2.1 N₂O₅ and NO₃ generation

Figure 1 shows a process flow diagram of the OFR-iN₂O₅ method. Separate flows containing NO₂ and O₃ were input to a perfluoroalkoxy (PFA) tube with a 2.54 cm o.d. (outer diameter), a 2.22 cm i.d. (inner diameter), and a 152.4 cm length that was operated as an LFR. Previous studies used a similar process to generate N₂O₅ (Wood et al., 2003; Boyd et al., 2015), although the LFR materials, flow rates, and reagent concentrations were different. A compressed gas cylinder containing 1.00±0.02 % NO₂ in N₂ (Praxair) was used to supply NO₂. While not used for this study, replacing NO₂ with NO to avoid NO₂-to-HNO₃ conversion inside the gas cylinder and increasing [O₃] accordingly achieves similar results. O₃ was generated by passing 1750–1800 cm³ min⁻¹ of pure O₂ through a custom O₃ chamber housing a mercury fluorescent lamp (GPH212T5VH, Light Sources, Inc.) or 500–1800 cm³ min⁻¹ O₂ through a corona discharge ozone generator (Enaly 1KNT). We used 1800 cm³ min⁻¹ of O₂ carrier gas flow through the LFR (Re ∼ 110, i.e., laminar flow) to achieve τ_LFR=20 s for reasons that are discussed in Sect. 3.1. The NO₂ mixing ratio entering the LFR, [NO₂]_0, LFR, was calculated from the NO₂ mixing ratio in the compressed gas mixture and the dilution ratio of 0–50 or 0–1300 cm³ min⁻¹ gas flow into O₂ which was controlled using mass flow controllers. The O₃ mixing ratio entering the LFR, [O₃]_0, LFR, was measured using a 2B Technologies 106-MFT or a Teledyne M452 flow-through O₃ analyzer when generated from the mercury lamp or corona discharge source, respectively. The output of the LFR was mixed with a carrier gas containing 3.8 L min⁻¹ synthetic air and then injected into a Potential Aerosol Mass (PAM) OFR (Aerodyne Research, Inc.), which is a horizontal 13.3 L aluminum cylindrical chamber operated in continuous flow mode (Kang et al., 2007; Lambe et al., 2011, 2019) with 6.5 L min ⁻¹ flow through the reactor. The mean residence time in the OFR (τ_OFR) was 120±34 s (±1σ), as obtained from measurements of 10 s pulsed inputs of NO₂ to the OFR obtained using a 2B Technologies Model 405 NO_x analyzer (Fig. S1). Across all experiments, the relative humidity in the OFR (RH_OFR) was controlled in the range of 7 %–85 % at 23–25 ^∘C by passing the carrier gas through a Nafion humidifier (Perma Pure LLC) or heated recirculating water bath (NESLAB Instruments, Inc.) prior to mixing with the LFR outflow. The O₃ mixing ratio at the exit of the OFR was measured with a 2B Technologies Model 106-M ozone analyzer.

Figure 1Process flow diagram of the OFR-iN₂O₅ technique used to generate nitrate radicals (NO₃).

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2.2 Photochemical model

We used the KinSim chemical kinetic solver to calculate concentrations of radical and oxidant species (Peng et al., 2015; Peng and Jimenez, 2017, 2019). The KinSim mechanism shown in Table S2 was adapted from Palm et al. (2017) to model NO₃ and N₂O₅ concentrations in the LFR and OFR. Inputs to the LFR-KinSim model were [O₃]_0, LFR, [NO₂]_0, LFR, RH=1 %, T=24 ^∘C, τ_LFR=20 s (modeled as plug flow, see Sect. 3.1), and first-order wall loss rates of NO₃ and N₂O₅ ($k_{{\mathrm{w}}_{\mathrm{LFR}},{\mathrm{NO}}_{\mathrm{3}}}$ and $k_{{\mathrm{w}}_{\mathrm{LFR}},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$). Inputs to the OFR-KinSim model were [O₃], [NO₂], [NO₃], and [N₂O₅] output from the LFR scaled by a measured dilution factor of 4.4; RH and T measured in the OFR; τ_OFR=120 s, $k_{{\mathrm{w}}_{\mathrm{OFR}},{\mathrm{NO}}_{\mathrm{3}}}$, and $k_{{\mathrm{w}}_{\mathrm{OFR}},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$; and input VOC tracer concentrations and their $k_{{\mathrm{NO}}_{\mathrm{3}}}$ values. Because the calculated N₂O₅ residence time in the OFR inlet (∼0.04 s) was short compared with the N₂O₅ decomposition timescale at T=23–25 ^∘C (∼20 s), potential thermal decomposition of N₂O₅ during the dilution step was not considered in the model.

3.1 LFR design considerations

The optimal LFR residence time (τ_LFR) was identified using model simulations of the injection of 300 ppm O₃ and NO₂ into the LFR followed by dilution and injection of the LFR output into an OFR operated with τ_OFR=120 s. Figure S2 plots the NO_3exp achieved in the OFR as a function of τ_LFR ranging from 1 to 60 s. Potential entry length effects that may have influenced results obtained below τ_LFR≈4–5 s were not considered in the model. Figure S2 shows that the maximum NO_3exp in the OFR was obtained at τ_LFR=20 s at room temperature (unheated case); other NO_3exp values were normalized to this condition. Below τ_LFR=20 s, NO_3exp was suppressed due to higher NO₂ levels entering the OFR. Above τ_LFR=20 s, NO_3exp was suppressed due to lower N₂O₅ levels entering the OFR because of more extensive LFR wall loss.

In traditional studies of NO₃ oxidative aging processes that are conducted at low pressure and short residence time (τ∼1 s), N₂O₅ is heated to generate a burst of NO₃ prior to injection into the system (Knopf et al., 2011). While not experimentally considered in this work, we modeled the NO_3exp achieved assuming complete thermal dissociation of N₂O₅ between the LFR and OFR – for example, by heating to 120 ^∘C for 300 ms (Wood et al., 2003). Figure S2 suggests that the effect of heating N₂O₅ on NO_3exp was most significant at short τ_LFR, where [N₂O₅] at the exit of the LFR was higher due to less wall loss and room-temperature decomposition. For example, at τ_LFR=8 s, the modeled NO_3exp was 2.8 times higher in the complete-dissociation case than in the unheated case, whereas NO_3exp increased by factors of 2.3 and 1.5 at τ_LFR=20 and 60 s. Thus, a combination of reducing τ_LFR and heating N₂O₅ at the exit of the LFR increases NO_3exp and should be explored for future advanced implementations of OFR-iN₂O₅.

3.2 Example OFR-iN₂O₅ characterization studies

Figure 2a shows time series of O₃ and NO₂ concentrations during an OFR-iN₂O₅ characterization experiment where RH_OFR=11 %, $[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{280}$ ppm, and $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0}$ to 320 ppm. Figure 2b shows time series of acetonitrile (C₂H₃N), butanal (C₄H₈O), thiophene (C₄H₄S), 2, 3-dihydrobenzofuran (C₈H₈O), and naphthalene-d₈ (C₁₀D₈) signals measured during the same period. Following NO₃ generation, the fractional decay of C₂H₃N, C₄H₈O, C₄H₄S, and C₈H₈O increased with increasing tracer $k_{{\mathrm{NO}}_{\mathrm{3}}}$, as expected. C₈H₈O was too reactive to measure any significant changes in its decay as a function of OFR-iN₂O₅ conditions, as shown in Fig. 2; however, maximum decay of C₄H₈O and C₄H₄S was observed at [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{0.7}$ in this experiment. Decay of naphthalene-d₈, which was influenced by both NO₃ and NO₂ concentrations (Table S1), was maximized at $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{0.3}$ to 1.1.

Figure 2Time series from a representative OFR-iN₂O₅ characterization experiment conducted at RH_OFR=11 % of (a) O₃ and NO₂ mixing ratios input to LFR (left axis) and O₃ measured at the exit of the OFR (right axis), and (b) VOC tracers measured with PTR-MS: acetonitrile (C₂H₃N), butanal (C₄H₈O), thiophene (C₄H₄S), 2, 3-dihydrobenzofuran (C₈H₈O), and naphthalene-d₈ (C₁₀D₈).

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To confirm that the VOC degradation shown in Fig. 2b was due to reaction with NO₃, Fig. 3 shows IBBCEAS measurements of NO₃ obtained in separate OFR-iN₂O₅ characterization experiments that used $[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{150}$–160 ppm and $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.75}$ and 2.0. The maximum IBBCEAS signal observed at λ=662 nm indicated the presence of NO₃, as is evident from comparison with the wavelength-dependent absorption cross section of NO₃ obtained by Orphal et al. (2003) and plotted in Fig. 3b. Additionally, Fig. S3 shows the relative rate coefficient obtained from the decay of C₄H₈O and C₄H₄S measured with PTR-MS. We measured a relative rate coefficient of 2.83, which is in agreement with a relative rate coefficient value of 3.22±0.95 calculated from C₄H₈O+NO₃ and C₄H₄S+NO₃ rate coefficients (Atkinson, 1991; D'Anna et al., 2001). Ions corresponding to peroxy butyl nitrate, nitrothiophene, and nitronaphthalene-d₇, which are known NO₃ oxidation products of C₄H₈O, C₄H₄S, and C₁₀D₈, respectively (Atkinson et al., 1990; Jenkin et al., 2003; Saunders et al., 2003; Cabañas et al., 2005), were also detected with PTR-MS. Tracer decay experiments similar to the measurements shown in Fig. 2 were repeated over [O₃]_0, LFR ranging from 10 to 7400 ppm, [NO₂]_0, LFR ranging from 0 to 7200 ppm, and RH_OFR ranging from 7 % to 85 %. For experiments where [O₃]${}_{\mathrm{0},\mathrm{LFR}}>\mathrm{6000}$ ppm, NO_3exp was calculated from the decay of o-xylene because (1) p-cymene has a large ionized fragment at ${\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{9}}^{+}$ (thus interfering with detection of toluene), (2) NO₃ oxidation products were generated that interfered with detection of oxygenated tracers (butanol, benzaldehyde, and butanal), and (3) the remaining tracers that were used were too reactive towards NO₃ to accurately constrain NO_3exp.

Figure 3(a) IBBCEAS measurements of NO₂ and NO₃ absorbance obtained from an OFR-iN₂O₅ characterization experiment conducted at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{150}$–160 ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.75}$ and 2.0. (b) Absorption cross sections of NO₂ and NO₃ (Vandaele et al., 1998; Orphal et al., 2003).

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Figure 4NO_3exp as a function of RH_OFR at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{250}$ ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{130}$ ppm. Horizontal lines represent N₂O₅ wall loss rate constants ranging from 0.01 to 0.08 s⁻¹ that were input to the OFR-iN₂O₅ KinSim mechanism (Table S2).

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3.3 Effect of RH_OFR, [O₃]_0, LFR, and [NO₂]_0, LFR on NO_3exp

Figure 4 shows NO_3exp as a function of RH_OFR at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{250}$ ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{130}$ ppm. Under these conditions, NO_3exp decreased from 1.2×10¹⁴ to 2.0×10¹³ molecules cm⁻³ s as RH_OFR increased from 11 % to 81 %. We hypothesize that this result is due to more efficient hydrolysis of N₂O₅ to HNO₃ on the wetted walls of the OFR at higher RH, thereby suppressing NO_3exp relative to values obtained at lower RH conditions. In an attempt to model this behavior, $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ values input to the model were adjusted as a function of RH_OFR. Figure 4 suggests that humidity-dependent $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ values ranging from 0.01 to 0.08 s⁻¹ were required to cover the range of measured NO_3exp . These values agreed within a factor of 2 or better with humidity-dependent $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ values ranging from 0.014 to 0.040 s⁻¹ measured by Palm et al. (2017) in a similar OFR and were applied in subsequent model calculations.

Figure 5 shows NO_3exp as a function of [O₃]_0, LFR for measurements with [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.5}\pm \mathrm{0.1}$ and RH${}_{\mathrm{OFR}}=\mathrm{11}\pm \mathrm{2}$ %. The equivalent ambient photochemical age shown on the right y axis was calculated assuming a 14 h average nighttime NO₃ mixing ratio of 30 ppt and a 10 h daytime NO₃ mixing ratio of 0 ppt (Asaf et al., 2010). NO_3exp increased with increasing [O₃]_0, LFR due to increased NO₃ production from higher [N₂O₅]. Over the range of measured conditions, increasing [O₃]_0, LFR from 33 to 7092 ppm increased NO_3exp from 6.4×10¹² to 4.0×10¹⁵ molecules cm⁻³ s⁻¹. The black line in Fig. 5 represents NO_3exp modeled using the mechanism shown in Table S2. Measured and modeled NO_3exp values agreed within a factor of 2 or better above [O₃]${}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{40}$ ppm, and the gain in NO_3exp as a function of [O₃]_0, LFR was highest between [O₃]${}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{10}$ and 300 ppm. Over this range of [O₃]_0, LFR, the NO₂ oxidation lifetime with respect to O₃ decreased from 115 to 4 s. Because τ_LFR=20 s, under this range of LFR conditions, the NO₂ lifetime in the LFR was long enough that high NO₂ levels exiting the LFR suppressed NO_3exp in the OFR. In contrast, increasing [O₃]_0, LFR from 300 to 7000 ppm decreased the NO₂ oxidation lifetime with respect to O₃ from 4 to 0.2 s, and [NO₂] exiting the LFR was too low to significantly affect NO_3exp. To support this hypothesis, Fig. 6 plots NO_3exp as a function of [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$ at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{250}\pm \mathrm{20}$ ppm and 6850±400 ppm. Here, we incorporated NO_3exp values obtained over RH_OFR=11 % to 81 % for better statistics, and normalized each NO_3exp value to the maximum NO_3exp obtained at the same RH. Figure 6 shows that at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{250}$ ppm, the maximum NO_3, exp was achieved at [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{0.5}$ to 0.7. Conversely, at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{6850}$ ppm, the maximum NO_3exp value was achieved at [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{1.2}$.

Figure 5NO_3exp as a function of [O₃]_0, LFR for measurements with [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.5}\pm \mathrm{0.1}$. Equivalent ambient photochemical age was calculated assuming a 14 h average nighttime NO₃ mixing ratio of 30 ppt and 10 h daytime average NO₃ mixing ratio of 0 ppt (Asaf et al., 2010). Model inputs were $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}=\mathrm{0.01}$ s⁻¹ and ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}=\mathrm{0.07}$ s⁻¹ ($[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}<\mathrm{1000}$ ppm) or 0.38 s⁻¹ ([O₃]${}_{\mathrm{0},\mathrm{LFR}}>\mathrm{1000}$ ppm). The shaded region encompasses model output scaled by factors of 0.5 and 2.

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Figure 6NO_3exp as a function of [NO₂]$\mathrm{0},\mathrm{LFR}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$ at fixed [O₃]_0, LFR values of 250±20 and 6850±400 ppm and RH_OFR=11 % to 81 %. NO_3exp values were normalized to the maximum NO_3exp value obtained at the same RH.

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In a related set of experiments, IBBCEAS measurements of the NO₂:NO₃ ratio at the exit of the OFR (obtained from Fig. 3a spectra) confirmed that significantly higher NO₂ levels were present in the OFR at higher [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$, as expected. For example, at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{150}$ ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{112}$ ppm, ${\mathrm{NO}}_{\mathrm{2}}:{\mathrm{NO}}_{\mathrm{3}}=\mathrm{28}$, whereas at $[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{160}$ ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{320}$ ppm, ${\mathrm{NO}}_{\mathrm{2}}:{\mathrm{NO}}_{\mathrm{3}}=\mathrm{613}$. NO₂:NO₃, along with NO₃:O₃ and NO₂:NO₃, has important implications for the fate of organic species in OFR-iN₂O₅ that are discussed in the following sections.

3.4 Model characterization of OFR-i${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}:{\mathrm{NO}}_{\mathrm{3}}:{\mathrm{O}}_{\mathrm{3}}$, NO₂:NO₃, and NO₂:O₂

To examine OFR-iN₂O₅ performance over a wider range of conditions, Fig. 7 plots the mean NO_3exp, [O₃], NO₃:O₃, NO₂:NO₃, and NO₂:O₂ values obtained with the model as a function of [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{10}$ to 10⁵ ppm (10 %), for [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.01}$, 0.1, 0.5, 1.0, 1.5, 1.8, and 2.0. Three observations are apparent from Fig. 7. First, at [O₃]${}_{\mathrm{0},\mathrm{LFR}}<\mathrm{1000}$ ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.01}$ to 1.8, the maximum NO_3exp increased with [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$ prior to decreasing at [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}>\mathrm{1.0}$ (Fig. 7a). Above [O₃]${}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{2000}$ ppm and below [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{2.0}$, NO_3exp was less sensitive to [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$. Second, the maximum NO₃:O₃ increased with increasing [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$ above [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{1000}$ ppm (Fig. 7c). Third, the [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{2.0}$ case demonstrated unique behavior relative to the other cases because residual O₃ exiting the LFR was low (<10 ppm) due to almost complete conversion of O₃ to O₂ inside the LFR (Fig. 7b). Consequently, the high residual [NO₂] suppressed NO_3exp by 1 to 2 orders of magnitude relative to [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}<\mathrm{2}$ cases (Fig. 7a) and generated enhanced NO₃:O₃, NO₂:NO₃, and NO₂:O₂ values. In addition, NO₂:NO₃ ratios obtained from IBBCEAS measurements at [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{150}$ to 160 ppm and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.75}$, 1.0 and 2.0 are shown in Fig. 7d. The measured NO₂:NO₃ values are comparable to, or lower than, the modeled NO₂:NO₃ values obtained under similar conditions and, therefore, broadly support using model results to further investigate the fate of (1) RO₂ formed from NO₃ oxidation of VOCs, (2) alkyl radicals that are reactive towards NO₂ and O₂, and (3) VOCs that are reactive towards O₃ and NO₃ in the following sections.

Figure 7Modeled (a) NO_3exp, (b) [O₃], (c) NO₃:O₃, (d) NO₂:NO₃, and (e) NO₂:O₂ as a function of [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{10}$ to 10⁵ ppm, for [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.01}$, 0.1, 0.5, 1.0, 1.5, 1.8, and 2.0. Model inputs were $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}=\mathrm{0.01}$ s⁻¹, NO₃R_ext=0.07 s⁻¹. IBBCEAS-measured NO₂:NO₃ values are plotted in (d).

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3.4.1 Fate of organic peroxy radicals (RO₂) formed from NO₃ + VOC reactions

Organic peroxy radicals (RO₂) react with NO, NO₂, NO₃, HO₂, or other RO₂ to generate alkoxy (RO) radicals, peroxynitrates (RO₂NO₂), hydroperoxides or organic peroxides, and may additionally undergo autooxidation via sequential isomerization and O₂ addition. To investigate the fate of RO₂ as a function of OFR-iN₂O₅ conditions, we applied the methodology of Peng et al. (2019) by calculating the fractional oxidative loss of a generic alkyl or acyl RO₂ to each of these species over the range of conditions shown in Fig. 7. Kinetic data from Orlando and Tyndall (2012) that were used in these calculations are summarized in Table S4. Under almost all OFR-iN₂O₅ conditions shown in Fig. 7, RO₂ reactions with NO, HO₂, and RO₂ were minor (<1 %) loss pathways compared with reaction with NO₂ and NO₃. We conducted a model sensitivity analysis in which the RO₂+RO₂ reaction rate was enhanced by increasing NO₃R_ext from 0.07 to 0.7 s⁻¹ and increasing the RO₂+RO₂ rate constant from $\mathrm{1}\times {\mathrm{10}}^{-\mathrm{11}}$ to $\mathrm{1}\times {\mathrm{10}}^{-\mathrm{10}}$ cm³ molecule⁻¹ s⁻¹ (Berndt et al., 2018a, b). Despite these perturbations, the relative contribution of RO₂+RO₂ reactions to total RO₂ loss remained <1 % across this range of OFR-iN₂O₅ conditions.

To investigate the relative importance of competing RO₂+NO₂ and RO₂+NO₃ pathways, we defined the fractional reactive loss of RO₂ due to NO₃, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$:

$$\begin{array}{}\text{(3)}& F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}={\displaystyle \frac{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]}{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}\left[{\mathrm{NO}}_{\mathrm{2}}\right]}}\end{array}$$

Figure 8a and b show F${}_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ calculated for alkyl and acyl RO₂, respectively. To simplify the analysis, we assumed that the thermal decomposition of RO₂NO₂ species formed from RO₂+NO₂ reactions was slow compared with τ_OFR. This assumption generates a lower limit F${}_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ value for the alkyl RO₂ case, where RO₂NO₂ decomposition occurs on timescales of seconds or less (Orlando and Tyndall, 2012) but has minimal influence on the acyl-RO₂ case due to higher thermal stability of peroxyl acyl nitrates. For alkyl RO₂, Fig. 8a shows that $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}=\mathrm{0.5}$ was achieved between [NO₂, O₃]${}_{\mathrm{0},\mathrm{LFR}}=$ (125 ppm, 250 ppm) and (3240 ppm, 1800 ppm). For acyl RO₂, due to faster reaction with NO₂, Fig. 8b shows that $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}=\mathrm{0.5}$ was achieved using [NO₂, O₃]${}_{\mathrm{0},\mathrm{LFR}}=$ (350 ppm, 700 ppm) to (1.1 %, 0.6 %).

To investigate the feasibility of generating OFR-iN₂O₅ conditions where RO₂ loss is dominated by autooxidation, we calculated the lifetime of alkyl and acyl RO₂ (${\mathit{\tau }}_{{\mathrm{RO}}_{\mathrm{2}}}$) over the range of OFR-iN₂O₅ conditions shown in Fig. 7 and Fig. 8a and b. As shown in Fig. 8d and e, maximum ${\mathit{\tau }}_{{\mathrm{RO}}_{\mathrm{2}}}$ values of ≈1.4 s (alkyl) and 0.4 s (acyl) were obtained at [NO₂]${}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{2}$ ppm and [O₃]${}_{\mathrm{0},\mathrm{LFR}}\approx \mathrm{200}$ ppm. At lower [O₃]_0, LFR, ${\mathit{\tau }}_{{\mathrm{RO}}_{\mathrm{2}}}$ decreased due to a faster RO₂+NO₂ reaction rate, and at higher [O₃]_0, LFR, ${\mathit{\tau }}_{{\mathrm{RO}}_{\mathrm{2}}}$ decreased due to a faster RO₂+NO₃ reaction rate. Because RO₂ autooxidation timescales range from 0.005 to 200 s depending on the specific RO₂ composition (Crounse et al., 2013), OFR-iN₂O₅ may achieve autooxidation-dominant conditions for some RO₂ but not for others.

Figure 8$F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ for (a) alkyl and (b) acyl RO₂, and (c) $F_{\mathrm{R}+{\mathrm{O}}_{\mathrm{2}}}$ over the same OFR-iN₂O₅ operating conditions and model inputs used to generate Fig. 7, with the corresponding lifetimes for (d) alkyl and (e) acyl RO₂.

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3.4.3 Fate of VOCs reactive towards O₃ and NO₃

We defined the fractional reactive loss of a VOC with respect to NO₃, $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}$:

$$\begin{array}{}\text{(5)}& F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}={\displaystyle \frac{k_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]}{k_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]+k_{\mathrm{VOC}+{\mathrm{O}}_{\mathrm{3}}}\left[{\mathrm{O}}_{\mathrm{3}}\right]}}\end{array}$$

and we established $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}=\mathrm{0.9}$ as the criterion for NO₃-dominated oxidative loss. Figure 9 plots NO₃:O₃ at which $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}=\mathrm{0.9}$ for several classes of organic compounds with published $k_{{\mathrm{NO}}_{\mathrm{3}}}$ and $k_{{\mathrm{O}}_{\mathrm{3}}}$ values greater than 10⁻¹⁶ and 10⁻¹⁹ cm⁻³ molecules⁻¹ s⁻¹, respectively. Therefore, this figure excludes compounds such as alkanes and monocyclic aromatics that react slowly with NO₃ and are essentially unreactive towards O₃ ($F_{{\mathrm{NO}}_{\mathrm{3}}}\approx \mathrm{1}$). NO₃:O₃ values that correspond to [NO₂]_0, LFR and [O₃]${}_{\mathrm{0},\mathrm{LFR}}=$ (2 ppm, 200 ppm), (150 ppm, 300 ppm), and (5400 ppm, 3000 ppm) are represented by horizontal bands with upper and lower limit values calculated assuming $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$ values of 0.01 and 0.08 s⁻¹ (Sect. 3.3). These LFR inputs generated OFR-iN₂O₅ conditions that maximize the RO₂ lifetime and NO₃:O₃ at $\left[{\mathrm{NO}}_{\mathrm{2}}\right]:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0.5}$ and 1.8, respectively (Figs. 7, 8). Figures 7 and 9 as well as kinetic data from the literature suggest that the injection of 2 ppm NO₂ and 200 ppm O₃ into the LFR was sufficient to achieve $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}\ge \mathrm{0.9}$ for phenols, PAHs with no double bonds, and mono- and sesquiterpenes with one double bond at low RH_OFR. Increasing [NO₂]_0, LFR to 150 ppm and [O₃]_0, LFR to 300 ppm additionally achieved $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}\ge \mathrm{0.9}$ for acenaphthylene, isoprene, and mono- and sesquiterpenes with one double bond at elevated RH_OFR. Further increasing [NO₂]_0, LFR to 5400 ppm and [O₃]_0, LFR to 3000 ppm achieved $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}\ge \mathrm{0.9}$ for ≥ C3 linear alkenes, unsaturated aldehydes, and mono- and sesquiterpenes with two double bonds at low RH_OFR. While $[{\mathrm{NO}}_{\mathrm{2}},{\mathrm{O}}_{\mathrm{3}}]=$ [20 %, 10 %] (not shown) achieved $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}\ge \mathrm{0.9}$ for (E)-3-penten-2-one and ethene, the corresponding NO_3exp ≈10¹⁴ molecules cm⁻³ s achieved at this condition (Fig. 7a) was insufficient to oxidize more than 1 %–2 % of the initial ethene concentration due to its slow NO₃ rate constant (Atkinson, 1991).

Figure 9NO₃:O₃ at which $F_{\mathrm{VOC}+{\mathrm{NO}}_{\mathrm{3}}}=\mathrm{0.9}$ for representative VOCs with $k_{{\mathrm{NO}}_{\mathrm{3}}}>{\mathrm{10}}^{-\mathrm{16}}$ and $k_{{\mathrm{O}}_{\mathrm{3}}}>{\mathrm{10}}^{-\mathrm{19}}$ cm³ molecules⁻¹ s⁻¹ (Manion et al., 2015). Horizontal bands represent upper and lower limit values calculated assuming $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}=\mathrm{0.01}$ and 0.08 s⁻¹.

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3.5 NO₃ estimation equation for OFR-iN₂O₅

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, thereby providing a simpler alternative to detailed photochemical models for experimental planning and analysis. Here, we expand on those studies by deriving an NO_3exp estimation equation for OFR-iN₂O₅. Model results obtained from the base case of the model – a VOC reacting with NO₃ at $\mathrm{2.5}\times {\mathrm{10}}^{-\mathrm{12}}$ cm³ molecule⁻¹ s⁻¹ as a surrogate for NO₃R_ext – were used to derive the following equation that allows for the estimation of NO_3exp for OFR-iN₂O₅:

Table 1Fit parameters for NO_3exp estimation equation (Eq. 6).

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The phase space of OFR-iN₂O₅ parameters for fitting Eq. (6) to the NO_3exp model results was defined as follows: [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{10}$–1000 ppm, [NO₂]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{10}$–1000 ppm, $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}\le \mathrm{2}$, ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}=\mathrm{1}$–200 s⁻¹, $k_{{\mathrm{w}}_{\mathrm{OFR}},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}=\mathrm{0.01}$–0.08 s⁻¹, T_OFR=0–40 ^∘C, and τ_OFR=60–300 s. The cases where [O₃]${}_{\mathrm{0},\mathrm{LFR}}>\mathrm{1000}$ ppm and/or [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}>\mathrm{2}$ were not considered due to less practical interest. We explored 11, 11, 7, 4, and 5 logarithmically evenly distributed values in the ranges of [O₃]_0, LFR, [NO₂]_0, LFR (11 values over 10–1000 ppm), ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}$, $k_{\mathrm{w},{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}}$, and τ_OFR, respectively. Due to significantly different chemical regimes in different parts of the phase space, fit coefficients that are reported in Table 1 were obtained by fitting the same functional form (Eq. 6) over three subphase spaces with the following additional constraints: (1) $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0}$–1 and ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}=\mathrm{20}$–200 s⁻¹; (2) $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{0}$–1 and ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}=\mathrm{1}$–20 s⁻¹; and (3) $[{\mathrm{NO}}_{\mathrm{2}}{]}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}=\mathrm{1}$–2. For these three subspaces, 10080, 13440, and 5880 respective model cases were simulated. In Eq. (6), the terms involving the coefficients g–j were included to reproduce the relationship between normalized NO_3exp and [NO₂]${}_{\mathrm{0},\mathrm{LFR}}:[{\mathrm{O}}_{\mathrm{3}}{]}_{\mathrm{0},\mathrm{LFR}}$ shown in Fig. 5. Logarithms of first- and second-order terms were successively added until no further fit quality improvement was achieved. Figure 10 compares NO_3exp estimated from Eq. (6) and calculated from the model described in Sect. 2.2. The mean absolute value of the relative deviation was 49 % which is comparable to results obtained for previous estimation equations with significant NO_y chemistry (Peng et al., 2018).

NO₃R_ext of a system will change over the course of multiple generations of NO₃ oxidation due to changes in kinetic rate coefficients between different species and NO₃ ($k_{{\mathrm{NO}}_{\mathrm{3}}}$). The sensitivity of Eq. (6) to changes in ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}$ depends in part on the relative magnitudes of ${{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}$ and the internal NO₃ reactivity, NO₃R_int, which is approximately equal to $k_{{\mathrm{NO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$[NO₂]. If ${\mathrm{NO}}_{\mathrm{3}}{\mathrm{R}}_{\mathrm{int}}\gg {{\mathrm{NO}}_{\mathrm{3}}\mathrm{R}}_{\mathrm{ext}}$, changes in NO₃R_ext would have minimal influence on Eq. (6). In one case study, we examined changes in NO₃R_ext following conversion of biogenic VOCs (BVOCs) to gas-phase carbonyl oxidation products with known $k_{{\mathrm{NO}}_{\mathrm{3}}}$ values. Table S5 compares $k_{{\mathrm{NO}}_{\mathrm{3}}}$ of isoprene to methyl vinyl ketone and methacrolein, α-pinene to pinonaldehyde, sabinene to sabinaketone, and 3-carene to caronaldehyde. At the limit where 100 % of each BVOC is converted to its carbonyl oxidation product, NO₃R_ext decreases by a factor of 200 or greater. Unsaturated organic nitrates that are generated from BVOC + NO₃ may also be reactive towards NO₃, but $k_{{\mathrm{NO}}_{\mathrm{3}}}$ for these species are not available. In another case study, we examined changes in NO₃R_ext following conversion of BVOCs to SOA. An effective $k_{{\mathrm{NO}}_{\mathrm{3}}}$ for SOA was calculated using the following equation adapted from Lambe et al. (2009):

where F_diff is a correction factor accounting for diffusion limitations to the particle surface in the transition regime (Fuchs and Sutugin, 1970),

γ is the fraction of collisions between NO₃ and SOA resulting in reaction; D_p is the surface area-weighted mean particle diameter; ρ_p is the particle density; N_A is Avogadro's number; $\stackrel{\mathrm{‾}}{c}$ is the mean molecular speed of NO₃ (3.2×10⁴ cm s ⁻¹ at T=298 K); M_SOA is the mean molecular weight of the SOA; and $D_{{\mathrm{NO}}_{\mathrm{3}}}$ = 0.08 cm² s⁻¹ is the NO₃ diffusion coefficient in air (Rudich et al., 1996). Figure S4 shows $k_{\mathrm{SOA}+{\mathrm{NO}}_{\mathrm{3}}}$ as a function of D_p ranging from 1 to 1000 nm, assuming ρ_p=1.4 g cm⁻³, M_SOA=250 g mol⁻¹ (Nah et al., 2016), and an upper limit γ=0.1 for BVOC-derived SOA (Ng et al., 2017). For reference, the range of slowest (isoprene) and fastest (humulene) known $k_{\mathrm{BVOC}+{\mathrm{NO}}_{\mathrm{3}}}$ are indicated by the vertical blue line on the y axis. At the limit where 100 % of a BVOC is converted to SOA, NO₃R_ext decreases by a factor of 10 or greater depending on $k_{\mathrm{BVOC}+{\mathrm{NO}}_{\mathrm{3}}}$ and D_p. Taken together, these results suggest that NO₃R_ext decreases following NO₃ oxidation of BVOCs to carbonyl oxidation products and/or SOA. In this case, inputting NO₃R_ext of the BVOC precursor to Eq. (6) generates a lower limit to NO_3exp over multiple generations of NO₃ oxidation. Results for other systems will depend on the $k_{{\mathrm{NO}}_{\mathrm{3}}}$ values of associated gas- and condensed-phase precursors and their oxidation products.

Figure 10NO_3exp calculated from the estimation equation (Eq. 6 and Table 1) as a function of NO_3exp calculated from the full OFR-iN₂O₅ KinSim mechanism (Table S2). Solid and dashed lines correspond to the 1:1 and the 1:3, and 3:1 lines, respectively.

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3.6 SOA generation from β-pinene + NO₃

To apply the OFR-iN₂O₅ technique to SOA formation studies, we generated SOA from β-pinene + NO₃ in the absence of seed particles using [O₃]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{300}$ ppm, [NO₂]${}_{\mathrm{0},\mathrm{LFR}}=\mathrm{150}$ ppm, and RH_OFR≈1 %. PTR-MS measurements confirmed the complete consumption of β-pinene, and numerous product ions were detected. The largest ions detected were (H⁺)C₉H₁₄O and (H⁺)C₁₀H₁₄ which may correspond to nopinone (C₉H₁₄O) and fragmentation or decomposition products of C₁₀H₁₇NO₄, respectively (Hallquist et al., 1999; Claflin and Ziemann, 2018). The mass yield of SOA ranged from 0.03 to 0.39 over β-pinene mixing ratios ranging from 20 to 400 ppbv that were injected into the OFR. These yield values are broadly consistent with previous environmental chamber studies (Ng et al., 2017) but are lower than chamber SOA yields obtained at the same β-pinene mixing ratio, presumably due to the absence of seed particles in the OFR (Lambe et al., 2015). To compare the results obtained using OFR-iN₂O₅ with a conventional environmental chamber method, Fig. 11a and b show HR-ToF-AMS spectra of SOA generated from NO₃ oxidation of β-pinene in the Georgia Tech chamber (Boyd et al., 2015) and in the OFR, along with a scatter plot of relative ion abundances present in the two spectra (Fig. 11c). The same spectra are presented on a logarithmic scale in Fig. S5. As is evident, β-pinene + NO₃ SOA generated in the chamber and OFR exhibit a high degree of similarity (linear regression slope =0.98 and r²=0.99). The largest ion signal was observed at NO⁺, which, along with the signal at ${\mathrm{NO}}_{\mathrm{2}}^{+}$ and ${\mathrm{NO}}^{+}:{\mathrm{NO}}_{\mathrm{2}}^{+}=\mathrm{6.7}$, is consistent with the formation of particulate organic nitrates (Farmer et al., 2010). Signals observed at CHO⁺, C₂H₃O⁺, and other C_xH_yO${}_{>\mathrm{1}}^{+}$ ions suggest the presence of other multifunctional oxidation products.

Figure 11AMS spectra of SOA generated from NO₃ oxidation of β-pinene in (a) the Georgia Tech environmental chamber (Boyd et al., 2015) and (b) OFR-iN₂O₅. The scatter plot in (c) shows spectra generated in the OFR and in the chamber plotted against each other.

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