp configurations. Integrated OH_exp values were obtained as a function of OFR185 conditions, and a photochemical box model was used to develop a system of OH_exp estimation equations that are applicable to OFR185 systems with $I_{\mathrm{185}}:I_{\mathrm{254}}\approx \mathrm{0.001}$ to 0.1.

2 Experimental

Experiments were conducted using an Aerodyne Potential Aerosol Mass (PAM) OFR, which is a horizontal aluminum cylindrical chamber (46 cm long × 22 cm ID) operated in continuous flow mode (Lambe et al., 2011). A simplified schematic is shown in Fig. S1 in the Supplement. The H₂O mixing ratio in the OFR was controlled by passing the carrier gas through a Nafion humidifier (Perma Pure LLC) or heated recirculating water bath (Neslab Instruments, Inc.) and then diluting with different levels of dry carrier gas at the OFR inlet. A photodetector (TOCON-C6, sglux GmbH) and a relative humidity and temperature (RH/T) sensor (SHT21, Sensiron) were mounted in the exit flange of the OFR. Across all experiments, [H₂O] ranged from 0.03 % (1 % RH at 25.3 ^∘C) to 3.9 % (88 % RH at 30.9 ^∘C). The O₃ mixing ratio at the exit of the OFR was measured with a UV ozone analyzer (106-M, 2B Technologies).

2.1 HO_x generation

HO_x was produced via Reactions (R1)–(R2) and (R4)–(R7). Photolysis of H₂O, O₂, and O₃ in the OFR was achieved using two low-pressure Hg fluorescent lamps (Light Sources, Inc.) that were isolated from the sample flow using type 214 quartz sleeves. Nitrogen purge gas was flowed over the lamps to prevent O₃ buildup between the lamps and sleeves. A fluorescent dimming ballast was used to regulate current applied to the lamps. The dimming voltage applied to the ballast ranged from 0.8 to 10 V direct current (DC). Below ∼0.8 V DC, the lamp output was unstable due to flickering, and 10 V DC was the maximum control voltage permitted by the ballast.

Figure 1 Low-pressure Hg fluorescent lamp types used in this study. Each lamp type contains 356 mm of quartz material that either transmits both λ=185 and 254 nm radiation (white, T₁₈₅=1), blocks λ=185 nm and transmits λ=254 nm radiation (grey, T₁₈₅=0), or blocks both λ=185 and 254 nm radiation (black, T₁₈₅=0).

Figure 1 shows the Hg fluorescent lamp configurations that were used in this study. Lamp type A is an ozone-producing low-pressure Hg germicidal fluorescent lamp (GPH436T5VH/4P, Light Sources Inc.) in which type 214 quartz that transmits λ=185 and 254 nm radiation is present along the entire 356 mm arc length. This lamp type is a standard component of the Aerodyne PAM OFR. The relative transmissivity of λ=185 nm radiation (T₁₈₅) in lamp type A is thus equal to 1. Lamp type B is equivalent to lamp type A with added segments of opaque heat shrink tubing applied to approximately 86 % of the arc length (T₁₈₅≈0.14; see also Fig. S2 in the Supplement) to reduce I₁₈₅ and I₂₅₄ to levels below what is achievable using the ballast dimming voltage. A different type of quartz is available (type 219) that blocks λ=185 nm and transmits λ=254 nm radiation (T₁₈₅=0). To cover the largest possible range of I₁₈₅:I₂₅₄, lamp types C, D, and E (GPH436T5L/VH/4P 90/10, GPH436T5L/VH/4P 96/4, and GPH436T5L/VH/4P 98.5/1.5; Light Sources, Inc.) fused one segment each of quartz with T₁₈₅=0 and T₁₈₅=1 to provide reduced I₁₈₅ relative to lamp type A while maintaining constant I₂₅₄. Finally, to evaluate the effect of lamp design at fixed T₁₈₅ and I₂₅₄, lamp types F and G contain the same ratios of T₁₈₅=0 and T₁₈₅=1 quartz as types C and D, but with 5 and 13 total segments instead of 2 segments. These different designs isolate the effect of discretized λ=185 nm irradiation across the entire arc length of the lamp vs. having all λ=185 nm radiation near the entrance of the OFR.

2.2 OH_exp characterization studies

OH_exp, defined here as the product of the average OH concentration and the mean OFR residence time (τ_OFR), was characterized by measuring the decay of carbon monoxide (CO) and/or sulfur dioxide (SO₂) tracers using Thermo 48i and 43i CO and SO₂ analyzers (e.g., Lambe et al., 2011). Tracer mixing ratios entering the reactor were 6–9 ppmv for CO and 288–629 ppbv for SO₂, each diluted from separate gas mixtures of 0.5 % CO or SO₂ in N₂ (Praxair). The corresponding total external OH reactivity (OHR_ext), which is the summed product of each tracer mixing ratio and its bimolecular OH rate coefficient, ranged from approximately 9 to 64 s⁻¹. Tracer concentrations were allowed to stabilize before initiating OH_exp measurements, during which steady-state levels of CO and/or SO₂ were obtained with the lamps turned off. Then, the lamps were turned on, and tracer concentrations were allowed to equilibrate before being measured at illuminated steady-state conditions.

In most experiments, the calculated mean residence time was τ_OFR=124 s, which was obtained from the ratio of the internal OFR volume (≈13 L) and the total sample and makeup flow rate through the OFR (6.4 L min⁻¹). This calculation implicitly assumes plug flow conditions, with associated uncertainty of approximately 10 % compared to an explicit residence time distribution measurement at a specific OFR condition (Li et al., 2015). Variability in OFR parameters (e.g., temperature, flow rate) may increase the uncertainty in this assumption across a continuum of conditions (Huang et al., 2017; Lambe et al., 2019). To characterize the uncertainty in our plug flow approximation across multiple sample flow conditions, we measured integrated OH exposure (OH_exp) values of 3.3×10¹¹, 7.8×10¹¹, and 2.0×10¹² molec. cm⁻³ s at sample flow rates of 12.5, 6.4, and 3.1 L min⁻¹, respectively, using the tracer decay method (Sect. 2.2) with the OFR operated at the same humidity and lamp intensity. Thus, perturbing the “plug flow” τ_OFR=124 s by a factor of 2 in either direction changed OH_exp by factors of 2.36 and 2.56. Based on these results, an upper-limit estimated uncertainty in τ_OFR and corresponding OH_exp is approximately 30 %.

2.3 Photochemical model

Table 1 OFR conditions input to photochemical model.

We used a photochemical model implemented in MATLAB and Igor Pro to calculate concentrations of radical/oxidant species produced in the reactor (Li et al., 2015). The KinSim chemical kinetic solver was used to compile the version of the model that was implemented in Igor Pro (Peng and Jimenez, 2019). Model input parameters are shown in Table 1, and reactions and associated kinetic rate coefficients that were included in the model are summarized in Table S1 in the Supplement (Peng and Jimenez, 2020a). For cases where [H₂O]≤0.1 % and the RH sensor accuracy became a limiting factor, we systematically adjusted the [H₂O] value that was input to the model to a value between 0.01 % and 0.1 % to achieve better agreement between measured and modeled OH_exp. I₂₅₄ and I₁₈₅ values input to the model were adjusted to match the measured OH_exp values as best as possible within the following constraints:

1. $I_{\mathrm{254},\text{max}}=(\mathrm{3.5}\pm \mathrm{0.7})\times {\mathrm{10}}^{\mathrm{15}}$ $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$ for two lamps operated at maximum output (Lambe et al., 2019).

2. At reduced lamp output, I₂₅₄ was calculated by multiplying I_254,max by the ratio of photodetector-measured irradiance values measured at maximum and reduced lamp output at λ=254 nm.

3. $I_{\mathrm{185},\text{max}}:I_{\mathrm{254},\text{max}}$ ⪅ 0.10 for lamp types A and B only (Spicer, 2013).

4. $I_{\mathrm{185},\text{max}}:I_{\mathrm{254},\text{max}}$ ⪅ 0.01, 0.004, and 0.0015 for lamp types C and F, D and G, and E, respectively.

5. At reduced lamp output, I₁₈₅ was calculated by multiplying I_185,max by the ratio of O₃ mixing ratios measured at maximum and reduced lamp output.

Within these constraints, the mean (±1σ) ratios of modeled to measured CO and SO₂ concentrations remaining at the exit of the OFR were 1.02±0.06 and $\mathrm{0.97}\pm \mathrm{0.17},$ respectively.

Figure 2 O₃ mixing ratio generated using OFR185 at $I_{\mathrm{254}}=(\mathrm{3.5}\pm \mathrm{0.7})\times {\mathrm{10}}^{\mathrm{15}}$ $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$ (lamp types A and C–G) and $I_{\mathrm{254}}=\mathrm{5.8}\times {\mathrm{10}}^{\mathrm{14}}$ $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$ (lamp type B) as a function of T₁₈₅ and [H₂O]. Error bars represent ±1σ of replicate O₃ measurements and ±2 mm uncertainty in lengths of individual T₁₈₅=0 and 1 segments.

3 Results and discussion

3.1 Influence of I₁₈₅ on [O₃] and OH_exp

Figure 2 shows [O₃] measured at the exit of the OFR as a function of T₁₈₅ with each lamp type operated at maximum UV output. Binned data are shown for conditions where $\left[{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\right]=\mathrm{0.15}\mathrm{\%}\pm \mathrm{0.11}$ %, 0.98 %±0.08 %, 1.74 %±0.23 %, and 3.42 %±0.30 %. At fixed [H₂O], [O₃] increased as a function of T₁₈₅. For example, [O₃] increased from 17.8 to 155 ppmv at [H₂O]=0.15 % and from 4.5 to 56 ppmv at [H₂O]=1.74 % as T₁₈₅ increased from 0.1 to 1. At fixed T₁₈₅ and I₂₅₄, [O₃] decreased with increasing [H₂O] due to faster O(¹D)+H₂O reaction rate following O₃ photolysis at λ=254 nm. Consequently, as [H₂O] increased from 0.15 % to 3.42 %, [O₃] decreased by a factor of 4–5 for lamp types A and C–G, whereas [O₃] decreased by a factor of 2 for lamp type B because of its reduced I₂₅₄ (Fig. 1). At [H₂O]=1.74 % and T₁₈₅=0.04 and 0.1, Fig. 2 shows that [O₃] generated using lamp types D and G was approximately 1.7 and 1.8 ppmv; here, lamp type D had one 15 mm quartz segment with T₁₈₅=1, whereas lamp type G had three 5 mm quartz segments with T₁₈₅=1. At the same OFR conditions, [O₃] generated using lamp types C and F was 4.5 and 2.7 ppmv; these lamps had one 35 mm and seven 5 mm quartz segments with T₁₈₅=1. Despite the discrepancy in measured [O₃], corresponding OH_exp obtained with lamp types C and F were 2.5×10¹² and 2.8×10¹² $\mathrm{molec}.{\mathrm{cm}}^{-\mathrm{3}}\mathrm{s}$, respectively. Thus, the worse agreement in [O₃] measured between lamp types C and F may be associated specifically with O₃ measurements from these experiments. We hypothesize that the OFR-volume-averaged I₁₈₅ is sufficient to describe associated HO_x production for these cases.

Figure 3 OH_exp generated using OFR185 ($\left[{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\right]=\mathrm{1.90}\mathrm{\%}\pm \mathrm{0.26}$ %) at minimum and maximum I₂₅₄ for each T₁₈₅ value. Corresponding photochemical age is shown on the right y axis assuming mean $\left[\mathrm{OH}\right]=\mathrm{1.5}\times {\mathrm{10}}^{\mathrm{6}}$ molec cm⁻³ (Mao et al., 2009). Error bars assume ±30 % uncertainty in OH_exp and ±2 mm uncertainty in lengths of individual T₁₈₅=0 and 1 segments.

Figure 3 plots OH_exp as a function of T₁₈₅ at $\left[{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\right]=\mathrm{1.90}\mathrm{\%}\pm \mathrm{0.26}$ %. The corresponding equivalent photochemical age shown on the right y axis assumes a 24 h average OH concentration of 1.5×10⁶ molec cm⁻³ (Mao et al., 2009). Results obtained with lamp types D and G, and C and F were averaged together at T₁₈₅=0.04 and 0.1, respectively, due to their similar OH_exp values. Over the range of T₁₈₅ shown in Fig. 3, excluding lamp type B, OH_exp increased by approximately a factor of 5 at $I_{\mathrm{254}}=(\mathrm{3.7}\pm \mathrm{0.6})\times {\mathrm{10}}^{\mathrm{15}}$ $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$) and a factor of 17 at $I_{\mathrm{254}}=(\mathrm{2.1}\pm \mathrm{0.3})\times {\mathrm{10}}^{\mathrm{14}}$ $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$. Maximum OH_exp also decreased by about a factor of 5 between lamp types A and B due to reduction in both I₂₅₄ and I₁₈₅ (not shown in Fig. 3). Similar trends were observed for OH_exp measurements at $\left[{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\right]=\mathrm{0.93}\mathrm{\%}\pm \mathrm{0.06}$ % and 3.42 %±0.30 %, but at $\left[{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\right]=\mathrm{0.09}\mathrm{\%}\pm \mathrm{0.07}$ %, the sensitivity of OH_exp to T₁₈₅ was weaker due to suppressed OH production at lower humidity.

3.2 I₁₈₅:I₂₅₄ determination and derivation of OH_exp estimation equations

Figure 4 plots I₁₈₅ as a function of I₂₅₄ for the Hg lamps used in this study and a different model of Hg lamps used in an earlier-generation PAM OFR (Li et al., 2015). As with OH_exp values shown in Fig. 3, I₁₈₅ and I₂₅₄ values obtained with lamp types D and G, and C and F were combined together into T₁₈₅=0.04 and 0.1 symbols following our hypothesis that the OFR-volume-averaged I₁₈₅ was sufficient to describe HO_x production. Linear fits applied to the data shown in Fig. 4 were used to calculate average I₁₈₅:I₂₅₄ values for lamp types A and B, C and F, D and G, and E. Lamp types A and B (red symbols) had the highest $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.0664}$, whereas lamp type E had the lowest $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.00167}$. $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.00561}$ for lamp types C and F (blue symbols) fell within the envelope of $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.004}$ to 0.012 characterized by Li et al. (2015), with a lower apparent sensitivity of I₁₈₅:I₂₅₄ to lamp power. This is presumably due to differences in the specific Hg lamp types, potential variability in lamp output within the same lamp type, and/or the method of dimming used in the two studies.

Figure 4 Calculated I₁₈₅ and I₂₅₄ values for the lamp types shown in Fig. 1. I₁₈₅:I₂₅₄ values were calculated from linear regression functions and used to derive OH_ex estimation equations. I₁₈₅ and I₂₅₄ values obtained by Li et al. (2015) in an earlier-generation PAM OFR are shown for reference.

Previous studies reported empirical OH exposure algebraic estimation equations for use with OFRs (Li et al., 2015; Peng et al., 2015, 2018; Lambe et al., 2019). These equations parameterize OH_exp as a function of readily measured experimental parameters, therefore providing a simpler alternative than detailed photochemical models for experimental planning and analysis. Here, we expand on those studies by deriving OH_exp estimation equations for the lamp types that were used in this study. We adapted the estimation equation format introduced by Li et al. (2015):

$$\begin{array}{}\text{(1)}& \begin{array}{rl}\log \left[{\mathrm{OH}}_{\text{exp}}\right]& =(a+(b+c\times {\mathrm{OHR}}_{\text{ext}}^d+e\times \log \left[{\mathrm{O}}_{\mathrm{3}}\right]\\ & \times {\mathrm{OHR}}_{\text{ext}}^f)\times \log [{\mathrm{O}}_{\mathrm{3}}]+\log [{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\left]\right)\\ & +\log \left({\displaystyle \frac{\mathit{\tau }}{\mathrm{124}}}\right).\end{array}\end{array}$$

This equation incorporates the following relationships between OH_exp and O₃, H₂O, τ, and OHR_ext identified by Li et al. (2015): (1) a power-law dependence of OH_exp on UV intensity and, accordingly, [O₃]; (2) a linear dependence of OH_exp on [H₂O] and τ; (3) OH suppression as a function of increasing OHR_ext. The fit coefficients a–f are lamp-specific.

Equation (1) was fit to data obtained from the base case of the model, with CO reacting with OH as a surrogate of OHR_ext, over the following OFR185 phase space: T=25 ^∘C, τ=124 s, OHR_ext=0.77 to 232 s⁻¹, [H₂O]=0.1 % to 3 %, I₂₅₄=10¹³ to 10¹⁶ $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$, and $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.00167}$, 0.00242, 0.00595, and 0.0664. For each I₁₈₅:I₂₅₄ value, we explored 10, 15, and 20 logarithmically evenly distributed values in the ranges of OHR_ext, [H₂O], and I₂₅₄, respectively. Figure 5 compares OH_exp estimated from Eq. (1) and calculated from the model for the $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.0664}$ case. Almost all of the equation-estimated and model OH_exp values agreed within a factor or 2 or better. The absolute value of the relative deviations increased above [H₂O]≈0.5 % and was largest at [H₂O]=3 %; the mean absolute value of the relative deviations was 28 %. Analogous plots for $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.00167}$, 0.00242, and 0.00595 cases are shown in Fig. S3 in the Supplement. For these other cases, the mean absolute values of the relative deviations were 20 %, 17 %, and 16 %, respectively. Equation (1) coefficients for lamps with the I₁₈₅:I₂₅₄ values reported here are presented in Table 2.

Figure 5 OH_exp calculated from the estimation equation (Eq. 1) as a function of OH_exp calculated from the full OFR185 KinSim mechanism (Table S1) for lamp types A and B. Solid and dashed lines correspond to the 1:1 and the 1:2 and 2:1 lines, respectively. Estimation equation fit coefficients are shown in Table 2.

Table 2 OH_exp estimation equation coefficients (±1σ) as defined in Eq. (1).

Figure 6 OH_exp estimation equation fit coefficients plotted as a function I₂₅₄:I₂₅₄. Trend lines were calculated from exponential regression functions with fit parameters that are presented in Table 3.

Table 3 Parameterization of Eq. (1) coefficients (±1σ): $y\mathrm{0}+A\times \exp [I_{\mathrm{185}}:I_{\mathrm{254}})\times \text{invTau}]$.

To generalize the results shown in Figs. 5 and S3 to OFR185 systems with other I₁₈₅:I₂₅₄ values, Fig. 6 plots fit coefficients a–f as a function of I₁₈₅:I₂₅₄. Each of these coefficients changes monotonically as a function of I₁₈₅:I₂₅₄, enabling the usage of simple exponential regression functions to parameterize the a–f values as a continuous function of I₁₈₅:I₂₅₄. Exponential function coefficients for the regression curves shown in Fig. 6 are presented in Table 3. Figure 7 compares the equation-estimated OH_exp (obtained using Eq. 1 with Table 3 fit coefficients) and the measured OH_exp obtained using the tracer decay method. The mean (±1σ) ratios of equation-estimated and measured OH_exp values were 0.94±0.55, $\mathrm{1.13}\pm \mathrm{0.48},\mathrm{1.03}\pm \mathrm{0.37},$ and 1.32±0.71 for $I_{\mathrm{185}}:I_{\mathrm{254}}=\mathrm{0.00167}$, 0.00242, 0.00595, and 0.0664.

Figure 7 OH_exp calculated from estimation equation (Eq. 1 and Table 2) as a function of OH_exp calculated from tracer decay method for Hg lamp types with I₂₅₄:I₂₅₄ values specified in the legend.

3.3 Influence of I₁₈₅ on HO₂, NO_x, and UV photolysis of aromatic volatile organic compounds

[HO₂] and [NO_x] (with N₂O present) increase along with [OH] as a function of I₁₈₅ (Fig. S4 in the Supplement). To isolate the effect of I₁₈₅ on related OFR photochemistry at fixed OH_exp, we investigated two OFR185 cases using ($I_{\mathrm{185}},I_{\mathrm{254}})=(\mathrm{3.33}\times {\mathrm{10}}^{\mathrm{12}}$, 1.96×10¹⁵) and (6.65×10¹², 1.01×10¹⁴) $\mathrm{photons}{\mathrm{cm}}^{-\mathrm{2}}{\mathrm{s}}^{-\mathrm{1}}$ that each generate a model-calculated OH${}_{\text{exp}}=\mathrm{5.0}\times {\mathrm{10}}^{\mathrm{11}}$ $\mathrm{molec}.{\mathrm{cm}}^{-\mathrm{3}}\mathrm{s}$ at base-case conditions of [H₂O]=2 %, τ=124 s, and OHR_ext=30 s⁻¹. These cases were designated as “low” and “high” I₁₈₅:I₂₅₄ cases. Thus, increasing I₁₈₅ by a factor of 2 enabled lowering I₂₅₄ by a factor of 20 to achieve equivalent OH_exp.

First, we investigated the resilience of each OFR185 case to OH suppression via OHR_ext. As OHR_ext was increased from 30 to 300 s⁻¹, OH_exp decreased from 5.0×10¹¹ to 7.9×10¹⁰ (low I₁₈₅:I₂₅₄) and 9.0×10¹⁰ (high I₁₈₅:I₂₅₄) $\mathrm{molec}.{\mathrm{cm}}^{-\mathrm{3}}{\mathrm{s}}^{\mathrm{1}}$. Thus, increasing I₁₈₅ decreased OH suppression by 15 %, primarily due to 30 % higher [HO₂] in the high I₁₈₅:I₂₅₄ case that increased the OH+HO₂ reaction rate and partially buffered the system against increasing OHR_ext. Second, we compared the ability of each OFR185 case to generate high-NO conditions in the presence of added [N₂O]. For example, at [N₂O]=2.7 %, $\mathrm{NO}:{\mathrm{HO}}_{\mathrm{2}}=\mathrm{1}$ and 0.4 at low and high I₁₈₅:I₂₅₄. While increasing [N₂O] from 2.7 % to 4.0 % achieved $\mathrm{NO}:{\mathrm{HO}}_{\mathrm{2}}=\mathrm{1}$ at high I₁₈₅:I₂₅₄, [NO₂] also increased from 50 to 100 ppbv. At higher UV intensity, a similar increase in [N₂O] could generate [NO₂]>1 ppm and promote artificially fast RO₂+NO₂ reactions compared to atmospheric conditions (Peng and Jimenez, 2017). Third, we compared relative timescales for OH oxidation and photolysis of representative aromatic volatile organic compounds (VOCs) that absorb λ=185 and 254 nm radiation. The fractional VOC loss due to photolysis, F_photolysis, was calculated using Eq. (2):

$$\begin{array}{}\text{(2)}& {\displaystyle }{F}_{\text{photolysis}}={\displaystyle \frac{{\mathit{\sigma }}_{\mathrm{185}}{I}_{\mathrm{185}}{\mathit{\varphi }}_{\mathrm{185}}+{\mathit{\sigma }}_{\mathrm{254}}{I}_{\mathrm{254}}{\mathit{\varphi }}_{\mathrm{254}}}{{\mathit{\sigma }}_{\mathrm{185}}{I}_{\mathrm{185}}{\mathit{\varphi }}_{\mathrm{185}}+{\mathit{\sigma }}_{\mathrm{254}}{I}_{\mathrm{254}}{\mathit{\varphi }}_{\mathrm{254}}+k_{\text{OH}}\left[\mathrm{OH}\right]}},\end{array}$$

where σ₁₈₅ and σ₂₅₄ are the VOC absorption cross sections at λ=185 and 254 nm, ϕ₁₈₅ and ϕ₂₅₄ are the VOC photolysis quantum yields, and k_OH is the bimolecular reaction rate coefficient with OH. Assuming upper limit ϕ₁₈₅=1 and ϕ₂₅₄=1 values, ${\mathit{\sigma }}_{\mathrm{185}}=\mathrm{2.8}\times {\mathrm{10}}^{-\mathrm{17}}$ cm², ${\mathit{\sigma }}_{\mathrm{254}}=\mathrm{8.9}\times {\mathrm{10}}^{-\mathrm{19}}$ cm² (Dawes et al., 2017), and $k_{\text{OH}}=\mathrm{1.28}\times {\mathrm{10}}^{-\mathrm{12}}$ ${\mathrm{cm}}^{\mathrm{3}}{\mathrm{molec}}^{-\mathrm{1}}{\mathrm{s}}^{-\mathrm{1}}$ (Atkinson, 1986), F_{photolysis, benzene}≤0.26 and 0.07 at low and high I₁₈₅:I₂₅₄. Similarly, F_{photolysis, toluene}≤0.07 and 0.04 at low and high I₁₈₅:I₂₅₄ (Atkinson, 1986; Serralheiro et al., 2015).

4 Conclusions

OFR185 is emerging as one of the most commonly used OFR methods by enabling efficient HO_x and NO_x generation over a range of oxidative aging timescales that are relevant to atmospheric processes. Important OFR185 parameters are I₁₈₅, I₂₅₄, [H₂O], [N₂O] (if NO_x generation is required), OHR_ext, and τ_OFR. However, I₁₈₅:I₂₅₄ is specific to the Hg lamp and/or OFR, as are associated calibration and estimation equations. To develop a general framework within which to evaluate and compare different OFR185 systems, we characterized OH_exp as a function of I₁₈₅, I₂₅₄, OHR_ext, and [H₂O] values, in the process using several novel low-pressure Hg lamp configurations to extend the range of achievable I₁₈₅:I₂₅₄. OH_exp estimation equations were developed for the Hg lamp types that were used, and corresponding estimation equation fit coefficients were parameterized as a function of I₁₈₅:I₂₅₄ to enable interpolation to other OFR185 systems that can employ the same Hg lamp type(s) over the range of [O₃], [H₂O], OHR_ext, and τ values parameterized here. Because low-pressure Hg germicidal fluorescent lamps are used in many industries (e.g., medical, HVAC, wastewater remediation), they are less expensive and more easily acquired than other Hg lamps. OHR_int, HO₂:OH, and F_photolysis were improved at higher I₁₈₅:I₂₅₄, whereas NO:HO₂ and NO:NO₂ were improved at lower I₁₈₅:I₂₅₄. Overall, our results suggest that optimal OFR185 performance is achieved by (1) maximizing I₁₈₅:I₂₅₄, (2) reducing OH_exp (if needed) through simultaneous reduction in I₁₈₅ and I₂₅₄ via electronically or mechanically dimming the lamp output (Fig. S2), and (3) increasing [N₂O] to offset higher [HO₂] if high-NO conditions are required, provided that [NO₂] does not exceed ≈1 ppm, in which case lower I₁₈₅:I₂₅₄ should be used. Future work will investigate the sensitivity of NO_x-dependent, OH-initiated oxygenated volatile organic compound and SOA formation processes to I₁₈₅:I₂₅₄.