2.1 Site description

The LANDEX intensive field campaign was conducted from 3 to 19 July 2017 at the Bilos field site in the Landes forest, southwestern France. The vegetation on the site was dominated by maritime pines (Pinus pinaster Aiton) presenting an average height of 10 m. The climate is temperate with a maritime influence due to the proximity of the Atlantic Ocean. This site is part of the European ICOS (Integrated Carbon Observation System) ecosystem infrastructure. A more detailed description of the site is available in Moreaux et al. (2011) and Kammer et al. (2018).

2.2 OH reactivity instruments

The LP-LIF instrument, referred to here as UL-FAGE (University of Lille-FAGE – Fluorescence Assay by Gas Expansion), measured the OH reactivity in the canopy, whereas the CRM instrument, referred to as LSCE-CRM, alternatively measured the OH reactivity at two heights (see Fig. 1b). Table 1 summarizes the performance of both instruments. The LP-LIF technique has a 3-fold better limit of detection than the CRM; however, the CRM has a larger dynamic range since it can measure OH reactivity up to 300 s⁻¹ without sample dilution. The overall systematic uncertainty (1σ) is around 15 % and 35 % for the LP-LIF and the CRM, respectively. The LSCE-CRM and UL-FAGE characteristics are given in the following paragraphs.

Figure 1Deployment of instruments at the measurement site. (a) The horizontal deployment and (b) the different sampling levels with respect to the average tree height.

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Table 1Performance of the two OH reactivity instruments deployed during the LANDEX campaign.

^a LOD: limit of detection; ^b without dilution.

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CRM-LSCE system characterization

Several tests were performed before, during and after the campaign to assess the performance of the instrument operated during the whole campaign. The PTR-MS was calibrated at the beginning and at the end of the field campaign, showing good stability under dry and wet conditions (slope of 15.5±0.9 (1σ)). Regular C1 measurements were made to check the stability of the initial pyrrole concentration all along the campaign. C1 was 70.7±4.0 (1σ) ppbv.

Small differences in humidity observed between C2 and C3 were considered while processing the raw data. In order to assess this correction, experiments were performed to assess the variability of C2 on humidity by contrasting the change in C2 (ΔC2) for various changes in the ratio of m∕z 37 to m∕z 19 (Δ (m∕z 37-to-m∕z 19 ratios)), with m∕z 37 and m∕z 19 being representative of H₃O⁺(H₂O) and H₃O⁺, respectively, and their ratio being proportional to humidity. During this campaign, three humidity tests were performed by varying the humidity in ambient air samples. These tests were in good agreement and showed a linear relationship between ΔC2 (ppbv) and Δ (m∕z 37-to-m∕z 19 ratio) with a slope of −89.18. The correction was applied as discussed in Michoud et al. (2015).

An important assumption to derive R_OH from Eq. (2) is to operate the instrument under pseudo-first-order conditions (i.e., [pyrrole] ≫ [OH]), which is not the case with current CRM instruments. To determine the correction factor for the deviation from pseudo-first-order kinetics, injections of known concentrations of isoprene ($k_{\mathrm{Isoprene}+\mathrm{OH}}=\mathrm{1}\times {\mathrm{10}}^{-\mathrm{10}}$ cm³ molecule⁻¹ s⁻¹, 1–120 ppbv) and α-pinene ($k_{\mathit{\alpha }\text{-}\mathrm{pinene}+\mathrm{OH}}=\mathrm{5.33}\times {\mathrm{10}}^{-\mathrm{11}}$ cm³ molecule⁻¹ s⁻¹, 3–190 ppbv) (Atkinson, 1985) were performed before and after the field campaign since they represent the dominant species in this forest ecosystem.

The measured OH reactivity obtained from these tests was then compared to the expected OH reactivity, leading to a correction factor that is dependent on the pyrrole-to-OH ratio. Therefore, standard OH reactivity experiments were conducted at different pyrrole-to-OH ratios ranging from 1.7 to 4.0, which encompass the ratio observed most of the time during the campaign. These tests led to a correction factor $\left(F\right)=-\mathrm{0.52}\times \left(\mathrm{pyrrole}\text{-}\mathrm{to}\text{-}\mathrm{OH}\right)+\mathrm{3.38}$.

NO mixing ratios were lower than 0.5 ppbv (corresponding to the detection limit of the NO_x monitor deployed during LANDEX) most of the time for the measurement time periods used in this study, and no correction was applied for the spurious formation of OH from the HO₂ + NO reaction. Similarly, for NO₂, no correction was applied due to the low ambient mixing ratio of 1.1±0.8 ppbv. Regarding O₃, no dependency was seen for LSCE-CRM based on previous experiments (Fuchs et al., 2017). Therefore, no correction was applied. The correction (D) on the reactivity values due to the dilution was around 1.46 during the campaign. Thus, the total OH reactivity may be expressed as

$$\begin{array}{}\text{(3)}& R_{\mathrm{OH}\mathrm{final}}=\left[{\displaystyle \frac{(\mathrm{C}\mathrm{3}-\mathrm{C}\mathrm{2}(\mathrm{corrected}\left)\right)}{(\mathrm{C}\mathrm{1}-\mathrm{C}\mathrm{3})}}\cdot kp\cdot \mathrm{C}\mathrm{1}\right]\cdot F\cdot D.\end{array}$$

Finally, overall uncertainties were estimated at 35 % (1σ) for the measured OH reactivity by the CRM (Zannoni, 2015).

Table 2 reports a summary of the corrections resulting from our tests and their impact on measurements. As shown in Table 2, the application of (F) for the deviation from pseudo-first-order kinetics induces the largest correction, with an absolute increase of 10.4 s⁻¹ on average. Furthermore, this factor (F) has the largest relative uncertainty, with ±36 % against ±2 % for the humidity correction factor.

Table 2Summary of corrections applied to raw reactivity data for LSCE-CRM. Correction coefficients are obtained from experiments performed before, during and after the field campaign.

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2.3 Ancillary measurements and corresponding locations

Measurements of VOCs (Table 3) were performed at different locations (Fig. 1) by a proton transfer reaction mass spectrometer (PTR-MS) and four online gas chromatographic (GC) instruments. Ozone scrubbers (copper tube impregnated with KI) and particle filters were added to the inlets of all GC sampling lines. Losses of BVOCs in these ozone scrubbers were investigated under similar sampling conditions in the absence and presence of O₃ (Mermet et al., 2019). The scrubbers exhibited less than 5 % losses for most non-oxygenated BVOCs, whereas in the presence of ozone, losses were relatively higher for some BVOCs but remained lower than 15 % (lower than 5 % for α- and β-pinene). High flow rates were applied in the sampling lines: 1 L min⁻¹ for GC instruments and 10 L min⁻¹ for the PTR-MS; therefore, the contact time between ambient BVOCs and the particle filters was extremely short and no significant losses are expected.

GC-BVOC1 is a gas chromatograph coupled to a flame ionization detector (airmoVOC C6–C12, Chromatotec) used by LSCE to monitor high-carbon VOCs (C6–C12) at 12 m of height with a time resolution of 30 min. Sampling was undertaken for 10 min. The instrument sampled ambient air with a flow rate of 60 mL min⁻¹. Once injected, the sample passed through a capture tube containing the adsorbent Carbotrap C for VOC preconcentration at room temperature; the capture tube is then heated up to 380 ^∘C and the sample is introduced into the separating column (MXT30CE; i.d. 0.28 mm, length 30 m, film thickness 1 µm), with hydrogen as the carrier gas. During the campaign, calibrations were performed with a certified standard containing a mixture of 16 VOCs (including eight terpenes) at a concentration level of 2 ppbv (National Physical Laboratory, Teddington, Middlesex, UK). Three calibrations were performed three times (at the beginning, in the middle and at the end of the campaign). As they were showing reproducible results (within 5 % for all the terpenes except cineole), a mean response factor per VOC was used to calibrate the measurements. Note that limonene and cymene had close retention times, which led to overlapping peaks, and for this reason only the sum of both compounds has been reported. For further details, refer to Gros et al. (2011). The sampling was done using a 13 m long sulfinert heated line ($\mathrm{1}/{\mathrm{8}}^{\prime \prime }$) connected to an external pump for continuous flushing.

GC-BVOC2 is an online thermodesorber system (Markes UNITY 1) coupled to a GC-FID (Agilent). It was used to monitor 20 C5–C15 BVOCs, including isoprene, α- and β-pinene, carenes, and β-caryophyllene at the 6 m height with a time resolution of 90 min. Ambient air was sampled at a flow rate of 20 mL min⁻¹ for 60 min through a sorbent trap (Carbotrap B) held at 20 ^∘C by a Peltier cooling system. The sample was thermally desorbed at 325 ^∘C and injected into BPX5 columns (60 m × 0.25 mm × 1 µm) using helium as a carrier gas (30 min). Calibrations were performed at the beginning, in the middle and at the end of the campaign with a certified standard mixture (National Physical Laboratory – NPL; Teddington, Middlesex, UK, 2014) containing 33 VOCs (including four BVOCs: α-pinene, β-pinene, limonene and isoprene) at a concentration of 4 ppbv each. The sampling was done using a 10 m long sulfinert line ($\mathrm{1}/{\mathrm{4}}^{\prime \prime }$) heated at 55 ^∘C and connected to an external pump to adjust the sampling flow rate at 1 L min⁻¹. The method has been optimized in terms of temperature of the thermodesorption, the column, the sampling volume and sampling line including a scrubber. Tests showed a low response for some compounds (i.e., sabinene, terpinolene, etc.); however, the most abundant compounds were well measured. More details about the optimization and the tests performed can be found in Mermet et al. (2019).

GC-NMHC is an online GC equipped with two columns and a dual FID system (Perkin Elmer^®) that was described in detail elsewhere (Badol et al., 2004). It was used to monitor 65 C₂–C₁₄ non-methane hydrocarbons (NMHCs), including alkanes, alkenes, alkynes and aromatics, at the 12 m height with a time resolution of 90 min. Ambient air was sampled at a flow of 15 mL min⁻¹ for 40 min through a Nafion membrane and through a sorbent trap (Carbotrap B and Carbosieve III) held at −30 ^∘C by a Peltier cooling system. The trap was thermodesorbed at 300 ^∘C and the sample was introduced in the GC system. The chromatographic separation was performed using two capillary columns with a switching facility. The first column, used to separate C₆–C₁₄ compounds, was a CP-Sil 5 CB (50 m × 0.25 mm × 1 µm), while the second column for C₂–C₅ compounds was a plot Al₂O₃∕Na₂SO₄ (50 m × 0.32 mm × 5 µm). Helium was used as a carrier gas. Calibrations were performed at the beginning, middle and end of the campaign with a certified standard mixture (NPL; Teddington, Middlesex, UK, 2016) containing 30 VOCs at a concentration level of 4 ppbv each. The sampling was done using a 13 m long sulfinert line ($\mathrm{1}/{\mathrm{4}}^{\prime \prime }$) heated at 55 ^∘C and connected to an external pump for continuous flushing at 2 L min⁻¹.

GC-OVOC is an online GC-FID (Perkin Elmer^®) used to monitor 16 C₃–C₇ oxygenated VOCs (OVOCs), including aldehydes, ketones, alcohols, ethers, esters and six NMHCs (BVOCs and aromatics). A detailed description can be found in Roukos et al. (2009). The measurements were performed at the 12 m height with a time resolution of 90 min. Ambient air was sampled at a flow rate of 15 mL min⁻¹ for 40 min through a water trap (cold finger, −30 ^∘C) and a quartz tube filled with Carbopack B and Carbopack X held at 12.5 ^∘C. VOCs were thermally desorbed at 280 ^∘C and injected into CP-Lowox columns (30 m × 0.53 mm × 10 µm) using helium as a carrier gas. Calibrations were performed three times during the campaign using a standard mixture (Apel Riemer, 2016) containing 15 compounds. This mixture was diluted with humidified zero air (RH = 50 %) to reach VOC levels of 3–4 ppbv. The sampling was done with the same sampling system as the GC-NMHC. The sulfinert material chosen for all GC sampling lines and used in the LSCE-CRM sampling system is recommended by ACTRIS (2014). High flows were set in the lines (residence time of less than 8 s) that were heated up to 50 ^∘C to minimize the losses of potential reactive species. Filters and scrubbers were changed twice for the GC-BVOC1 and one time for the other GC instruments.

The PTR-MS (PTR-QiToFMS, IONICON Analytic GmbH) sequentially measured trace gases at four levels (L1 = 12 m, L2 = 10 m, L3 = 8 m, L4 = 6 m) with a cycle of 30 min (6 min at each level and 6 min of zero air). The drift tube was operated at a pressure of 3.8 mbar, a temperature of 70 ^∘C and an E∕N ratio of 131 Td. Four identical sampling lines of 15 m were used to sample ambient air at each height. The lines (PFA, $\mathrm{1}/{\mathrm{4}}^{\prime \prime }$ o.d.) were heated at 50 ^∘C and were constantly flushed at 10 L min⁻¹ using an additional pump and rotameters. Indeed, Kim et al. (2009) tested losses of β-caryophyllene in similar operating conditions. The authors varied the temperature from zero to 40 ^∘C, showing that losses of β-caryophyllene are negligible above 20 ^∘C. The residence time was lower than 2 s.

Teflon filters were used to filter particles at the entrance of the sampling lines. The PTR-MS drew ambient air at a flow rate of 300 mL min⁻¹ from the different lines using Teflon solenoid valves and a 1.5 m long inlet (PEEK, $\mathrm{1}/{\mathrm{16}}^{\prime \prime }$ o.d.) heated at 60 ^∘C. Zero air was generated using a gas calibration unit (GCU; IONICON Analytic GmbH) containing a catalytic oven and connected to L1. Ion transmissions were calibrated over the 21–147 Da mass range every 3 d using the GCU and a certified calibration mixture provided by IONICON (15 compounds at approximately 1 ppmv, including methanol, acetaldehyde, acetone, aromatic compounds and chlorobenzenes). Measurements of methanol, acetonitrile, acetaldehyde, acetone, isoprene, methacrolein + methylvinylketone + fragment ISOPOOH, methylethylketone, the sum of monoterpenes, the sum of sesquiterpenes, acetic acid, nopinone and pinonaldehyde, obtained from levels 1 and 4 corresponding to the levels at which OH reactivity measurements were performed, are discussed in this article. Sesquiterpenes, acetic acid, nopinone and pinonaldehyde measurements were not corrected for fragmentation in the drift tube, and we cannot rule out the detection of other isomers at these masses such as glycolaldehyde for acetic acid measurements.

Inorganic traces gases (O₃ and NO_x) were measured by commercial analyzers deployed by IMT-Lille-Douai (L1 to L4 for O₃) and EPOC (L4 for NO_x). The nitrate radical (NO₃) was measured using an IBB-CEAS instrument (incoherent broadband cavity absorption spectroscopy) developed by the LISA (Laboratoire Interdisciplinaire des Systèmes Atmosphériques) research group and deployed for the first time on site during the LANDEX field campaign. Meteorological parameters such as temperature, relative humidity, global radiation, vertical turbulence, wind speed and wind direction were monitored using sensors already available at the ICOS measurement site. More details can be found in Kammer et al. (2018).

Table 3Summary of supporting measurements performed inside and/or above the canopy.

^a These compounds were not considered in the calculation of the weighted k rate constant for the reaction of monoterpenes with OH. Nopinone, linalool and β-caryophyllene had relatively low contributions to OH reactivity that were around 0.02, 0.37 and 0.18 s⁻¹ on average, respectively. Maximum contributions did not exceed 2.2 s⁻¹ for linalool and 1.5 s⁻¹ for β-caryophyllene. ^b Fragmentation was not corrected for, and reported concentrations are likely lower limits. ^c Potential interferences from isomeric compounds such as glycolaldehyde.

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2.4 OH reactivity calculation

As different instruments were available to quantify VOCs at different locations (Fig. 1 and Table 3), a selection of the data used to calculate the OH reactivity (Eq. 1) was made based on data availability for the different instruments (Table S1 in the Supplement). Since measurements from the PTR-MS instrument cover the whole campaign and were performed at the same heights as OH reactivity measurements, these measurements, including methanol, acetonitrile, acetaldehyde, acetone, isoprene, methacrolein + methylvinylketone + fragment ISOPOOH, methylethylketone and the sum of monoterpenes, were selected to calculate the OH reactivity and to evaluate the potential missing OH reactivity at both levels. However, using only this set of data presents some limitations.

1. The PTR-MS only measures the sum of monoterpenes (m∕z 137$+m/z\mathrm{81}$), while the detected monoterpenes are speciated by the GCs.

2. It was observed that isoprene measurements at m∕z 69 were disturbed by the fragmentation of some terpenic species (Kari et al., 2018; Tani, 2013), which led to a significant impact on the nighttime measurements when isoprene was low.

3. Some NMHCs and OVOCs measured by GC at the 12 m height were not measured by the PTR-MS. This requires an assessment of the contribution of these additional species to the total OH reactivity for both heights.

To overcome these limitations, several tests were made to evaluate the reliability of the PTR-MS data to calculate the OH reactivity.

1. In order to use the sum of monoterpenes measured by the PTR-MS to calculate the total OH reactivity, it was necessary to determine a weighted rate constant for the reaction of monoterpenes with OH. After checking the consistency between the two GCs (BVOC1 and BVOC2; see Sect. S2) and comparing the sum of monoterpenes measured by each GC to the PTR-MS measurements (simultaneous measurements at the same height; Fig. S2b and c), the weighted rate constant was calculated as the sum of the rate constants of each OH + monoterpene reaction multiplied by the average contribution of each specific monoterpene to the sum. The contribution of each monoterpene was calculated by dividing the concentration of the eight speciated monoterpenes that were measured by both GCs (α-pinene, β-pinene, myrcene, Δ-carene, p-cimene, limonene + cymene, cineol) by their total concentration (Fig. S3a). The weighted rate constant is defined as

   $$\begin{array}{}\text{(5)}& k_{\mathrm{OH},\mathrm{weighted}}=\sum _i{k}_{\mathrm{OH}+X_i}{F}_i,\end{array}$$

   where F_i represents the contribution of each individual species to the total concentration of monoterpenes, and $k_{\mathrm{OH}+X_i}$ is the corresponding rate constant with OH. The reaction rate constant of the different trace species quantified in the field were taken from the literature (Atkinson et al., 2006). The OH reactivity of monoterpenes measured by PTR-MS was calculated according to the following equation:

   $$\begin{array}{}\text{(6)}& R_{\mathrm{OH}\text{-}\mathrm{monoterpenes}}=k_{\mathrm{OH},\mathrm{weighted}}\times \left[\mathrm{MT}\right],\end{array}$$

   where [MT] represents the sum of monoterpenes measured by PTR-MS.

   The calculated OH reactivity inside and above the canopy (Fig. S3b and e) from (i) the use of the weighted OH reaction rate constant and the total concentration of monoterpenes measured by GC and (ii) the use of individual species and their associated rate constants is in relatively good agreement as shown by the scatter plots. At a slope of 0.95, R²=0.99 has been obtained using the monoterpenes measured with the GC-BVOC1 at 12 m (Fig. S3c); at a slope of 0.94, R²=1.0 was obtained using the same eight compounds commonly monitored with GC-BVOC1 but measured at 6 m with GC-BVOC2 (Fig. S3f). When replacing the total concentration of monoterpenes measured by GCs by the PTR-MS measurements, slopes of 1.22 and 1.19 were obtained at 12 and 6 m heights, respectively (Fig. S3d and g). This increase in the slope values is likely due to an underestimation of the total monoterpene concentration by the GC instruments since these instruments only measured the most abundant monoterpenes present at the site. We cannot rule out a small overestimation of monoterpenes by the PTR-MS since fragments from other species such as sesquiterpenes could be detected at the monoterpene m∕z. However, this interference should be negligible due to the low concentration of ambient sesquiterpenes. These results are in agreement with the scatter plots comparing the sum of monoterpenes measured by GC and by PTR-MS (slopes of 1.29 and 1.10 at the 12 and 6 m heights, respectively; see Fig. S2b and c). Thus, the PTR-MS data were used to calculate the OH reactivity from monoterpenes for both heights, with a weighted reaction rate constant of $\mathrm{76}\times {\mathrm{10}}^{-\mathrm{12}}$ cm³ molecule⁻¹ s⁻¹ at the 12 m height and $\mathrm{77.9}\times {\mathrm{10}}^{-\mathrm{12}}$ cm³ molecule⁻¹ s⁻¹ at the 6 m height.

2. As mentioned above, some monoterpenes have been observed to fragment at m∕z 69.0704, which would result in an interference for isoprene measurements. In order to use the PTR-MS data for this species (the only instrument measuring isoprene at 12 m), the contribution of monoterpenes to m∕z 69 has been estimated by comparing the GC-BVOC2 and PTR-MS measurements of isoprene performed at 6 m. This comparison showed that approximately 4 % of the monoterpene concentration measured by PTR-MS had to be subtracted from that measured at m∕z 69.0704 to get a good agreement between the PTR-MS and GC-BVOC2 measurements of isoprene, as shown in Fig. S4a.

3. A large range of NMHCs and OVOCs was measured at the 12 m height only by GC-NMHC and GC-OVOC (Table 3). Butanol (from scanning mobility particle sizer – SMPS – exhausts) was also checked and found to be negligible at 12 m and highly and rapidly variable at 6 m (short peaks). NO and NO₂ were only measured at the 6 m height. The mean NO mixing ratio was below the LOD for the measurement period, and NO₂ was around 1.1±0.8 ppbv on average. Thus, it was chosen not to take these species into account in the OH reactivity calculations. However, sensitivity tests were performed in order to compute their relative contribution to OH reactivity (see Sect. 3.5 and Figs. S5 and S6). Regarding methane and carbon monoxide, an estimation was made given their relatively low k reaction rate coefficient with OH, resulting in mean concentration values of 2000 and 150 ppbv, respectively.

The above limitations are summarized in Table S7. The data used to calculate the OH reactivity have been resampled to 1 min based on a linear interpolation (see Table 3 for the respective time resolution of the different instruments). This time base was chosen to be comparable to the time resolution of the UL-FAGE reactivity instrument in order to keep the dynamics in OH reactivity variability.

3.1 Comparison between LSCE-CRM and UL-FAGE measurements

3.1.1 Intercomparison of LSCE-CRM and UL-FAGE OH reactivity measurements at the same location

A direct comparison between the LSCE-CRM and UL-FAGE reactivity instruments was done during the last 2 d of the campaign (Fig. 2). The sampling line of LSCE-CRM was moved to be collocated with the sampling line of UL-FAGE. Both instruments were measuring at the same location inside the canopy level, above the UL container at 5 m of height. In this way the comparison between the two instruments was made possible while minimizing variabilities that could be related to the heterogeneity in ambient air. During this period, similar values were measured by both instruments, as shown in Fig. 2, with total OH reactivity ranging between 5 and 69 s⁻¹. The lowest values were observed during daytime.

Figure 4(a) Time series of total OH reactivity measured by the UL-FAGE and LSCE-CRM instruments from 13 to 18 July 2017 (a). Dark blue symbols represent the measured reactivity by UL-FAGE; green and yellow symbols represent the measured reactivity by LSCE-CRM inside and above the canopy, respectively. (b) The sum of monoterpenes (MTs) and isoprene measured with the PTR-MS in the field for the same period. Dark blue and light blue dots correspond to isoprene concentrations at 6 and 12 m of height, respectively. Orange and yellow dots represent monoterpene concentrations at 6 and 12 m of height, respectively.

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When OH reactivity measurements from LSCE-CRM are plotted versus OH reactivity measurements from UL- FAGE (Fig. 3), the linear regression exhibits an R² of 0.87. Applying the orthogonal distance regression technique, which takes into account the uncertainties on LSCE-CRM and UL-FAGE measurements, a slope of 1.28±0.02 and an intercept of 0.96±0.23 s⁻¹ are obtained. These results indicate that both instruments respond similarly (within 30 %) to changes in OH reactivity with a relatively low intercept. This intercept can be due to an overestimation of LSCE-CRM measurements, an underestimation of the UL-FAGE measurements or both. Nevertheless, it stays within the range of uncertainties. It is worth noting that the higher points of OH reactivity observed in Fig. 3 correspond to the period from 19:30 to 20:00 LT (local time) on 18 July when the ambient relative humidity increased quickly by 20 %, which was not seen on previous days and may have interfered with LSCE-CRM OH reactivity measurements.

3.1.2 LSCE-CRM and UL-FAGE OH reactivity measurements at two different locations inside the canopy

From 13 to 15 July midday (first period) and from 17 to 18 July midday (second period), the two instruments were sampling at the same height but from different horizontal locations within the canopy (with sequential within- and above-canopy measurements for CRM during the second period). The horizontal distance between the two inlets was around 10 m as shown in Fig. 1. Similar trends in OH reactivity are seen between the two datasets, even if the first period was associated with a clear vertical stratification (Fig. 4, green frame), leading to higher concentrations of monoterpenes within the canopy, whereas the second period was characterized by a higher vertical mixing (mean $u^{*}\approx \mathrm{0.3}$ m s⁻¹) leading to similar concentrations of monoterpenes at the two heights (Fig. 4, dashed green–yellow frame). These observations are linked to the vertical turbulence that influences BVOC levels inside and above the canopy, resulting in a more or less important vertical stratification, as discussed in Sect. 3.2.

At the same height but different horizontal locations, the linear regression of LSCE-CRM data plotted against UL-FAGE data (not shown) indicates a good correlation with an R² of 0.87. Using the orthogonal distance regression technique, both datasets show a good agreement with a slope of 1.28±0.02 and an intercept of $-\mathrm{2.63}\pm \mathrm{0.15}$ (first and second period). Compared to the results at the same location (vertical and horizontal), the slopes and the correlation coefficients are the same. Only the intercept differs slightly ($-\mathrm{2.63}\pm \mathrm{0.15}$ compared to 0.96±0.23). This change could be related to air mass inhomogeneities, which could be systematically less reactive at one location compared to the other one. From these observations, we can conclude that reactivity measurements performed at different horizontal locations are consistent and that inhomogeneities in ambient air can lead to differences on the order of several seconds.

Figure 5Variability of measured OH reactivity by LSCE-CRM and UL-FAGE inside and above the canopy with (a) global radiation (black), (b) temperature (red), friction velocity (green), and (c) monoterpene and isoprene concentrations. Yellow stripes indicate stable nighttime atmospheric conditions (S nights with mean $u^{*}\le \mathrm{0.2}$ m s⁻¹), and blue stripes indicate unstable nighttime conditions (U nights with mean $u^{*}\ge \mathrm{0.4}$ m s⁻¹). Class SU includes nights with stable and unstable atmospheric conditions (blue + yellow stripes). Wn and Wd stand for warm nights and warm days, respectively. Cn and Cd stand for cooler nights and cooler days, respectively. Red dashes and black dashes indicate the temperature thresholds to distinguish warm and cool days and nights, respectively. Green dashes indicate the friction velocity threshold to distinguish stable and unstable nights.

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3.2 Measured OH reactivity and meteorological parameters

Figure 5a and b shows the variability of total OH reactivity measured inside and above the canopy by LSCE-CRM and UL-FAGE, together with global radiation, temperature and friction velocity. Considering the whole campaign, the measured OH reactivity at both heights shows a diurnal trend ranging between LOD (3 s⁻¹) and 99 s⁻¹ inside canopy and between LOD and 70 s⁻¹ above the canopy, with maximum values of OH reactivity mostly recorded during nights. These OH reactivity levels are larger than other measurements performed in forested environments (Yang et al., 2016; Dusanter and Stevens, 2017), with maximum values of approximately 80 s⁻¹ reported for a tropical forest (Edwards et al., 2013).

The predominant meteorological parameter that had a role in OH reactivity levels was friction velocity. It traduces the vertical turbulence intensity that was high during the day (mean daytime $u^{*}\ge \mathrm{0.4}$ m s⁻¹) and lower during most nights (mean nighttime $u^{*}\le \mathrm{0.2}$ m s⁻¹). Based on this parameter, nighttime OH reactivity (between 21:00 and 06:00 LT of the next day) was separated into three classes:

• class S involves stable atmospheric conditions (mean $u^{*}\le \mathrm{0.2}$ m s⁻¹),

• class U involves unstable atmospheric conditions (mean $u^{*}\ge \mathrm{0.4}$ m s⁻¹), and

• class SU involves stable and unstable conditions during the same night.

The lower vertical turbulence intensity, observed for S nights as well as for some hours of SU nights, led to a lower boundary layer (Saraiva and Krusche, 2013) and a significant nocturnal stratification within the canopy, with higher concentrations of primary compounds within the canopy (Fig. 5c). These stable atmospheric conditions, together with no photochemical oxidation of BVOCs, resulted in higher total OH reactivity during these nights due to higher BVOC concentration even though the emissions are lower compared to daytime (Simon et al., 1994).

Another important parameter to consider is ambient temperature, which is known to enhance BVOC emissions during the day when stomata are open and which also plays a role for nighttime emissions due to permeation, even though stomata are closed in the dark (Simon et al., 1994). Considering temperature, two subclasses can be added to nighttime OH reactivity classification: the subclass Wn corresponding to warm nights (nights with mean T≥18.9 ^∘C, which is the mean nighttime temperature over the whole campaign) and the subclass Cn that includes cooler nights (nights with mean T<18.9 ^∘C). Thus, comparing S/Wn nights and S/Cn nights, it can be seen that, for similar turbulent conditions, the magnitude of the measured OH reactivity was temperature dependent. Indeed, higher OH reactivity values were linked to higher ambient temperatures: the nights of 4–5, 6–7 and 16–17 July (S/Wn) were characterized by an average temperature of 21 ^∘C compared to 16.6 ^∘C for the nights with lower OH reactivity (S/Cn).

Regarding the period when measurements were done simultaneously at both heights (15 to 18 July, LSCE-CRM above the canopy and UL-FAGE within the canopy), we can analyze the effect of turbulence on the above- and within-canopy differences. For the night of 16–17 July (S/Wn) when the vertical turbulence was relatively low, total OH reactivity measured above the canopy (LSCE-CRM) was lower than the one measured inside the canopy by a mean factor of 1.6 (UL-FAGE reactivity) despite similar general trends. For the night of 17–18 July (SU/Wn), stable atmospheric conditions started to settle at the beginning of the night (20:30 LT), inducing a similar stratification to that observed on the previous nights. However, this situation did not last the whole night since these stable conditions were disturbed by higher turbulences around 21:00 LT. This led to a decrease in OH reactivity values going to similar levels inside and above the canopy. A similar event occurred during the night of 18–19 July, when three OH reactivity peaks showed up that were not correlated with the variation of turbulence intensity or with temperature changes. However, it is worth noting that during this night, intense wind, rain and thunder occurred, which could have led to the observed bursts of BVOCs (Nakashima et al., 2013), leading to distinct peaks of BVOCs and total OH reactivity and thus relatively high total OH reactivity compared to other nights from the same class.

Total OH reactivity also increased during the day, although to a lesser extent than during nighttime, and reached a daytime maximum of up to 74.2 s⁻¹ inside the canopy and 69.9 s⁻¹ above the canopy, following the same trends as temperature and solar radiation. Temperature appeared to be an important driving factor of total OH reactivity during daytime hours; therefore, daytime OH reactivity was divided into two classes: class Wd with warm conditions (mean daytime T≥24 ^∘C) and class Cd with cooler temperatures (mean daytime T<24 ^∘C), as indicated in Fig. 5. The solar radiation also played a role in daytime OH reactivity since it is responsible for initiating the emission of some compounds like isoprene that are light and temperature dependent. Thus, with the first rays of sunlight, the emission and the concentration of isoprene increased, leading to an increase in total OH reactivity.

Examining BVOC profiles (Fig. 5c), we can see how the variability of primary BVOC concentrations can explain the day–night variability of total OH reactivity. Indeed, monoterpenes, which are the main emitted compounds in this ecosystem, were influenced by vertical turbulence and nighttime temperature, exhibiting a diurnal profile with maxima during stable nights and minima during daytime. Under stable atmospheric conditions (class S), monoterpene concentrations started to increase at the beginning of the night (between 20:00 and 21:00 LT), corresponding to the time of day when the turbulence intensity started to drop and the nocturnal boundary layer started to build up. Maximum mixing ratios were reached in the middle of the night, corresponding to a lower dilution in the atmosphere and a lower oxidation rate (low OH concentrations, nitrate radical mixing ratios lower than the LOD of 3 ppt min⁻¹ most of the time and BVOC chemistry with ozone generally slower than during daytime; Fuentes et al., 2002). Finally, the monoterpene concentration dropped as soon as the first sunlight radiation broke the stable nocturnal boundary layer, inducing lower levels of OH reactivity. Under these conditions, the concentration of monoterpenes inside the canopy was higher than above the canopy, showing a clear stratification consistent with the differences seen in total OH reactivity at the different heights. In contrast, during turbulent night hours (class U and SU), the concentration of monoterpenes was lower inside the canopy and similar to that observed above, leading to lower and closer nighttime OH reactivity at both measurement heights.

Figure 6Variability of measured ROH (LSCE-CRM) and calculated ROH (PTR-MS) at 6 and 12 m of height.

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In the end, even though BVOC emissions are more intense during the day (Simon et al., 1994), the higher turbulence observed compared to nighttime led to a faster mixing within the canopy and thus similar levels of isoprene and monoterpenes inside and above the canopy. These daytime levels were lower than those observed at night for monoterpenes and higher for isoprene, the latter being light and temperature dependent.

To conclude, these observations show that, on one hand, lower turbulence inducing stable atmospheric conditions during the night explains the observed stratification in terms of monoterpene levels and thus in terms of OH reactivity levels within the canopy; on the other hand, higher turbulence during daytime leads to higher mixing within the canopy and vertical homogeneity, with similar BVOC concentrations and OH reactivity levels at both heights. Diurnal average values of total OH reactivity for inside- and above-canopy measurements are given in Table S9.

3.3 Measured and calculated R_OH within and above the canopy

Figure 6 shows that there is a good covariation of the measured total OH reactivity by the CRM instrument with the values calculated from the PTR-MS data (22 %–24 % (2σ)). However, a certain fraction of the measured total OH reactivity remains unexplained by the considered compounds (Table 3). Diurnal variations of OH reactivity were observed within the canopy during the major part of the campaign, with maximum values recorded during most nights and averages of 19.2±12.8 and 19.3±16.3 s⁻¹ measured by the LSCE-CRM and UL-FAGE instruments, respectively. This diurnal cycle was also observed above the canopy where the average total OH reactivity was 16.5±12.3 s⁻¹, which is higher than observations made in other temperate coniferous forests (Ramasamy et al., 2016) where the reported OH reactivity ranges from 4 to 13 s⁻¹ (campaign average).

During the first part of the campaign (3–10 July), when the LSCE-CRM was measuring alone inside the canopy, total OH reactivity varied between LOD (3 s⁻¹ at 3σ) and 76.9 s⁻¹, while the calculated reactivity ranged between 1.4 and 60 s⁻¹. During the second period (13–15 and 17–18 July), similar maxima were recorded by the LSCE-CRM (74.2 s⁻¹) and the UL-FAGE instruments (78.9 s⁻¹), when both were measuring at two different locations within the canopy. Regarding the calculated OH reactivity, it varied between 2.6 and 59.3 s⁻¹. During this same period, the FAGE instrument measured alone within the canopy from 15 to 17 July and recorded total OH reactivity values ranging between 3.6 and 99.2 s⁻¹; however, the PTR-MS data were not taken into account for the period from 16 July 15:00 LT to 17 July 12:00 LT due to an electrical failure. Finally, during the last 2 d (18–19 July), total OH reactivity showed a particular behavior as mentioned in Sect. 3.2. It started to increase in the afternoon, reached a maximum at the beginning of the night, which was suddenly broken by turbulences, and showed three peaks during the night corresponding to more stable conditions observed for both the measured and calculated reactivity.

Regarding above-canopy measurements, the measured OH reactivity varied between LOD and 35.7 s⁻¹ between 10 and 12 July, whereas the calculated reactivity varied between 1.2 and 14.5 s⁻¹. A similar trend was observed for the second period of measurements performed above the canopy (15–18 July) during which higher OH reactivity was recorded with a maximum of 69.9 s⁻¹, which is 1.7 times higher than the calculated OH reactivity (40.8 s⁻¹).

3.4 Contribution of VOCs (PTR-MS) to calculated OH reactivity within and above the canopy

Figure 7 shows the breakdown of trace gases to the calculated OH reactivity during daytime and nighttime at the two heights, taking into account the whole measurement period (campaign average). We note that primary BVOCs (monoterpenes, isoprene) are by far the main contributors to the calculated OH reactivity, representing 92 %–96 % of the calculated OH reactivity on average.

Figure 7The components of calculated OH reactivity within and above the canopy during daytime and nighttime.

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Monoterpenes exhibited the most prominent contribution to the calculated OH reactivity. These species had a similar contribution within and above the canopy but significant differences between daytime (68 %–65 %) and nighttime (92 %–89 %). Next to monoterpenes, isoprene had a maximum contribution during daytime and represented on average 25 %–27 % of the calculated OH reactivity, followed by acetaldehyde (3 %) and MACR + MVK + ISOPOOH (2 %–3 %) at both measurement heights. However, during nighttime, isoprene accounted for only 4 %–6 % of the OH reactivity measured within and above the canopy, with acetaldehyde contributing approximately 2 % and MACR + MVK + ISOPOOH around 1 %.

Thus, we can conclude that no substantial difference in the atmospheric chemical composition existed between the two sampling heights, even when we only consider stable nights (the monoterpene relative contribution is around 92 % inside and above the canopy).

3.5 Description and investigation of potential missing OH reactivity during the LANDEX campaign

The missing OH reactivity was calculated as a difference between the total OH reactivity measured by LSCE-CRM, since it was operated over the whole campaign and at both heights, and the OH reactivity calculated from PTR-MS data. It is worth noting that a scatter plot of the LSCE-CRM and UL-FAGE data led to a slope of 1.28 and an intercept of 0.96 s⁻¹ (Sect. 3.1.1), indicating higher OH reactivity values measured by the CRM instrument. Considering OH reactivity values measured by the CRM instrument may therefore maximize the missing OH reactivity. In the following, the analysis of the missing OH reactivity was performed when it was higher than both the LOD of 3 s⁻¹ (3σ) and 35 % of the measured OH reactivity (uncertainty on the CRM measurements; see Sect. 2.2).

Figure 8 shows (a) the variability of the missing OH reactivity within and above the canopy, together with ambient temperature, (b) friction velocity (red), and ozone mixing ratios within (yellow) and above (blue) the canopy. The ozone variability is discussed below as ozone chemistry can dominate the nighttime chemistry of BVOCs observed at this site (σ-pinene, β-pinene) (Fuentes et al., 2002; Kammer et al., 2018).

Figure 8Missing OH reactivity inside and above the canopy together with (a) temperature, (b) friction velocity (red), ozone mixing ratios inside (yellow) and above (blue) the canopy, (c) relative humidity (clear blue), MACR + MVK + ISOPOOH (dark blue) and acetic acid (green) inside the canopy, (d) nopinone (yellow) and pinonaldehyde (purple) inside the canopy, and (e) sesquiterpenes inside (blue) and above (green) the canopy.

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The concentration of OH was 4.2×10⁶ molecules cm⁻³ on average during daytime, with a maximum of 4.3×10⁷ molecules cm⁻³, and around 1.5×10⁶ molecules cm⁻³ on average during nighttime (data available between 13 and 19 July). However, a potential artifact on OH radical measurements, leading to a possible overestimation of OH radical concentrations, could not be ruled out. Regarding ozone, its mixing ratio showed a diurnal cycle with maximum values during the day (max ≈ 60 ppbv, mean ≈ 29 ppbv) that were similar within and above the canopy due to efficient mixing and lower levels during nights, with an average of 18 ppbv inside the canopy, while levels were higher by 1–10 ppbv on average above the canopy. Considering OH and O₃ average mixing ratios, the α-pinene lifetime was estimated to be 1.2 and 4 h, respectively, during the day and 3.6 and 5.8 h, respectively, during the night. At maximum OH and O₃ mixing ratios during daytime, the α-pinene lifetime was reduced to 7.4 min and 2 h, respectively. Thus, OH chemistry remained dominant compared to the ozonolysis of the main emitted compounds at this site (i.e., α-pinene). An article on the reactivity of monoterpenes with OH, ozone and nitrate for this campaign is in preparation (Kenneth Mermet, personal communication, 2019).

When comparing measurements of OH reactivity with calculations based on PTR-MS data (see Table 3), an average of 38 % (7.2 s⁻¹) and 48 % (6.1 s⁻¹) remained unexplained inside and above the canopy, respectively.

Considering other measurements performed inside the canopy (6 m) and not included in the OH reactivity calculations, such as NO, NO₂, ozone and butanol (leakage from SMPS), and assuming constant concentrations of CO (150 ppbv) and methane (2000 ppbv), their contribution can reach 3.0 s⁻¹ on average (maximum around 7 s⁻¹) at this level. This said, the mean missing OH reactivity was finally around 4.2 s⁻¹ (22 %) inside the canopy for the whole measurement period.

Regarding other measurements performed above the canopy, online chromatographic instruments (Table 3) provided information on other oxygenated VOCs (seven compounds) and non-methane hydrocarbons (36 compounds). These compounds could explain 0.48 s⁻¹ on average (0.43 s⁻¹ from NMHC and 0.05 s⁻¹ from OVOC measured by GC) of the missing OH reactivity between 10 and 12 July. However, after 14 July, the GC measuring OVOC stopped working, but NMHCs alone could account for 0.5 s⁻¹ of missing OH reactivity on average. While O₃ was measured at 12 m, no NO_x measurements were performed at this height; however, their contribution at the 6 m height was 0.3 s⁻¹ on average, suggesting only a small contribution to the missing OH reactivity. Methane and CO were also considered, assuming the same mixing ratios as inside. Finally, looking at butanol measured by the PTR-MS at the 12 m height, a maximum mean contribution of 0.3 s⁻¹ was assessed for the nights of 10–11 July. Hence, considering OVOCs, NMHC, O₃, CO, CH₄ and butanol, the mean missing OH reactivity above the canopy level was around 4.3 s⁻¹ (33 %). However, this missing fraction exhibited a diurnal variability at both heights, which is worth discussing in detail. A summary of mean missing OH reactivity values at both heights is presented in Table 4.

Table 4Summary of the measured OH reactivity and the missing OH reactivity inside and above the canopy during the day and the night, taking into account only PTR-MS data or all the data available at each height for OH reactivity calculations. These averages are calculated for the periods when CRM, PTR-MS and other instrumental data are available.

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Figure 9Daytime missing OH reactivity binned by ambient temperature for the 6 m height for temperatures ranging from 292 to 308 K. Error bars represent the standard deviation on missing OH reactivity calculated for each temperature bin.

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3.5.2 Nighttime missing OH reactivity

On average, the highest nighttime missing OH reactivity inside the canopy (13.1 s⁻¹) was observed on the stable and warm night of 4–5 July, whereas during stable and cool as well as unstable and warm nights, no significant missing OH reactivity was found (< LOD). Interestingly, the stable and warm night of 6–7 July did not show significant missing OH reactivity, meaning that the missing fraction inside the canopy during night was not only influenced by meteorological parameters, even if, as shown before, BVOC concentrations and total OH reactivity were. So what was the difference between these two nights with similar meteorological conditions?

Checking monoterpene oxidation product variabilities (nopinone and pinonaldehyde), both nights exhibited higher concentration levels of these species; however, their contribution to OH reactivity remained relatively low and did not exceed 1 s⁻¹ on average for both nights, keeping in mind that this is a lower limit of their contribution (since the reported measurements do not account for potential fragmentation in the PTR-MS). Thus, only a small part of the missing fraction can be explained by these species. Interestingly, isoprene, acetic acid and MVK + MACR + ISOPOOH exhibited higher concentration levels during the night of 4–5 July, which was not the case for the night of 6–7 July. Indeed, these species marked relatively high nocturnal and inside-canopy levels. When looking at air mass backward trajectories (Fig. 10), the night of 4–5 July was characterized by an air mass originally coming from the ocean, which spent at least 48 h above the continent before reaching the site. This could have led to the enrichment of the air mass with species emitted by the widely spread Landes forests and their oxidation products. Thus, the significant missing OH reactivity observed during the mentioned night is likely related to unconsidered compounds of biogenic origin characterized by a similar behavior to that of isoprene, acetic acid and MVK + MACR + ISOPOOH, which accumulated in the stable nocturnal boundary layer. In contrast, air masses spent approximately 12–18 h above the continent during 6–7 July, with more time above the ocean. Marine air masses are generally known to be clean, with relatively low levels of reactive species. Even though the night of 5–6 July shows similar air mass backward trajectories to the night of 4–5 July, the higher turbulence during this night prevents the accumulation of reactive species (including long-lived oxidation products) due to a higher boundary layer height, lowering the reactivity and the missing OH reactivity (Fig. 10).

Figure 10Air mass backward trajectories for the nights of 4–5 and 6–7 July. Red lines represent air masses arriving around midnight UTC (around 02:00 LT – local time) to the site. The time difference between two points is 6 h.

Regarding above-canopy measurements (10–12 and 15–18 July), the nighttime average missing OH reactivity was 5.6 s⁻¹ (all the nights were characterized by stable and cool atmospheric conditions). Monoterpene oxidation products had similar concentration levels above and inside the canopy. Their maximum contribution was around 0.4 s⁻¹ on average for the SU/W night of 17–18 July. Therefore, these monoterpene nighttime oxidation products are only responsible for a small fraction of the missing OH reactivity observed above the canopy during the night. Sesquiterpenes (SQTs) exhibited a similar temporal trend as monoterpenes, showing higher mixing ratios during nighttime. Interestingly, sesquiterpene mixing ratios were higher inside the canopy compared to above and the difference was significant during stable nights. O₃ mixing ratios during these nights decreased to very low levels. Plotting the ratio SQT(above) ∕ MTs(above) with the ratio SQT(inside) ∕ MTs(inside) shows a good linear correlation with a slope of 0.73 and an R² of 0.6. Knowing that sesquiterpenes are highly reactive with ozone (Ciccioli et al., 1999), which can dominate the chemistry during dark hours, this observation suggests that a larger fraction of these species (≈30 %) could be consumed by ozonolysis above the canopy, leading to the formation of unidentified secondary compounds. However, sesquiterpenes were present at relatively low concentrations (maximum of 0.25 and 0.11 ppbv inside and above the canopy, respectively). Assuming that all sesquiterpenes are β-caryophyllene and considering that 30 % are transformed into first-generation oxidation products through ozonolysis reactions, the maximum mixing ratio of these products would be around 0.07 ppbv each, assuming a yield of 1. However, it was reported by Winterhalter et al. (2009) that oxidation products of β-caryophyllene were much less reactive (100 times) than their precursor. Thus, the contribution of sesquiterpene nighttime oxidation products to the missing OH reactivity is likely negligible.

Finally, it is worth noting that Holzinger et al. (2005) reported the emission of highly reactive BVOCs in a coniferous forest, which is 6–30 times the emission of monoterpenes in the studied Ponderosa pine forest. This large fraction of BVOCs is subject to oxidation by ozone and OH, leading to unidentified, non-accounted for secondary molecules. These oxidation products can participate in the growth of new particles. Indeed, new particle formation episodes were recently reported on this site (Kammer et al., 2018).

To summarize, higher daytime missing OH reactivity was observed for warm days (Wd) inside and above the canopy, exhibiting a dependency on temperature profiles and showing that trace gases leading to the missing OH reactivity could be linked to an enhancement of primary species as well as secondary product formation. Regarding nighttime missing OH reactivity, higher levels were seen for the stable and warm night of 4–5 July, showing that these conditions could have been favorable for the accumulation of long-lived species (primary and secondary species) during the transport of the air mass from nearby forests.