Exploring the hydrogen abstraction pathway in HOM formation from α-pinene photooxidation systems under varying NO conditions

Highly oxygenated organic molecules (HOM) are formed via autoxidation during •OH-initiated oxidation of α-pinene. We investigated the relative contributions of OH-addition and hydrogen (H)-abstraction to HOM formation from α-pinene photooxidation under varying nitrogen oxide (NO) conditions. HOM molecules were detected by a nitrate chemical ionization mass spectrometer (CIMS). In the absence of NO, C₁₀H₁₇O_x• peroxy radicals and related termination products (e.g. C₁₀H₁₈O_x) dominated the HOM spectrum, accounting for > 70 % of total HOM. The presence of NO substantially altered HOM products, particularly by rapid formation of C₁₀H₁₅O_x•-related HOM, like C₁₀H₁₅NO₈. The ratio of C₁₀H₁₅NO_x to C₁₀H₁₇NO_x increased from 0.34 to 0.84 as the RO₂• loss rate via reaction with NO increased from 0.18 to 1.06 s⁻¹. Under high-NO conditions, C₁₀H₁₅O_x•-related HOM contributed up to 34 % to total HOM from α-pinene oxidation systems. The H-abstraction channel proved to be the source of C₁₀H₁₅O_x•-related HOM. Fuzzy c-means clustering indicated that C₁₀H₁₅O_x•-related HOM exhibited the fastest formation rate among the identified HOM groups, consistent with first-generation products. Comparison with pinonaldehyde oxidation, obtained by normalizing HOM yields to pinonaldehyde turnover, suggests that pinonaldehyde contributed ∼ 5 % of HOM in α-pinene systems, excluding secondary oxidation as the dominant source. Detection of C₁₀H₁₅NO₄ under high-NO conditions by propylamine-CIMS indicates the formation of C₁₀H₁₅O₃• peroxy radicals, formed by alkoxy radical decomposition and six-membered ring opening in the H-abstraction channel. This study highlights the role of the H-abstraction pathway in •OH-initiated α-pinene oxidation under NO-influenced conditions and provides new constraints on detailed HOM formation mechanisms.

1 Introduction

Secondary organic aerosols (SOA) in the atmosphere contribute significantly to particulate matter in the PM₁ size range and can affect regional air quality, human health and global radiative forcing (Jimenez et al., 2009; Peng et al., 2016). The oxidation of biogenic volatile compounds (BVOC) contributes significantly to the total formed SOA. Highly oxygenated organic compounds (HOM) are produced via autoxidation of peroxy radicals (RO₂•). They typically contain at least six oxygen atoms and consequently show a low to extremely low volatility. This enables them to nucleate or to condense onto existing particles and thus to significantly contribute to SOA formation (Bianchi et al., 2019; Mentel et al., 2015; Ehn et al., 2014).

α-pinene is globally the most abundant monoterpene and has been shown to form significant amounts of HOM when it is oxidized by O₃ or by hydroxyl radicals (•OH) (Ehn et al., 2014; Berndt et al., 2016). •OH initiated oxidation of α-pinene occurs via the OH-addition pathway (∼ 90 %) and the hydrogen (H)-abstraction pathway (∼ 10 %), leading to the formation of peroxy radicals (RO₂•) with the formulas C₁₀H₁₇O₃• and C₁₀H₁₅O₂•, respectively (Vereecken et al., 2007).

The OH-addition pathway is known to effectively form HOM with fast rates when the four-membered ring is broken (Berndt, 2021; Xu et al., 2019). However, the potential role of the H-abstraction pathway for HOM formation has received far less attention and was thought to be negligible until Shen and coworkers suggested its significant contribution to HOM (Shen et al., 2022). In addition, Luo et al. (2023) also highlighted the significance of H-abstraction to HOM formation from limonene photooxidation in the presence of NO. Therefore, the H-abstraction pathway can be a potential explanation for the observations in Hyytiälä where C₁₀H₁₅O_x•-related HOM species (e.g. C₁₀H₁₅NO₈) are dominant and have been identified as fingerprints of daytime oxidation (Yan et al., 2016). However, confirmation and clarification of the contribution of the H-abstraction pathway to HOM formation are still required. In the study by Shen et al. (2022) the potential role of the conversion of pinonaldehyde to C₁₀H₁₅O_x• species was only estimated and not directly measured. The extent to which pinonaldehyde oxidation contributes to C₁₀H₁₅O_x•-related HOM in α-pinene systems remains uncertain.

The detection of C₁₀H₁₅O_x• related products with fewer than five oxygen atoms suggests contributions from H-abstraction pathways. However, Shen et al. (2022) reported only HOM with oxygen numbers greater than six, and NO was present in all experiments performed (Shen et al., 2022). The presence or absence of NO can have a significant impact, as it strongly influences the rearrangement of alkoxy radicals, which is crucial to facilitate HOM formation via the H-abstraction pathway (Shen et al., 2022). Therefore, systematic experiments under varying NO conditions, especially including NO-free conditions, are essential for confirming the occurrence of the H-abstraction pathway, to elucidate its product distribution, and to clarify its role in HOM and subsequent SOA formation in α-pinene photooxidation systems.

Competition between termination pathways of C₁₀H₁₇O_x• and C₁₀H₁₅O_x• peroxy radicals determines the product distributions (Mentel et al., 2015). All products are separated into various families based on the numbers of carbon atoms, hydrogen atoms and oxygen atoms. The C₁₀H₁₈O_x family, which includes compounds containing ten carbon atoms, eighteen hydrogen atoms, and a varying number of oxygen atoms with hydroperoxide or alcohol functions, can only be attributed to C₁₀H₁₇O_x• via termination either by HO₂• radicals (Reaction R1) or by other peroxy radicals (R^′O₂•) via Reaction (R3) (Baker et al., 2024). Similarly, the organic nitrate C₁₀H₁₇NO_x family is formed via the reaction of C₁₀H₁₇O_x• with NO (Reaction R6) (Berndt, 2021). The C₁₀H₁₄O_x family, comprising carbonyl containing compounds, is generated either via reactions of C₁₀H₁₅O_x• with R^′O₂• or via self-termination (Reaction R8) (Rissanen et al., 2014). In the atmosphere, alkoxy radicals with the formula C₁₀H₁₅O_x−1• are mainly products of the reaction between C₁₀H₁₅O_x• and NO (Reaction R2) (Berndt, 2021). The C₁₀H₁₅NO_x family is uniquely attributed to the reaction of C₁₀H₁₅O_x• peroxy radicals with NO (Reaction R6). Thus, the appearance of HOM with formulas C₁₀H₁₈O_x and C₁₀H₁₇NO_x can clearly be traced to C₁₀H₁₇O_x•, while C₁₀H₁₄O_x and C₁₀H₁₅NO_x are clearly related to C₁₀H₁₅O_x•. The formation of the C₁₀H₁₆O_x family can occur via multiple pathways including termination reactions of C₁₀H₁₅O_x• with HO₂• or R^′O₂•, termination of C₁₀H₁₇O_x• with R^′O₂•, or self-termination of C₁₀H₁₇O_x•. The important initiation of reactions is shown in Fig. S1 in the Supplement, and termination pathways of RO₂• and corresponding products are shown below in Fig. 1.

$$\begin{array}{}\text{(R1)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+{\mathrm{HO}}_{\mathrm{2}}\mathrm{•}\to \text{ROOH }+{\mathrm{O}}_{\mathrm{2}}\text{(R2)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+{\mathrm{HO}}_{\mathrm{2}}\mathrm{•}\to \mathrm{RO}\mathrm{•}+\mathrm{•}\mathrm{OH}+{\mathrm{O}}_{\mathrm{2}}\text{(R3)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+{\mathrm{R}}^{\prime }{\mathrm{O}}_{\mathrm{2}}\mathrm{•}\to \mathrm{R}=\mathrm{O}+{\mathrm{R}}^{\prime }\mathrm{OH}+{\mathrm{O}}_{\mathrm{2}}\text{(R4)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+{\mathrm{R}}^{\prime }{\mathrm{O}}_{\mathrm{2}}\mathrm{•}\to \mathrm{RO}\mathrm{•}+{\mathrm{R}}^{\prime }\mathrm{O}\mathrm{•}+{\mathrm{O}}_{\mathrm{2}}\text{(R5)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+{\mathrm{R}}^{\prime }{\mathrm{O}}_{\mathrm{2}}\mathrm{•}\to {\mathrm{ROOR}}^{\prime }+{\mathrm{O}}_{\mathrm{2}}\text{(R6)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+\mathrm{NO}\to {\mathrm{RONO}}_{\mathrm{2}}\text{(R7)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}+\mathrm{NO}\to \mathrm{RO}\mathrm{•}+{\mathrm{NO}}_{\mathrm{2}}\text{(R8)}& {\displaystyle }{\mathrm{RO}}_{\mathrm{2}}\mathrm{•}\to \mathrm{R}=\mathrm{O}+\mathrm{•}\mathrm{OH}\end{array}$$

Figure 1 A schematic diagram illustrating •OH initiated α-pinene photooxidation through either the OH-addition pathway or the H-abstraction pathway. The unique peroxy radicals and their subsequent termination reactions are also depicted. The potential closure products are categorized into different families based on their formation pathways and the number of hydrogen atoms in molecules.

In our study we conducted systematic experiments of α-pinene photooxidation under varying NO concentrations to investigate the role of the H-abstraction pathway in HOM formation. We focused on early reaction stages, which means the first minutes after •OH generation by photolysis of H₂O₂, to capture primary generation product distributions, during which only autoxidation and reaction with NO were the two dominant reaction pathways for RO₂•. Fuzzy c-means clustering was applied to distinguish C₁₀H₁₅O_x•-related HOM from other products based on their formation pathways. To elucidate the role of secondary oxidation, pinonaldehyde was investigated under identical conditions, and the results were compared with the α-pinene system. Both less oxygenated and highly oxygenated products were detected by employing propylamine and nitrate reagents in chemical ionization mass spectrometry (CIMS).

Combination of the results provides complementary and consistent insights into product distributions and formation pathways, highlighting the importance of the H-abstraction pathway to produce HOM and subsequently contribute to SOA formation. The results demonstrate that the H-abstraction pathway is an important process generating product distributions highlighting its potential contribution to HOM and SOA formation.

2 Experiments and Methods

2.1 Simulation experiments in SAPHIR STAR

The experiments were conducted in SAPHIR-STAR, a 2 m³ continuously stirred tank reactor (SAPHIR: Simulation of Atmospheric PHotochemistry In a large Reaction chamber; STAR: – STirred Atmospheric flow Reactor). The details about its basic concept of operation have been described previously (Mentel et al., 2009; Baker et al., 2024). For this study the system was operated with a total flow rate of 60 L min⁻¹, of which 32 L min⁻¹ were injected into the reactor, resulting in a residence time of approximately 60 min. Experimental conditions were maintained at 20 °C and at 20 % relative humidity. •OH radicals were produced by photolyzing hydrogen peroxide (H₂O₂) using two 254 nm UV-C lamps (TUV 16W 4P SE, Philips). The photolysis rate was controlled by reducing the photon flux by covering part of the lamps with adjustable bellows. H₂O₂ vapor was introduced into SAPHIR-STAR by bubbling a 0.2 L min⁻¹ nitrogen (N₂) flow through a 30 % $w/w$ H₂O₂ solution (Sigma-Aldrich).

In the α-pinene photooxidation experiments a continuous flow of 0.012 L min⁻¹ from an α-pinene cylinder (with a concentration of ∼ 48.8 ppm) was injected into SAPHIR-STAR resulting in an initial concentration of ∼ 10 ppbv. For pinonaldehyde photooxidation, liquid pinonaldehyde (Orgentis chemicals, 99.7 %) was introduced by use of a syringe pump (Fusion 4000, Chemyx Inc.) at a flow rate of 0.145 µL h⁻¹ resulting in a concentration of 5 ppbv of pinonaldehyde in the reactor. NO was injected into the chamber from a gas cylinder (20.09 ppm NO in N₂ 5.0) via a mass flow controller for initial concentrations before reaction ranging between 0 and 7.5 ppbv.

To investigate the role of the H-abstraction pathway in α-pinene photooxidation we focused on the early stages of the reaction systems before secondary-generation products become important. All precursors were injected into the dark chamber. Once all precursors were well mixed and showed stable concentrations, the UV-C lamps were switched on for •OH production by photolysis. The UV-C lamps were then kept on at a constant setting for one hour to initialize the photooxidation of α-pinene, followed by three hours without irradiation to allow for products to be flushed out and for precursors to recover initial concentrations. This cycle was repeated three times for every system investigated.

Since the first hour of photooxidation is crucial for investigating the relative role of the H-abstraction channel compared to the •OH-addition channel, a cycle of one hour of photolysis followed by three hours without irradiation was repeated three times for each experimental condition listed in Table 1, as mentioned before. The time series of α-pinene, H₂O₂, and C₁₀H₁₇O₇• peroxy radicals over a total run of the experiments (14 h) are shown in Fig. S2, together with an example of the experimental procedure. During the final cycle the lamps were kept on continuously for 6 h to allow the system to reach steady state.

2.2 Methods and instrumentation

2.2.1 Instrumentation

The α-pinene concentration was measured by a proton-transfer-reaction mass spectrometry (PTR-TOF-MS, Ionicon Analytik GmbH). Gas concentrations of NO and NO_x were detected by a NO monitor (nCLD899, Eco Physics GmbH) coupled to a self-built photolytic NO₂ converter, with a detection limit of 0.05 ppb for NO measurement. O₃ was measured by O₃ monitor (O342e, Envea GmbH), with a detection limit of 0.2 ppb. H₂O was monitored by a Picarro CRDS analyzer (G2401, Picarro Inc.).

An Eisele type inlet (Eisele and Tanner, 1993) was coupled to an atmospheric-pressure-interference time-of-flight mass spectrometer (Api-TOF-MS, Tofwerk AG) using a positive reagent ion (C₃H₇NH${}_{\mathrm{3}}^{+}$) produced from propylamine (C₃H₇NH₂, ≥ 99 %, Sigma Aldrich) to ionize less oxygenated organic products in gas phase (Berndt et al., 2018). The resolution of this instrument was ∼ 3500 m/Δ m FWHM for $m/z$ larger than 150. C₃H₇NH${}_{\mathrm{3}}^{+}$ has the advantage that it does cluster with pinonaldehyde very efficiently, which is vital for investigating α-pinene related systems since pinonaldehyde is a major oxidation product and a representative for the OH-addition pathway. The sensitivity of pinonaldehyde (Fig. S3) was calibrated in propylamine CIMS (amine-CIMS) by using a Liquid Calibration Unit (LCU, Ionicon Analytik GmbH). The long TOF-MS (LTOF, Tofwerk AG), with a resolution ∼ 8500 m/Δ m FWHM for masses larger than 200 Th, was coupled with a multi-scheme ionization inlet (MION, Karsa Oy) (Rissanen et al., 2019). The MION inlet allows switching between bromide and nitrate modes, utilizing Br⁻ and NO${}_{\mathrm{3}}^{-}$ as reagent ions, respectively. The Br⁻ was mainly used to detect HO₂• radicals and their relative change (Albrecht et al., 2019). NO${}_{\mathrm{3}}^{-}$ CIMS has been shown to efficiently detect HOM (Ehn et al., 2014). Wang et al. have also shown that HOM(NO${}_{\mathrm{3}}^{-}$) clusters with four or more oxygen atoms in the HOM have a higher bonding strength than the H₂SO₄(NO${}_{\mathrm{3}}^{-}$) cluster (Wang et al., 2024), possibly due to the multiple hydroperoxyl and hydroxy functional groups present in HOM molecules (Bianchi et al., 2019). This indicates a relative sensitivity at the collision limit for these highly functionalized HOM. Since the study by Wang and coworkers (Wang et al., 2024) used the same setup regarding instrument and inlet we assumed that the MION-CIMS used in this study detects HOM compound with same sensitivity. All CIMS data were processed using the IGOR Pro (WaveMetrics) based Tofware v3.3.0.

2.2.2 •OH concentration, VOC turnover, and HO₂• concentration during steady stages

The concentration of •OH radicals was calculated based on the reacted fraction of α-pinene or pinonaldehyde under steady-state conditions (Eq. 1) (Kiendler-Scharr et al., 2009). When the system is in steady-state, all parameters are stable and injection rates equal loss rates. In Eq. (1), F represents the total flow through the chamber, and V is the volume of chamber. [VOC]₀ and [VOC]_SS represent the VOC concentration in the dark and in steady state (SS), respectively. k_OH denotes the rate constants for the reaction of α-pinene or pinonaldehyde with •OH radicals, which are $\mathrm{5.4}\times {\mathrm{10}}^{-\mathrm{11}}$ cm³ s⁻¹ and $\mathrm{4.0}\times {\mathrm{10}}^{-\mathrm{11}}$ cm³ s⁻¹ at 20 °C, respectively (Atkinson and Arey, 2003; Rolletter et al., 2020). The estimated •OH concentrations are listed in Table 1. The turnover of VOC by •OH can be calculated using Eq. (2), which represents the amount of VOC consumed by the reaction with •OH (Baker et al., 2024).

$$\begin{array}{}\text{(1)}& {\displaystyle }{\left[{}^{\mathrm{•}}\mathrm{OH}\right]}_{\mathrm{SS}}={\displaystyle \frac{\frac{F}{V}\times \frac{{\left[\mathrm{VOC}\right]}_{\mathrm{0}}-{\left[\mathrm{VOC}\right]}_{\mathrm{ss}}}{{\left[\mathrm{VOC}\right]}_{\mathrm{ss}}}}{k_{\mathrm{OH}}}}\text{(2)}& {\displaystyle }{\mathrm{turnover}}_{\mathrm{voc}}=k_{\mathrm{OH}}\times {\left[\mathrm{VOC}\right]}_{\mathrm{SS}}\times {[}^{\mathrm{•}}\mathrm{OH}{]}_{\mathrm{SS}}\end{array}$$

To determine the concentrations of HO₂• in the chamber, a series of isoprene photooxidation experiments was conducted, and a corresponding box model based on the MCM v3.3.1 chemistry, which used the same boundary conditions, was applied. More details about the box model can be found in Baker et al. (2024).

In these experiments, a continuous flow of 0.050 L min⁻¹ of isoprene from a gas cylinder (11.8 ± 0.24 ppmv, Linde GmbH) was introduced into the chamber to achieve a target concentration of 10 ppbv before reactions. The UV-C lamps were then adjusted in five different steps by varying the lamp bellows to vary [•OH]_SS. The box model reproduced both [isoprene]_SS and [•OH]_SS for each step within the measurement uncertainties (Fig. S4). A linear fit was applied to normalized HO₂• $/$ [Br${}^{-}+$(H₂O)Br⁻] (in normalized counts per second, ncps) and modelled [HO₂•]_SS (Fig. S5), and the slope was used as calibration factor for calculating HO₂• levels in subsequent experiments (Table 1).

2.2.3 Fuzzy c-means clustering (FCM)

Hierarchical clustering has been shown to be an effective dimensionality-reduction technique to identify major ion groups and patterns of chemical behaviour in mass spectrometry previously (Koss et al., 2020). Wu et al. demonstrated the application of the soft clustering method FCM for simplifying complex mass spectrometric data to reveal chemical and kinetic characteristics of chemical reaction systems (Wu et al., 2024). Here, FCM was applied to mass spectrometric data to explore the formation rates of a variety of products along competing reaction channels. The time series of 156 ions in the early reaction stage (up to 3000 s after start of the photooxidation processes) were selected for clustering. Herein, the major ions from both nitrate-CIMS and amine-CIMS were combined. Normalization was applied to all ions used since their typical time behaviour was more important for clustering than their absolute abundance. Initially, all ion signals were normalized relative to their corresponding reagent ions. Subsequently, Frobenius normalization was applied to each ion across the entire time range.

Three parameters were calculated to help to constrain the optimal number of clusters: sum of squared errors (SSE), distortions, and silhouette coefficient (Wu et al., 2024; Campello and Hruschka, 2006). The FCM algorithm was run 50 times for each cluster number in a range between 2 to 13. The results of the three parameters as function of cluster numbers are shown in Fig. S6. The four-cluster solution was then selected for the subsequent analysis.

2.2.4 Model simulations to determine early reaction stages

To clarify the role of the H-abstraction channel under various NO conditions, the early reaction stages are particularly relevant. In this study, we define early reaction stages using three criteria: (i) α-pinene turnover exceeds that of pinonaldehyde by a factor of ten, (ii) under conditions with NO, biomolecular reactions of RO₂• are dominated by RO₂• + NO and contribute more than 90 % to the biomolecular loss of RO₂•, and (iii) α-pinene oxidation is primarily driven by •OH rather than O₃. Therefore, model simulations for α-pinene photooxidation systems were conducted to quantitively identify early reaction stages.

As shown in Fig. 2a-1 to c-1, the model captures the evolution of the system quite well but generally underestimates NO and O₃ concentrations. These discrepancies may arise from differences in the temporal resolution of measurements and simulations, physical effects introduced by the activation of the UV-C lamps (e.g., changing emissions due to temperature adjustment of the UV-C lamp), and uncertainties in the chemical mechanism including reactions currently not represented in the model framework. The HO₂• and RO₂• concentrations obtained from the simulations are shown in Fig. 2a-2 to c-2 together with the relative contributions of RO₂• + NO, RO₂• + HO₂• and RO₂• + NO to the total biomolecular RO₂• loss. Here, the reaction rates were calculated with $k_{\left[{\mathrm{RO}}_{\mathrm{2}}\right]\left[{\mathrm{HO}}_{\mathrm{2}}\right]}$ = 1.85 × 10⁻¹¹ cm³ s⁻¹, $k_{\left[{\mathrm{RO}}_{\mathrm{2}}\right]\left[\mathrm{NO}\right]}=\mathrm{1}$ × 10⁻¹¹ cm³ s⁻¹ and $k_{\left[{\mathrm{RO}}_{\mathrm{2}}\right]\left[{\mathrm{RO}}_{\mathrm{2}}\right]}=\mathrm{5}$ × 10⁻¹² cm³ s⁻¹ (Baker et al., 2024; Berndt et al., 2016; Jenkin et al., 1997).

Based on these results, reactions occurring within the first 100, 100, and 320 s after initiation of reactions by starting the photooxidation were defined as early stages for the NO-free, low-NO, and high-NO conditions, respectively. The average bimolecular reaction rates for RO₂• + NO were negligible for cases without NO and increased to 0.179 and 1.06 s⁻¹ for the low-NO and high-NO conditions, respectively. For RO₂• + HO₂• reactions, the rates were 0.0055, 0.011, and 0.0075 s⁻¹, while those for RO₂• + RO₂• reactions were 0.0075, 0.0029, and 0.0014 s⁻¹ for NO-free, low-NO, and high-NO conditions, respectively. The contribution of RO₂• + NO was higher than 90 % under both low-NO and high-NO conditions.

For pinonaldehyde photooxidation under high-NO conditions, the reactions within the early reaction stages were also investigated to facilitate comparison with the corresponding α-pinene system under high NO conditions. Besides H-abstraction channel, reactions of α-pinene with O₃, and pinonaldehyde with •OH may also contribute to the formation of C₁₀H₁₅O_x• radicals. Therefore, the loss of α-pinene due to oxidation by O₃ formed as a byproduct of other reactions, and the loss of pinonaldehyde via •OH oxidation, were evaluated. The results are shown in detail in Fig. S7. In both cases, losses are smaller than 1 % of the reaction rate of α-pinene with •OH, which further supports our definition of early reaction stage being reasonable and representative.

Figure 2 Results of model simulations for α-pinene photooxidation under NO-free (a-1, a-2), low-NO (b-1, b-2), and high-NO conditions (c-1, c-2). The upper panels show modelled and measured concentrations of α-pinene, NO, and O₃ across the three conditions, while the lower panels show the corresponding radical concentrations of RO₂• and HO₂•. In addition, the lower panels illustrate the relative contributions of RO₂• + HO₂•, RO₂• + RO₂• and RO₂• + NO reactions to the total bimolecular reaction rate of RO₂• (shown as RO₂• fate on the right y-axis).

3 Results and Discussion

3.1 C₁₀H₁₅O_x•-related HOM form rapidly from α-pinene photooxidation in the presence of NO

All major products from α-pinene photooxidation that were detected by NO${}_{\mathrm{3}}^{-}$ CIMS, in the absence or presence of NO, were categorized into the following groups: C₁₀ monomers, C₂₀ dimers, and C_<10 fragments. The time series for contributions of each group varies for the three different NO conditions, as shown in Fig. 3a-1, a-2, a-3. A comparison under varying NO conditions is critical to elucidate the role of NO, since the presence of NO, ring opening and isomerization of alkoxy radicals, can be crucial in facilitating HOM formation from H-abstraction channel (Shen et al., 2022). The analysis was focused on the early reaction stages to explore the importance of the H-abstraction channel for HOM formation. The average product distributions of C₁₀ monomers from early stage α-pinene photooxidation without and with NO are illustrated by Kendrik Mass Defect (KMD) plots as a function of oxygen number (Kendrick Mass = oxygen, Fig. 3b-1, b-2, b-3). Each plot features seven distinct families, with compounds in each family sharing the same carbon and hydrogen numbers but differing in oxygen content.

Table 1 Overview of experimental conditions: pinonaldehyde photooxidation without NO (NO-free) and with high NO (high-NO), α-pinene photooxidation without NO (NO-free), with low NO (low-NO), and with high NO (high-NO). The precursor concentrations in the dark and in photooxidation steady-state conditions are shown at t₀ and t_S, respectively. The measurement uncertainty is less than 5 %. Details about how to estimate •OH and HO₂• radicals are given in Sect. 2.2.

Under α-pinene NO-free conditions the relative contributions of C₁₀H₁₈O_x, C₁₀H₁₆O_x, and C₂₀H₃₄O_x are 56.1 %, 11.1 %, and 8.8 %, respectively, and they are all closed-shell products related to C₁₀H₁₇O_x•. The first generation peroxy radicals with four-membered ring-opening, C₁₀H₁₇O₃•, can undergo fast unimolecular reactions with rates of 4 ± 2 s⁻¹ (Xu et al., 2019). As shown in Fig. 3b-1, species with the formula C₁₀H₁₇O₇• are identified as predominantly HOM peroxy radicals and are detected in the nitrate mass spectra as C₁₀H₁₇O₇(NO${}_{\mathrm{3}}^{-}$). This agrees with previous observations by Berndt (2021), which were obtained using four different reagent ions, and calculations by Piletic and Kleindienst (2022). Additional HO₂• radicals were produced in our experiments via the reaction of the •OH radical with H₂O₂. The increasing contribution of C₁₀H₁₈O_x (Fig. 3(1-a)) indicates a significant termination reaction of RO₂• by HO₂•. Among the C₁₀H₁₈O_x family, C₁₀H₁₈O₇ shows the highest intensity and is likely produced via termination reactions of C₁₀H₁₇O₇• with HO₂•, which aligns with the dominance of C₁₀H₁₇O₇• in the C₁₀H₁₇O_x• family. Compounds with six oxygen atoms but one hydrogen less or one more hydrogen than C₁₀H₁₇O₇• (i.e., C₁₀H₁₆O₆ and C₁₀H₁₈O₆) are significant contributors to the total product signal of the C₁₀H₁₇O_x• family. Compounds with the formula C₁₀H₁₆O₆ likely have carbonyl functionalities and are formed either directly from Reaction (R3) or from self-termination reactions of C₁₀H₁₇O₇• (Reaction R8) (Iyer et al., 2018; Jenkin et al., 2019). In contrast, C₁₀H₁₈O₆ species are possibly alcohols formed from C₁₀H₁₇O₇• via Reaction (R3), or more likely hydroperoxides derived from C₁₀H₁₇O₆• via Reaction (R1) (Iyer et al., 2018; Jenkin et al., 2019).

In addition to C₁₀ HOM monomers, C₂₀ accretion products also significantly contribute to HOM, particularly species containing 8, 10, or 12 oxygen atoms (Fig. S8). These have been identified as the most abundant dimers from •OH initiated α-pinene photooxidation system by Berndt et al. (2016). C₂₀H₃₄O${}_{(\mathrm{8},\mathrm{10},\mathrm{12})}$ can be mechanistically explained by recombination of C₁₀H₁₇O₇• with C₁₀H₁₇O₃•, C₁₀H₁₇O₅•, or another C₁₀H₁₇O₇• via Reaction (4). C₁₀H₁₇O${}_{(\mathrm{3},\mathrm{5},\mathrm{7})}\mathrm{•}$ have previously been reported to dominate the product spectrum (Lee et al., 2023; Berndt, 2021). In summary, the obtained HOM distributions in the base case without NO addition are consistent with previous studies and show mechanistically reasonable results.

When reactions are initiated in the presence of NO, the product distribution in the early reaction stage (when the RO₂• + NO regime is dominant) differs significantly from the NO-free case. The C₁₀H₁₇NO_x product family, identified as organic nitrates, accounts for 29.9 % and 29.4 % of the total signal under low-NO and high-NO conditions, respectively, and dominates C₁₀H₁₇O_x•-related HOM families. The most abundant species with formulas C₁₀H₁₇NO${}_{(\mathrm{6},\mathrm{7},\mathrm{8})}$ most likely represent organic nitrates formed via reactions of C₁₀H₁₇O${}_{(\mathrm{5},\mathrm{6},\mathrm{7})}\mathrm{•}$ with NO. As the initial NO concentration increases, contribution from C₁₀H₁₇O₆•-derived HOM, such as C₁₀H₁₇NO₇ and C₁₀H₁₈O₆, also rises (Figure S9). When the initial NO increased from 0.91 ± 0.05 to 7.4 ± 0.05 ppbv, the ratios of C₁₀H₁₇NO₇ to C₁₀H₁₇NO₈ and C₁₀H₁₈O₆ to C₁₀H₁₈O₇ are enhanced from 0.32 to 0.43 and from 0.63 to 0.78, respectively. At least one alkoxy step (Reaction R7) is necessary to produce peroxy radicals with even number, e.g. C₁₀H₁₇O₆•. Therefore, the presence NO promotes the occurrence of alkoxy steps in HOM formation. Compared to the NO-free condition, the contributions of accretion products with C₂₀H₃₄O_(9,11) are also promoted. For forming C₂₀H₃₄O_(9,11) via Reaction (R5) an C₁₀H₁₇O_x• peroxy radical with an even oxygen number must be involved, e.g. C₁₀H₁₇O₄• and C₁₀H₁₇O₆•, further highlighting the role of alkoxy steps in HOM formation.

The C₁₀H₁₇O₇• peroxy radicals exhibit a rapid formation rate and a high molar yield in α-pinene photooxidation systems, but their formation shows only a weak dependence on the NO concentration. Although •OH addition to α-pinene yields three structural first-generation isomers (C₁₀H₁₇O₃•), only the isomer that undergoes an opening of the four-membered ring is capable of subsequent autoxidation (Lee et al., 2023; Piletic and Kleindienst, 2022; Berndt et al., 2016). The ring-opened C₁₀H₁₇O₃• can then undergo two rapid unimolecular autoxidation steps, with a rate constant of approximately 4 s⁻¹ for the first step which will lead to the formation of C₁₀H₁₇O₅•, and a rate constant around 10 s⁻¹ for the subsequent step and formation of C₁₀H₁₇O₇• species (Piletic and Kleindienst, 2022; Xu et al., 2019). The fast autoxidation steps of the resulting peroxy radicals imply that the characteristic timescales of their formation are significantly shorter than those of competing bimolecular reactions with NO, RO₂•, and HO₂•, whose maximum effective rates in our experiments range from 0.02 to 1.88 s⁻¹ under NO-free, low-NO, and high-NO conditions, respectively. Therefore, biomolecular reactions, particularly involving NO, cannot effectively compete with the rapid autoxidation reactions, which can explain the observed weak NO dependence of the formation of C₁₀H₁₇O₇•-related HOM.

In addition to the change of C₁₀H₁₇O_x•-related HOM, C₁₀H₁₅O_x•-related HOM families emerge once NO is introduced into the system (Fig. 3a-2, a-3, b-2, b-3). Under the NO-free condition, neither the C₁₀H₁₅NO_x nor the C₁₀H₁₄O_x family contributes more than 2 %. However, under low-NO conditions their contribution rises to 10.3 % and 4.7 %, respectively, and further increases to 24.9 % and 6.0 % under high-NO conditions. The most abundant peroxy radical in C₁₀H₁₅O_x• family is C₁₀H₁₅O₇•, as shown in Fig. S9, followed by C₁₀H₁₅O₉•, which is consistent with the closed shell organic nitrate distribution, where C₁₀H₁₅NO₈ is most prevalent, followed by C₁₀H₁₅NO₁₀. The C₁₀H₁₄O_x family contains products with carbonyl groups and is formed via self-termination reactions of peroxy radicals. In addition to C₁₀ monomers, accretion products with the formula C₂₀H₃₀O_x and C₂₀H₃₂O_x, derived from Reaction (R5), also increase with elevated NO concentrations. Here at least one peroxy radical containing 15 hydrogen atoms must be involved (Berndt et al., 2018). Compared to monomers, formation of accretion products is reduced since in early stages of the reactions the RO₂• + R^′O₂• pathway is less important compared to the NO + RO₂• pathway or the RO₂• autoxidation. The intensity of C₁₀H₁₆O₇ increases gradually until it surpasses that of C₁₀H₁₆O₆, which in the absence of NO is the most abundant peak. C₁₀H₁₆O₇ is likely a carbonyl compound (or family), formed either through Reaction (R3), or from self-termination. Both need C₁₀H₁₇O₈• as precursors. However, analysis of the closed-shell families C₁₀H₁₈O_x and C₁₀H₁₇NO_x indicates the absence of significant formation of C₁₀H₁₇O₈• in presence of NO, which also aligns with Berndt (2021) and Piletic and Kleindienst (2022). C₁₀H₁₆O₇ may also be a hydroperoxide produced via Reaction (R2), or an alcohol produced via Reaction (R3), with C₁₀H₁₅O₇• peroxy radicals or C₁₀H₁₅O₈• peroxy radicals serving as precursors. Products formed via RO₂• + RO₂• reactions were produced at significantly slower rates than other species, as will be discussed in Sect. 3.2. Additionally, when C₁₀H₁₅O₇• reacts with another RO₂• radical, it forms C₁₀H₁₆O₆ or C₁₀H₁₄O₆ rather than C₁₀H₁₆O₇. Thus, Reaction (R3) was considered to not contribute significantly, and the reaction of C₁₀H₁₅O₇• with HO₂• most likely accounts for the significant increase of C₁₀H₁₆O₇. Similarly, species in the C₁₀H₁₆O_x family that have more than seven oxygen atoms can be attributed to reactions of C₁₀H₁₅O_x• peroxy radicals with HO₂•.

Contributions from peroxy radicals containing more than seven oxygen atoms to the C₁₀H₁₇O_x• family are negligible, which is consistent with the number of oxygen atoms in the observed closed-shell products. This phenomenon is in accordance with previous measurement studies and calculations (Berndt, 2021; Lee et al., 2023). In the NO-free condition, compounds with more than seven oxygen atoms account for only 0.1 % of the C₁₀H₁₈O_x family, whereas for the low-NO and high-NO cases, compounds with more than eight oxygen atoms contribute 5 % and 6 %, respectively, to the C₁₀H₁₇NO_x family. Although the fate of the C₁₀H₁₇O₇• family remains unclear, the product distributions indicate that it is unlikely for them to undergo further steps of autoxidation. However, the C₁₀H₁₅O_x•-related HOM are more oxidized than C₁₀H₁₇O_x•-related HOM. For example, species containing more than eight oxygen atoms in C₁₀H₁₅NO_x account for 43.6 % and 46.7 % in low-NO and high-NO cases, respectively, of the entire C₁₀H₁₅NO_x family. Obviously C₁₀H₁₅O_x• isomers with x≥7 are formed, which exhibit faster unimolecular reaction rates than the accessible isomers of the C₁₀H₁₇O_x• peroxy radicals.

The distribution of the less oxygenated compounds was investigated based on the mass spectra detected by amine-CIMS, which are shown in Fig. S10. Consistent with nitrate-CIMS, C₁₀H₁₅O_x•-related less oxygenated compounds also emerge with elevated NO concentrations, particularly those in the C₁₀H₁₄O_x and C₁₀H₁₅NO_x families. The dominant compounds in the C₁₀H₁₅NO_x family are C₁₀H₁₅NO₄, C₁₀H₁₅NO₆, and C₁₀H₁₅NO₈, which should be derived from Reaction (R6) involving peroxy radicals with the formula C₁₀H₁₅O₃•, C₁₀H₁₅O₅•, and C₁₀H₁₅O₇•, respectively. Among the major compounds in the C₁₀H₁₅NO_x family detected by amine-CIMS only highly oxidized species with the formular C₁₀H₁₅NO₈ can also be detected in significant amounts by nitrate-CIMS. The same phenomenon can also be seen for C₁₀H₁₇O_x•-related compounds. In amine-CIMS, the first-generation peroxy radical, C₁₀H₁₇O₃•, and its associated closed-shell products play a dominant role in the combined C₁₀H₁₇O_x•, C₁₀H₁₈O_x, and C₁₀H₁₇NO_x families, although the C₁₀H₁₇O₇• related products are dominant in the mass spectrum obtained by nitrate-CIMS. For example, C₁₀H₁₇NO₄ and C₁₀H₁₈O₃ are major peaks, which are produced mechanistically through termination reactions of C₁₀H₁₇O₃• by NO and by HO₂• radicals, respectively. C₁₀H₁₇NO₄ contributes 56.6 % and 57.5 % to the C₁₀H₁₇NO_x family in low-NOand high-NO conditions, respectively. C₁₀H₁₈O₃accounts for 70.6 %, 77.6 %, and 69.3 % in NO-free, low-NO, and high-NO cases, respectively.

Only ring-opened C₁₀H₁₇O₃• can undergo fast autoxidation reactions to produce HOM, and they account only for approximately 25 % of the sum of C₁₀H₁₇O₃• peroxy radicals derived from the •OH addition pathway (Xu et al., 2019, Vereecken et al., 2007). However, the nitrate-CIMS technique ionizes specifically highly oxygenated organics (Ehn et al., 2014), resulting in the detection of only a narrow range of highly oxidized compounds derived from four-membered ring-opened C₁₀H₁₇O₃•. The amine-CIMS is efficient in detecting a large range of products including less oxygenated compounds (O < 7) (Berndt, 2021). C₁₀H₁₆O₂(C₃H₇NH${}_{\mathrm{3}}^{+}$) is found to be the strongest peak among the entire mass spectrum in amine-CIMS, indicating the production of pinonaldehyde. Pinonaldehyde can be formed from the formation of alkoxy radicals via reactions of four-membered ring-retained C₁₀H₁₇O₃• peroxy radicals and NO, which then undergo a fast opening of the six-membered ring, shown in Fig. S1 (Rolletter et al., 2019). The inclusion of amine-CIMS extends the detection of products, especially less oxygenated ones, which could indicate the fate of four-membered ring-retained C₁₀H₁₇O₃• peroxy radicals.

Alkoxy radicals derived from reactions involving C₁₀H₁₇O_x• peroxy radicals and NO can undergo an H-shift, β-scissions, or a HO₂• loss step; however, none of these reactions are expected to form C₁₀H₁₅O_x• peroxy radicals directly. For example, neither H-migration nor β-scission will lead to the loss of a hydrogen atom without a carbon-carbon bond cleavage, and the resulting alkyl radicals will react efficiently with O₂ to regenerate C₁₀H₁₇O_x• radicals and thus continue the autooxidation cycle. The HO₂• loss channel is expected to be a minor pathway for the considered alkoxy radicals, because intramolecular H-migration and β-scission are more favorable, as previously shown by Vereecken and Peeters (2009, 2010). If the HO₂• loss channel would happen, it would lead to the formation of carbonyl and HO₂•, losing one hydrogen atom forming C₁₀H₁₆O_x products. While the C₁₀H₁₆O_x products could, in principle, react with •OH radical again to produce C₁₀H₁₅O_x• intermediates, this process would occur on the timescale of secondary chemistry and is therefore unlikely to contribute significantly during the early reaction stages. Pinonaldehyde was dominant among C₁₀H₁₆O_x compounds, but its oxidation cannot become a significant source of C₁₀H₁₅O_x•-related HOM, which will be discussed in Sect. 3.2.

In conclusion, the product distributions from α-pinene photooxidation during early reaction stages were investigated. Compared to NO-free conditions, the presence of NO enhanced the formation of organic nitrates and promoted alkoxy formation steps, which lead to significant formation of C₁₀H₁₅O_x•-related compounds. These were detected by both nitrate-CIMS and amine-CIMS. The initial peroxy radicals leading to C₁₀H₁₅O_x• related products were identified as C₁₀H₁₅O₃•, which cannot arise from ozonolysis of α-pinene since α-pinene was dominantly oxidized by •OH radicals rather than O₃, as shown in Fig. S7. Also, C₁₀H₁₅O₄• peroxy radicals are mechanistically expected to be first generation peroxy radicals in ozonolysis. Potential mechanisms that could be responsible for the emergence of C₁₀H₁₅O_x•-related HOM will be discussed in the following section.

Figure 3 Panels (a-1), (a-2), (a-3) show stacked figures with the contributions of each product family from α-pinene photooxidation as a function of reaction time. The families representing major monomers (C = 10), fragments (C < 10), and accretion products (C = 20) are included in panels (a-1, a-2, a-3). Time series of concentrations for α-pinene, NO, and O₃ are also presented. The corresponding panels (b-1), (b-2), (b-3) located below each stack plot show the Kendrik Mass Defect (KMD, based on oxygen weight) of monomer compounds as a function of oxygen number, with marker sizes indicating the relative abundance of each compound in the early reaction stages (0–100, 0–100, and 0–320 s after reaction starts for (b-1), (b-2), and (b-3), respectively. This is also indicated by the vertical white lines in (a-1), (a-2), and (a-3)). Compounds aligned along the same horizontal dashed lines are in same family, with the same carbon and hydrogen number but differing in oxygen number. Family names are shown on the right. The texts in panels (b-1), (b-2), (b-3) highlight major compounds with high abundance and their possible formation pathways. NO conditions increase in the panels from left to right, from NO-free and low-NO to high-NO, respectively. The products here are detected by nitrate-CIMS.

3.2 C₁₀H₁₅O_x•-related HOM attributed to H-abstraction pathway

In α-pinene oxidation systems three possible pathways can lead to monomers with 15 hydrogen atoms: either the H-abstraction channel by •OH radicals (Shen et al., 2022) , ozonolysis (Berndt, 2022), or oxidation of pinonaldehyde (Rolletter et al., 2020). However, as mentioned in Sects. 2.2 and 3.1, ozone was not present at the beginning of each transient cycle and accounted for less than 1 % of α-pinene reaction loss. Thus ozonolysis cannot explain the significant formation of C₁₀H₁₅O_x•-related products, in particular •OH-initiation yields more HOM than ozonolysis (Berndt, 2022). To explore potential formation pathways of C₁₀H₁₅O_x•-related compounds, FCM clustering was applied to the time series of 156 ions detected by nitrate-CIMS and amine-CIMS. Selection criteria for the ions were contributions of more than 0.1 % to the total ion signal in nitrate-CIMS or more than 1 % to the total ion signal in amine-CIMS.

A four-cluster solution was identified as the optimal solution. Corresponding results are presented in Fig. 4. Figure 4a shows the time series of the four cluster centers and the contributing ions. Compounds in cluster 1 form immediately after reactions are initiated by switching on the UV-C lamps, followed by cluster 2 and cluster 3, while compounds in cluster 4 show the longest formation time. The cluster centers peak at approximately 160, 210, 290, and 850 s, respectively. As shown in Fig. 4b, the major monomers in the C₁₀H₁₈O_x family and the major dimers (C₂₀H₃₀O_x, C₂₀H₃₂H₁₂, C₂₀H₃₄O₈) are all found in cluster 4, which exhibits the slowest formation rate among four clusters. These compounds originate from biomolecular termination reactions (Reactions R1 and R5), with their production rates largely controlled by the concentration of HO₂• and RO₂• radicals. NO is already in a steady state in the dark chamber, while HO₂• and RO₂• only begin to accumulate after the UV-C lamps are switched on. Therefore, the RO₂• fate is immediately affected by NO. Consequently, under high-NO conditions, RO₂• + NO reactions as well as autoxidation play a dominant role for the RO₂• in early reactions, and only these two reactions are discussed in following sections.

Compounds in cluster 1 and cluster 2 initially increase dramatically before decreasing again, and cluster 1 exhibits a steeper decline than cluster 2. The bulk chemical properties and the average carbon oxidation state ($\stackrel{\mathrm{‾}}{\mathrm{OSc}}$) of each cluster are calculated (Kroll et al., 2011; Wu et al., 2021) and plotted as a function of average carbon (n_C) or nitrogen atoms (n_N) (Fig. S11). The early-generation cluster 1 exhibits the highest $\stackrel{\mathrm{‾}}{\mathrm{OSc}}$ and n_N but the lowest n_C, indicating that it mainly consists of highly oxidized nitrogen-containing compounds and fragments (C < 10). This characteristic distinguishes cluster 1 from the other clusters. It is determined by the emerging of •OH radicals at elevated NO concentration in the early stage.

The formation rate of each cluster can be interpreted kinetically as “typical” chemical formation rate (Wu et al., 2024). The compounds C₁₀H₁₅NO₇(NO${}_{\mathrm{3}}^{-}$) and C₁₀H₁₅NO₈(NO${}_{\mathrm{3}}^{-}$) are assigned to cluster 1 with the fastest formation rate; thus, they are likely first-generation products. Pinonaldehyde (C₁₀H₁₆O₂(C₃H₇NH${}_{\mathrm{3}}^{+}$)), a possible precursor, can only be found in cluster 3, which has a much slower formation rate. This clearly shows that pinonaldehyde oxidation cannot explain the production of C₁₀H₁₅NO₇(NO${}_{\mathrm{3}}^{-}$) or C₁₀H₁₅NO₈(NO${}_{\mathrm{3}}^{-}$). C₇H₉NO₈(NO${}_{\mathrm{3}}^{-}$), which is detected in high amount in nitrate-CIMS and also attributed to cluster 1, goes through a fragmentation step and is possibly formed via decomposition of C₁₀H₁₅O_x•, followed by further autoxidation and termination reactions by NO (Shen et al., 2022). Therefore, H-abstraction is probably the reason for the significant formation of C₁₀H₁₅O_x•-related HOM, and not the oxidation of pinonaldehyde. The fragment C₇H₁₁NO₈(NO${}_{\mathrm{3}}^{-}$) is formed faster than the organic nitrate monomer C₁₀H₁₇NO₈(NO${}_{\mathrm{3}}^{-}$), even though both are initialized by the OH-addition channel. This relation differs from that of C₇H₉NO₈(NO${}_{\mathrm{3}}^{-}$) and C₁₀H₁₅NO₈(NO${}_{\mathrm{3}}^{-}$).

As shown in Fig. S12, the summed abundance of C₇ families (C₇H₍₁₀,₁₂₎O_x, C₇H₍₉,₁₁₎NO_x) increases accordingly with the increase of the NO injection, accounting for 0.01 %, 0.3 % and 2.4 % of HOM compounds under NO-free, low-NO and high-NO conditions, respectively. It indicates increasing alkoxy radical decomposition with increasing NO. C₇ fragments are formed via dissociation of C₁₀ alkoxy radicals derived from bimolecular reactions of C₁₀ peroxy radicals with NO. Thus, the presence of NO promotes C₇ fragments by enhancing the production of alkoxy radicals (Vereecken and Peeters, 2000; Berndt, 2021). The most abundant families, C₇H₉NO_x and C₇H₁₁NO_x, likely originate from decomposition of C₁₀H₁₅O_x−1• and C₁₀H₁₇O_x−1•, respectively. Under high-NO conditions, the ratio of C₇H₉NO_x to C₁₀H₁₅NO_x reaches 0.22, substantially higher than the ratio of C₇H₁₁NO_x to C₁₀H₁₇NO_x (0.05). This observation suggests that either alkoxy radical formation or subsequent decomposition is more efficient in the H-abstraction pathway than in the •OH-addition pathway.

Evidence for alkoxy radical H-migration is provided by the enhanced formation of C₁₀H₁₇NO₇ and C₂₀H₃₄O₁₁ under high-NO conditions (Fig. S9). According to the oxygen-parity framework, C₁₀H₁₇O_x• radicals formed solely through autoxidation are expected to contain an odd number of oxygen atoms, whereas an alkoxy H-migration step changes the oxygen parity from odd to even (Kang et al., 2025). Thus, the observed increase in C₁₀H₁₇NO₇ and C₂₀H₃₄O₁₁ suggests the formation of C₁₀H₁₇O₆• peroxy radicals through at least one alkoxy radical isomerization step.

To initialize the HOM chain via •OH-addition pathway, four-membered rings must open from chemically activated tertiary radicals, which is independent of NO (Xu et al., 2019). In contrast, HOM formation through the H-abstraction pathway requires NO. The initially formed peroxy radicals (C₁₀H₁₅O₂•) are unlikely to undergo autoxidation directly; therefore, their reaction with NO to form alkoxy radicals (C₁₀H₁₅O•), followed by the ring-opening step producing C₁₀H₁₅O₃• radicals, is necessary to initiate the HOM formation chain.

As autoxidation proceeds, higher oxygenated peroxy radicals, such as C₁₀H₁₇O_x• and C₁₀H₁₅O_x•, are formed from C₁₀H₁₇O₃• and C₁₀H₁₅O₄•, respectively. These peroxy radicals can further react with NO, yielding either the corresponding alkoxy radicals (C₁₀H₁₇O_x−1• and C₁₀H₁₅O_x−1•) or organic nitrates (C₁₀H₁₇NO_x+1 and C₁₀H₁₅NO_x+1). However, the branching ratio between these two channels remains poorly constrained, contributing substantial uncertainty to the role of NO in HOM formation. Once formed, the fate of alkoxy radicals strongly depends on alkoxy radical structure (Vereecken and Peeters, 2010, 2009). Regardless of whether the precursor RO₂• radicals originate from the •OH-addition or H-abstraction pathway, their subsequent chemistry can proceed through alkoxy radical intermediates. Therefore, the fate of these alkoxy radicals ultimately determines the impact of NO on HOM formation.

Overall, NO does influence HOM formation through alkoxy-radical chemistry. By promoting the conversion of RO₂• radicals to RO• radicals, NO initializes pathways for H-migration and decomposition that would otherwise be less important. The extent to which these processes enhance or suppress HOM formation depends strongly on the structures of both the peroxy and alkoxy radicals, introducing substantial uncertainty into the overall role of NO.

To assess the robustness of the clustering results, additional cluster analysis was performed using a three- and five-cluster solution, as shown in Figs. S13 and S14, respectively. Consistent conclusions are obtained regardless of the number of clusters specified. In particular, the key compounds derived from the H-abstraction channel, such as C₁₀H₁₅NO₈(NO${}_{\mathrm{3}}^{-}$) and C₇H₉NO₈(NO${}_{\mathrm{3}}^{-}$), were consistently assigned to the first cluster with the fastest formation rate. The stability of the clustering results was further evaluated by performing 50 clustering runs with random initializations and using the four-cluster solution. The distributions of the cluster assignments were then analysed, as shown in Fig. S15, with particular attention given to the key compounds discussed above, including C₁₀H₁₅NO₈(NO${}_{\mathrm{3}}^{-}$), C₇H₉NO₈(NO${}_{\mathrm{3}}^{-}$), and C₁₀H₁₆O₂(C₃H₇NH${}_{\mathrm{3}}^{+}$). The results demonstrate the high stability of the cluster solution, as these compounds were consistently assigned to the same cluster across all runs. For example, C₁₀H₁₅NO₈(NO${}_{\mathrm{3}}^{-}$) was assigned to the first cluster in all runs. Therefore, the clustering results and insights derived from them are highly robust and reliable.

Figure 4 Results of fuzzy c-means clustering for the dominant peaks (156 in total) during early reaction stages of α-pinene photooxidation under high NO condition are shown in panel (a). The four-cluster solution is shown here. The time series of the cluster centers are displayed as colored solid lines, while individual species are shown in gray lines. Panel (b) illustrates the cluster apportionment of selected major products detected in both nitrated-CIMS (N) and amine-CIMS (A).

Figure 5 Panel (a-1) displays concentrations and turnover of α-pinene and pinonaldehyde (Pinal) during α-pinene photooxidation under high-NO conditions, while panel (a-2) represents concentrations and turnover of pinonaldehyde during pinonaldehyde photooxidation under high-NO conditions. Major product distributions are shown in panels (b-1) to (b-4). Signals detected in the pinonaldehyde experiment are normalized by the ratio of the pinonaldehyde turnover during the pinonaldehyde experiment to the respective turnover during the α-pinene experiment. The products are separated into four groups: (A)C₁₀H₁₄O_x, (A)C₁₀H₁₅NO_x, (N)C₁₀H₁₄O_x, (N)C₁₀H₁₅NO_x, of which (A) and (N) represent signals derived from amine-CIMS and nitrate-CIMS, respectively. Data from α-pinene oxidation and pinonaldehyde oxidation are colored in pink and blue, respectively.

To assess the potential contribution of pinonaldehyde oxidation to the formation of C₁₀H₁₅O_x•-related HOM, pinonaldehyde photooxidation experiments were conducted under NO-free and high-NO conditions, analogous to conditions applied in the α-pinene systems (Table 1). Time series of α-pinene and its oxidation product pinonaldehyde during the first 30 min of the α-pinene (high-NO) experiment are shown in Fig. 5a-1. The corresponding turnovers of α-pinene and pinonaldehyde are presented in Fig. 5a-1. Similarly, the concentration of pinonaldehyde and its turnover during the pinonaldehyde (high-NO) experiment are shown in Fig. 5a-2. •OH concentrations were estimated using model simulations.

During the early stage, the average pinonaldehyde turnover was 8.9 × 10⁶ molecules cm⁻³ s⁻¹ under pinonaldehyde (high-NO) condition, approximately 6 times higher than the turnover (1.5 × 10⁶ molecules cm⁻³ s⁻¹) under α-pinene (high-NO) condition. To facilitate comparison between the two experiments, we calculated the pinonaldehyde turnover for both experiments and compared it to obtain the ratio between the α-pinene (high-NO) and pinonaldehyde (high-NO) experiments (hereafter referred to as the pinonaldehyde turnover ratio). Signals measured in the pinonaldehyde experiment were subsequently normalized by this ratio to account for differences in the pinonaldehyde turnover between the two systems. The distributions of major C₁₀H₁₅O_x•-related products observed during α-pinene and pinonaldehyde experiments are presented in Fig. 5b. Major products belonging to the C₁₀H₁₄O_x and C₁₀H₁₅NO_xfamily detected by both nitrate-CIMS (N) and amine-CIMS (A) are included. The data shown in Fig. 5b represents averages over the first 320 s following the initiation of oxidation, corresponding to the early reaction stage, during which RO₂• + NO accounts for more than 90 % of the total bimolecular RO₂• loss rate. In addition, all product signals here as well as all data used for comparison in the following paragraphs have been normalized by the pinonaldehyde turnover.

The pinonaldehyde turnover ratio during the first 320 s exceeded 5 and was substantially higher during the first 100 s. This large difference arose because pinonaldehyde was present at the start of the pinonaldehyde system and was therefore immediately available for reaction with •OH. In contrast, under the α-pinene system, pinonaldehyde had to be formed through α-pinene oxidation before it could undergo further reactions, resulting in a much lower pinonaldehyde turnover during the early stages of the experiment. For C₁₀H₁₅O_x•-related HOM detected by nitrate-CIMS the summed signals of the C₁₀H₁₄O_x and C₁₀H₁₅NO_x families under the α-pinene system exceed those of pinonaldehyde system by factors of approximately 20 and 10, respectively. Although pinonaldehyde oxidation contributes to the formation of C₁₀H₁₅O_x•-related HOM under high-NO conditions, this contribution is minor and can explain approximately 5 % of the HOM yield observed from α-pinene oxidation. Similarly, for less oxygenated products measured by amine-CIMS, the summed concentrations of the C₁₀H₁₄O_x and C₁₀H₁₅NO_x families were 98 times higher in the α-pinene system, and 6.7 times higher in the pinonaldehyde system.

For highly oxygenated compounds detected by nitrate-CIMS, species containing more than 10 oxygen atoms account for 11 % of the C₁₀H₁₄O_x family and for 13 % of the C₁₀H₁₅NO_x family under α-pinene high-NO condition, whereas their contributions are much lower for pinonaldehyde high-NO conditions (3 % and 5 %, respectively). The initial NO concentrations (Table 1) for two conditions are comparable, and consequently autoxidation rates can be the reason responsible for the differences in HOM distributions. Therefore, for peroxy radicals with more than 9 oxygen atoms the autoxidation chain of C₁₀H₁₅O_x• peroxy radicals in pinonaldehyde photooxidation experiments are more easily terminated by NO than for peroxy radicals resulting from α-pinene photooxidation systems. The difference in the product distributions is significant between the two systems. In the pinonaldehyde system, the intensity of the C₁₀H₁₄O_x family is lower than that of the C₁₀H₁₅NO_x family, indicating that the branching towards Reaction (R8) undergoing self-termination is lower than the branching towards Reaction (R6).

For less oxygenated compounds detected by amine-CIMS, the most abundant species in C₁₀H₁₅NO_x family, C₁₀H₁₅NO₄,derived from the reaction of C₁₀H₁₅O₃• peroxy radicals with NO in the α-pinene system, does not appear in the pinonaldehyde system. Primary peroxy radicals with a formula of C₁₀H₁₅O₄• are formed when •OH abstracts a hydrogen atom, mainly from the aldehyde group in pinonaldehyde (Rolletter et al., 2020; Fantechi et al., 2002), as a prerequisite for autoxidation. C₁₀H₁₅O₃• form when •OH radicals abstract a hydrogen atom from α-pinene molecules, followed an alkoxy step (Reaction R7), a 6-membered ring opening, and an O₂ molecule addition (Shen et al., 2022). Here, the appearance of C₁₀H₁₅NO₄ could be explained by termination reactions between C₁₀H₁₅O₃• and NO in the α-pinene system, when the H-abstraction pathway indeed exists. The species detected in amine-CIMS with the formula C₁₀H₁₅NO₆ are significantly higher than other compounds in the C₁₀H₁₅NO_x family in the pinonaldehyde system. These species are less important than C₁₀H₁₅NO₄ in the α-pinene system. If an alkoxy radical is formed by the reaction of C₁₀H₁₅O₄• with NO, followed by H-shift and an O₂ addition, C₁₀H₁₅O₅• peroxy radicals will be produced and can be terminated by NO to produce C₁₀H₁₅NO₆ species. This is a potential pathway which could be responsible for the significant formation of C₁₀H₁₅NO₆ in the pinonaldehyde system.

In summary, clustering analysis shows that C₁₀H₁₅NO_x HOM species exhibit the fastest formation rate among all HOM compounds. They are also faster than first-generation product generation (e.g., pinonaldehyde) in the OH-addition channel under α-pinene high-NO conditions. The intensity of C₁₀H₁₅O_x• related HOM (C₁₀H₁₄O_x and C₁₀H₁₅NO_x) in the α-pinene photooxidation system is still 11 times higher than those under the pinonaldehyde system although the pinonaldehyde turnover is corrected. In addition, product distributions observed in the α-pinene system differ substantially from those in the pinonaldehyde system, regardless of whether they are detected by nitrate-CIMS or amine-CIMS. Therefore, these findings demonstrate that the H-abstraction pathway, rather than the oxidation of pinonaldehyde, accounts for the formation of C₁₀H₁₅O_x• related HOM in the α-pinene system.

4 Conclusions and Atmospheric Implications

In this study a series of photooxidation experiments was conducted under varying NO levels to investigate the effect of NO on the C₁₀H₁₅O_x•-related HOM formation in the α-pinene photooxidation system, and to unravel potential formation pathways of these products. The chemical system was brought to a steady state in the dark. Afterwards, reactions were initiated by photolyzing H₂O₂. We focused on early reaction stages (the first hundreds of seconds), during which RO₂• + NO reactions and RO₂• autoxidation reactions dominate the fate of the RO₂•. C₁₀H₁₇O_x•-related HOM were dominantly formed via the OH-addition pathway. Product distributions, potential formation pathways, and rates of C₁₀H₁₇O_x•-related HOM have been widely and extensively studied (Berndt, 2021; Xu et al., 2019), and our findings on C₁₀H₁₇O_x•-related HOM formation are consistent with previous studies. However, potential formation pathways, distributions, and NO dependence of C₁₀H₁₅O_x•-related compounds derived from the H-abstraction channel were missing systematic exploration so far.

To constrain the potential contribution of H-abstraction to HOM formation, regimes with and without NO were directly compared, and a separate experiment of pinonaldehyde oxidation under similar conditions was performed. Additionally, measurements by amine-CIMS were used to detect less oxygenated products. Since the photolysis of H₂O₂ was utilized to produce •OH radicals, O₃ did not accumulate during the early reaction stages; therefore, ozonolysis did not interfere with the •OH pathways. FCM analysis proves that the dominant species in the C₁₀H₁₅NO_x family, C₁₀H₁₅NO₈ and C₇H₉NO₈, can be distinguished from other species by their fast formation rates in the presence of NO. Pinonaldehyde oxidation can only contribute ∼ 5 % to HOM formation in α-pinene oxidation systems. Therefore, neither ozonolysis nor secondary oxidation can explain the significant formation of C₁₀H₁₅O_x•-related HOM, which points to H-abstraction from α-pinene by •OH being the most likely formation pathway.

As illustrated in Figs. S12 and S16, the contribution of the H-abstraction pathway to HOM formation increases with elevated NO levels compared to the OH-addition pathway. The ratios between major C₁₀H₁₅O_x•-related HOM and C₁₀H₁₇Ox•-related HOM rise with increasing NO concentrations. The ratios of C₁₀H₁₅O_x• peroxy radicals to C₁₀H₁₇O_x• peroxy radicals are 0.06, 0.83, and 1.42 under NO-free, low-NO, and high-NO conditions, respectively. Similarly, the ratio of C₁₀H₁₅NO_x to C₁₀H₁₇NO_x increases from 0.34 to 0.84 when NO levels increase from low to high. The sum of the major C₁₀H₁₅O_x•-related closed-shell HOM (C₁₀H₁₄O_x and C₁₀H₁₅NO_x) contributes 24 % and 34 % under low-NO and high-NO conditions, while being negligible under NO-free conditions.

The contribution of H-abstraction related products observed in this study was lower than that reported by Shen and coworkers, who reported a contribution of more than 70 % (Shen et al., 2022). The experiments of the two studies were conducted in different chambers, under different conditions, and with different instrumentations applied. The two chambers could introduce different wall losses of HOM, which can further differ between less oxygenated HOM and highly oxygenated HOM.

In this study the •OH radicals were generated via photolysis of H₂O₂, whereas HONO photolysis was the main source of •OH in Shen et al. (2022). The consecutive reaction between •OH and H₂O₂ in our experiments led to an enhanced production of HO₂• radicals, resulting in an HO₂• concentration of ∼ 4 × 10⁸ molecule cm⁻³, which was approximately one order of magnitude higher than that reported by Shen et al. (2022). Also, the NO concentration in Shen et al. was constantly around ∼ 20 ppb, while it was only ∼ 7 ppb before reaction in our study, resulting in a concentration of ∼ 1 ppb during the early-reaction stage. Despite these differences, the contributions of RO₂• + NO reactions were dominant in both studies, accounting for more than 95 % of RO₂• biomolecular losses under high-NO conditions. In addition, we did only consider early reaction stages, during which secondary oxidation was negligible and RO₂• chemistry was dominated by autoxidation and RO₂• + NO reactions. Therefore, the chemical system can be expected to be similar for both studies, at least under high-NO conditions. However, the constantly higher level of NO in the study by Shen et al. (2022) may continuously promote the H-abstraction channel.

The discrepancy may also arise from differences in measurement techniques: Shen et al. (2022) used an Eisele type inlet (Mauldin et al., 1999), while in this study a MION inlet was used (Rissanen et al., 2019). To compare both inlet systems, we conducted an experiment where both types of inlets were simultaneously used to measure the same oxidation systems. The reference system was α-pinene oxidation by •OH, with the addition of NO (Fig. S17). The results indicate that the MION inlets detects a larger fraction of lesser oxygenated HOM (less than eight oxygen atoms) than the Eisele inlet, and that these HOM dominate the C₁₀H₁₇O_x•-related HOM family, while the relative contribution of higher oxygenated HOM (more than eight oxygen atoms) was larger in the study by Shen et al. (2022). When only the highly oxygenated HOM compounds are considered, which were analysed by Shen and coworkers, the relative contribution of C₁₀H₁₅O_x•-related HOM to the total HOM does increase to 64 %. Therefore, a deeper comparison of the two inlet types may be of interest in the future but is outside the scope of this study.

These findings indicate that the HOM yield attributed to the H-abstraction pathway compared to the OH-addition pathway can be significant and should not be overlooked when assessing the importance of HOM in SOA formation, especially in the presence of NO. Comprehensive product distributions formed through the H-abstraction pathway in α-pinene oxidation are illustrated in this study. HOM species with the formula C₁₀H₁₅NO₈, recognized as characteristic indicators of •OH initiated monoterpene oxidation in the presence of NO under daytime atmospheric conditions (Yan et al., 2016; Kulmala et al., 2013), may potentially be attributed to the H-abstraction pathway. However, even the NO concentrations in the low-NO case of this study were higher than those typically observed at Hyytiälä. Therefore, the significance of the H-abstraction pathway under typical low-NO boreal forest conditions remains uncertain and requires further investigation. In contrast, the contribution of the H-abstraction channel to HOM formation is expected to be more pronounced in megacities supposed to be under high NO conditions. The presence of NO not only terminates peroxy radicals and suppresses HOM formation, but also propagates the oxidative radical chain through the formation of alkoxy radicals and their subsequent isomerization, a process that under high NO levels can even compete with autoxidation (Kang et al., 2025). These reactions lead to effective NO-NO₂ conversion and can then become a crucial step to promote O₃ formation. The occurrence of alkoxy steps can also directly be related to O₃ formation in addition to O₃ sensitivity via the ratio of organic nitrate and non-nitrate HOM (Zhang et al., 2024). However, branching ratios towards alkoxy radical formation remain uncertain, when RO₂• reacts with NO. Further investigation is needed to clarify the role of alkoxy processes in both SOA and O₃ formation.