HO_x and NO_x production in oxidation flow reactors via photolysis of isopropyl nitrite, isopropyl nitrite-d₇, and 1,3-propyl dinitrite at λ = 254, 350, and 369 nm

Oxidation flow reactors (OFRs) are an emerging technique for studying the formation and oxidative aging of organic aerosols and other applications. In these flow reactors, hydroxyl radicals (OH), hydroperoxyl radicals (HO₂), and nitric oxide (NO) are typically produced in the following ways: photolysis of ozone (O₃) at λ=254 nm, photolysis of H₂O at λ=185 nm, and via reactions of O(¹D) with H₂O and nitrous oxide (N₂O); O(¹D) is formed via photolysis of O₃ at λ=254 nm and/or N₂O at λ=185 nm. Here, we adapt a complementary method that uses alkyl nitrite photolysis as a source of OH via its production of HO₂ and NO followed by the reaction NO + HO₂ → NO₂ + OH. We present experimental and model characterization of the OH exposure and NO_x levels generated via photolysis of C₃ alkyl nitrites (isopropyl nitrite, perdeuterated isopropyl nitrite, 1,3-propyl dinitrite) in the Potential Aerosol Mass (PAM) OFR as a function of photolysis wavelength (λ=254 to 369 nm) and organic nitrite concentration (0.5 to 20 ppm). We also apply this technique in conjunction with chemical ionization mass spectrometer measurements of multifunctional oxidation products generated following the exposure of α-Pinene to HO_x and NO_x obtained using both isopropyl nitrite and O₃ + H₂O + N₂O as the radical precursors.

1 Introduction

Hydroxyl (OH) radicals govern the concentrations of most atmospheric organic compounds, including those that lead to secondary organic aerosol (SOA) formation. The relative importance of different primary OH precursors varies in different parts of the atmosphere and may include contributions from O(¹D)–H₂O reactions, hydrogen peroxide (H₂O₂), methyl peroxide (CH₃OOH), nitrous acid (HONO) photolysis, and ozone–alkene reactions. Additionally, ozone-hydroperoxy (HO₂) reactions and NO–HO₂ reactions recycle HO₂ back to OH (Lee et al., 2016; Mao et al., 2009). For decades, a handful of radical precursors have been used to generate OH radicals in the laboratory to initiate SOA production under controlled conditions. Environmental chambers most commonly photolyze nitrous acid (HONO), methyl nitrite (CH₃ONO), or hydrogen peroxide (H₂O₂) at λ>310 nm to mimic SOA production, over experimental timescales of hours to days, simulating up to 2 days of equivalent atmospheric exposure (Atkinson et al., 1981; Chhabra et al., 2011; Finewax et al., 2018; Matsunaga and Ziemann, 2010).

Oxidation flow reactors (OFRs) photolyze H₂O and O₃ at λ=185 and 254 nm over experimental timescales of minutes, simulating multiple days of equivalent atmospheric exposure (Lambe et al., 2012; Peng et al., 2015). Recent application of $\mathrm{O}{(}^{\mathrm{1}}\mathrm{D})+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+{\mathrm{N}}_{\mathrm{2}}\mathrm{O}$ reactions to study NO_x-dependent SOA formation pathways facilitated characterization of oxidation products generated over a range of low- to high-NO_x conditions (Lambe et al., 2017; Peng et al., 2018). Potential limitations of the method include (1) the inability to unambiguously deconvolve contributions from multiple oxidants (O₃, OH, NO₃), which may compete with each other under certain conditions and for specific unsaturated precursors; (2) required use of 254 nm photolysis, which may enhance photolytic losses that compete with OH oxidation, especially for species that are characterized by strong absorption/quantum yield at 254 nm and low-OH reactivity (Peng et al., 2016); (3) optimal high-NO_x application at OH exposures corresponding to multiple equivalent days of oxidative aging rather than 1 day or less.

Here, we adapt a complementary method that uses alkyl nitrite photolysis to generate an alkoxy radical (RO⋅) and NO. In the presence of air, RO⋅ reacts with O₂ to generate a carbonyl product (R'O) and a hydroperoxyl (HO₂) radical, and NO and HO₂ subsequently react to generate OH and NO₂. Using this method, O₃ is not required to generate OH radicals, and insignificant amounts of O₃ or NO₃ are generated as byproducts. We present experimental and model characterization of OH and NO_x levels that are generated as a function of photolysis wavelength, and organic nitrite concentration and composition. We furthermore carried out chemical ionization mass spectrometer measurements to compare nitrogen-containing photooxidation products obtained from the reaction of α-Pinene with radicals generated via alkyl nitrite photolysis or the $\mathrm{O}{(}^{\mathrm{1}}\mathrm{D})+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+{\mathrm{N}}_{\mathrm{2}}\mathrm{O}$ reaction.

2 Experiment

2.1 Alkyl nitrite preparation

Figure 1 shows molecular structures of the alkyl nitrites that were used. Isopropyl nitrite (iPrONO; Pfaltz and Bauer, >95 % purity) was used without additional purification. Perdeuterated isopropyl nitrite (iPrONO-d₇) and 1,3-propyl dinitrite [1,3-Pr(ONO)₂] were synthesized from the action of HONO on isopropanol-d₈ or 1,3-propanediol, respectively, as described elsewhere (Carrasquillo et al., 2014; Noyes, 1933). Briefly, sodium nitrite (>99.999 %, Sigma-Aldrich) and alcohol were combined in a 1.1:1.0 molar ratio and stirred with a magnetic stirrer inside a round-bottom flask. Sulfuric acid was added dropwise to the flask – thereby generating HONO upon reaction with sodium nitrite – until a 0.5:1.0 acid : alcohol molar ratio was achieved. The resulting clear yellow liquid was dried over sodium sulfate, neutralized with excess sodium bicarbonate and then stored in amber vials and refrigerated at 4 ^∘C until use (within 1 week of synthesis in this work). Under these storage conditions, the nominal shelf life of iPrONO and similar organic nitrites is approximately 2 years (Robert Milburn, personal communication, 29 October 2018).

Figure 1 Molecular structures of isopropyl nitrite, isopropyl nitrite-d₇ (d = deuterium = ²H), and 1,3-propyl dinitrite.

A syringe pump was used to introduce iPrONO, iPrONO-d₇, and 1,3-Pr(ONO)₂ through a 10.2 cm length of 0.0152 cm ID teflon tubing at liquid flow rates ranging from 0.016 to 0.63 µL min⁻¹. The liquid organic nitrite was evaporated into a 1 L min⁻¹ N₂ carrier gas at the end of the tubing. The flow containing organic nitrite vapor was then mixed with a 7 L min⁻¹ synthetic air carrier gas at the reactor inlet.

The organic nitrite mixing ratio entering the reactor, r_RONO, was equal to $\frac{Q_{\mathrm{RONO},\mathrm{g}}}{Q_{\mathrm{carrier}}}$, where Q_RONO,g was the volumetric flow rate of organic nitrite vapor (L min⁻¹) and Q_carrier was the volumetric flow rate of carrier gas (L min⁻¹). Q_RONO,g was calculated using the ideal gas law as applied by Liu et al. (2015):

$$\begin{array}{}\text{(1)}& Q_{\mathrm{RONO},\mathrm{g}}=Q_{\mathrm{RONO},\mathrm{l}}\times {\displaystyle \frac{\mathit{\rho }}{\mathrm{MW}}}\times {\displaystyle \frac{RT}{P}}\times \mathrm{0.01},\end{array}$$

where Q_RONO,l (µL min⁻¹) is the volumetric flow of organic nitrite liquid, ρ (g cm⁻³) and MW (g mol⁻¹) are the organic nitrite liquid density and molecular weight, R (8.314 J mol⁻¹ K⁻¹) is the universal gas constant, T (K) is temperature, P (hPa) is pressure, and 0.01 is a lumped pressure, volume and density unit conversion factor.

2.2 Alkyl nitrite photolysis

Alkyl nitrites were photolyzed inside a Potential Aerosol Mass (PAM) oxidation flow reactor (Aerodyne Research, Inc.), which is a horizontal 13.3 L aluminum cylindrical chamber (46 cm long ×22 cm ID) operated in continuous flow mode (Lambe et al., 2017), with 5.1±0.3 L min⁻¹ flow through the reactor unless stated otherwise. The relative humidity (RH) in the reactor was controlled in the range of 31 %–63 % at 21–32 ^∘C using a Nafion humidifier (Perma Pure LLC), with corresponding H₂O volumetric mixing ratios of approximately 1.5 %–1.7 %. Four UV lamps centered at λ=254 nm (GPH436T5L; Light Sources, Inc.), 350 nm (F436T5/BL/4P-350; Aerodyne Research, Inc.), or 369 nm (F436T5/BLC/4P-369; Aerodyne Research, Inc.) were used. Emission spectra obtained from the primary manufacturer (Light Sources, Inc. or LCD Lighting, Inc.) are shown in Fig. S1 in the Supplement. A fluorescent dimming ballast (IZT-2S28-D, Advance Transformer Co.) was used to regulate current applied to the lamps. The UV irradiance was measured using a photodetector (TOCON-GaP6, sglux GmbH) and was varied by changing the control voltage applied to the ballast between 1.6 and 10 VDC.

NO and NO₂ mixing ratios were measured using a NO_x analyzer (Model 405 nm, 2B Technologies), which quantified [NO₂] (ppb) from the measured absorbance at λ=405 nm, and [NO] (ppb) by reaction with O₃ to convert to NO₂. Alkyl nitrites introduced to the reactor with the lamps turned off consistently generated signals in both the NO and NO₂ measurement channels of the NO_x analyzer, possibly due to impurities and/or species generated via iPrONO+O₃ reactions inside the analyzer. For example, background NO and NO₂ mixing ratios increased from 0 to 1526 and 0 to 1389 ppb as a function of injected [iPrONO] =0 to 18.7 ppm with the lamps off (Fig. S2). We attempted to correct [NO] and [NO₂] for this apparent alkyl nitrite interference by subtracting background signals measured in the presence of alkyl nitrite with lamps off, to no avail, because background signals (alkyl nitrite present with lamps off) were large compared to signals obtained with alkyl nitrite present with lamps on. Instead, we constrained [NO] and [NO₂] using the photochemical model discussed in Sect. 2.4.

2.2.1 Actinic flux calibration

To quantify the actinic flux I in the reactor for each lamp type, we measured the rate of NO₂ photolysis as a function of UV irradiance (Fig. S4). Measurements were conducted in the absence of oxygen to avoid O₃ formation. The first-order NO₂ photolysis rate ($j_{{\mathrm{NO}}_{\mathrm{2}}}$) was calculated using Eq. (2):

$$\begin{array}{}\text{(2)}& j_{{\mathrm{NO}}_{\mathrm{2}}}=\ln \left({\displaystyle \frac{{\mathrm{NO}}_{\mathrm{2},\mathit{\tau }}}{{\mathrm{NO}}_{\mathrm{2},\mathrm{0}}}}\right){\displaystyle \frac{\mathrm{1}}{{\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{2}}}}},\end{array}$$

where NO_2,0 and NO_2,τ were the steady-state NO₂ mixing ratios measured at the exit of the reactor with the lamps turned off and on, respectively. The mean NO₂ residence time in the reactor, ${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{2}}}$, was characterized using 10 s pulsed inputs of NO₂. To mimic the effect of axial dispersion induced by temperature gradients from the lamps being turned on (Huang et al., 2017; Lambe et al., 2011), residence time distributions were measured in the presence of four lamps centered at λ=658 nm (F436T5/4P-658; Aerodyne Research, Inc.), where the NO₂ quantum yield is zero (Gardner et al., 1987). NO₂ residence time distributions are shown in Fig. S3, where ${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{2}}}$ ranged from 120±34 s (±1σ; lamps off) to 98±63 s (±1σ; lamps on) in a manner that is consistent with previous observations (Huang et al., 2017; Lambe et al., 2011). Assuming ${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{2}}}=\mathrm{98}$ s, maximum $j_{{\mathrm{NO}}_{\mathrm{2}}}$ values were 0.12, 0.36, and 0.50 min⁻¹ following photolysis at full lamp power at λ=254, 350, and 369 nm, respectively.

Table 1 Absorption cross section (σ_A,λ; cm²) or A + B bimolecular rate constant (k_A+B, cm³ molec⁻¹ s⁻¹) reference values.

¹ This work. ² Raff and Finlayson-Pitts (2010). ³ Atkinson et al. (2004). ⁴ Raff and Finlayson-Pitts (2010). ⁵ Estimated from k_iPrONO+OH scaled by relative rate constants of n-C₃H₈ + OH and n-C₃D₈ + OH (Nielsen et al., 1988, 1991). ⁶ Raff et al. (2005). ⁷ Burkholder et al. (2015). ⁸ Orlando and Tyndall (2012).

Corresponding I₂₅₄, I₃₅₀, and I₃₆₉ values were calculated using a photochemical model implemented in the KinSim chemical kinetics solver (Peng et al., 2015; implemented within Igor Pro 7, WaveMetrics Inc.) that incorporated the following reactions:

$$\begin{array}{}\text{(R1)}& {\displaystyle }& {\displaystyle }{\mathrm{NO}}_{\mathrm{2}}+h\mathit{\nu }\to \mathrm{NO}+\mathrm{O}{(}^{\mathrm{3}}\mathrm{P})\text{(R2)}& {\displaystyle }& {\displaystyle }\mathrm{O}{(}^{\mathrm{3}}\mathrm{P})+\mathrm{NO}\to {\mathrm{NO}}_{\mathrm{2}}\text{(R3)}& {\displaystyle }& {\displaystyle }{\mathrm{O}{(}^{\mathrm{3}}\mathrm{P})+\mathrm{NO}}_{\mathrm{2}}\to {\mathrm{NO}}_{\mathrm{3}}\text{(R4)}& {\displaystyle }& {\displaystyle }{\mathrm{O}{(}^{\mathrm{3}}\mathrm{P})+\mathrm{NO}}_{\mathrm{2}}\to \mathrm{NO}+{\mathrm{O}}_{\mathrm{2}}.\end{array}$$

NO₂ absorption cross sections were averaged across the 254, 350, and 369 nm lamp emission spectra, respectively (Table 1) (Atkinson et al., 2004) and input to the model. Maximum $I_{\mathrm{254}}=\mathrm{8.6}\times {\mathrm{10}}^{\mathrm{16}}$, $I_{\mathrm{350}}=\mathrm{6.3}\times {\mathrm{10}}^{\mathrm{15}}$, and $I_{\mathrm{369}}=\mathrm{6.5}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹ were obtained. While I₃₅₀ and I₃₆₉ values were in agreement with values calculated from lamp manufacturer specifications ($I_{\mathrm{350}}=\mathrm{5.8}\times {\mathrm{10}}^{\mathrm{15}}$ and $I_{\mathrm{369}}=\mathrm{6.2}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹) within uncertainties, I₂₅₄ obtained from our calibration was ∼13 times larger than expected. We hypothesize that this discrepancy was due to the presence of additional minor mercury lines (e.g., λ∼313, 365, 405) that induce NO₂ photolysis and that were not fully accounted for using Eq. (2) or the manufacturer spectra (Fig. S1). Thus, we instead assume maximum $I_{\mathrm{254}}=\mathrm{6.5}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹ based on manufacturer specifications.

2.2.2 OH exposure calibration

The OH exposure (OH_exp) obtained from alkyl nitrite photolysis, that is, the product of the OH concentration and mean residence time, was calculated from the addition of between 280 and 420 ppb SO₂ at the reactor inlet. Over the course of these experiments, NO_x generated from alkyl nitrite photolysis significantly interfered with the SO₂ mixing ratio measured with an SO₂ analyzer (Model 43i, Thermo Scientific); a representative example is shown in Fig. S5. To circumvent this issue, we measured the initial SO₂ mixing ratio, [SO_2,0], prior to alkyl nitrite photolysis, then used an Aerosol Chemical Speciation Monitor (ACSM; Aerodyne Research, Inc.) to measure the concentration of particulate sulfate generated from SO₂+OH reactions.

To relate the measured [SO_2,0] and sulfate to OH_exp, we conducted an offline calibration where 493 ppb SO₂ was added to the reactor and OH was generated via O₃+ hν₂₅₄ → O(¹D)+O₂ followed by O(¹D)+H₂O → 2OH in the absence of NO_x (OFR254 mode). The reactor was operated at the same residence time and humidity that was used in alkyl nitrite experiments, although we note that humidity will not change the response of the ACSM to sulfuric acid aerosols. Because no particulate ammonia was present aside from trace background levels, we assumed an ACSM collection efficiency of unity for the sulfate particles. SO₂ decay and particulate sulfate formation were measured across a range of UV irradiance and [O₃], from which a calibration equation relating sulfate to OH_exp was obtained (Fig. S6) and applied to alkyl nitrite photolysis experiments. In a separate experiment conducted with 2.2 ppm of iPrONO input to the reactor at $I_{\mathrm{369}}=\mathrm{6.5}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹, we verified that the mass of particulate sulfate detected by the ACSM responded linearly to a change in the input mixing ratio of SO₂ between 200 and 473 ppb (Fig. S7). This suggests that the sulfate particles were large enough for efficient transmission through the inlet lens of the ACSM across the range of OH_exp used in our experiments. While not applicable in this work, we note that heterogeneous uptake of SO₂ into organic aerosol may bias OH exposure measurements (Ye et al., 2018).

2.3 Chemical ionization mass spectrometer (CIMS) measurements

In a separate set of experiments, mass spectra of gas-phase α-Pinene photooxidation products were obtained with an Aerodyne high-resolution time-of-flight chemical ionization mass spectrometer using nitrate as the reagent ion (${\mathrm{NO}}_{\mathrm{3}}^{-}$-HRToF-CIMS, hereafter abbreviated as ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS) (Ehn et al., 2012; Eisele and Tanner, 1993). Nitrate (${\mathrm{NO}}_{\mathrm{3}}^{-}$) and its higher-order clusters (e.g., ${\mathrm{HNO}}_{\mathrm{3}}{\mathrm{NO}}_{\mathrm{3}}^{-}$) generated from X-ray ionization of HNO₃ were used as the reagent due to their selectivity to highly oxidized organic compounds, including species that contribute to SOA formation (Ehn et al., 2014; Krechmer et al., 2015; Lambe et al., 2017). The ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS sampled the reactor output at 10.5 L min⁻¹. α-Pinene oxidation products were detected as adduct ions of ${\mathrm{NO}}_{\mathrm{3}}^{-}$. In these experiments, the reactor was operated with a residence time of approximately 80 s to accommodate the undiluted ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS inlet flow requirement. OFR369-i(iPrONO) and OFR369-i(iPrONO-d₇) were operated using $I_{\mathrm{369}}=\mathrm{6.5}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹ and >7 ppm alkyl nitrite; in these experiments, α-Pinene was evaporated into the carrier gas by flowing 1 sccm N₂ through a bubbler containing liquid α-Pinene. Assuming the N₂ flow was saturated with α-Pinene vapor, we estimate ∼500 ppb α-Pinene was introduced to the OFR based on its vapor pressure at room temperature and known dilution ratio into the main carrier gas. In a separate experiment, OFR254-iN₂O was operated using $I_{\mathrm{254}}=\mathrm{3.2}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹ and 5 ppm O₃ + 1 % H₂O + 3.2 % N₂O. Here, α-Pinene was introduced by flowing 1 sccm of a gas mixture containing 150 ppm α-Pinene in N₂ into the main carrier gas (this gas mixture was unavailable for the iPrONO photolysis experiments); the calculated α-Pinene mixing ratio that was introduced to the OFR was ∼16 ppb.

2.4 Photochemical model

We used the KinSim OFR photochemical model to calculate concentrations of radical/oxidant species produced (Peng and Jimenez, 2017; Peng et al., 2015). In addition to NO+HO₂ → OH+NO₂ and other reactions included in Peng and Jimenez (2017), the following reactions were added for this study:

$$\begin{array}{}\text{(R5)}& {\displaystyle }& {\displaystyle }\mathrm{iPrONO}+h\mathit{\nu }\to {\mathrm{iC}}_{\mathrm{3}}{\mathrm{H}}_{\mathrm{7}}\mathrm{O}\cdot +\mathrm{NO}\text{(R6)}& {\displaystyle }& {\displaystyle }\mathrm{iPrONO}+h\mathit{\nu }\to {\mathrm{CH}}_{\mathrm{3}}\mathrm{CHO}+{\mathrm{CH}}_{\mathrm{3}}\cdot +\mathrm{NO}\text{(R7)}& {\displaystyle }& {\displaystyle }{\mathrm{CH}}_{\mathrm{3}}\cdot +{\mathrm{O}}_{\mathrm{2}}\to {\mathrm{CH}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{2}}\cdot \text{(R8)}& {\displaystyle }& {\displaystyle }{\mathrm{iC}}_{\mathrm{3}}{\mathrm{H}}_{\mathrm{7}}\mathrm{O}\cdot +{\mathrm{O}}_{\mathrm{2}}\to \left({\mathrm{CH}}_{\mathrm{3}}{)}_{\mathrm{2}}\mathrm{C}\right(\mathrm{O})+{\mathrm{HO}}_{\mathrm{2}}\text{(R9)}& {\displaystyle }& {\displaystyle }({\mathrm{CH}}_{\mathrm{3}}{)}_{\mathrm{2}}\mathrm{CO}+\mathrm{OH}\to \mathrm{Products}\text{(R10)}& {\displaystyle }& {\displaystyle }\mathrm{iPrONO}+\mathrm{OH}\to \left({\mathrm{CH}}_{\mathrm{3}}{)}_{\mathrm{2}}\mathrm{C}\right(\mathrm{O})+\mathrm{NO}\text{(R11)}& {\displaystyle }& {\displaystyle }{\mathrm{CH}}_{\mathrm{3}}\mathrm{CHO}+\mathrm{OH}\to {\mathrm{CH}}_{\mathrm{3}}\mathrm{CO}\cdot +{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\text{(R12)}& {\displaystyle }& {\displaystyle }{\mathrm{CH}}_{\mathrm{3}}\mathrm{CO}\cdot +{\mathrm{O}}_{\mathrm{2}}\to {\mathrm{CH}}_{\mathrm{3}}\mathrm{C}\left(\mathrm{O}\right){\mathrm{O}}_{\mathrm{2}}\cdot \text{(R13)}& {\displaystyle }& {\displaystyle }{\mathrm{CH}}_{\mathrm{3}}\mathrm{C}\left(\mathrm{O}\right){\mathrm{O}}_{\mathrm{2}}\cdot +\mathrm{NO}\to {\mathrm{CH}}_{\mathrm{3}}\cdot +{\mathrm{NO}}_{\mathrm{2}}+{\mathrm{CO}}_{\mathrm{2}}\text{(R14)}& {\displaystyle }& {\displaystyle }{\mathrm{CH}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{2}}\cdot +\mathrm{NO}\to {\mathrm{CH}}_{\mathrm{3}}\mathrm{O}\cdot +{\mathrm{NO}}_{\mathrm{2}}\text{(R15)}& {\displaystyle }& {\displaystyle }{\mathrm{CH}}_{\mathrm{3}}\mathrm{O}\cdot +{\mathrm{O}}_{\mathrm{2}}\to \mathrm{HCHO}+{\mathrm{HO}}_{\mathrm{2}}\text{(R16)}& {\displaystyle }& {\displaystyle }\mathrm{HCHO}+\mathrm{OH}\to \mathrm{HCO}\cdot +{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\text{(R17)}& {\displaystyle }& {\displaystyle }\mathrm{HCO}\cdot +{\mathrm{O}}_{\mathrm{2}}\to \mathrm{CO}+{\mathrm{HO}}_{\mathrm{2}}.\end{array}$$

Model input parameters included pressure, temperature, [H₂O], [iPrONO], mean residence time, actinic flux, and absorption cross sections and bimolecular rate constants shown in Table 1. We assumed the quantum yield of Reaction (2) to be 0.50 above 350 nm (Raff and Finlayson-Pitts, 2010). We assumed the quantum yield of Reaction (2) to be 0.04 above 350 nm (value for t-butyl nitrite) (Calvert and Pitts, 1966), suggesting minimal influence of CH₃O₂⋅ and CH₃C(O)O₂⋅ that are generated via Reactions (2), (2), and (2) following iPrONO decomposition to CH₃⋅ and CH₃CHO⋅ via Reaction (2). At 254 nm, the influence of CH₃O₂ and CH₃C(O)O₂⋅ on ensuing photochemistry may be more significant. This is due to a higher quantum yield of Reaction (2) at 254 nm, which is estimated to be 0.86 under a vacuum (Calvert and Pitts, 1966). Assuming that all 254 nm photons initiate photolysis, the quantum yield of Reaction (2) is 0.14. Due to collisional deactivation at 1 atm that prevents i-C₃H₇O⋅ decomposition, the quantum yield of Reaction (2) at λ=254 nm and 1 atm is expected to be higher than 0.14. Because quantum yield measurements were unavailable at these conditions, we applied an upper-limit quantum yield of 0.50 as applicable at λ>350 nm and 1 atm (Raff and Finlayson-Pitts, 2010). We calculated a corresponding nominal quantum yield of 0.32 by averaging the lower- and upper-limit values of 0.14 and 0.50, resulting in a quantum yield of 0.68 for Reaction (2).

We assumed that the residence time distribution of iPrONO in the reactor was similar to the residence time distribution of NO₂. To model iPrONO photolysis at λ=254 nm, we extended the range of previously measured σ_iPrONO values by measuring the gas-phase absorption cross sections of iPrONO (purified via four freeze-pump-thaw cycles prior to measurement) down to λ=220 nm using a custom-built absorption cell (Raff and Finlayson-Pitts, 2010). Results at λ=220 to 436 nm are shown in Fig. S1 and are in agreement with previous work (Raff and Finlayson-Pitts, 2010) over the range of overlap at λ=300 to 450 nm.

To account for uncertainties associated with the assumptions we made for quantum yield values, as well as uncertainties in other kinetic parameters, temperature, residence time, actinic flux, and organic nitrite concentration, we performed Monte Carlo uncertainty propagation (BIPM et al., 2008) as described previously (Peng and Jimenez, 2017; Peng et al., 2015). All uncertain kinetic parameters were assumed to follow lognormal distributions unless stated otherwise below. Uncertainties in rate constants and cross sections newly included in this study were adopted from Burkholder et al. (2015) if available. The relative uncertainty in the rate constant of Reaction (2) was estimated to be 40 % based on the dispersion of rate constant measurements of published RO₂+NO reactions. We assumed the random samples of the quantum yields of Reactions (2) and (2) at 254 nm and Reaction (2) at 369 nm followed uniform distributions in the range of [0.50, 0.86], [0.14, 0.50] and [0, 0.20], respectively. We assumed uncertainties of 5 K and 20 s in temperature and residence time (normal distributions assumed) and relative uncertainties of 50 %, 100 %, and 25 % in actinic flux at 369 nm, actinic flux at 254 nm, and organic nitrite concentration.

3 Results and discussion

We first characterized OH_exp and NO_x by separately varying the photolysis wavelength (Sect. 3.1.1) and input organic nitrite concentration to the reactor (Sect. 3.1.2), with the goal of identifying optimal OFR conditions for OH and NO_x generation via iPrONO photolysis. Second, we synthesized novel alkyl nitrites and compared their performances to iPrONO (Sect. 3.2). Third, we parameterized OH_exp and NO₂ production in a set of algebraic equations to guide selection of OFR experimental conditions. Finally, we compared ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS spectra of photooxidation products generated from reaction of α-Pinene with radicals produced via alkyl nitrite photolysis and $\mathrm{O}{(}^{\mathrm{1}}\mathrm{D})+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+{\mathrm{N}}_{\mathrm{2}}\mathrm{O}$ reactions.

Figure 2 Measured and modeled (a–b) OH exposure, (c–d) NO mixing ratio, and (e–f) NO₂ mixing ratio values as a function of actinic flux (I) following photolysis of 1.9 ppm isopropyl nitrite (iPrONO) at λ=254 (OFR254-i(iPrONO)), 350 (OFR350-i(iPrONO)), or 369 nm (OFR369-i(iPrONO)) in the PAM oxidation flow reactor. Error bars for measurements represent ±50 % uncertainty in OH_exp and I values.

3.1 OH_exp and NO_x generated from iPrONO photolysis

3.1.1 Effect of photolysis wavelength

Figure 2 shows OH_exp, [NO], and [NO₂] obtained as a function of actinic flux following photolysis of 1.9 ppm of iPrONO injected into the reactor at λ=254, 350, or 369 nm. These systems are hereafter designated as OFR254-i(iPrONO), OFR350-i(iPrONO), and OFR369-i(iPrONO), respectively; similar nomenclature is adapted for other alkyl nitrites. In these notations, the numbers following “OFR” are the photolysis wavelengths (in nm), and the “i” preceding the parentheses means initial injection of the radical precursor compound noted in the parentheses. Modeled OH_exp, NO, and NO₂ values for the OFR254-i(PrONO) and OFR369-i(PrONO) modes are shown in Fig. 2 at the same nominal operating conditions.

At a fixed photolysis wavelength, OH_exp, NO, and NO₂ increased with increasing actinic flux. Measured and modeled OH_exp values were in agreement within uncertainties at λ=369 nm. At λ=254 nm, model OH_exp results were higher than the measurements, perhaps due to uncertainty in assumptions that were necessary to model OFR254-i(iPrONO) (Sect. 2.4). Higher NO₂ concentrations were modeled at λ=254 nm than at λ=369 nm because more iPrONO was photolyzed and the NO₂ yield was only weakly dependent on the fate of i-C₃H₇O⋅. For example, NO is converted to NO₂ either via reaction with HO₂ obtained via Reaction (2) or CH₃O₂⋅ and CH₃C(O)O₂⋅ obtained via Reaction (2). However, the effect of photolysis wavelength on NO and OH_exp was different. Specifically, the highest NO concentration and OH_exp were achieved via OFR369-i(iPrONO). OH_exp achieved via OFR369-i(iPrONO) was slightly higher than OH_exp attained using OFR350-i(iPrONO), likely because photolysis of both iPrONO and NO₂, the reaction of which with OH suppresses OH_exp, is more efficient at λ=369 nm than at λ=350 nm (Fig. S1 and Table 1). Further, the NO and OH yields achieved via OFR254-i(iPrONO) were suppressed due to significant (>73 %) decomposition of iC₃H₇O⋅ (Calvert and Pitts, 1966). The dependence of OH, NO, and NO₂ on the quantum yields of Reactions (2) and (2) was confirmed by sensitivity analysis of uncertainty propagation inputs and outputs as described in Sect. 2.4. OH_exp and NO were strongly anticorrelated with the quantum yield of Reaction (2), whereas the correlation between NO₂ and the quantum yield of Reaction (2) was negligible.

The products of this decomposition, i.e., CH₃CHO and CH₃⋅, both have adverse effects with regard to our experimental goals: CH₃CHO is reactive toward OH and can thus suppress OH; the RO₂⋅ formed through this reaction, CH₃C(O)O₂⋅, consumes NO and generates NO₂ but does not generate OH; CH₃⋅ rapidly converts to CH₃O₂⋅, which also consumes NO and generates NO₂ but does not directly produce OH. Importantly, Fig. 2 suggests that it is preferable to photolyze alkyl nitrites at λ>350 because optimal OH_exp and NO:NO₂ were attained via OFR369-i(iPrONO). Moreover there is added risk of significant unwanted photolysis of organics via OFR254-i(iPrONO) (Peng et al., 2016).

3.1.2 Effect of alkyl nitrite concentration

Figure 3 shows measured OH_exp and modeled NO_x concentrations obtained from photolysis of 0.5 to 20 ppm iPrONO at $I_{\mathrm{369}}\approx \mathrm{7}\times {\mathrm{10}}^{\mathrm{15}}$ photons cm⁻² s⁻¹. [NO] and [NO₂] increased with increasing [iPrONO], as expected. For [iPrONO] ≤5 ppm, OH_exp increased with increasing [iPrONO] because the rate of OH production increased faster than the rate of OH destruction from reaction with iPrONO and NO₂. The model results showed that for [iPrONO] >5 ppm, the opposite was true and OH_exp plateaued or decreased. A maximum OH${}_{\exp }=\mathrm{7.8}\times {\mathrm{10}}^{\mathrm{10}}$ molecules cm⁻³ s was achieved via photolysis of 10 ppm iPrONO, with corresponding modeled [NO] and [NO₂] values of 148 and 405 ppb respectively. Modeled NO₃ concentrations were negligible in OFR369-i(iPrONO) (≤1 ppt) because there was no O₃ present and NO₃ production via HNO₃+OH → NO₃+H₂O reactions was insignificant.

Figure 3 Measured and modeled (a) OH exposure, (b) NO mixing ratio, and (c) NO₂ mixing ratio values obtained using OFR369-i(iPrONO) at $I_{\mathrm{369}}=\mathrm{7}\times {\mathrm{10}}^{\mathrm{15}}$ ph cm⁻² s⁻¹ as a function of the iPrONO mixing ratio. Error bars for measurements represent ±50 % uncertainty in OH_exp and estimated ±30 % uncertainty in the iPrONO mixing ratio values.

3.2 OH_exp generated from photolysis of perdeuterated iPrONO and 1,3-propyl dinitrite

Although OH${}_{\exp }=\mathrm{7.8}\times {\mathrm{10}}^{\mathrm{10}}$ molecules cm⁻³ s (approximately 0.6 days of equivalent atmospheric OH exposure) may be suitable for some OFR applications, it may be insufficient to simulate multigenerational oxidative aging of precursors with OH rate constants slower than $\sim {\mathrm{10}}^{-\mathrm{11}}$ cm³ molecule⁻¹ s⁻¹. We attempted to synthesize three C₃ alkyl nitrites that we hypothesized could generate higher OH_exp than iPrONO: perdeuterated isopropyl nitrite (iPrONO-d₇), 1,3-propyl dinitrite [1,3-Pr(ONO)₂], and hexafluoroisopropyl nitrite (HFiPrONO). We successfully synthesized 1,3-Pr(ONO)₂ and iPrONO-d₇, but were unable to synthesize HFiPrONO (Sect. 3.5.3). Figure 4 shows OH_exp attained from photolysis of 1.2 ppm 1,3-Pr(ONO)₂ and 1.7 ppm iPrONO-d₇ as a function of I₃₆₉, along with the model output for OFR369-i(iPrONO) shown for reference. At these organic nitrite concentrations and I₃₆₉ values, maximum OH_exp measurements were 1.1×10¹¹ (iPrONO-d₇), 4.0×10¹⁰ (iPrONO), and 1.8×10¹⁰ molecules cm⁻³ s [1,3-Pr(ONO)₂], respectively. At maximum I₃₆₉ and after correcting for the different iPrONO, iPrONO-d₇, and 1,3-Pr(ONO)₂ concentrations that were used (Fig. 3), OH${}_{\exp ,\mathrm{iPrONO}-{\mathrm{d}}_{\mathrm{7}}}$ ≈ 2.9 × OH_exp,iPrONO and OH${}_{\exp ,\mathrm{1},\mathrm{3}-\mathrm{Pr}(\mathrm{ONO}{)}_{\mathrm{2}}}$ ≈ 0.81 × OH_exp,iPrONO.

Figure 4 Measured and modeled OH exposure values measured as a function of I₃₆₉ following photolysis of perdeuterated isopropyl nitrite (iPrONO-d₇) and 1,3-propyl dinitrite (1,3-Pr(ONO)₂). Modeled OH_exp values obtained from OFR369-i(iPrONO-d₇) and OFR369-i(iPrONO) (Fig. 2d) are shown for reference. Error bars for measurements represent ±50 % uncertainty in OH_exp and I values.

We hypothesize that higher OH_exp obtained from OFR369-i(iPrONO-d₇) relative to OFR369-i(iPrONO) was due to ∼2.6 times lower OH reactivity of iPrONO-d₇ relative to iPrONO (Nielsen et al., 1988, 1991) and 6 times lower OH reactivity of acetone-d₆ relative to acetone (Raff et al., 2005). This hypothesis is supported by the modeled OH_exp attained via OFR369-i(iPrONO-d₇), which is in agreement with measured OH_exp within uncertainties and is 41 % higher than modeled OH_exp attained via OFR369-i(iPrONO). Model simulations revealed that this effect was most pronounced near the reactor inlet (e.g., at low residence time), where the local OH concentration was higher than elsewhere in the reactor because NO_x was very low, resulting in higher sensitivity of [OH] to the OH reactivity of the specific organic nitrite that was used. On the other hand, OFR369-i(1,3-Pr(ONO)₂) was less efficient than OFR369-i(iPrONO). In this case, it is possible that higher NO₂ production during 1,3-Pr(ONO)₂ photolysis and/or production of more reactive intermediates (e.g., malonaldehyde) offset any benefit gained from faster OH production via photolysis of both -ONO groups or more efficient photolysis of one -ONO group (Wang and Zu, 2016).

3.3 OH_exp and NO₂ estimation equations for OFR369-i(iPrONO) and OFR369-i(iPrONO-d₇)

Previous studies reported empirical OH_exp algebraic estimation equations for OFR185 and OFR254 (Li et al., 2015; Peng et al., 2015). These equations parameterize OH_exp as a function of readily measured experimental parameters, therefore providing a simpler alternative to detailed photochemical models that aids in experimental planning and analysis. Here, we expand on those studies by deriving OH_exp and NO₂ estimation equations for OFR369-i(iPrONO) and OFR369-i(iPrONO-d₇). Model results (14 641 model runs in total) obtained from the base case of the model (SO₂ as surrogate of external OH reactivity, OHR_ext) were used to derive the following equations that allow an estimation the OH exposure for OFR369-i(iPrONO) and OFR369-i(PrONO-d₇):

$$\begin{array}{ll}{\displaystyle }& {\displaystyle }\log \left({\mathrm{OH}}_{\exp }\right)=\log \left(I_{\mathrm{369}}\right)-\mathrm{0.0026728}{\mathrm{OHR}}_{\mathrm{ext}}\\ {\displaystyle }& {\displaystyle }+\mathrm{0.46017}\log \left(\right[\mathrm{iPrONO}\left]\right)+\mathrm{1.1928}\log \left(\mathit{\tau }\right)\\ {\displaystyle }& {\displaystyle }+\mathrm{0.35317}\log \left(\right[\mathrm{iPrONO}\left]\right)\log \left({\mathrm{OHR}}_{\mathrm{ext}}\right)\\ {\displaystyle }& {\displaystyle }-\mathrm{0.11109}\log \left({\mathrm{OHR}}_{\mathrm{ext}}\right)\log \left(\mathit{\tau }\right)-\mathrm{0.015606}\\ \text{(3)}& {\displaystyle }& {\displaystyle }\log \left(I_{\mathrm{369}}\right)\log \left(\right[\mathrm{iPrONO}\left]\right)\log \left(\mathit{\tau }\right)-\mathrm{7.6164}{\displaystyle }& {\displaystyle }\log \left({\mathrm{OH}}_{\exp }\right)=\mathrm{0.85558}\log \left(I_{\mathrm{369}}\right)-\mathrm{0.0029546}\\ {\displaystyle }& {\displaystyle }{\mathrm{OHR}}_{\mathrm{ext}}+\mathrm{0.61837}\log \left(\right[\mathrm{iPrONO}-d_{\mathrm{7}}\left]\right)\\ {\displaystyle }& {\displaystyle }+\mathrm{1.2115}\log \left(\mathit{\tau }\right)+\mathrm{0.36081}\log \left(\right[\mathrm{iPrONO}-d_{\mathrm{7}}\left]\right)\\ {\displaystyle }& {\displaystyle }\log \left({\mathrm{OHR}}_{\mathrm{ext}}\right)-\mathrm{0.15501}\log \left({\mathrm{OHR}}_{\mathrm{ext}}\right)\log \left(\mathit{\tau }\right)\\ {\displaystyle }& {\displaystyle }-\mathrm{0.017061}\log \left(I_{\mathrm{369}}\right)\log \left(\right[\mathrm{iPrONO}-d_{\mathrm{7}}\left]\right)\log \left(\mathit{\tau }\right)\\ \text{(4)}& {\displaystyle }& {\displaystyle }-\mathrm{5.1541},\end{array}$$

where OH_exp, I₃₆₉, OHR_ext, [iPrONO or iPrONO-d₇], and τ are in units of molecules cm⁻³ s, photons cm⁻² s⁻¹, s⁻¹, ppm, and s, respectively. Fit coefficients were obtained by fitting Eqs. (3) and (4) to OH_exp model results over the following range of OFR parameters: ([iPrONO/iPrONO-d₇]; 0.2–20 ppm), I₃₆₉ (1×10¹⁵–2×10¹⁶ photons cm⁻² s⁻¹), OHR_ext (1–200 s⁻¹), and residence time, τ, between 30 and 200 s. We explored 11 logarithmically evenly distributed values in these ranges for each parameter and thus performed simulations for 14641 model cases in total. To determine the functional form of Eqs. (3) and (4), we used the sum of the logarithms of first-, second- and third-order terms of the four parameters and iteratively removed the terms with very small fit coefficients until further removal of the remaining terms significantly worsened the fit quality.

Figure 5a and c compare OH_exp estimated from Eqs. (3) and (4) and calculated from the model described in Sect. 2.4. The mean absolute value of the relative deviation is 29 %, indicating that the estimation equations are typically producing results within the inherent model uncertainties. Care should be taken to not use the equations away from the range in which they were derived, as much larger errors are possible when extrapolating.

Figure 5 Comparison of OH_exp and NO₂ values obtained from estimation equations and photochemical model for (a–b) OFR369-i(iPrONO) and (c–d) OFR369-i(iPrONO-d₇).

While several techniques are available to monitor NO₂, interferences from other nitrogen-containing species are well known and may create issues similar to those shown in Fig. 2f. NO₂ production and loss rates are primarily governed by the alkyl nitrite concentration, actinic flux, and residence time in the OFR. These parameters were experimentally constrained (Sect. 2.2.2). Thus, we derived NO₂ estimation equations for OFR369-i(iPrONO) (Eq. 5) and OFR369-i(iPrONO-d₇) (Eq. 6) as a function of [RONO], I₃₆₉, and τ, to all of which NO₂ production is proportional, over the same phase space used to fit Eqs. (3) and (4):

$$\begin{array}{ll}{\displaystyle }\log \left({\mathrm{NO}}_{\mathrm{2}}\right)& {\displaystyle }=\log \left(I_{\mathrm{369}}\right)+\log \left(\right[\mathrm{iPrONO}\left]\right)\\ \text{(5)}& {\displaystyle }& {\displaystyle }+\log \left(\mathit{\tau }\right)-\mathrm{6.2198}{\displaystyle }\log \left({\mathrm{NO}}_{\mathrm{2}}\right)& {\displaystyle }=\log \left(I_{\mathrm{369}}\right)\\ \text{(6)}& {\displaystyle }& {\displaystyle }+\log \left(\right[\mathrm{iPrONO}-d_{\mathrm{7}}\left]\right)+\log \left(\mathit{\tau }\right)-\mathrm{6.2607}.\end{array}$$

Figure 5b and d compare NO₂ estimated from Eqs. (3) and (4) and calculated from the model described in Sect. 2.4. The mean absolute value of the relative deviation between NO₂ estimated by Eqs. (5) and (6) and NO₂ computed by the photochemical model is 19 %. The mean model NO:NO₂ fraction is approximately 0.33 (Figs. 2–3).

3.4 ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS spectra of organic nitrates generated from α-Pinene + OH/OD reactions via OFR369-i(iPrONO), OFR369-i(iPrONO-d₇), and OFR254-iN₂O

To evaluate the efficacy of OFR369-i(iPrONO), OFR369-i(iPrONO-d₇), and OFR254-iN₂O for generating HO_x under high-NO_x photooxidation conditions, we obtained ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS spectra of α-Pinene + OH/OD nitrogen-containing oxidation products generated using each method, with experimental conditions described in Sect. 2.3. Calculated OH exposures for OFR369-i(iPrONO), OFR369-i(iPrONO-d₇), and OFR254-iN₂O were 2.9×10¹⁰, 5.9×10¹⁰ and 5.0×10¹¹ molecules cm⁻³ s, respectively, in the absence of OH consumption due to α-Pinene. These calculated steady-state OH_exp values decreased to 8.5×10⁸, 6.8×10⁸ and 4.6×10¹¹ molecules cm⁻³ s after accounting for OH consumption. This suggests that most of the OH that was produced in these OFR369-i(iPrONO/iPrONO-d₇) experiments was consumed by α-Pinene and its early-generation photooxidation products. OH suppression relative to 254 nm photons, O₃, and O is not a concern in OFR369-i(iPrONO), unlike in OFR254-iN₂O (Peng et al., 2016).

Figure 6 NO${}_{\mathrm{3}}^{-}$-CIMS spectra of nitrogen-containing α-Pinene photooxidation products with ${\mathrm{C}}_{\mathrm{7}-\mathrm{9}}{\mathrm{H}}_{\mathrm{9},\mathrm{11},\mathrm{13},\mathrm{15}}{\mathrm{NO}}_{\text{5-10}}$ (C₇, C₈, C₉), ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15},\mathrm{17}}{\mathrm{NO}}_{\mathrm{4}-\mathrm{14}}$ (C₁₀), ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{8},\mathrm{10}}{\mathrm{DNO}}_{\mathrm{8}-\mathrm{14}}$ (C₈D), ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{14},\mathrm{16}}{\mathrm{DNO}}_{\mathrm{7}-\mathrm{14}}$ (C₁₀D) or ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16},\mathrm{18}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}-\mathrm{13}}$ (C₁₀ dinitrate) formulas generated via (a) OFR369-i(iPrONO) (b) OFR254-iN₂O (H₂O = 1 %, N₂O = 3.2 %). (c, d) OFR369-i(iPrONO-d₇) and observed in ambient measurements at (e) Centreville, Alabama, United States (Massoli et al., 2018) and (f) Hyytiälä, Finland (Yan et al., 2016). O_x indicates the number of oxygen atoms in corresponding signals (excluding three oxygen atoms per nitrate functional group).

Exposure of α-Pinene to OH/OD generated via OFR369-iPrONO, OFR369-iPrONO-d₇, and OFR254-iN₂O produced C₇–C₁₀ organic nitrate and C₁₀ dinitrate signals that are shown in Fig. 6a–d. ([(NO₃)C₇H₉NO₈]⁻ and [(NO₃)C₇H₁₁NO₈]⁻ signals at $m/z=\mathrm{297}$ and 299 are excluded due to significant intra- and inter-experiment variability for unknown reasons). Figure 6 shows that many of the same products are observed independent of radical precursors. The Fig. 6a spectrum (OFR369-i(iPrONO)) is shifted to a lower oxygen-to-carbon ratio relative to Fig. 6b (OFR254-iN₂O) and c (OFR369-i(iPrONO-d₇)), consistent with the lower OH_exp achieved with OFR369-i(iPrONO) compared to OFR369-i(iPrONO-d₇) and OFR254-iN₂O. For example, [(NO₃)C₁₀H₁₅NO₇]⁻ was the largest C₁₀ nitrate signal observed via OFR369-i(iPrONO), whereas [(NO₃)C₁₀H₁₅NO₈]⁻ was the largest C₁₀ nitrate signal observed via OFR369-i(iPrONO-d₇) and OFR254-iN₂O. Qualitatively similar trends were observed for C₇–C₉ organic nitrates and C₁₀ dinitrates across the three systems.

Two additional features are of note in Fig. 6. First, a series of ion signals at $m/z=\mathrm{312}$, 328, 344, 360, 376, 392, 408 and 340, 356, 372, 388, 402, 420 were observed at higher levels via OFR369-i(iPrONO-d₇) relative to OFR369-i(iPrONO). These ions are plotted separately in Fig. 6d. The most plausible explanation is the additional contribution of [(NO₃)C₈H₁₀DNO₈₋₁₄]⁻ and [(NO₃)C₁₀H₁₄DNO₇₋₁₄]⁻ ions that retain -OD functionality following initial addition of OD (rather than OH) to α-Pinene. There is evidence of other deuterium-containing ions in Fig. 6b that are either less prominent or more difficult to resolve from other ions at the same integer mass. Second, C₁₀ dinitrates were present in all three spectra, with the highest dinitrate fractions observed in Fig. 6b (0.090) and c (0.081), and the lowest dinitrate fraction observed in Fig. 6a (0.056). Dinitrates are presumably generated from α-Pinene following (1) two OH reactions followed by two RO₂+ NO termination reactions or (2) one NO₃ reaction followed by one RO₂+ NO termination reaction. Previous application of OFR254-iN₂O could not exclude the contribution of α-Pinene + NO₃ reactions, with NO₃ radicals generated from NO₂ + O₃ and other reactions (Lambe et al., 2017). However, generation of dinitrates via OFR369-i(iPrONO-d₇), which produced negligible NO₃, suggests that dinitrates are not an artifact of unwanted α-Pinene + NO₃ reactions.

The ability of OFR369-i(iPrONO) and OFR369-i(iPrONO-d₇) to mimic polluted atmospheric conditions can be evaluated by comparing signals observed in Fig. 6a–c with ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS spectra obtained in Centreville, AL, USA (Massoli et al., 2018) and in Hyytiälä, Finland (Yan et al., 2016) that are shown in Fig. 6e–f. Both measurement locations are influenced by local biogenic emissions mixed with occasional anthropogenic outflow. Figure 6e and f were obtained on 25 June 2013 (07:30–11:00 Centreville time) and 11 April 2012 (10:00–13:00 Hyytiälä time). The mean NO mixing ratios during these periods were 0.53±0.17 (Centreville) and 0.27±0.09 ppb (Hyytiälä). In Centreville, a terpene nitrate source factor that peaked during the early morning contained C₉H₁₅NO_5-7, ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15},\mathrm{17}}{\mathrm{NO}}_{\mathrm{6}-\mathrm{10}}$, and C₁₀H_14,16N₂O_8-11 compounds (Massoli et al., 2018). The largest C₁₀ nitrate and dinitrate species in that factor were C₁₀H₁₅NO₆, C₁₀H₁₅NO₈, C₁₀H₁₆N₂O₉ and C₁₀H₁₆N₂O₁₀. In Hyytiälä, concentrations of C₁₀H₁₅NO_7-11 and C₁₀H₁₆N₂O_8-11 peaked in the morning and early afternoon. Elevated C₁₀ dinitrate levels during the daytime in Hyytiälä (Fig. 6f) suggest their formation from monoterpenes following two OH reactions followed by two RO₂+ NO termination reactions, as proposed earlier.

Overall, Fig. 6 shows that many of the C₇–C₁₀ nitrogen-containing compounds observed in Centreville and Hyytiälä were generated via OFR369-i(iPrONO), OFR369-i(iPrONO-d₇), and OFR254-iN₂O. Due to the local nature of the ambient terpene emissions at the Centreville and Hyytiälä sites, the associated photochemical age was presumably <1 day. Thus, while the ambient ${\mathrm{NO}}_{\mathrm{3}}^{-}$-CIMS spectra at those sites were more complex and contained contributions from precursors other than α-Pinene, the oxidation state of the ambient terpene-derived organic nitrates was more closely simulated via OFR369-i(iPrONO) or OFR369-i(iPrONO-d₇), where the largest C₁₀ nitrates and dinitrates were C₁₀H₁₅NO₇ and C₁₀H₁₆N₂O₉ (OFR369-i(iPrONO); Fig. 6a), and C₁₀H₁₅NO₈, C₁₀H₁₅NO₉ and C₁₀H₁₆N₂O₉ (OFR369-i(iPrONO-d₇); Fig. 6c). By comparison, C₁₀H₁₅NO₈ and C₁₀H₁₆N₂O₁₁ were the largest nitrate and dinitrate species generated via OFR254-iN₂O (Fig. 6b).

3.5 Anticipated performance of alternative high-NO_x HO_x precursors in OFRs

3.5.1 Methyl nitrite (MeONO)

MeONO is commonly used as an OH radical source in environmental chamber studies (Atkinson et al., 1981; Chhabra et al., 2011; Finewax et al., 2018; Matsunaga and Ziemann, 2010). To evaluate its potential use in OFRs, we examined previous measurements in an environmental chamber equipped with blacklights ($j_{{\mathrm{NO}}_{\mathrm{2}}}=\mathrm{0.27}$ min⁻¹, assumed 350 nm wavelength), where photolysis of 10 ppm MeONO generated [OH] $\sim \mathrm{2}\times {\mathrm{10}}^{\mathrm{8}}$ molecules cm⁻³ for a few minutes (Atkinson et al., 1981). In our OFR, $j_{{\mathrm{NO}}_{\mathrm{2}},\max }=\mathrm{0.36}$ min⁻¹ at λ=350 nm. Thus, over 98 s exposure time, we anticipate OH${}_{\exp }\approx \mathrm{2}\times {\mathrm{10}}^{\mathrm{10}}$ molecules cm⁻³ s would be obtained via photolysis of 10 ppm MeONO in OFRs. This is lower than the OH_exp attained via photolysis of 10 ppm iPrONO even after correcting for different $j_{{\mathrm{NO}}_{\mathrm{2}}}$ values in the different studies. Lower OH_exp achieved from MeONO photolysis is presumably due to the higher reactivity of formaldehyde, the primary photolysis product of MeONO, relative to acetone, the primary photolysis product of iPrONO at 369 nm (Raff and Finlayson-Pitts, 2010). Along with less efficient OH production, MeONO must be synthesized, trapped at low temperature, and stored under a vacuum. Thus, there is no advantage to using OFR350-iMeONO (or OFR350-MeONO-d₄) in OFRs relative to OFR369-i(iPrONO) or OFR369-i(iPrONO-d₇).

3.5.2 Nitrous acid (HONO)

HONO is also commonly used as an OH radical source in environmental chamber studies. To evaluate its potential application in OFRs, we examined previous measurements in an environmental chamber equipped with blacklights, where photolysis of 3–20 ppm HONO generated initial [OH] $\approx \mathrm{6}\times {\mathrm{10}}^{\mathrm{7}}$ molecules cm⁻³ (Cox et al., 1980), which is 3.3 times lower than [OH] obtained from photolysis of comparable levels of MeONO (Sect. 3.5.1). Lower OH_exp achieved from HONO photolysis is presumably due to higher OH reactivity of HONO relative to MeONO/iPrONO. Additionally, HONO is difficult to prepare without NO₂ impurities (Febo et al., 1995) that may cause additional OH suppression. For these reasons, we believe that there is no advantage to using HONO as a HO_x precursor in OFRs.

3.5.3 Hexafluoroisopropyl nitrite (HFiPrONO)

HFiPrONO has been synthesized from O-nitrosation of hexafluoroisopropanol (Andersen et al., 2003; Shuping et al., 2006). We predict that OFR369-i(HFiPrONO) should attain higher OH_exp than OFR369-i(iPrONO) and OFR369-i(iPrONO-d₇) due to similar photolysis rates (Andersen et al., 2003) and ∼200 times lower OH reactivity of HFiPrONO/hexafluoroacetone relative to iPrONO/acetone (Atkinson et al., 1992; Tokuhashi et al., 1999). Simple modeling calculations suggest that application of OFR369-i(HFiPrONO) may achieve up to a week of equivalent OH exposure. We made several unsuccessful attempts to synthesize HFiPrONO and other fluorinated alkyl nitrites with a procedure similar to that used by Andersen et al. (2003) and Shuping et al. (2006). The synthesis product was blue (not yellow) in color when trapped or stored in nitrogen, generated negligible OH upon irradiation in the reactor, and evolved into brown vapor in the presence of air or upon warming to room temperature (Fig. S8). These observations suggest the formation of N₂O₃, which we hypothesize was formed in solution from the reactions

$$\begin{array}{}\text{(R18)}& {\displaystyle }& {\displaystyle }{\mathrm{H}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{4}}+\mathrm{2}{\mathrm{NaNO}}_{\mathrm{2}}\to \mathrm{2}\mathrm{HONO}+{\mathrm{Na}}_{\mathrm{2}}\left({\mathrm{SO}}_{\mathrm{4}}\right)\text{(R19)}& {\displaystyle }& {\displaystyle }\mathrm{2}\mathrm{HONO}\to \mathrm{NO}+{\mathrm{NO}}_{\mathrm{2}}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\text{(R20)}& {\displaystyle }& {\displaystyle }\mathrm{NO}+{\mathrm{NO}}_{\mathrm{2}}\rightleftharpoons {\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.\end{array}$$

This pathway may have been favored if the O-nitrosation of hexafluoroisopropanol was slow compared to non-fluorinated alcohols.

4 Conclusions

Recently, we developed new methods that enable NO_x-dependent photooxidation studies in OFRs using O(¹D) $+{\mathrm{N}}_{\mathrm{2}}\mathrm{O}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ reactions via O₃ photolysis at λ=254 nm and/or H₂O+N₂O photolysis at 185 nm (OFR254-iN₂O and OFR185-iN₂O) (Lambe et al., 2017; Peng et al., 2018). Alkyl nitrite photolysis is an established method that facilitates high-NO_x photooxidation studies in modern OFRs. Here, we adapted alkyl nitrite photolysis for new OFR applications by characterizing the photolysis wavelength, nitrite concentration, and nitrite composition that result in optimal HO_x and NO_x generation capabilities. Based on our results, we recommend photolysis of 5–10 ppm isopropyl nitrite at λ≈365–370 nm photolysis wavelength and I>10¹⁵ photons cm⁻² s⁻¹. If the user has the resources to synthesize iPrONO-d₇, better performance is expected relative to iPrONO. Alkyl nitrite photolysis at λ=254 nm is not recommended. Taken together, OFR254/185-iN₂O and OFR369-i(iPrONO/iPrONO-d₇) are complementary methods that provide additional flexibility for NO_x-dependent OFR studies. OFR254/185-iN2O generate variable-NO_x photooxidation conditions (NO:HO₂ ≈0–100) and are suitable for the characterization of multigenerational oxidative aging processes at up to OH_exp≈ (5–10)×10¹¹ molecules cm⁻³ s (∼5–10 equivalent days). OFR369-i(iPrONO/iPrONO-d7) generate high-NO photooxidation conditions (NO:HO₂ ≈10–10 000; NO:NO₂ ≈0.2–0.7) with minimal O₃ and NO₃ formation at longer photolysis wavelength than OFR254/185-iN₂O. We anticipate that alkyl nitrite photolysis is advantageous for the characterization of first-generation, high-NO_x photooxidation products of most precursors at up to OH${}_{\exp }\approx \mathrm{1}\times {\mathrm{10}}^{\mathrm{11}}$ molecules cm⁻³ s (1 equivalent day), which is comparable to environmental chambers investigating high-NO_x conditions. The generation of OD (rather than OH) via OFR369-i(iPrONO-d₇) may be useful in photooxidation studies of unsaturated precursors due to the shift in the m∕z of the addition products, though at the potential expense of generating more complex distributions of oxidation products. Potential disadvantages of the OFR369-i(iPrONO) method are (1) restriction to high-NO photochemical conditions, (2) restriction to OH_exp of 1 equivalent day or less, (3) additional complexity involved with integration of the alkyl nitrite source (compared to ${\mathrm{O}}_{\mathrm{3}}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}+{\mathrm{N}}_{\mathrm{2}}\mathrm{O}$), (4) additional cost and complexity to retrofit a specific OFR design with blacklights, and (5) that it acts as an interference that precludes NO_x measurements by chemiluminescence detection. Future work will evaluate the ability of each method to mimic polluted atmospheric conditions in specific source regions.