The FIGAERO system enables simultaneous characterisation of gas- and particle-phase species by sampling gases through one inlet while collecting particulate matter on a filter via a separate sampling port (Bannan et al., 2019; Lopez-Hilfiker et al., 2014). The instrument was operated in negative-ion mode using I- as the reagent ion, generated by passing CH3I and N2 over a 210Po radioactive source. It was run in a cyclic mode consisting of the following procedure: (i)

30 min of gas-phase sampling and simultaneous particle collection onto a PTFE filter (2.0 µm pore size, Zefluor; filters were pre-heated to 200 °C to remove potential contaminants) both at 1 Lmin-1. During this step, the instrument was flushed with N2 for 0.5 min every 4.5 min to obtain the gas-phase instrument background signal.

To account for background species in the chamber, background measurements were conducted weekly. During these measurements, all components (SOA precursors, seed particles, CO, and NOx) were injected under the same conditions as in the regular experiments, while the chamber was kept in the dark. Data obtained during these background measurements were subtracted from the corresponding gas- and particle-phase data acquired during the “experiment” phase.

The FIGAERO-CIMS data were analysed using the Tofware package (v4.0.0) in Igor Pro 7.0.8 (WaveMetrics ©). I-, H2OI-, CH2O2I-, and I3- were used for mass-to-charge calibration (calibration error≤3ppm). High-resolution peak identification and fitting were performed in the m/z range of 200–550 (iodide adducts), which contained the vast majority of the total signal. Owing to the lack of available calibration standards and potential variability in instrument sensitivity across different oxygenated organic compounds, quantitative analysis using I--CIMS remains challenging (Lee et al., 2014). As a result, a uniform instrument sensitivity was assumed for all detected products. Additional uncertainties arise from the thermal decomposition in the FIGAERO. As shown in Fig. S2 in the Supplement, several compounds with relatively low carbon numbers exhibited comparatively high average carbon oxidation state (OSc‾) values and elevated maximum desorption temperature (Tmax). However, these species together accounted for less than 10 % of the total signal, indicating that the impact of thermal decomposition on the chemical composition was limited.

The Vocus PTR-ToF-MS provides high-sensitivity and fast-response measurements of organic compounds without the need for pre-concentration or chromatographic separation. Compared to traditional PTR-MS, the Vocus employs a focusing ion-molecule reactor (IMR) consisting of a glass tube that is mounted inside a radio frequency (RF) quadrupole, with an axial electric field applied along the tube. This design enhances ion transmission efficiency and suppresses the clustering of ions with water molecules, thereby improving sensitivity and lowering the limit of detection (Jensen et al., 2023; Krechmer et al., 2018; Yuan et al., 2017).

In our experiments, the ion source was supplied with a 20 sccm flow of water vapor. The IMR was operated at 60 °C and 2.0 mbar, with an axial voltage of approximately 568 V and an RF amplitude of 450 V at 1.3 MHz. The reduced electric field strength (E/N) was 141 Td. Measurements were conducted on a 5 min cycle, consisting of 4 min of sampling followed by 1 min of instrumental background measurement. Instrument calibration was conducted daily. The calibration curve for α-pinene is presented in Fig. S3 in the Supplement. Owing to the absence of an n-dodecane calibration standard, direct quantification was not feasible. Moreover, n-dodecane undergoes extensive fragmentation during ionisation, and its protonated molecular ion signal is subject to interference from overlapping species. Therefore, alternative approaches were adopted for its quantification: (i) the initial mixing ratios were taken as the target values (160 ppb in the single-precursor system and 80 ppb in the mixed-precursor system), and (ii) the relative consumption of n-dodecane was inferred from the temporal evolution of the C10H21+ fragment ion (Fig. S4 in the Supplement). However, interference from other oxidation products or fragments cannot be fully excluded and may have led to an overestimation of SOA particle mass yields. Nevertheless, this uncertainty is unlikely to affect the overall trends or relative differences in yields.

A detailed description of the C-ToF-AMS can be found in Drewnick et al. (2009). Ionization efficiency (IE) and relative ionization efficiency (RIE) calibrations were carried out using size-selected NH4NO3 and (NH4)2SO4 particles. The average IE of NH4NO3 was determined to be 2.75×10-7ionsmolecule-1, while the RIE for NH4+ and SO42- were 4.71±0.24 and 1.13±0.01, respectively. These values are comparable to those reported in the literature (Canagaratna et al., 2007; Lannuque et al., 2023).

In this study, the organic aerosol (OA)/sulfate correction method was applied to correct for chamber wall losses in the SOA particle mass concentrations measured by AMS (Wang et al., 2018). This method assumes that the loss rate constants of OA and seed aerosols are identical, and that seed concentrations are affected solely by wall loss. The corrected particle mass concentration is given by: 2COA,corr(t)=COA(t)Cseed(t)Cseed(0) where COA(t)/Cseed(t) represents the SOA-to-sulfate ratio derived from AMS measurements, and Cseed(0) denotes the sulfate concentration at the beginning of the experiment.

SOA particle mass yields (YSOA) for each system were derived from SOA particle mass concentrations measured by AMS and precursor concentrations measured by PTR. It is defined as the mass of SOA particles formed per unit of precursor consumed (Gao et al., 2022): 3YSOA=ΔSOAΔprecursor For the single-precursor systems, Δprecursor (µgm-3) denotes the consumption of α-pinene or n-dodecane, whereas in the mixed-precursor system it refers to the total consumption of α-pinene and n-dodecane. In this study, the SOA particle mass yield refers to the overall yield and is calculated as the ratio of the total SOA particle mass formed to the total precursor consumed at the end of the experiment.

The initial O3 concentration in the chamber was negligible. Upon illumination, O3 gradually accumulated, peaking at 38.5 ppb approximately two hours after lights on in the absence of CO, and then declined over time (Fig. 1a). In the presence of CO, the peak O3 concentration (60.7 ppb) was observed near the end of the experiment.

The initial α-pinene/NOx ratio in the α-pinene experiments was approximately 0.8 (Table 1). In the absence of CO, NOx concentrations declined during the first two hours of the reaction and subsequently stabilised. In contrast, in the presence of CO, NOx declined continuously throughout the experiment (Fig. S5). α-Pinene was almost entirely consumed within three hours under both conditions (Fig. 1b). Notably, the initial consumption rate was lower in the presence of CO. After approximately two hours, however, the decay rate increased and eventually converged with that observed in the absence of CO.

Compared to the experiment without CO, SOA particle mass increased more slowly in the presence of CO, resulting in substantially lower SOA particle mass concentrations (Fig. 1c). In both cases, the concentrations stabilised during the final hour of the reaction. By the end of the experiment, SOA particle mass concentrations reached 41.0 µgm-3 in the absence of CO and 17.8 µgm-3 in its presence. Correspondingly, the α-pinene SOA particle mass yield decreased from 0.14–0.08.

Owing to the absence of data from the final two FIGAERO cycles in the α-pinene experiment with CO, the analysis of SOA particle composition was based on the second cycle, corresponding to two hours of reaction, by which time substantial SOA mass had already formed.

Figure 2a presents the high-resolution mass spectra of particle-phase compounds from α-pinene experiments conducted with and without CO, together with their differences. The products were mainly distributed within the molecular mass range of 150–280 Da. Under both conditions, C8H10O5 was the most abundant compound. Based on elemental composition, the compounds were classified into CHO and CHON groups. CHON species accounted for 21 % of the total signal in the absence of CO and 28 % in its presence.

The compounds can be categorised into three classes based on carbon number: monomers, fragments, and accretion products (Fig. 2b). Monomers derived from α-pinene consisted of C10 products, whereas fragment compounds contained fewer than 10 carbon atoms and accretion products contained more than 10. Fragments dominated under both conditions, accounting for 55 % and 60 % of the total signal in the absence and presence of CO, respectively. Within the CHO group, fragments contributed more than 60 %, with a substantial proportion distributed in the C7–C9 range (Fig. S11 in the Supplement). Monomers accounted for 36 % and 31 % in the absence and presence of CO, respectively, and were the dominant class within the CHON group, accounting for more than 50 %. The presence of CO led to a lower proportion of C10 CHO compounds (e.g., C10H16O4-6) and a higher proportion of C10 CHON compounds (e.g., C10H15NO7-8) (Fig. 2a). The overall fraction of accretion products remained constant at 9 % under both conditions. However, the proportion of C16–C24 accretion products was lower in the presence of CO (Fig. 2c).

The major RO2 radicals derived from α-pinene react via the Reactions (R1) and (R2) pathways to form the C10H14On, C10H16On, and C10H18On families. As shown in Fig. 3, in the absence of CO these species accounted for 11.0 %, 13.6 %, and 4.0 % of the CHO group, respectively, and decreased to 9.0 %, 9.5 %, and 2.4 % in its presence.

In the n-dodecane experiments, O3 concentrations were generally higher in the absence of CO than in its presence (Fig. 1a). The temporal evolution of O3 differed markedly between the two conditions. In the absence of CO, O3 had nearly reached its peak by the end of the experiment, whereas in the presence of CO it continued to increase throughout the experiment. Despite these differences in formation rates and peak timing, the final O3 concentrations in both systems converged to similar levels, approaching 100 ppb.

The initial n-dodecane/NOx ratio was approximately 0.9 (Table 1). NOx concentrations declined steadily throughout the experiment under both conditions (Fig. S5). In the presence of CO, the decay rate of n-dodecane was lower (Fig. 1b). By the end of the experiment, 37 % of the initial n-dodecane remained unreacted in the absence of CO, whereas 47 % remained when CO was present.

In the absence of CO, the final SOA particle mass concentration reached 122.9 µgm-3, corresponding to a mass yield of 0.17 (Exp. 5). In the presence of CO, it reached 20.5 µgm-3, with a yield of 0.04.

Compared to α-pinene, particle-phase products derived from n-dodecane exhibited generally higher molecular mass distributions, primarily within the range of 210–310 Da (Fig. 4a). In the absence of CO, the most abundant species were C12H25NO5, C12H24O5, and C12H26O3, whereas in the presence of CO, C12H25NO4, C12H23NO7, and C12H25NO6 dominated. CHON compounds accounted for 37 % and 43 % of the total signal in the absence and presence of CO, respectively.

In the n-dodecane systems, compounds containing 12 carbon atoms were classified as monomers, those with fewer than 12 as fragments, and those with more than 12 as accretion products (Fig. 4b). Monomers dominated under both conditions, accounting for 57 % of the total signal in the absence of CO and 48 % in its presence. Within the CHON group, monomers accounted for more than 70 %. The presence of CO led to a lower proportion of C12 CHO compounds (e.g., C12H24O5 and C12H26O3) and a higher proportion of C12 CHON compounds (e.g., C12H25NO4 and C12H23NO7) (Fig. 4a). However, a few exceptions were observed. For example, a series of highly oxygenated C13 CHO compounds, such as C13H24O9 and C13H22O10, accounted for a higher fraction in the presence of CO, whereas C12H25NO5 accounted for a higher fraction in the absence of CO. Fragments accounted for 30 % and 37 % in the absence and presence of CO, respectively. While the overall fraction of accretion products was comparable under both conditions, the presence of CO reduced the fraction of C16–C24 accretion products (Fig. 4c).

The major RO2 radicals derived from n-dodecane react via the Reactions (R1) and (R2) pathways to form the C12H24On and C12H26On families. As shown in Fig. 3, in the absence of CO these species accounted for 12.6 % and 10.4 % of the CHO group, respectively, and decreased to 5.6 % and 6.1 % in its presence.

During the first hour of the reaction, O3 concentrations were comparable in the absence and presence of CO (Fig. 1a). Thereafter, O3 levels became higher in the presence of CO. In both cases, O3 concentrations peaked during the final hour, reaching 81.1 ppb without CO and 101.6 ppb with CO.

The initial precursor/NOx ratio was approximately 0.8 (Table 1). NOx concentrations declined steadily throughout the reaction under both conditions (Fig. S5). In the mixture, the presence of CO led to lower decay rates for both α-pinene and n-dodecane compared to the experiment without CO (Fig. 1b). Nevertheless, α-pinene was fully consumed within three hours in both cases. By the end of the experiment, 25 % of the initial n-dodecane remained unreacted without CO, whereas 51 % remained with CO.

In the absence of CO, the final SOA particle mass concentration reached 63.9 µgm-3, corresponding to a mass yield of 0.11 (Exp. 9). In the presence of CO, it reached 58.8 µgm-3, with a yield of 0.16.

Compared to single-precursor systems, the mixed-precursor system exhibited a broader molecular mass distribution, primarily ranging from 150–330 Da (Fig. 5a). In the absence of CO, C8H10O5, C8H12O6, and C12H24O5 showed the highest signal intensities, whereas in the presence of CO, C12H24O5, C8H10O5, and C12H25NO6 were most abundant. CHON compounds accounted for 30 % and 29 % of the total signal in the absence and presence of CO, respectively.

In the mixture, compounds with fewer than 10 carbon atoms were classified as fragments, while those containing more than 12 carbon atoms were considered accretion products. Fragments dominated under both conditions and accounted for 40 % of the total signal in each case. Accretion products accounted for 12 % and 11 % in the absence and presence of CO, respectively. Except for C15 species, the fractions of C13–C24 products decreased slightly in the presence of CO. In addition, the presence of CO resulted in an increased proportion of C12 CHO compounds (e.g., C12H26O4 and C12H24O5), and a reduced proportion of C10 CHON compounds (e.g., C10H15NO7) (Fig. 5a). Overall, changes in carbon number distribution were less pronounced in the mixed-precursor system than in the single-precursor systems (Fig. S11).

The bottom panel of Fig. 3 shows the relative contributions of C10H14On, C10H16On, C10H18On, C12H24On, and C12H26On to the CHO products in the mixture. In the presence of CO, the fractions of C10H14On and C10H18On decreased from 6.9 % and 1.2 % to 5.5 % and 0.8 %, respectively, whereas those of C10H16On, C12H24On, and C12H26On increased from 7.6 %, 5.0 %, and 3.1 % to 10.4 %, 6.6 %, and 4.7 %, respectively.

The presence of CO led to several consistent changes in the chemical composition of SOA particles in both the α-pinene and n-dodecane systems, including an increased relative contribution of the CHON group and fragment species and a reduced fraction of C16–C24 accretion products (Figs. 2 and 4). In addition, the relative contributions of representative RO2+RO2 termination products (C10H14On and C12H24On) within the CHO group decreased (Fig. 3). These observations provide evidence for a similar shift in RO2 fate in the presence of CO in both systems. However, owing to the limitations of I--CIMS measurements, the absolute contributions in individual reaction pathways cannot be fully constrained. The following discussion is therefore based partly on relative changes.

Organic nitrate concentrations were estimated from AMS measurements using the method described by Kiendler-Scharr et al. (2016). The results show that, in the single-precursor systems, the presence of CO led to a pronounced reduction in organic nitrate concentrations (Fig. S13 in the Supplement). This reduction can be attributed to two main factors. First, CO competes with SOA precursors for available OH (Figs. 1b and S6). Second, CO enhances HO2 formation, increasing the importance of the RO2+HO2 pathway and thereby altering RO2 reaction branching. In addition, lower NO concentrations were observed in the presence of CO (Fig. S5), consistent with enhanced conversion of NO to NO2 via the HO2+NO reaction. The increase in HO2 and decrease in NO reduced the likelihood of RO2 reacting with NO. Despite this absolute reduction, FIGAERO-CIMS results showed that the relative contributions of the CHON group and fragment products increased in the presence of CO (Figs. 2 and 4). CHON products are primarily formed through the RO2+NO→RONO2 pathway, and fragment species originate from the fragmentation of RO radicals (Atkinson, 2000; Ziemann and Atkinson, 2012). Owing to the rapid reaction of RO2 with NO and the high branching towards RO formation, reactions of RO2 with NO represent an important source of RO radicals under NOx conditions (Orlando et al., 2003; Ziemann and Atkinson, 2012). These observations therefore indicate that, in the presence of CO, the contribution of RO2+NO reactions decreased, but to a lesser extent than competing RO2 termination pathways.

AMS measurements showed a decrease in SOA particle mass concentrations in the presence of CO (Fig. 1c). In addition to OH scavenging, another important factor is that CO enhances competition between RO2+RO2 and RO2+HO2 reactions, thereby reducing the formation of accretion products (Baker et al., 2024; McFiggans et al., 2019; Peräkylä et al., 2023). Despite this reduction, CO did not significantly alter the overall fraction of accretion products. However, the relative contribution of C16–C24 species decreased (Figs. 2c and 4c), accompanied by an increase in C11–C15 species in the α-pinene system and C13–C14 species in the n-dodecane system. Accretion products with lower carbon numbers are expected to form via pathways that involve fragmentation of RO radicals (Kang et al., 2025), and their increased relative contribution is consistent with the elevated fraction of fragment products discussed above. In contrast, longer-chain accretion products are more likely to originate from RO2+RO2 reactions involving non-fragmented C10/C12 RO2 radicals, including reactions between non-fragmented RO2 radicals and fragmented RO2 radicals (<C10), or between two non-fragmented RO2 radicals, yielding C20 and C24 accretion products in the α-pinene and n-dodecane systems, respectively. Combined with the reduced fractions of C10H14On and C12H24On families (Fig. 3), these observations indicate that CO preferentially suppressed RO2+RO2 chemistry, particularly pathways forming longer-chain accretion products.

Overall, in the single-precursor systems, CO reduced the contributions of both RO2+RO2 and RO2+NO reactions. However, reactions of RO2 with NO decreased to a lesser extent than competing RO2 termination pathways, and the reduction in RO2+RO2 termination was more pronounced for longer-chain accretion products than for shorter-chain ones.

Compared with the single-precursor systems, the influence of CO on SOA chemical composition differed in the mixed-precursor system. Specifically, (i) the presence of CO did not significantly alter the relative contributions of the CHON group and fragment species (Fig. 5a and b); (ii) the fractions of C13–C24 accretion products (excluding C15) slightly decreased (Fig. 5c); and (iii) within the CHO group, the fraction of the C10H14On family decreased, whereas that of the C12H24On family increased (Fig. 3).

In the mixed-precursor system, organic nitrate concentrations exhibited little variation in the presence of CO (Fig. S13), consistent with the largely unchanged relative contribution of the CHON group and fragment species. This suggests that the contribution of RO2+NO reactions was not substantially reduced under CO conditions.

SOA particle mass concentrations and the fraction of accretion products both decreased slightly in the presence of CO (Figs. 1c and 5), suggesting a slight reduction in the contribution of RO2+RO2 termination.

Moreover, CO led to a lower fraction of the C10H14On family in the mixture, consistent with the trend observed in the α-pinene single-precursor system. In contrast to the n-dodecane single-precursor system, however, the relative contribution of the C12H24On family increased in the presence of CO in the mixture. Together with the increase in the fraction of C12 species and decrease in that of C10 species (Fig. S11), these observations may indicate that CO affected RO2+RO2 termination involving α-pinene-derived RO2 more strongly than that involving n-dodecane-derived RO2.

Overall, in the mixed-precursor system, the influence of CO on RO2 termination pathways was less pronounced than in the single-precursor systems and may have affected n-dodecane- and α-pinene-derived RO2 to different extents.

Although the underlying mechanism cannot be fully resolved in this study, the observed changes in product distributions provide important evidence for shifts in RO2 reaction pathways in the mixed-precursor system under different conditions. As α-pinene and n-dodecane were used as representative precursors, these findings may be specific to the present system. Future chamber studies covering a broader range of precursor combinations are therefore needed to assess the generality of the observed behaviour.