The Vocus PTR-TOF measurements were performed from 8 to 20 July 2018 in the Landes forest (44∘29′39.69′′ N, 0∘57′21.75′′ W), as part of the CERVOLAND field campaign. The sampling site is situated at the European Integrated Carbon Observation System (ICOS) station at Bilos in southwestern France along the Atlantic coast, ∼40 km southwest from the nearest urban area of the Bordeaux metropole. Both population density and industrial emissions are low in this area. Due to the proximity of the Atlantic Ocean, the site has a strong maritime influence. The forest is largely composed of maritime pines (Pinus pinaster Aiton) and has an average height of ∼10 m. Monoterpenes are known to be strongly emitted in the forest (Simon et al., 1994), which provides a good place for BVOC characterization. More detailed descriptions of the site have been provided in earlier studies (Moreaux et al., 2011; Kammer et al., 2018; Bsaibes et al., 2019).

Compared to conventional PTR instrument, the Vocus PTR-TOF used in this study is mainly differentiated in the following aspects:

a new chemical ionization source with a low-pressure reagent-ion source and focusing ion–molecule reactor (FIMR);

The temperature, relative humidity (RH), wind speed, and ambient pressure were continuously monitored at 3.4 m a.g.l., whereas the solar radiation was measured at 15.6 m a.g.l. from a mast located at the site. The mixing ratios of nitrogen oxides (NOx) and ozone (O3) were measured at 4 m a.g.l. with UV absorption and chemiluminescence analyzers, respectively. All data are reported in Coordinated Universal Time (UTC).

Data analysis was performed using the software package “Tofware” (https://www.tofwerk.com/software/tofware/, last access: 12 February 2020) that runs in the Igor Pro environment (WaveMetrics, OR, USA). Tofware enables time-dependent mass calibration, baseline subtraction, and assignment of a molecular formula to the identified ions by high-resolution analysis. Signals were averaged over 30 min before mass calibration. Due to the high resolving power of the LTOF (long time of flight) mass analyzer, isobaric ions were more clearly separated. Examples of peak identification are given in Fig. S1 in the Supplement.

The Vocus was calibrated twice a day during the campaign with a mixture (70 ppb each) of terpenes (m/z 137: α-/β-pinene + limonene; m/z 135: p-cymene) that was diluted using UHP N2. Similar to conventional PTR instruments, the sensitivities of different VOCs in the Vocus PTR-TOF are linearly related to their proton-transfer reaction rate constants (k) when ion transmission efficiency and fragmentation ions are considered (Sekimoto et al., 2017; Krechmer et al., 2018). Krechmer et al. (2018) have shown that within the Vocus PTR-TOF, the transmission efficiencies of ions >m/z 100 Th (thomson units) reach up to 99 %. Therefore, the influence of fragmentation correction should be included in this study. According to terpene calibrations, the residual fractions were on average 66 % and 55 % for protonated monoterpenes and p-cymene, respectively, after their fragmentation within the instrument. Based on the corrected sensitivities for fragmentation and the k values of monoterpenes and p-cymene, an empirical relationship between the sensitivity and k was built from the scatter plots using linear regression: sensitivity (cps ppb-1)=828.9×k, where cps represents counts per second (Fig. S2). Once k is available, the sensitivity of a compound can be predicted. It should be noted that the established relationship in this study is not applicable to other conditions or instruments. Some studies found that isoprene may fragment significantly to m/z 41 (Keck et al., 2008; Schwarz et al., 2009). However, with the ambient data in this work, isoprene seems not to fragment much to C3H5+, and they correlate poorly with each other (Fig. S3). Therefore, the fragmentation of isoprene is not considered for its quantification. Sesquiterpenes and some terpene oxidation products were found to fragment to varying degrees (Kim et al., 2009; Kari et al., 2018). Due to the lack of calibrations using other terpenes or terpene oxidation products, their fragmentation patterns within the Vocus PTR-TOF are not known in this work. Therefore, all the other terpenes and terpene oxidation products were quantified without consideration of fragment ions, which should be regarded as the lower limit of their ambient concentrations.

Rate constants for the proton-transfer reactions have only been measured for a subset of compounds. To quantify terpenes and their oxidation products, we used the method proposed by Sekimoto et al. (2017) to calculate the rate constants of different compounds with the polarizability and permanent dipole moment of the molecule. According to Sekimoto et al. (2017), the polarizability and dipole moment of a molecule can be obtained based on the molecular mass, elemental composition, and functionality of the compound. For a class of VOCs with the same number of electronegative atoms, their polarizabilities can be well described using their molecular mass (Sekimoto et al., 2017). For VOCs containing a specific functional group, it is found that their dipole moments are relatively constant based on results in the CRC Handbook of Chemistry and Physics (Lide, 2005). Since no isomer information is provided by mass spectrometry alone, it is challenging to figure out the functionality of different compounds. Therefore, the polarizability and dipole moment of the compounds observed in this study were estimated only based on the molecular mass and elemental composition. In this work, based on the physical properties of various compounds in CRC Handbook of Chemistry and Physics (Lide, 2005) and the results in Sekimoto et al. (2017), we built the functions between polarizability (α) and molecular mass (MR) for different groups of VOCs and calculated the average dipole moment (μ) for each group. For example, the polarizabilities of hydrocarbons were approximated as α=0.142 MR-0.3, and the dipole moment was approximated to be zero. For the non-nitrate oxygenated compounds with one oxygen, α=0.133 MR-1.2, and the dipole moment was averaged to be 1.6.

It should be noted that uncertainties are introduced to the calculated sensitivities in the following factors. Firstly, the small difference between the rate coefficients of monoterpenes and p-cymene may lead to large uncertainty in the established linear regression function between sensitivity and k. Calibrations with more VOC compounds should be performed in future work to cover a larger range of k values. Secondly, as mentioned above, the theoretically calculated sensitivities of sesquiterpenes, diterpenes, and terpene oxidation products may be underestimated to varying extents without the consideration of their fragment ions. Further, some low-volatility compounds may experience wall losses inside the inlet tubing and the instrument and therefore have worse transmissions. The method in this work may overestimate the sensitivities of these low-volatility compounds. In addition to proton-transfer reactions, some VOCs can be ionized through ligand switching reactions with water cluster ((H2O)nH3O+) (Tani et al., 2004), thus increasing their sensitivity. However, with the calibration standards used in this study, it is hard to estimate the effect of ligand switching ionization. Lastly, uncertainties come from the estimation of polarizability and dipole moment of a molecule. With the method used in this study, the sensitivity is calculated to be within 50 % error when only the elemental composition of a compound is known (Sekimoto et al., 2017).

Figure 1 displays the time variations of meteorological conditions and trace gases during the observation period. The weather was mostly sunny, with solar radiation varying from 400 to 800 W m-2 during daytime, indicating strong photochemical activity. The ambient temperature and RH varied regularly every day. On average, the temperature was 22.8±5.9 ∘C (mean±SE), ranging from 12.1 to 35.0 ∘C, which is favorable for BVOC emissions in the forest. The average RH was 70.5%±19.0 % during the campaign. Generally, the air masses were quite stable within the canopy. The wind speed never exceeded 1 m s-1, indicating the major influence of local sources on atmospheric processes in this study.

The O3 levels fluctuated dramatically between day and night during the campaign. The average O3 diurnal cycle showed that O3 concentration peaked up to ∼50 ppb in the daytime. However, during most of the nights, O3 concentration dropped below 2 ppb. Considering the high nighttime concentration of terpenes observed by the previous study at this site in the same season (Kammer et al., 2018), the low O3 level at night suggests the large consumption of O3 by terpenes. Such reactions of terpenes with O3 can produce low-volatility organic compounds, thus contributing to SOA formation (Presto et al., 2005; Jokinen et al., 2014). In addition, plant surface uptake is likely another important ozone sink in the canopy (Goldstein et al., 2004).

The NO concentration was generally low during the campaign, below detection limit (i.e., <0.5 ppb) most of the time. However, clear NO plumes were sometimes observed in the early morning (Fig. 1e). The NO concentration peak at 04:00 is probably the combination of local emission sources and a low boundary layer. With the increasing sunlight afterwards, the NO concentration started to decrease. A similar diel pattern of NO2 was observed by the previous study at this site (Kammer et al., 2018). The lower NO2 concentration during daytime is likely explained by dilution with increasing boundary layer height and NO2 photolysis.

While Krechmer et al. (2018) and Riva et al. (2019a) have described the novel setup and performance of the Vocus PTR-TOF and its application during a lab study, the instrument capability has not been fully explored in an ambient environment. Based on the CERVOLAND deployment, we provide here the first overview of gas-phase molecules measured by the Vocus PTR-TOF in the forest. For a better visualization of the complex data set from real atmosphere, mass defect plots (averaged over the whole campaign) are shown in Fig. 2 with the difference between the exact mass and the nominal mass of a compound plotted against its exact mass. With the addition of hydrogen atoms, the mass defect increases, while the addition of oxygen atoms decreases the mass defect. Therefore, changes in the mass defect plot help to provide information on chemical transformation such as oxidation.

The mass defect plot in Fig. 2a is colored according to the retrieved elemental composition, with the black circle indicating unidentified molecules. The size of the markers is proportional to the logarithm of the peak area of the molecule. During the campaign, the Vocus PTR-TOF detected large amounts of (O)VOCs, with elemental composition categories of CH, CHO, CHN, CHS, CHON, CHOS, and others. For hydrocarbons, multiple series with different carbon numbers were measured, especially those compounds containing 5 (“C5”) to 10 carbon atoms (“C10”), 15 carbon atoms (“C15”), and 20 carbon atoms (“C20”). Some of the C5–C9 ions can be fragments of terpenes and their oxidation products (Tani et al., 2003; Tani, 2013; Kim et al., 2009; Kari et al., 2018). For ions <35 Th, the detection efficiency is much reduced due to a high-pass band filter of the big segmented quadrupole (BSQ; Krechmer et al., 2018). Compared to conventional PTR instruments, the observation of larger hydrocarbon molecules by the Vocus PTR-TOF is mainly caused by the much lower wall losses and increased detection efficiency. Hydrocarbon signals were largely contributed by monoterpene (C10H16H+) and its major fragment (C6H8H+), indicating the monoterpene-dominated environment in the Landes forest (Kammer et al., 2018). According to previous studies, monoterpene emissions in the Landes forest are dominated by α-pinene and β-pinene (Simon et al., 1994; Kammer et al., 2018). The identified compound with the elemental composition of C4H9+ ranked the third largest peak in hydrocarbons. A detailed discussion about C4H9+ ions can be found in the Supplement.

In addition to the emitted precursors, the Vocus PTR-TOF detected various VOC reaction products and intermediates. Similar to the PTR3 measurements in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber (Breitenlechner et al., 2017), many oxygenated compounds from terpene reactions with varying degrees of oxidation were observed in this study. However, as a potential limitation of the instrument, no dimers in the atmosphere were identified by the Vocus PTR-TOF, consistent with the results from a previous laboratory deployment (Riva et al., 2019a).

Figure 2b compares the daytime and nighttime variations of different molecules, with the marker sized by the signal difference between day and night. The daytime periods cover from 04:30 to 19:30, and the nighttime periods are from 19:30 to 04:30 of the next day (both are UTC time; local time equals UTC time + 2). The data points are colored in pink when the nighttime signal of the compound is larger than its daytime signal and in blue when the daytime signal is higher. Patterns in the figure clearly show the difference in the diurnal variations of gas molecules with different oxidation degrees. For example, most hydrocarbons are characterized with higher concentrations at night, which is largely caused by the stable nocturnal boundary layer. The more oxidized compounds with higher oxygen numbers are generally more abundant during the day due to enhanced photochemistry, whereas the concentrations of the less oxidized compounds are mostly higher at night. Details on the diurnal profiles of different oxidation products and their formation mechanisms are provided in Sect. 3.4.

The characterizations of isoprene, monoterpenes, sesquiterpenes, and the rarely reported diterpenes are investigated in this study (Figs. 3, 4). On the global scale, isoprene is the most emitted BVOC species. It has been well established that photooxidation of isoprene in the atmosphere contributes to SOA formation through the multiphase reactions of isoprene-derived oxidation products (Claeys et al., 2004; Henze and Seinfeld, 2006; Surratt et al., 2010). However, recent advances in isoprene chemistry found that isoprene can impact both particle number and mass of monoterpene-derived SOA by scavenging hydroxyl and peroxy radicals (Kiendler-Scharr et al., 2009; Kanawade et al., 2011; McFiggans et al., 2019). During the CERVOLAND campaign, the average mixing ratio of isoprene was 0.6 ppb, consistent with the mean value of 0.4 ppb reported for the LANDEX campaign during summer 2017 at the same site (Mermet et al., 2019). These values are much lower than that in the southeastern United States (Xiong et al., 2015) and Amazon rainforest (Wei et al., 2018) but higher than observations in the boreal forest at the SMEAR II station in Finland (Hellén et al., 2018). Isoprene emissions are strongly light dependent (Monson and Fall, 1989; Kaser et al., 2013). Therefore, a pronounced diurnal pattern of isoprene was observed with maximum mixing ratios occurring during daytime and minima at night. It has been shown that the attribution of C5H9+ ions to isoprene with PTR instruments can be influenced by the fragmentation of many other compounds, i.e., cycloalkane and 2-methyl-3-buten-2-ol (MBO) (Karl et al., 2012; Gueneron et al., 2015). For example, using an E/N ratio (the reduced electric field) of 106 Td (townsend units) in the PTR-MS with a quadrupole mass analyzer, 71 % of the parent MBO fragmented to C5H9+ ions (Warneke et al., 2003). However, in this study, the C5H9+ signal was around 10 times as high as the C5H11O+ signal, and both ions correlated poorly with each other (Fig. S4; r2=0.33). This information demonstrates that the fragmentation of MBO does not likely have a significant influence on the attribution of C5H9+ ions to isoprene in this work.

As expected, monoterpenes showed the highest mixing ratios among all the terpenes, with an average value of 6.0 ppb. On 9 July, a heavy monoterpene episode occurred at night, with the monoterpene mixing ratio reaching as high as 41.2 ppb. Comparatively, the average monoterpene level observed in this work is similar to the measurements performed in 2015 and 2017 at the same site (Kammer et al., 2018; Mermet et al., 2019) and more than 10 times higher than that observed in the boreal forest at SMEAR II in summer (Hakola et al., 2012; Hellén et al., 2018). The high concentration of monoterpenes indicates the potential significance of monoterpene-related aerosol chemistry in the Landes forest. Unlike the light dependence of isoprene emissions, monoterpene emissions are found to be mainly controlled by temperature (Hakola et al., 2006; Kaser et al., 2013). At night, monoterpenes can be continuously emitted and accumulated within the boundary layer. Therefore, monoterpenes showed the opposite diel pattern to isoprene and peaked during nighttime. During daytime, the concentration of monoterpenes dropped to around 0.9 ppb, due to the increased atmospheric mixing after sunrise and the rapid photochemical consumptions.

A study in Hyytiälä concluded that sesquiterpenes, due to their higher reactivity, could play a more important role in O3 chemistry than monoterpenes, even though the concentration of sesquiterpenes was much lower (Hellén et al., 2018). However, the short lifetimes of sesquiterpenes also mean that their concentrations will be highly dependent on the sampling location at a given site. Some studies also proposed that sesquiterpene oxidation products are linked to atmospheric new particle formation (Bonn and Moortgat, 2003; Boy et al., 2007). Despite the potential importance of sesquiterpenes in aerosol chemistry, the available data on ambient sesquiterpene quantification remains still quite limited. In this work, the mixing ratios of sesquiterpenes were found to vary from 8.9 to 408.9 ppt in the Landes forest, with an average of 64.5 ppt during the observations. This sesquiterpene level is comparable to that reported by Mermet et al. (2019) in summer 2017 at the same site and observations by Jardine et al. (2011) in Amazonia but higher than previous measurements at the SMEAR II station (Hellén et al., 2018). Kim et al. (2009) show that different sesquiterpenes fragment on monoterpene parent and fragment ions to varying degrees inside PTR instruments. Without the consideration of sesquiterpene fragmentation, the quantification of sesquiterpenes in this work may be underestimated. As shown in Fig. 4, sesquiterpenes displayed a similar diurnal pattern with monoterpenes, consistent with observations in other areas (Jardine et al., 2011; Hellén et al., 2018).

While diterpenes are present in all plants in the form of phytol, for a long time they have been thought to be not released by vegetation due to their low volatility (Keeling and Bohlmann, 2006). In 2004, von Schwartzenberg et al. (2004) reported for the first time the release of plant-derived diterpenes into the air. A recent study found that the emission rate of diterpenes by Mediterranean vegetation was in the same order of magnitude as monoterpenes and sesquiterpenes (Yáñez-Serrano et al., 2018). For the first time, this study reports the ambient concentration of diterpenes in a forest. According to the Vocus PTR-TOF measurements, the average mixing ratio of diterpenes was around 2 ppt in the Landes forest. Considering the low volatility of diterpenes and their potential wall losses inside the inlet tubing and the instrument, the diterpene concentration might be higher. Similar to monoterpenes and sesquiterpenes, diterpenes presented peak concentrations at night and lower levels during the day. Although the amounts of diterpenes in the atmosphere are hundreds to thousands of times lower than those of monoterpenes and sesquiterpenes, diterpenes potentially play a role in atmospheric chemistry due to their unsaturated structure and high molecular weight (Matsunaga et al., 2012). Up to now, there is no report on the possible atmospheric implications of diterpenes, which should deserve more attention in the future.

Considering the similar atmospheric behaviors of monoterpenes, sesquiterpenes, and diterpenes in this study, it is questioned if the observed sesquiterpenes and diterpenes are real signals in the atmosphere or generated by monoterpenes in the instrument. Bernhammer et al. (2018) have shown that secondary association reactions of protonated isoprene with isoprene can form monoterpenes within the PTR reaction chamber. Figure 5 illustrates the scatter plots among monoterpenes, sesquiterpenes, and diterpenes, colored by time of the day. At night, both sesquiterpenes and diterpenes correlated well with monoterpenes. However, their correlation with monoterpenes got weaker during daytime as the data points became more scattered. This suggests that the observations of sesquiterpenes and diterpenes are real emissions in the atmosphere. Comparatively, sesquiterpenes and diterpenes showed a strong correlation with each other through the whole day (r2=0.85).