Technical note: Gas-phase nitrate radical generation via irradiation of aerated ceric ammonium nitrate mixtures

Abstract

We present a novel photolytic source of gas-phase ${\mathrm{NO}}_{\mathrm{3}}$ suitable for use in atmospheric chemistry studies that has several advantages over traditional sources that utilize ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ reactions and/or thermal dissociation of dinitrogen pentoxide (${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$). The method generates ${\mathrm{NO}}_{\mathrm{3}}$ via irradiation of aerated aqueous solutions of ceric ammonium nitrate (CAN, $\left({\mathrm{NH}}_{\mathrm{4}}{)}_{\mathrm{2}}\mathrm{Ce}\right({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}$) and nitric acid (${\mathrm{HNO}}_{\mathrm{3}}$) or sodium nitrate (${\mathrm{NaNO}}_{\mathrm{3}}$). We present experimental and model characterization of the ${\mathrm{NO}}_{\mathrm{3}}$ formation potential of irradiated CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures containing [CAN] $=$ 10${}^{\mathrm{-}\mathrm{3}}$ to 1.0 M, [${\mathrm{HNO}}_{\mathrm{3}}$] $=$ 1.0 to 6.0 M, [${\mathrm{NaNO}}_{\mathrm{3}}$] $=$ 1.0 to 4.8 M, photon fluxes ($I$) ranging from 6.9 $\times$ 10${}^{\mathrm{14}}$ to 1.0 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, and irradiation wavelengths ranging from 254 to 421 nm. ${\mathrm{NO}}_{\mathrm{3}}$ mixing ratios ranging from parts per billion to parts per million by volume were achieved using this method. At the CAN solubility limit, maximum [${\mathrm{NO}}_{\mathrm{3}}$] was achieved using [${\mathrm{HNO}}_{\mathrm{3}}$] $\approx$ 3.0 to 6.0 M and UVA radiation (${\mathit{\lambda }}_{max}$ $=$ 369 nm) in CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures or [${\mathrm{NaNO}}_{\mathrm{3}}$] $\ge$ 1.0 M and UVC radiation (${\mathit{\lambda }}_{max}$ $=$ 254 nm) in CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures. Other reactive nitrogen (${\mathrm{NO}}_{\mathrm{2}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$, ${\mathrm{HNO}}_{\mathrm{2}}$, ${\mathrm{HNO}}_{\mathrm{3}}$, ${\mathrm{HNO}}_{\mathrm{4}}$) and reactive oxygen (${\mathrm{HO}}_{\mathrm{2}}$, ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$) species obtained from the irradiation of ceric nitrate mixtures were measured using a ${\mathrm{NO}}_x$ analyzer and an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). To assess the applicability of the method for studies of ${\mathrm{NO}}_{\mathrm{3}}$-initiated oxidative aging processes, we generated and measured the chemical composition of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) from the $\mathit{\beta }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reaction using a Filter Inlet for Gases and AEROsols (FIGAERO) coupled to the HR-ToF-CIMS.

1 Introduction

The importance of ${\mathrm{NO}}_{\mathrm{3}}$ as a nighttime atmospheric oxidant is well established . ${\mathrm{NO}}_{\mathrm{3}}$ is generated via the reaction ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ $\to$ ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{O}}_{\mathrm{2}}$, followed by achievement of temperature-dependent equilibrium between ${\mathrm{NO}}_{\mathrm{3}}$, ${\mathrm{NO}}_{\mathrm{2}}$, and dinitrogen pentoxide (${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$). ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ also hydrolyzes efficiently to ${\mathrm{HNO}}_{\mathrm{3}}$ on aqueous surfaces . Thus, any investigation of the influence of ${\mathrm{NO}}_{\mathrm{3}}$ chemistry in a specific source region necessarily must account for the local temperature, humidity, and particle surface area along with other factors. Despite these complications, for decades, laboratory studies investigating gas-phase ${\mathrm{NO}}_{\mathrm{3}}$ chemistry have utilized the same ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ reactions and/or ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ thermal decomposition to produce ${\mathrm{NO}}_{\mathrm{3}}$ as it occurs in the atmosphere and accommodated the inherent limitations associated with ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$, namely, that it must be stored under cold and dry conditions until use. Few viable alternative methods for the generation of gas-phase ${\mathrm{NO}}_{\mathrm{3}}$ have been identified. Reactions between fluorine atoms and nitric acid (F $+$ ${\mathrm{HNO}}_{\mathrm{3}}$ $\to$ HF $+$ ${\mathrm{NO}}_{\mathrm{3}}$) or chlorine atoms and chlorine nitrate (Cl $+$ ${\mathrm{ClNO}}_{\mathrm{3}}$ $\to$ ${\mathrm{Cl}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$) require handling and/or synthesizing hazardous halogen-containing compounds . F and Cl can also compete with ${\mathrm{NO}}_{\mathrm{3}}$ for the oxidation of target analytes, as can ${\mathrm{O}}_{\mathrm{3}}$ if its reaction with ${\mathrm{NO}}_{\mathrm{2}}$ is used as the ${\mathrm{NO}}_{\mathrm{3}}$ source.

In the 1960s and 1970s, following earlier research into the properties of ceric solutions , Thomas Martin and coworkers discovered that irradiating solutions containing ceric ammonium nitrate (CAN, $\left({\mathrm{NH}}_{\mathrm{4}}{)}_{\mathrm{2}}\mathrm{Ce}\right({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}$) generates aqueous ${\mathrm{NO}}_{\mathrm{3}}$ . In $\gtrsim$ 6 M nitric acid (${\mathrm{HNO}}_{\mathrm{3}}$), CAN is thought to dissociate primarily into ${\mathrm{NH}}_{\mathrm{4}}^{+}$ cations and hexanitratocerate ($\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}^{\mathrm{2}-}$) anions . The $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}^{\mathrm{2}-}$ is subsequently reduced to $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{5}}^{\mathrm{2}-}$ upon irradiation by ultraviolet light, and ${\mathrm{NO}}_{\mathrm{3}}$ is generated as a primary photolysis product. A similar process occurs in other solvents, although the ensuing ceric composition in solution is complex and influenced by several factors. For example, in glacial acetic acid (${\mathrm{CH}}_{\mathrm{3}}\mathrm{COOH}$), CAN dissociates into primarily $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{4}}$ . Additionally, ceric ions containing complexed hydroxyl (OH) or ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$, ${\mathrm{CH}}_{\mathrm{3}}\mathrm{COOH}$, or acetonitrile (${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$) molecules are formed in aqueous, acetic acid, or ${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ media, respectively . Higher solution acidity and/or CAN concentration appears to promote the formation of $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}^{\mathrm{2}-}$ and ceric nitrate dimers . The following generalized mechanism was proposed by to describe ceric nitrate photochemistry: $$\begin{array}{}\text{R1}& {\displaystyle }{\mathrm{Ce}}^{\left(\mathrm{IV}\right)}+h\mathit{\nu }\to {\mathrm{Ce}}^{\left(\mathrm{III}\right)}+{\mathrm{NO}}_{\mathrm{3}},\text{R2}& {\displaystyle }{\mathrm{Ce}}^{\left(\mathrm{III}\right)}+{\mathrm{NO}}_{\mathrm{3}}\to {\mathrm{Ce}}^{\left(\mathrm{IV}\right)},\text{R3}& {\displaystyle }{\mathrm{NO}}_{\mathrm{3}}+{\mathrm{NO}}_{\mathrm{3}}\to {\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}},\text{R4}& {\displaystyle }{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}+\mathrm{2}{\mathrm{Ce}}^{\left(\mathrm{IV}\right)}\to \mathrm{2}{\mathrm{NO}}_{\mathrm{2}}+{\mathrm{O}}_{\mathrm{2}}+\mathrm{2}{\mathrm{Ce}}^{\left(\mathrm{III}\right)},\text{R5}& {\displaystyle }{\mathrm{NO}}_{\mathrm{3}}+{\mathrm{NO}}_{\mathrm{2}}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{2}{\mathrm{HNO}}_{\mathrm{3}},\end{array}$$ where ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ represents ceric nitrates as diverse as $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{4}}$, $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}^{\mathrm{2}-}$, $\left({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{5}}\mathrm{CeOCe}\right({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{5}}^{\mathrm{4}-}$, and $\left({\mathrm{H}}_{\mathrm{2}}\mathrm{O}{)}_{\mathrm{3}}\right({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{3}}\mathrm{CeOCe}\left({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{3}}\right({\mathrm{H}}_{\mathrm{2}}\mathrm{O}{)}_{\mathrm{3}}$ that are potentially formed in solution . Similarly, ${\mathrm{Ce}}^{\left(\mathrm{III}\right)}$ represents cerous nitrates such as $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{3}}$ and $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{5}}^{\mathrm{2}-}$. The rate of Reaction (R2) is [${\mathrm{HNO}}_{\mathrm{3}}$]-dependent , and the dinitrogen hexaoxide (${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$) intermediate was proposed on the basis of supporting observations without direct measurements .

CAN is used routinely as an oxidizing agent in organic synthesis due to its widespread availability, low cost, high oxidative potential, and low toxicity . However, its usage in atmospheric chemistry to date is limited to studies of ${\mathrm{NO}}_{\mathrm{3}}$-initiated oxidative aging processes in solution; e.g., . Given the potential simplicity of irradiating ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ mixtures relative to synthesizing and storing ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ under cold and dry conditions or reacting ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ under carefully controlled conditions, ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation could in principle enable more widespread studies of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation chemistry, which is understudied compared to OH chemistry . Here, for the first time, we investigated the use of ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation as a source of gas-phase ${\mathrm{NO}}_{\mathrm{3}}$. First, we designed a photoreactor that generates gas-phase ${\mathrm{NO}}_{\mathrm{3}}$ from irradiated CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures. Second, we characterized ${\mathrm{NO}}_{\mathrm{3}}$ concentrations achieved over a range of reactor operating conditions and mixture composition. Third, we characterized gas-phase reactive nitrogen and reactive oxygen species generated following ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation. Fourth, we demonstrated application of the method to generate and characterize oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) from the $\mathit{\beta }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reaction.

2 Methods

2.1 Photoreactor design and operation

Figure  shows a schematic of the experimental setup used in this study. A zero-air carrier gas flow of 0.5 L min${}^{-\mathrm{1}}$ was bubbled through a gas dispersion line consisting of 6.35 mm o.d. $\times$ 4.8 mm i.d. fluorinated ethylene propylene (FEP) tubing into approximately 10 mL of aqueous CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ or CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures placed at the bottom of a 12.7 mm o.d. $\times$ 11.1 cm i.d. FEP tube. The FEP tube was surrounded by low-pressure mercury fluorescent lamps installed vertically in a custom enclosure. These lamps had a 35.6 cm illuminated length. At these operating conditions, the calculated gas transit time in the illuminated portion of the reactor was approximately 3 s. After exiting the photoreactor, the carrier gas flow was passed through a filter holder (Savillex, 401-21-47-10-21-2) containing a 47 mm PTFE membrane filter (Pall Gelman, R2PJ047) to transmit ${\mathrm{NO}}_{\mathrm{3}}$ while removing stray droplets from the sample flow. At the end of each experiment, the lamps were turned off, the gas dispersion line was removed from the top of the reactor, and the FEP tubing and filter holder were flushed with distilled ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ to remove residual Ce${}^{\left(\mathrm{III}\right)}$ precipitate. Initial studies were conducted using a cavity-attenuated phase-shift (CAPS) ${\mathrm{NO}}_{\mathrm{2}}$ monitor operating at $\mathit{\lambda }$ $=$ 405 nm and a second retrofitted CAPS monitor operating at $\mathit{\lambda }$ $=$ 630 nm, which established that ${\mathrm{NO}}_{\mathrm{2}}$ and ${\mathrm{NO}}_{\mathrm{3}}$ were produced from irradiated ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$. Subsequent studies described in the next section used a 2B Technologies model 405 analyzer to measure NO and ${\mathrm{NO}}_{\mathrm{2}}$ .

Figure 1

Overview of experiments conducted in this study. Aqueous mixtures of ceric ammonium nitrate (CAN) and nitric acid (${\mathrm{HNO}}_{\mathrm{3}}$) or sodium nitrate (${\mathrm{NaNO}}_{\mathrm{3}}$) were irradiated in a photoreactor to generate nitrate radicals (${\mathrm{NO}}_{\mathrm{3}}$) in solution. Air was bubbled through the solution to evaporate ${\mathrm{NO}}_{\mathrm{3}}$ and other volatile photolysis products into the gas phase. The photoreactor effluent was then (i) injected into a dark oxidation flow reactor (OFR) along with a VOC mixture to characterize [${\mathrm{NO}}_{\mathrm{3}}$] via tracer decay measurements using a Vocus proton transfer reaction time-of-flight mass spectrometer (PTR-ToF) (ii) sampled with an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) (iii) injected into a dark OFR to characterize $\mathit{\beta }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products with a Filter Inlet for Gases and AEROsols (FIGAERO) coupled to the HR-ToF-CIMS. Supporting measurements were obtained using a ${\mathrm{NO}}_x$ analyzer.

[Figure omitted. See PDF]

Depending on the specific experiment, lamps with peak emission output centered at $\mathit{\lambda }$ $=$ 254, 313, 369, or 421 nm, respectively (GPH436TL/4P, Light Sources, Inc.; F436T5/NBUVB/4P-313, F436T5/BLC/4P-369, F436T5/SDI/4P-421, LCD Lighting, Inc.) were used. Emission spectra from the manufacturer 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. To quantify the photon flux $I_{\mathit{\lambda }}$ in the photoreactor for studies that used $\mathit{\lambda }$ $=$ 254, 313, or 369 nm radiation, we measured the rate of externally added ${\mathrm{O}}_{\mathrm{3}}$ ($\mathit{\lambda }$ $=$ 254 nm) or ${\mathrm{NO}}_{\mathrm{2}}$ photolysis ($\mathit{\lambda }$ $=$ 313 or 369 nm) as a function of lamp voltage under dry conditions (RH $<$ 5 %). The photon flux was not quantified in studies that used $\mathit{\lambda }$ $=$ 421 nm radiation. ${\mathrm{NO}}_{\mathrm{2}}$ photolysis measurements were conducted in the absence of oxygen to avoid ${\mathrm{O}}_{\mathrm{3}}$ formation. Photon flux values were then calculated using methods described in ; maximum values of $I_{\mathrm{254}}$ $=$ 1.0 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, $I_{\mathrm{313}}$ $=$ 6.0 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, and $I_{\mathrm{369}}$ $=$ 7.0 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ were obtained.

2.2 Characterization studies

In one set of experiments, the 0.5 L min${}^{-\mathrm{1}}$ photoreactor effluent was mixed with a 6.5 L min${}^{-\mathrm{1}}$ zero-air carrier gas and injected into a dark Potential Aerosol Mass oxidation flow reactor (OFR; Aerodyne Research, Inc.), which is a horizontal 13 L conductive Teflon-coated aluminum cylindrical chamber operated in continuous flow mode. Approximately 6.5 L min${}^{-\mathrm{1}}$ of sample flow was pulled from the reactor, resulting in a calculated mean residence time in the OFR (${\mathit{\tau }}_{\mathrm{OFR}}$) of approximately 120 s. To constrain ${\mathrm{NO}}_{\mathrm{3}}$ mixing ratios, a mixture of 10 VOC tracers with ${\mathrm{NO}}_{\mathrm{3}}$ reaction rate coefficients ($k_{{\mathrm{NO}}_{\mathrm{3}}}$) ranging from 3.01 $\times$ 10${}^{-\mathrm{19}}$ to 2.69 $\times$ 10${}^{-\mathrm{11}}$ cm${}^{\mathrm{3}}$ molec.${}^{-\mathrm{1}}$ s${}^{-\mathrm{1}}$ at $T$ $=$ 298 K (Table S1) was injected through a 10.2 cm length of 0.0152 cm i.d. Teflon tubing at a liquid flow rate of 0.94 $\mathrm{\mu }$L h${}^{-\mathrm{1}}$ using a syringe pump. The tracer mixture was then evaporated into a 1 L min${}^{-\mathrm{1}}$ zero-air carrier gas prior to injection into the OFR. The total external ${\mathrm{NO}}_{\mathrm{3}}$ reactivity (${\mathrm{NO}}_{\mathrm{3}}\mathrm{R}$${}_{\mathrm{ext}}$), which is the summed product of each tracer mixing ratio and its $k_{{\mathrm{NO}}_{\mathrm{3}}}$, was approximately 5 s${}^{-\mathrm{1}}$. VOCs with proton affinities greater than that of ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ were chosen to enable their measurement with a Tofwerk–Aerodyne Vocus proton transfer reaction time-of-flight mass spectrometer (hereafter referred to as “Vocus PTR”) operated using ${\mathrm{H}}_{\mathrm{3}}{\mathrm{O}}^{+}$ reagent ion chemistry and $\sim$ 8000 (Th $/$ Th) resolving power. ${\mathrm{NO}}_{\mathrm{3}}$ mixing ratios were calculated from the measured decrease in VOC mixing ratios using the Vocus PTR. Here, we assumed that the total concentration of reacted VOCs was equal to the concentration of ${\mathrm{NO}}_{\mathrm{3}}$ injected into the OFR. Because ${\mathrm{NO}}_{\mathrm{3}}$ may additionally react with organic peroxy radicals (${\mathrm{RO}}_{\mathrm{2}}$) generated from VOC $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reactions as well as OVOCs, these calculated ${\mathrm{NO}}_{\mathrm{3}}$ concentrations represent lower limits. Modeling calculations suggest that the fractional consumption of ${\mathrm{NO}}_{\mathrm{3}}$ by ${\mathrm{RO}}_{\mathrm{2}}$ ranged from $<$ 0.01 to 0.17 over the range of conditions that were studied (Fig. S2). A subset of OVOCs generated from VOC $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reactions that had proton affinities greater than that of ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ were also detected with the Vocus PTR.

In a separate set of experiments, the photoreactor effluent was diluted into 4 L min${}^{-\mathrm{1}}$ zero-air carrier gas and sampled with an Aerodyne iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS; hereafter referred to as “CIMS”; ) and the ${\mathrm{NO}}_x$ analyzer. The CIMS was operated at a $\sim$ 4000 (Th $/$ Th) resolving power. Iodide-adduct reagent ion chemistry was used due to its high sensitivity and selectivity towards nitrogen oxides and multifunctional organic nitrates . To demonstrate application of the method to study ${\mathrm{NO}}_{\mathrm{3}}$-initiated oxidative aging processes, the chemical composition of $\mathit{\beta }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}$ gas- and condensed-phase oxidation products was measured with a Filter Inlet for Gases and AEROsols (FIGAERO) coupled to the CIMS . Gas sampling and simultaneous particle collection were performed for 1 min intervals, followed by thermal desorption of the particle sample from a PTFE filter membrane (15 min ramp from room temperature to 200 ${}^{\circ }$C, 10 min holding time, 8 min cooldown to room temperature).

2.3 Photochemical model

To supplement our measurements, and to characterize aqueous-phase concentrations of species produced in the photoreactor that were not measured, we developed a photochemical box model that was implemented in the KinSim chemical kinetic solver . The KinSim mechanism shown in Table S2 contains reactions to model concentrations of ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$, ${\mathrm{Ce}}^{\left(\mathrm{III}\right)}$, NO, ${\mathrm{NO}}_{\mathrm{2}}$, ${\mathrm{NO}}_{\mathrm{3}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$, ${\mathrm{HNO}}_{\mathrm{2}}$, ${\mathrm{HNO}}_{\mathrm{3}}$, ${\mathrm{HNO}}_{\mathrm{4}}$, H, O, OH, ${\mathrm{HO}}_{\mathrm{2}}$, and ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$. We assumed that ${\mathrm{HNO}}_{\mathrm{3}}$ that was present in solution prior to irradiation completely dissociated into ${\mathrm{H}}^{+}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$. When possible, we used condensed-phase rate coefficients in the mechanism. For reactions that we assumed occurred but did not have published condensed-phase rate coefficients (e.g., ${\mathrm{NO}}_{\mathrm{3}}$ $+$ OH $\to$ ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{HO}}_{\mathrm{2}}$), we used published gas-phase rate coefficients instead with no modifications aside from unit conversion. Gas-phase wall loss rates of ${\mathrm{NO}}_x$, ${\mathrm{NO}}_y$, and ${\mathrm{HO}}_x$ species were not explicitly considered in the mechanism. UV–Vis extinction cross sections (${\mathit{\sigma }}_{\mathrm{ext}}$) of CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures were separately obtained between $\mathit{\lambda }$ $=$ 200 and 600 nm using an Agilent Cary 5000 UV–Vis–NIR spectrophotometer. Because of the high absorptivity and concentrations of the mixtures, samples were prepared in a 0.01 mm short-path-length cuvette (20/C-Q-0.01, Starna) to minimize saturation of the photodetector relative to a cuvette with a standard 10 mm path length. Even with the cuvette that was used, CAN dilution was necessary in some cases in order to obtain ${\mathit{\sigma }}_{\mathrm{ext}}$ without photodetector saturation at shorter wavelengths. Spectra were obtained as a function of [CAN] (0.047 to 0.526 M), [${\mathrm{HNO}}_{\mathrm{3}}$] (0 to 6.0 M), and [${\mathrm{NaNO}}_{\mathrm{3}}$] (0 to 4.0 M) to cover the approximate range of mixture compositions that were characterized in Sect. . The ${\mathit{\sigma }}_{\mathrm{ext}}$ values of the mixture were then calculated using the Beer–Lambert law and applied in the KinSim mechanism. Model outputs were obtained over a total experimental time of 14400 s at 1 s intervals.

3 Results and discussion

The maximum ${\mathrm{NO}}_{\mathrm{3}}$ quantum yield (${\mathit{\varphi }}_{{\mathrm{NO}}_{\mathrm{3}}}$) of UVA-irradiated CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures is obtained at 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ ; thus, this mixture composition served as the basis from which additional characterization studies were conducted. We found that 0.5 M CAN was the approximate solubility limit in 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ at 25 ${}^{\circ }$C. Because 1.1 M CAN is the solubility limit in ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ and CAN is nearly insoluble in ${\mathrm{HNO}}_{\mathrm{3}}$ , 0.7 M CAN is the estimated solubility limit in 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ in the absence of changes in ceric nitrate composition in solution. Thus, the reduction in CAN solubility (0.7 M $\to$ 0.5 M) observed in our studies was presumably associated with significant conversion of CAN to dimeric ceric nitrates in 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ .

3.1

${\mathrm{NO}}_{\mathrm{3}}$ characterization studies

Figure a shows time series of thiophene (${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{4}}\mathrm{S}$), 2,3-dihydrobenzofuran (${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{8}}\mathrm{O}$), cis-3-hexenyl acetate (${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{14}}{\mathrm{O}}_{\mathrm{2}}$), isoprene (${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{8}}$), dimethyl sulfide (${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}\mathrm{S}$), 2,5-dimethylthiophene (${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{8}}\mathrm{S}$), $\mathit{\alpha }$-pinene (${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}$), and guaiacol (${\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}{\mathrm{O}}_{\mathrm{2}}$) concentrations following injection into the OFR and exposure to ${\mathrm{NO}}_{\mathrm{3}}$ generated in the photoreactor from irradiation of a mixture of 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ at $I_{\mathrm{369}}$ $=$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$. Here, concentrations of each VOC were first normalized to the acetonitrile concentration to correct for changes in the syringe pump output over time and then normalized to the VOC concentration prior to ${\mathrm{NO}}_{\mathrm{3}}$ exposure. Aside from ${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{8}}\mathrm{S}$, whose relative decay was less pronounced than expected (Table S1), and butanal (${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{8}}\mathrm{O}$, not shown), whose signal decreased by approximately 30 % and did not recover for reasons that are unclear, the oxidative loss of each tracer increased with increasing $k_{{\mathrm{NO}}_{\mathrm{3}}}$. Maximum tracer consumption was observed at the beginning of the experiment due to maximum ${\mathrm{NO}}_{\mathrm{3}}$ production from ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation. As the experiment progressed and ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ was reduced to ${\mathrm{Ce}}^{\left(\mathrm{III}\right)}$, the ${\mathrm{NO}}_{\mathrm{3}}$ concentration and corresponding VOC oxidative loss decreased. Compared to the other VOCs, the initial increase in ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}$ and ${\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}{\mathrm{O}}_{\mathrm{2}}$ concentrations over the first 2 h was delayed because of their higher $k_{{\mathrm{NO}}_{\mathrm{3}}}$ values that resulted in $>$ 95 % consumption and lower sensitivity to changes in [${\mathrm{NO}}_{\mathrm{3}}$] in the initial stage of the experiment. To confirm that VOC degradation shown in Fig. a was due to reaction with ${\mathrm{NO}}_{\mathrm{3}}$, Fig. S3 shows the relative ${\mathrm{NO}}_{\mathrm{3}}$ rate coefficients obtained from the decay of ${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{4}}\mathrm{S}$, ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{8}}\mathrm{O}$, and ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{14}}{\mathrm{O}}_{\mathrm{2}}$ measured with the Vocus PTR. We measured relative rate coefficients of 3.59 between ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{8}}\mathrm{O}$ and ${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{4}}\mathrm{S}$ and 6.92 between ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{14}}{\mathrm{O}}_{\mathrm{2}}$ and ${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{4}}\mathrm{S}$, which are in agreement with relative rate coefficient values of 3.44 $\pm$ 1.20 and 7.68 $\pm$ 2.84 calculated from their absolute ${\mathrm{NO}}_{\mathrm{3}}$ rate coefficients . Time series of ions corresponding to nitrothiophene (${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{3}}{\mathrm{NO}}_{\mathrm{2}}\mathrm{S}$), ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{7}}{\mathrm{NO}}_{\mathrm{4}\text{–}\mathrm{6}}$ and ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{5},\mathrm{6}}$ organic nitrates, and nitroguaiacol (${\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{7}}{\mathrm{NO}}_{\mathrm{4}}$), which are known ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products of ${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{4}}\mathrm{S}$, ${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{8}}$, ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}$, and ${\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}{\mathrm{O}}_{\mathrm{2}}$ , along with ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{5},\mathrm{7}}{\mathrm{NO}}_{\mathrm{4}\text{–}\mathrm{6}}$ and ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{13}}{\mathrm{NO}}_{\mathrm{5}\text{–}\mathrm{6}}$ ions that may be associated with ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products of ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{8}}\mathrm{O}$ and ${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{14}}{\mathrm{O}}_{\mathrm{2}}$, respectively, were anticorrelated with those of their respective VOC precursors (Fig. S4). Tracer decay experiments, similar to the one shown in Fig. S3, were used to obtain results that are discussed in more detail in Sect. , , and .

Figure 2

Example results from an experiment in which a mixture of 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ was irradiated to generate ${\mathrm{NO}}_{\mathrm{3}}$ (${\mathit{\lambda }}_{max}$ $=$ 369 nm, I${}_{\mathrm{369}}$ $=$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$) that was injected into the OFR along with a reactive VOC tracer mixture. (a) Time series of the fractional consumption of VOC tracers measured with the Vocus following irradiation: thiophene (${\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{4}}\mathrm{S}$), 2,3-dihydrobenzofuran (${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{8}}\mathrm{O}$), cis-3-hexenyl-1-acetate (${\mathrm{C}}_{\mathrm{8}}{\mathrm{H}}_{\mathrm{14}}{\mathrm{O}}_{\mathrm{2}}$), isoprene (${\mathrm{C}}_{\mathrm{5}}{\mathrm{H}}_{\mathrm{8}}$), dimethyl sulfide (${\mathrm{C}}_{\mathrm{2}}{\mathrm{H}}_{\mathrm{6}}\mathrm{S}$), 2,5-dimethylthiophene (${\mathrm{C}}_{\mathrm{6}}{\mathrm{H}}_{\mathrm{8}}\mathrm{S}$), $\mathit{\alpha }$-pinene (${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}$), and guaiacol (${\mathrm{C}}_{\mathrm{7}}{\mathrm{H}}_{\mathrm{8}}{\mathrm{O}}_{\mathrm{2}}$). Signals of each tracer were normalized to their initial concentrations prior to ${\mathrm{NO}}_{\mathrm{3}}$ exposure and to acetonitrile concentrations to account for changes in the syringe pump output. (b) Time series of [${\mathrm{NO}}_{\mathrm{3}}$] calculated from (a) and Table S1.

[Figure omitted. See PDF]

3.2 Effect of irradiation wavelength

Figure a shows normalized [${\mathrm{NO}}_{\mathrm{3}}$] values obtained following irradiation of mixtures containing CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ or 4.8 M ${\mathrm{NaNO}}_{\mathrm{3}}$ as a function of irradiation wavelength. In CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures, [${\mathrm{NO}}_{\mathrm{3}}$] was a factor of 2.4–3.5 higher following irradiation at $\mathit{\lambda }$ $=$ 369 compared to the other wavelengths. On the other hand, [${\mathrm{NO}}_{\mathrm{3}}$] decreased with increasing irradiation wavelength following irradiation of CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures; at $\mathit{\lambda }$ $=$ 254 nm, [${\mathrm{NO}}_{\mathrm{3}}$] was 3.2–42 times higher than at the other irradiation wavelengths that were used. These differences in [${\mathrm{NO}}_{\mathrm{3}}$] were larger than the differences in calibrated photon flux values at the maximum output of each lamp type ($\pm$40 %; Sect. ). Different ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ in CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures may have influenced these trends, as suggested by their UV–Vis spectra (Fig. b). The ${\mathit{\sigma }}_{\mathrm{ext}}$ curves of CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures were generally larger, broader, and redshifted relative to those of CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures, with the extent of red shifting increasing with larger [${\mathrm{HNO}}_{\mathrm{3}}$], possibly due to higher yields of $\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}^{\mathrm{2}-}$ and/or ceric nitrate dimers . For $\mathit{\lambda }$ $>$ 250 nm, CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures had ${\mathit{\sigma }}_{\mathrm{ext},\max }$ values between $\mathit{\lambda }$ $=$ 306–311 nm, whereas CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ solutions had ${\mathit{\sigma }}_{\mathrm{ext},\max }$ values at $\mathit{\lambda }$ $=$ 296 nm. However, if [${\mathrm{NO}}_{\mathrm{3}}$] was simply proportional to ${\mathit{\sigma }}_{\mathrm{ext}}$, irradiation of CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures at $\mathit{\lambda }$ $=$ 313 nm should have produced the highest [${\mathrm{NO}}_{\mathrm{3}}$]; this was not the case. Instead, model calculations suggest that higher [${\mathrm{NO}}_{\mathrm{2}}$] obtained from significantly faster photolysis of ${\mathrm{HNO}}_{\mathrm{3}}$ at $\mathit{\lambda }$ $=$ 254 and 313 nm relative to $\mathit{\lambda }$ $>$ 350 nm suppressed ${\mathrm{NO}}_{\mathrm{3}}$ downstream of the photoreactor when shorter irradiation wavelengths were used (, Table S2). At a photon flux of 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, model-calculated [${\mathrm{NO}}_{\mathrm{3}}$] values were within $\pm$13 % of each other for irradiation wavelengths ranging from $\mathit{\lambda }$ $=$ 254 to 369 nm. However, higher [${\mathrm{NO}}_{\mathrm{2}}$] values obtained following ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation at $\mathit{\lambda }$ $=$ 254 and 313 nm suppressed ${\mathrm{NO}}_{\mathrm{3}}$ by $>$ 96 % relative to the $\mathit{\lambda }$ $=$ 369 nm case during 120 s of simulated ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reactions in the OFR. Thus, although the measured ${\mathrm{NO}}_{\mathrm{3}}$ suppression at these other irradiation wavelengths was less substantial than the model output, the measurement and model trends, along with achievement of maximum [${\mathrm{NO}}_{\mathrm{3}}$] following $\mathit{\lambda }$ $=$ 254 nm irradiation of CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures that had lower [${\mathrm{HNO}}_{\mathrm{3}}$], qualitatively support this explanation for the wavelength-dependent ${\mathrm{NO}}_{\mathrm{3}}$ yields observed in CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures.

Figure 3

(a) [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from irradiated CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN and 4.8 M ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures as a function of irradiation wavelengths. Results were normalized to [${\mathrm{NO}}_{\mathrm{3}}$] achieved with irradiation of CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures at $\mathit{\lambda }$ $=$ 369 nm or CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures at $\mathit{\lambda }$ $=$ 254 nm. Error bars represent $\pm$1$\mathit{\sigma }$ uncertainty in binned [${\mathrm{NO}}_{\mathrm{3}}$] values. (b) Extinction cross sections (${\mathit{\sigma }}_{\mathrm{ext}}$) of CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures (for details see Sect. ). The black dot (note the superscripted 1 in the legend) corresponds to data from .

[Figure omitted. See PDF]

3.3 Effect of mixture composition

To characterize the influence of individual reagents on ${\mathrm{NO}}_{\mathrm{3}}$ formation, tracer decay experiments similar to the measurements shown in Fig.  were repeated as a function of [CAN], [${\mathrm{HNO}}_{\mathrm{3}}$], and [${\mathrm{NaNO}}_{\mathrm{3}}$]. Figure a shows [${\mathrm{NO}}_{\mathrm{3}}$] obtained from irradiated 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ solutions containing 0.001 to 0.5 M CAN ($I_{\mathrm{369}}$ $=$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$) and irradiated 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ solutions containing 0.5 to 1.0 M CAN ($I_{\mathrm{254}}$ $=$ 1 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$). Results were normalized to [${\mathrm{NO}}_{\mathrm{3}}$] achieved with solutions containing 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$. Control experiments conducted with irradiated 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ or 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ solutions at $I_{\mathrm{254}}$ $=$ 1 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ in the absence of CAN suggest that a fraction of the ${\mathrm{NO}}_{\mathrm{3}}$ obtained in CAN mixtures was generated via the reactions ${\mathrm{HNO}}_{\mathrm{3}}$ $+$ $h\mathit{\nu }$ $\to$ OH $+$ ${\mathrm{NO}}_{\mathrm{2}}$ and ${\mathrm{HNO}}_{\mathrm{3}}$ $+$ OH $\to$ ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$. The remaining ${\mathrm{NO}}_{\mathrm{3}}$ was clearly obtained from CAN irradiation because [${\mathrm{NO}}_{\mathrm{3}}$] increased with increasing [CAN], as expected from Reaction (R1). Overall, [${\mathrm{NO}}_{\mathrm{3}}$] increased by approximately a factor of 3 as [CAN] was increased from 0.001 to 0.5 M in 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$.

Figure 4

[${\mathrm{NO}}_{\mathrm{3}}$] obtained from (a) irradiated 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ solutions containing 0.001 to 0.5 M CAN ($I_{\mathrm{369}}$ $=$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$), and irradiated 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ solutions containing 0.5 to 1.0 M CAN ($I_{\mathrm{254}}$ $=$ 1 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$). (b) Irradiated 0.5 M CAN solutions containing 1.0 to 6.0 M [${\mathrm{HNO}}_{\mathrm{3}}$] or 1.0 to 4.8 M [${\mathrm{NaNO}}_{\mathrm{3}}$] at the same $I_{\mathrm{369}}$ and $I_{\mathrm{254}}$ values used to obtain results shown in (a). Results were normalized to [${\mathrm{NO}}_{\mathrm{3}}$] achieved with mixtures of 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$. Error bars represent estimated $\pm$35 % uncertainty in [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures; $\pm$15 % uncertainty in [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures; and $\pm$10 % uncertainty in [CAN], [${\mathrm{HNO}}_{\mathrm{3}}$], and [${\mathrm{NaNO}}_{\mathrm{3}}$] values.

[Figure omitted. See PDF]

Figure b shows [${\mathrm{NO}}_{\mathrm{3}}$] obtained in irradiated solutions containing 0.5 M $\mathrm{CAN}$ as a function of [${\mathrm{HNO}}_{\mathrm{3}}$] ranging from 1.0 to 6.0 M or [${\mathrm{NaNO}}_{\mathrm{3}}$] ranging from 1.0 to 4.8 M at the same $I_{\mathrm{369}}$ and $I_{\mathrm{254}}$ values used to obtain results shown in Fig. a. Irradiated CAN solutions containing 3.0 and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ generated the same [${\mathrm{NO}}_{\mathrm{3}}$] concentrations within measurement uncertainties, presumably because the ${\mathrm{NO}}_{\mathrm{3}}$ quantum yield (${\mathit{\varphi }}_{{\mathrm{NO}}_{\mathrm{3}}}$) ranged from 0.92–1.00 over this range of acidity . [${\mathrm{NO}}_{\mathrm{3}}$] decreased by a factor of 2 as [${\mathrm{HNO}}_{\mathrm{3}}$] was decreased from 3.0 to 1.0 M, consistent with a reduction in ${\mathit{\varphi }}_{{\mathrm{NO}}_{\mathrm{3}}}$ from 0.92 to 0.46 . On the other hand, in irradiated CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures with uncharacterized ${\mathit{\varphi }}_{{\mathrm{NO}}_{\mathrm{3}}}$, [${\mathrm{NO}}_{\mathrm{3}}$] was constant within measurement uncertainties between 1.0 and 4.8 M ${\mathrm{NaNO}}_{\mathrm{3}}$.

Other mixture components that were tested or considered included substitution of ${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ in place of ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ and ${\mathrm{HNO}}_{\mathrm{3}}$, ammonium nitrate (${\mathrm{NH}}_{\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}$) instead of ${\mathrm{NaNO}}_{\mathrm{3}}$, ceric potassium nitrate (${\mathrm{K}}_{\mathrm{2}}\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}$) instead of CAN, and addition of sodium persulfate (${\mathrm{Na}}_{\mathrm{2}}{\mathrm{S}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{8}}$) to generate additional ${\mathrm{NO}}_{\mathrm{3}}$ via ${\mathrm{S}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{8}}^{\mathrm{2}-}$ $+$ $h\mathit{\nu }$ $\to$ 2${\mathrm{SO}}_{\mathrm{4}}^{-}$ followed by ${\mathrm{SO}}_{\mathrm{4}}^{-}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}^{-}$ $\to$ ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ . CAN $/$ ${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ mixtures are commonly used in organic synthesis applications, perhaps even more so than CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ . In limited testing, CAN $/$ ${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ appeared to generate significantly less ${\mathrm{NO}}_{\mathrm{3}}$ than CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ or CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$, possibly due to lower ${\mathit{\varphi }}_{{\mathrm{NO}}_{\mathrm{3}}}$ of irradiated ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$–${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ complexes and/or suppression of ${\mathrm{NO}}_{\mathrm{3}}$ due to its reaction with ${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ in solution. ${\mathrm{K}}_{\mathrm{2}}\mathrm{Ce}({\mathrm{NO}}_{\mathrm{3}}{)}_{\mathrm{6}}$ is less widely available and less water soluble than CAN and therefore was not considered further. Irradiation of CAN $/$ ${\mathrm{NH}}_{\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures generated similar [${\mathrm{NO}}_{\mathrm{3}}$], but we prefer ${\mathrm{NaNO}}_{\mathrm{3}}$ due to its lower volatility. Finally, ternary mixtures containing 0.5 M CAN $+$ 2.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ $+$ 0.5 M ${\mathrm{Na}}_{\mathrm{2}}{\mathrm{S}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{8}}$ irradiated at $\mathit{\lambda }$ $=$ 254 nm generated negligible additional ${\mathrm{NO}}_{\mathrm{3}}$ compared to binary CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures.

3.4 Effect of photon flux

Figure  shows normalized [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from irradiated mixtures of (1) 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ ($\mathit{\lambda }$ $=$ 369 nm) and (2) 0.5 M CAN and 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ ($\mathit{\lambda }$ $=$ 254 nm) as a function of photon flux ranging from 6.9 $\times$ 10${}^{\mathrm{14}}$ to 7.5 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$. Results for both CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ and CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures were normalized to [${\mathrm{NO}}_{\mathrm{3}}$] achieved with 0.5 M CAN, 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$, and $I_{\mathrm{369}}$ $=$ 6.8 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$. Symbols are colored by the ${\mathrm{NO}}_{\mathrm{3}}$ lifetime (${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{3}}}$), defined here as the time it took for [${\mathrm{NO}}_{\mathrm{3}}$] to experience one e-folding decay relative to the maximum [${\mathrm{NO}}_{\mathrm{3}}$] that was measured. Figure  shows that [${\mathrm{NO}}_{\mathrm{3}}$] increased with increasing photon flux, consistent with the fact that it is a primary photolysis product, along with a concurrent decrease in ${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{3}}}$ due to faster reduction of ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ to ${\mathrm{Ce}}^{\left(\mathrm{III}\right)}$. For the CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ system, [${\mathrm{NO}}_{\mathrm{3}}$] increased by a factor of 1.5 as $I_{\mathrm{369}}$ was increased from 6.9 $\times$ 10${}^{\mathrm{14}}$ to 6.8 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, in agreement with the model-calculated increase in [${\mathrm{NO}}_{\mathrm{3}}$] within measurement uncertainty. ${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{3}}}$ decreased from 9 to 5 h. For the CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ system, [${\mathrm{NO}}_{\mathrm{3}}$] increased by a factor of 1.9 as $I_{\mathrm{254}}$ was increased from 1.0 $\times$ 10${}^{\mathrm{15}}$ to 7.5 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, and ${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{3}}}$ decreased from 10 to 3 h.

Figure 5

Normalized [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from irradiated mixtures of 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ ($\mathit{\lambda }$ $=$ 369 nm) or 0.5 M CAN and 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ ($\mathit{\lambda }$ $=$ 254 nm) as a function of photon flux ranging from 6.9 $\times$ 10${}^{\mathrm{14}}$ to 7.5 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$. Results were normalized to [${\mathrm{NO}}_{\mathrm{3}}$] achieved with 0.5 M CAN, 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$, and $I_{\mathrm{369}}$ $=$ 6.8 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$. Symbols are colored by the time it took for [${\mathrm{NO}}_{\mathrm{3}}$] to experience one e-folding decay relative to the maximum [${\mathrm{NO}}_{\mathrm{3}}$] that was measured (${\mathit{\tau }}_{{\mathrm{NO}}_{\mathrm{3}}}$). Error bars represent estimated $\pm$35 % uncertainty in [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures, $\pm$15 % uncertainty in [${\mathrm{NO}}_{\mathrm{3}}$] values obtained from CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures, and $\pm$30 % uncertainty in photon flux values.

[Figure omitted. See PDF]

To examine concentrations of ${\mathrm{NO}}_{\mathrm{3}}$ and a subset of additional gas-phase photolysis products obtained over a wider range of conditions, Fig.  plots model-calculated $\left[{\mathrm{NO}}_{\mathrm{3}}\right]$, ${\mathrm{NO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$, ${\mathrm{HO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$, and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ values as a function of photon flux ranging from 1 $\times$ 10${}^{\mathrm{14}}$ to 1 $\times$ 10${}^{\mathrm{17}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ following $\mathit{\lambda }$ $=$ 254, 313, 369, and 421 nm irradiation of a mixture of 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$. Figure a also plots the measured [${\mathrm{NO}}_{\mathrm{3}}$] obtained from irradiation of a mixture of 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ at $I_{\mathrm{369}}$ $=$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ (Fig. ) after correcting for dilution between the photoreactor and the OFR (Sect. ) as well as application of a ${\mathrm{NO}}_{\mathrm{3}}$ wall loss rate coefficient of 0.2 s${}^{-\mathrm{1}}$ within the photoreactor . At this photon flux value, the model-calculated [${\mathrm{NO}}_{\mathrm{3}}$] $=$ 1.4 ppmv agrees with [${\mathrm{NO}}_{\mathrm{3}}$] $=$ 1.7 $\pm$ 0.6 ppmv obtained from measurements. When considering only the primary photochemical process (Reactions R1–R5), maximum $\left[{\mathrm{NO}}_{\mathrm{3}}\right]$ values within $\pm$10 % of each other were achieved at photon fluxes ranging from 5 $\times$ 10${}^{\mathrm{15}}$ ($\mathit{\lambda }$ $=$ 313 nm) to 4 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ ($\mathit{\lambda }$ $=$ 421 nm). $\left[{\mathrm{NO}}_{\mathrm{3}}\right]$ values decreased at higher photon flux values due to conversion of ${\mathrm{NO}}_{\mathrm{3}}$ to ${\mathrm{NO}}_{\mathrm{2}}$ via photolysis. As shown in Fig. b, significant additional ${\mathrm{NO}}_{\mathrm{2}}$ production was obtained via ${\mathrm{HNO}}_{\mathrm{3}}$ photolysis at shorter irradiation wavelengths above $I$ $\approx$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, resulting in ${\mathrm{NO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ $>$ 10 ($\mathit{\lambda }$ $=$ 254 nm) and 1 ($\mathit{\lambda }$ $=$ 313 nm). Given additional reaction time downstream of the photoreactor, high ${\mathrm{NO}}_{\mathrm{2}}$ may suppress ${\mathrm{NO}}_{\mathrm{3}}$ (Sect. ) and increase ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ beyond the range of values shown in Fig. c. We also calculated OH : ${\mathrm{NO}}_{\mathrm{3}}$ and ${\mathrm{HO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ following irradiation of CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures over the range of conditions shown in Fig. . Aqueous OH : ${\mathrm{NO}}_{\mathrm{3}}$ $\approx$ 0.1 and did not change significantly as a function of photon flux or irradiation wavelength, and aqueous ${\mathrm{HO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ values ranged from 0.05 ($\mathit{\lambda }$ $=$ 254 nm) to 0.25 ($\mathit{\lambda }$ $\ge$ 369 nm). While OH influenced aqueous-phase chemistry inside the photoreactor via formation of reactive oxygen species (Sect. ), OH probably did not influence downstream gas-phase chemistry due to significant wall losses inside the photoreactor: assuming a lower-limit OH wall loss rate coefficient of 5 s${}^{-\mathrm{1}}$ , the estimated OH penetration efficiency through the reactor was less than 10${}^{-\mathrm{6}}$.

Figure 6

Model-calculated (a) $\left[{\mathrm{NO}}_{\mathrm{3}}\right]$, (b) ${\mathrm{NO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$, (c) ${\mathrm{HO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$, and (d) ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ values in solution as a function of photon flux ranging from 1 $\times$ 10${}^{\mathrm{14}}$ to 1 $\times$ 10${}^{\mathrm{17}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ following $\mathit{\lambda }$ $=$ 254, 313, 369, and 421 nm irradiation of a mixture containing 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$. [${\mathrm{NO}}_{\mathrm{3}}$] obtained from measurements shown in Fig.  is plotted in (a). For details see Sect.  and Table S2.

[Figure omitted. See PDF]

3.5 Characterization of reactive nitrogen and reactive oxygen photolysis products

Figure  shows time series of reactive nitrogen and reactive oxygen species detected following irradiation of the same mixture of 0.5 M CAN and 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$ ($I_{\mathrm{254}}$ $\approx$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$), shown here because the signal-to-noise ratio in CIMS measurements of irradiated CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures was generally better than in measurements of irradiated CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures due to reagent ion depletion by ${\mathrm{HNO}}_{\mathrm{3}}$. A time series of [${\mathrm{NO}}_{\mathrm{3}}$] obtained separately from VOC tracer decay measurements under similar irradiation conditions is also shown. The ${\mathrm{NO}}_{\mathrm{2}}$ and ${\mathrm{NO}}_{\mathrm{3}}$ mixing ratios reached maximum values of 26 and 58 ppbv shortly after the lights were turned on (Fig. a), suggesting an initial ${\mathrm{NO}}_{\mathrm{2}}$ : ${\mathrm{NO}}_{\mathrm{3}}$ $\approx$ 0.45 (Fig. ). Multiple reactions may generate ${\mathrm{NO}}_{\mathrm{2}}$, including Reaction (R3), ${\mathrm{HNO}}_{\mathrm{3}}$ photolysis, and/or ${\mathrm{NO}}_{\mathrm{3}}$ photolysis as well as other reactions listed in Table S2. While ${\mathrm{NO}}_{\mathrm{2}}$ and/or ${\mathrm{HNO}}_{\mathrm{2}}$ photolysis generated NO, its concentration was negligible in these experiments.

Figure 7

Time series of (a) ${\mathrm{NO}}_{\mathrm{2}}$ and ${\mathrm{NO}}_{\mathrm{3}}$, (b) ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$, (c) ${\mathrm{HNO}}_{\mathrm{2}}$ and ${\mathrm{HNO}}_{\mathrm{4}}$, and (d) ${\mathrm{HO}}_{\mathrm{2}}$ and ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ detected following irradiation of a mixture containing 0.5 M CAN and 1.0 M ${\mathrm{NaNO}}_{\mathrm{3}}$. ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$, ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$, ${\mathrm{HO}}_{\mathrm{2}}$, and ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ were detected as ${\mathrm{I}}^{-}$ adducts, and ${\mathrm{HNO}}_{\mathrm{2}}$ and ${\mathrm{HNO}}_{\mathrm{4}}$ were detected as both ${\mathrm{I}}^{-}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ adducts with HR-ToF-CIMS. CIMS signals detected as iodide adducts were normalized to the ${\mathrm{I}}^{-}$ signal prior to the start of the experiment, and CIMS signals detected as nitrate adducts were normalized to the maximum ${\mathrm{NO}}_{\mathrm{3}}^{-}$ obtained during the experiment (see Fig. S5).

[Figure omitted. See PDF]

Figure b shows time series of ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}^{-}$ and ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}^{-}$ signals measured with the CIMS. ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}^{-}$ was formed from ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ $\to$ ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ reactions in the photoreactor and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ $+$ ${\mathrm{I}}^{-}$ $\to$ ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}^{-}$ reactions in the CIMS ion molecule reactor (IMR). As expected, ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}^{-}$ followed a similar profile as ${\mathrm{NO}}_{\mathrm{2}}$ and ${\mathrm{NO}}_{\mathrm{3}}$. ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}^{-}$ was either generated from ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ $\to$ ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$ reactions in the photoreactor followed by ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$ $+$ ${\mathrm{I}}^{-}$ $\to$ ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}^{-}$ reactions in the IMR or from the following series of reactions in the IMR: ${\mathrm{HNO}}_{\mathrm{3}}$ $+$ ${\mathrm{IO}}^{-}$ $\to$ ${\mathrm{NO}}_{\mathrm{3}}^{-}$ $+$ HOI, HOI $+$ ${\mathrm{NO}}_{\mathrm{3}}^{-}$ $\to$ ${\mathrm{INO}}_{\mathrm{3}}$ $+$ ${\mathrm{OH}}^{-}$, and ${\mathrm{INO}}_{\mathrm{3}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}^{-}$ $\to$ ${\mathrm{IN}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}^{-}$ . To further explore the plausibility of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$ formation in this system, we conducted a theoretical investigation of the gas-phase ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ $\to$ ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{6}}$ reaction and found that this reaction is exothermic, even more so than ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ $\to$ ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$. Additional details regarding this analysis are provided in Sect. S1.

Figure c shows time series of ${\mathrm{IHNO}}_{\mathrm{2}}^{-}$, ${\mathrm{HNO}}_{\mathrm{2}}{\mathrm{NO}}_{\mathrm{3}}^{-}$, ${\mathrm{IHNO}}_{\mathrm{4}}^{-}$, and ${\mathrm{HNO}}_{\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}^{-}$. These ions are associated with nitrous acid (${\mathrm{HNO}}_{\mathrm{2}}$) and peroxynitric acid (${\mathrm{HNO}}_{\mathrm{4}}$), respectively . Because rapid formation of ${\mathrm{HNO}}_{\mathrm{2}-\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}^{-}$ ions was observed following ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation, and because ${\mathrm{IO}}_x^{-}$ signals were relatively low (Sect. S2.1), we hypothesize that ${\mathrm{I}}^{-}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ and/or ${\mathrm{I}}^{-}$ $+$ ${\mathrm{HNO}}_{\mathrm{3}}$ reactions were the main source of ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and that subsequent competitive ${\mathrm{NO}}_{\mathrm{3}}^{-}$ $+$ ${\mathrm{HNO}}_{\mathrm{2}-\mathrm{4}}$ and ${\mathrm{I}}^{-}$ $+$ ${\mathrm{HNO}}_{\mathrm{2}-\mathrm{4}}$ reactions in the IMR generated both ${\mathrm{IHNO}}_{\mathrm{2}-\mathrm{4}}^{-}$ and ${\mathrm{HNO}}_{\mathrm{2}-\mathrm{4}}{\mathrm{NO}}_{\mathrm{3}}^{-}$. ${\mathrm{HNO}}_{\mathrm{4}}$ was generated following the reactions ${\mathrm{HNO}}_{\mathrm{3}}$ $+$ $h\mathit{\nu }$ $\to$ OH $+$ ${\mathrm{NO}}_{\mathrm{2}}$, OH $+$ ${\mathrm{NO}}_{\mathrm{3}}$ $\to$ ${\mathrm{HO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{2}}$, and ${\mathrm{HO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{2}}$ $\to$ ${\mathrm{HNO}}_{\mathrm{4}}$. This hypothesis is supported by the similarity between ${\mathrm{NO}}_{\mathrm{2}}$ and ${\mathrm{IHNO}}_{\mathrm{4}}^{-}$ time series coupled with the relatively constant concentrations of ${\mathrm{HO}}_{\mathrm{2}}$ generated via OH $+$ OH $\to$ ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ and OH $+$ ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$ $\to$ ${\mathrm{HO}}_{\mathrm{2}}$ $+$ ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ reactions. ${\mathrm{H}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$, detected as ${\mathrm{IH}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}^{-}$, also behaved similarly as ${\mathrm{IHO}}_{\mathrm{2}}^{-}$ (Fig. d). ${\mathrm{HNO}}_{\mathrm{2}}$ had a different temporal profile than the other nitrogen oxides: ${\mathrm{IHNO}}_{\mathrm{2}}^{-}$ increased throughout the experiment, and ${\mathrm{HNO}}_{\mathrm{2}}{\mathrm{NO}}_{\mathrm{3}}^{-}$ increased and then decreased. We hypothesize that ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{2}}$ $\to$ ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ $+$ ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ $\to$ ${\mathrm{HNO}}_{\mathrm{2}}$ $+$ ${\mathrm{HNO}}_{\mathrm{3}}$ reactions were the main source of ${\mathrm{HNO}}_{\mathrm{2}}$ (Sect. S2.2).

Figure S13 shows time series of the same ions plotted in Fig.  following irradiation of a solution containing 0.5 M CAN and 3.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ ($I_{\mathrm{369}}$ $\approx$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$). Here, 3.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ was used because 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ depleted the CIMS reagent ion too much (${\mathrm{IHNO}}_{\mathrm{3}}^{-}$ : ${\mathrm{I}}^{-}$ $\approx$ 15) to achieve signal-to-noise ratio that was sufficient for comparison to CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures (${\mathrm{IHNO}}_{\mathrm{3}}^{-}$ : ${\mathrm{I}}^{-}$ $\approx$ 3). The same gas-phase nitrogen oxides and reactive oxygen species were observed in this reaction system as with the irradiated CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixture. The relative yields of each compound plotted in Figs.  and S13 were within a factor of 3 of each other, although signals of nitrogen oxides and reactive oxygen species obtained from irradiated CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures decreased at a slower rate than the same compounds obtained from irradiated CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures. These trends may be due to different ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ composition (Fig.  and Sect. ) and/or an enhanced rate of ${\mathrm{Ce}}^{\left(\mathrm{III}\right)}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ $\to$ ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ reactions in ${\mathrm{HNO}}_{\mathrm{3}}$ relative to ${\mathrm{NaNO}}_{\mathrm{3}}$ (Reaction R2).

3.6

OVOC–SOA generation from $\mathit{\beta }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}$

To demonstrate proof of principle for ${\mathrm{NO}}_{\mathrm{3}}$-initiated oxidative aging studies, we generated ${\mathrm{NO}}_{\mathrm{3}}$ via irradiation of a mixture of 0.5 M CAN and 3.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ ($I_{\mathrm{369}}$ $=$ 7 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$), reacted it with $\mathit{\beta }$-pinene in a dark OFR, and obtained FIGAERO-CIMS spectra of gas- and condensed-phase $\mathit{\beta }$-pinene $+$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products (Sect. ). Figure a shows a spectrum of gas-phase $\mathit{\beta }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products detected between $m/Q$ $=$ 320 and 420, where the majority of the signal was observed; signals shown are unmodified (M $+$ I)${}^{-}$ formulas. The largest ion detected was at $m/Q$ $=$ 356 (${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{5}}^{-}$), which represents a major first-generation dicarbonyl nitrate oxidation product with a relative abundance of 0.31 and a calculated saturation vapor pressure of 2 $\times$ 10${}^{-\mathrm{7}}$ atm ($C^{\ast }$ $=$ 1900 $\mathrm{\mu }$g m${}^{-\mathrm{3}}$; ). Other ions corresponding to first-generation hydroxycarbonyl nitrate (${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{5}}^{-}$, $C^{\ast }$ $=$ 95 $\mathrm{\mu }$g m${}^{-\mathrm{3}}$), tricarbonyl nitrate (${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{6}}^{-}$, $C^{\ast }$ $=$ 35 $\mathrm{\mu }$g m${}^{-\mathrm{3}}$), hydroxydicarbonyl nitrate (${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{6}}^{-}$, $C^{\ast }$ $=$ 4.7 $\mathrm{\mu }$g m${}^{-\mathrm{3}}$), and hydroxycarbonyl nitrate acid (${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{7}}^{-}$, $C^{\ast }$ $=$ 0.29 $\mathrm{\mu }$g m${}^{-\mathrm{3}}$) products were detected in addition to ${\mathrm{IC}}_{\mathrm{9}}{\mathrm{H}}_{\mathrm{13}}{\mathrm{NO}}_{\mathrm{5}}^{-}$ and a suite of additional previously characterized ${\mathrm{C}}_{\mathrm{8}}$ and ${\mathrm{C}}_{\mathrm{9}}$ organic nitrates . The ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}^{-}$ hydroxy dinitrate, which was also previously observed in FIGAERO-CIMS spectra of $\mathit{\alpha }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ SOA , was generated via an unknown reaction pathway. Overall, the high molar yield and vapor pressure of ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{5}}$ are consistent with it having the highest relative abundance in the gas phase (Fig. a), whereas the other ${\mathrm{C}}_{\mathrm{10}}$ $\mathit{\beta }$-pinene oxidation products were semivolatile under our experimental conditions.

Figure 8

HR-ToF-CIMS spectra of gas-phase $\mathit{\beta }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products obtained following $\mathit{\beta }$-pinene reaction with ${\mathrm{NO}}_{\mathrm{3}}$ generated via (a) irradiation of a mixture of 0.5 M CAN and 3.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ and subsequent injection into the OFR (b) thermal decomposition of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ injected into the Georgia Tech environmental chamber. Signals shown are unmodified (M $+$ I)${}^{-}$ formulas.

[Figure omitted. See PDF]

Figure a shows a spectrum of condensed-phase $\mathit{\beta }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products obtained with the FIGAERO-CIMS; signals were averaged over the entire thermal desorption cycle and are plotted on logarithmic scale and represent unmodified (M $+$ I)${}^{-}$ formulas. To aid interpretation of the major features of the spectrum, bands of ion signals corresponding to ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_x^{-}$, ${\mathrm{IC}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_x^{-}$, and ${\mathrm{IC}}_{\mathrm{30}}{\mathrm{H}}_{\mathrm{47}}{\mathrm{N}}_{\mathrm{3}}{\mathrm{O}}_x^{-}$ oxidation products were highlighted and colored by the number of oxygen atoms in their chemical formulas. Here, the largest ion detected was at $m/Q$ $=$ 372 (${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{6}}^{-}$), which is the condensed-phase component of the same tricarbonyl nitrate detected in the gas phase (Fig. a). ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{5}}^{-}$ and ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_{\mathrm{7}-\mathrm{9}}^{-}$ signals were also detected. The second-largest ion signal was measured at $m/Q$ $=$ 571 (${\mathrm{IC}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{9}}^{-}$), an acetal dimer obtained from the condensed-phase reaction of two ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{5}}$ monomers followed by ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ elimination . Similar accretion reactions between other ${\mathrm{C}}_{\mathrm{10}}$ organic nitrates yielded ${\mathrm{IC}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{8}}^{-}$ and ${\mathrm{IC}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{10}-\mathrm{13}}^{-}$ signals. Likewise, reactions between ${\mathrm{C}}_{\mathrm{10}}$ monomers and ${\mathrm{C}}_{\mathrm{20}}$ dimers generated ${\mathrm{C}}_{\mathrm{30}}$ trimers detected between $m/Q$ $=$ 768–864 (${\mathrm{IC}}_{\mathrm{30}}{\mathrm{H}}_{\mathrm{47}}{\mathrm{N}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}-\mathrm{18}}^{-}$). The largest trimer-related ion, ${\mathrm{IC}}_{\mathrm{30}}{\mathrm{H}}_{\mathrm{47}}{\mathrm{N}}_{\mathrm{3}}{\mathrm{O}}_{\mathrm{12}}^{-}$, was generated from ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{4}}$ $+$ ${\mathrm{C}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{NO}}_{\mathrm{9}}$ $-$ ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ or ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{5}}$ $+$ ${\mathrm{C}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{NO}}_{\mathrm{8}}$ $-$ ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ reactions . A fourth cluster of ion signals at $m/Q$ $>$ 984 was also observed. Unambiguous assignment of chemical formulae to these signals was challenging due to the limited range of the CIMS $m/z$ calibration and lack of available information about $\mathrm{C}$${}_{>\mathrm{30}}$ $\mathit{\beta }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products. However, it seems plausible that these signals are associated with tetramers.

Figure 9

FIGAERO–HR-ToF-CIMS spectra of condensed-phase $\mathit{\beta }$-pinene $/$ ${\mathrm{NO}}_{\mathrm{3}}$ oxidation products obtained following $\mathit{\beta }$-pinene reaction with ${\mathrm{NO}}_{\mathrm{3}}$ generated via (a) irradiation of a mixture of 0.5 M CAN and 3.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ and subsequent injection into an OFR and (b) thermal decomposition of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ injected into the Georgia Tech environmental chamber. Signals shown are unmodified (M $+$ I)${}^{-}$ formulas. Bands of ion signals corresponding to ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_x$, ${\mathrm{C}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_x$, and ${\mathrm{C}}_{\mathrm{30}}{\mathrm{H}}_{\mathrm{47}}{\mathrm{N}}_{\mathrm{3}}{\mathrm{O}}_x$ oxidation products are highlighted and colored by the number of oxygen atoms in their chemical formulas.

[Figure omitted. See PDF]

To compare our results with those obtained using a conventional ${\mathrm{NO}}_{\mathrm{3}}$ generation method (room-temperature ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ thermal decomposition) in an environmental chamber study, Figs. b and b show reference gas- and condensed-phase FIGAERO-${\mathrm{I}}^{-}$-CIMS spectra of OVOCs and SOA generated from ${\mathrm{NO}}_{\mathrm{3}}$ oxidation of $\mathit{\beta }$-pinene in the Georgia Tech environmental chamber . The spectra obtained here and by exhibit an overall high degree of similarity, with linear correlation coefficients of 0.87 and 0.96 between the respective gas- and condensed-phase spectra. Clusters of ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{15}}{\mathrm{NO}}_x^{-}$, ${\mathrm{IC}}_{\mathrm{20}}{\mathrm{H}}_{\mathrm{32}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_x^{-}$, and ${\mathrm{IC}}_{\mathrm{30}}{\mathrm{H}}_{\mathrm{47}}{\mathrm{N}}_{\mathrm{3}}{\mathrm{O}}_x^{-}$ ion signals are present in both Fig. a and b. The main differences between the gas-phase spectra shown in Figs. a and a were the different abundances of ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{4}}^{-}$, a first-generation hydroxynitrate product , and ${\mathrm{IC}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}^{-}$. Because ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{4}}$ is formed from ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{RO}}_{\mathrm{2}}$ reactions and is sufficiently volatile ($C^{\ast }$ $=$ 750 $\mathrm{\mu }$g m${}^{-\mathrm{3}}$) to partition into the gas phase , differences in gas-phase ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{17}}{\mathrm{NO}}_{\mathrm{4}}$ and ${\mathrm{C}}_{\mathrm{10}}{\mathrm{H}}_{\mathrm{16}}{\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{7}}$ yields were likely related to differences in the relative importance of ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{RO}}_{\mathrm{2}}$ versus ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reaction pathways in the study by compared to this work.

To further investigate the fate of ${\mathrm{RO}}_{\mathrm{2}}$ generated from VOC $+$ ${\mathrm{NO}}_{\mathrm{3}}$ reactions as a function of CAN irradiation conditions, we calculated the fractional oxidative loss of generic alkyl and acyl ${\mathrm{RO}}_{\mathrm{2}}$ species due to reactions with ${\mathrm{HO}}_{\mathrm{2}}$, ${\mathrm{NO}}_{\mathrm{3}}$, and ${\mathrm{NO}}_{\mathrm{2}}$ ($F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$) using Eqs. ()–(): $$\begin{array}{}\text{1}& {\displaystyle }\begin{array}{rl}& F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}=\\ & {\displaystyle \frac{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}\left[{\mathrm{HO}}_{\mathrm{2}}\right]}{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}\left[{\mathrm{HO}}_{\mathrm{2}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}\left[{\mathrm{NO}}_{\mathrm{2}}\right]}},\end{array}\text{2}& {\displaystyle }\begin{array}{rl}& F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}=\\ & {\displaystyle \frac{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]}{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}\left[{\mathrm{HO}}_{\mathrm{2}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}\left[{\mathrm{NO}}_{\mathrm{2}}\right]}},\end{array}\text{3}& {\displaystyle }\begin{array}{rl}& F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}=\\ & {\displaystyle \frac{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}\left[{\mathrm{NO}}_{\mathrm{2}}\right]}{k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}\left[{\mathrm{HO}}_{\mathrm{2}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}\left[{\mathrm{NO}}_{\mathrm{3}}\right]+k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}\left[{\mathrm{NO}}_{\mathrm{2}}\right]}}.\end{array}\end{array}$$ Here, $k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$, $k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$, and $k_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$ are reaction rate coefficients for the corresponding ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{HO}}_{\mathrm{2}}$, ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{3}}$, and ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{NO}}_{\mathrm{2}}$ forward reactions whose values are summarized in Table S3. Several simplifying assumptions were made. First, we assumed that ${\mathrm{RO}}_{\mathrm{2}}$ $+$ NO reactions were negligible. Second, we did not consider ${\mathrm{RO}}_{\mathrm{2}}$ isomerization–autoxidation and ${\mathrm{RO}}_{\mathrm{2}}$ $+$ ${\mathrm{RO}}_{\mathrm{2}}$ reactions that are influenced by external factors. Third, we set $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$ $=$ 0 for alkyl-${\mathrm{RO}}_{\mathrm{2}}$-generated ${\mathrm{RO}}_{\mathrm{2}}{\mathrm{NO}}_{\mathrm{2}}$, which thermally decomposes on timescales of seconds or less . Fourth, we assumed that vapor wall losses of acyl-${\mathrm{RO}}_{\mathrm{2}}$-generated ${\mathrm{RO}}_{\mathrm{2}}{\mathrm{NO}}_{\mathrm{2}}$ were a minor ${\mathrm{RO}}_{\mathrm{2}}$ sink because the OFR residence time (${\mathit{\tau }}_{\mathrm{OFR}}$ $\approx$ 120 s, Sect. ) was significantly shorter than their estimated wall loss timescale (${\mathit{\tau }}_{\mathrm{wall}}$ $\approx$ 400 s; ). Figure  shows calculated $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$, and $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$ values for alkyl-${\mathrm{RO}}_{\mathrm{2}}$ and acyl-${\mathrm{RO}}_{\mathrm{2}}$ as a function of photon flux over the range of ${\mathrm{NO}}_{\mathrm{3}}$ generation conditions presented in Fig. . For alkyl-${\mathrm{RO}}_{\mathrm{2}}$, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$ decreased and $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ increased with increasing photon flux and decreasing irradiation wavelength. On the other hand, for acyl-${\mathrm{RO}}_{\mathrm{2}}$, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$ increased, while $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$ and $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ decreased over the same irradiation conditions. Overall, at the optimal ${\mathrm{NO}}_{\mathrm{3}}$ generation conditions (e.g., $\mathit{\lambda }=\mathrm{369}$ nm and $I_{\mathrm{369}}$ $\approx$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$), our calculations suggest that $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$ $\approx$ $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ for alkyl-${\mathrm{RO}}_{\mathrm{2}}$ (Fig. c) and that $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$ $\approx$ $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$ $\approx$ $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$ for acyl-${\mathrm{RO}}_{\mathrm{2}}$ (Fig. g).

Figure 10

Fractional oxidative loss of alkyl and acyl organic peroxy radicals (${\mathrm{RO}}_{\mathrm{2}}$) due to reaction with ${\mathrm{HO}}_{\mathrm{2}}$, ${\mathrm{NO}}_{\mathrm{3}}$, and ${\mathrm{NO}}_{\mathrm{2}}$ ($F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{HO}}_{\mathrm{2}}}$, $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{3}}}$, and $F_{{\mathrm{RO}}_{\mathrm{2}}+{\mathrm{NO}}_{\mathrm{2}}}$) generated following (a, e) $\mathit{\lambda }$ $=$ 254, (b, f) $\mathit{\lambda }$ $=$ 313, (c, g) $\mathit{\lambda }$ $=$ 369, and (d, h) $\mathit{\lambda }$ $=$ 421 nm irradiation of a mixture containing 0.5 M CAN and 6.0 M ${\mathrm{HNO}}_{\mathrm{3}}$ as a function of photon flux ranging from 1 $\times$ 10${}^{\mathrm{14}}$ to 1 $\times$ 10${}^{\mathrm{17}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$.

[Figure omitted. See PDF]

4 Conclusions

${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation complements ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ reactions and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ thermal dissociation as a customizable photolytic ${\mathrm{NO}}_{\mathrm{3}}$ source. Important method parameters were [CAN], [${\mathrm{HNO}}_{\mathrm{3}}$], or [${\mathrm{NaNO}}_{\mathrm{3}}$]; UV intensity; and irradiation wavelength. By contrast, important parameters for ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ and ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ based methods are [${\mathrm{O}}_{\mathrm{3}}$], [${\mathrm{NO}}_{\mathrm{2}}$], temperature, and humidity. Because ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation already generates ${\mathrm{NO}}_{\mathrm{3}}$ in aqueous solution, its performance is not hindered by humidity to the same extent (if at all) as ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$-based methods, where hydrolysis of ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ to ${\mathrm{HNO}}_{\mathrm{3}}$ decreases the efficacy of the source. Additionally, the ${\mathrm{NO}}_{\mathrm{3}}$ $+$ ${\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ reaction rate in solution or on surfaces is slow in relation to other ${\mathrm{NO}}_{\mathrm{3}}$ loss pathways . Another advantage of ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation is that it does not involve the use of ${\mathrm{O}}_{\mathrm{3}}$ as a reagent; therefore it eliminates the possibility of competing ${\mathrm{O}}_{\mathrm{3}}$ and ${\mathrm{NO}}_{\mathrm{3}}$ oxidation of compounds that are reactive towards both oxidants if ${\mathrm{NO}}_{\mathrm{2}}$ $+$ ${\mathrm{O}}_{\mathrm{3}}$ reactions and/or online ${\mathrm{N}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5}}$ synthesis are used as the ${\mathrm{NO}}_{\mathrm{3}}$ source . To identify optimal operating conditions for maximizing [${\mathrm{NO}}_{\mathrm{3}}$], we characterized concentrations of ${\mathrm{NO}}_{\mathrm{3}}$ at [CAN] $=$ 10${}^{-\mathrm{3}}$ to 1 M, [${\mathrm{HNO}}_{\mathrm{3}}$] $=$ 1.0 to 6.0 M, [${\mathrm{NaNO}}_{\mathrm{3}}$] $=$ 1.0 to 4.8 M, a photon flux of 6.9 $\times$ 10${}^{\mathrm{14}}$ to 1.0 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$, and irradiation wavelengths of $\mathit{\lambda }$ $=$ 254, 313, 369, or 421 nm. With CAN $/$ ${\mathrm{HNO}}_{\mathrm{3}}$ mixtures, maximum [${\mathrm{NO}}_{\mathrm{3}}$] was achieved with [CAN] $\approx$ 0.5 M, [${\mathrm{HNO}}_{\mathrm{3}}$] $\approx$ 3.0 to 6.0 M, and $I_{\mathrm{369}}$ $=$ 8 $\times$ 10${}^{\mathrm{15}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ (4.3 mW cm${}^{-\mathrm{2}}$). With CAN $/$ ${\mathrm{NaNO}}_{\mathrm{3}}$ mixtures, maximum [${\mathrm{NO}}_{\mathrm{3}}$] was achieved with [CAN] $\approx$ 1.0 M, [${\mathrm{NaNO}}_{\mathrm{3}}$] $\ge$ 1.0 M, and $I_{\mathrm{254}}$ $\approx$ 1 $\times$ 10${}^{\mathrm{16}}$ photons cm${}^{-\mathrm{2}}$ s${}^{-\mathrm{1}}$ (7.8 mW cm${}^{-\mathrm{2}}$). Thus, for applications such as environmental chambers or OFR studies of ${\mathrm{NO}}_{\mathrm{3}}$-initiated oxidative aging processes, where significant ${\mathrm{NO}}_{\mathrm{3}}$ production over relatively short time periods is beneficial, irradiation of concentrated ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ solutions at high photon flux is advantageous. Other applications that require sustained ${\mathrm{NO}}_{\mathrm{3}}$ production at lower concentrations and/or over longer time periods may benefit from using lower [${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$] and photon flux. Overall, because ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ irradiation generates ${\mathrm{NO}}_{\mathrm{3}}$ at room temperature using widely available, low-cost reagents and light sources (including high-power light-emitting diodes in addition to, or instead of, UV fluorescent lamps), it is easier to apply than other ${\mathrm{NO}}_{\mathrm{3}}$ generation techniques – especially in field studies – and it may therefore enable more widespread studies of ${\mathrm{NO}}_{\mathrm{3}}$ oxidation chemistry. Adapting a photoreactor to operate with continuous injection of fresh ${\mathrm{Ce}}^{\left(\mathrm{IV}\right)}$ or alternative photolytic ${\mathrm{NO}}_{\mathrm{3}}$ precursors (e.g., ) rather than in batch mode, as was done here, may further enhance its performance and will be investigated in future work.

Code and data availability

Data presented in this paper are available upon request. The KinSim mechanism used in this paper is included with the Supplement. The KinSim kinetic solver is freely available at https://gitlab.com/JimenezGroup/KinSim_Code .

The supplement related to this article is available online at: https://doi.org/10.5194/acp-23-13869-2023-supplement.

Author contributions

ATL, BB, and PL conceived and planned the experiments. ATL, BB, MWA, and PL carried out the experiments. ATL conceived, planned, and carried out the KinSim model simulations. NO and PMZ conceived, planned, and carried out the quantum chemical calculations. ATL, BB, MT, NO, PMZ, MSC, DRW, and PL contributed to the interpretation of the results. ATL took the lead in writing the paper. All authors provided feedback on the paper.

Competing interests

At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Disclaimer

Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors.

Acknowledgements

Andrew T. Lambe thanks Anita Avery, Jordan Krechmer, Timothy Onasch (Aerodyne), and Shreya Suri (Georgia Tech) for experimental assistance; Evgeni Glebov (Russian Academy of Sciences) for sharing published UV–Vis spectra of CAN $/$ ${\mathrm{CH}}_{\mathrm{3}}\mathrm{CN}$ mixtures; and the following colleagues for helpful discussions: Harald Stark, Manjula Canagaratna (Aerodyne), Steve Brown (NOAA CSL), Hartmut Herrmann (TROPOS), William Brune (Pennsylvania State University), Tyson Berg (Colorado State University), Lasse Moormann (Max Planck Institute for Chemistry), Uta Wille (University of Melbourne), and Burkhard Koenig (University of Regensburg). The authors thank the anonymous referee and Sergey A. Nizkorodov for their constructive comments during the paper review process.

Financial support

This work was supported by the Atmospheric Chemistry Program of the US National Science Foundation (grant nos. AGS-2131368, AGS2148439, AGS-2131458, AGS-2131084, and AGS-2147893).

Review statement

This paper was edited by Sergey A. Nizkorodov and reviewed by Sergey A. Nizkorodov and one anonymous referee.