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

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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):

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 1Absorption 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).

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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.

2.3 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:

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.

3.1 OH_exp and NO_x generated from iPrONO photolysis

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 3Measured 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.

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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 4Measured 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.

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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):

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 5Comparison 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).

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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):

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 6NO${}_{\mathrm{3}}^{-}$-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).

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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