The capabilities of the recently developed Vocus
proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) are
reported for the first time based on ambient measurements. With the
deployment of the Vocus PTR-TOF, we present an overview of the observed
gas-phase (oxygenated) molecules in the French Landes forest during
summertime 2018 and gain insights into the atmospheric oxidation of
terpenes, which are emitted in large quantities in the atmosphere and play
important roles in secondary organic aerosol production. Due to the greatly
improved detection efficiency compared to conventional PTR instruments, the
Vocus PTR-TOF identifies a large number of gas-phase signals with elemental
composition categories including CH, CHO, CHN, CHS, CHON, CHOS, and others.
Multiple hydrocarbons are detected, with carbon numbers up to 20.
Particularly, we report the first direct observations of low-volatility
diterpenes in the ambient air. The diurnal cycle of diterpenes is similar to
that of monoterpenes and sesquiterpenes but contrary to that of isoprene.
Various types of terpene reaction products and intermediates are also
characterized. Generally, the more oxidized products from terpene oxidations
show a broad peak in the day due to the strong photochemical effects, while
the less oxygenated products peak in the early morning and/or in the
evening. To evaluate the importance of different formation pathways in
terpene chemistry, the reaction rates of terpenes with main oxidants (i.e.,
hydroxyl radical, OH; ozone,
Organic aerosol (OA) constitutes a large fraction of atmospheric particles, having significant impacts on climate change, air quality, and human health (Maria et al., 2004; IPCC, 2013; Mauderly and Chow, 2008). On a global scale, secondary OA (SOA) is the largest source of OA, formed through the oxidation of volatile organic compounds (VOCs) (Jimenez et al., 2009). Biogenic VOCs (BVOCs) are released into the atmosphere in high amounts, with an annual global budget being 760 Tg C (Sindelarova et al., 2014). On average, SOA production from biogenic precursors ranges from 2.5 to 44.5 Tg C annually, which is much larger than that from anthropogenic sources (Tsigaridis and Kanakidou, 2003). Over the past decades, a considerable number of studies have been conducted to investigate the atmospheric chemistry of BVOCs (Kanakidou et al., 2005; Henze and Seinfeld, 2006; Hatfield and Huff Hartz, 2011; Calfapietra et al., 2013; Jokinen et al., 2015; Ng et al., 2017). However, an incomplete understanding of BVOC characteristics and their oxidation processes in the atmosphere remains and yields large uncertainties in quantitative estimates of air quality and climate effects of atmospheric aerosols (Carslaw et al., 2013; Zhu et al., 2019).
Terpenes make up the main fraction of BVOCs (Guenther et al., 1995),
encompassing isoprene (
With a high time response and sensitivity, proton-transfer-reaction mass
spectrometry (PTR-MS) has been widely used to study the emissions and
chemical evolution of VOCs in the atmosphere (Yuan et al., 2017). However,
due to the relatively low sensitivity, previous PTR-MS instruments were not
optimized to detect low-volatility compounds. For example, only a few
ambient PTR-MS observations of sesquiterpenes are available (Kim et al.,
2009; Jardine et al., 2011). Correspondingly, it is not surprising that
ambient observations of diterpenes, which are generally considered to be
non-volatile compounds, have never been reported. In addition, the existing
PTR-MS is often not sensitive enough to quantify terpene oxidation products
at atmospherically relevant concentrations (Yuan et al., 2017). To address
these instrumental limitations, two new versions of PTR spectrometers were recently
developed: the PTR3 (Breitenlechner et al., 2017) and the Vocus PTR-TOF (Krechmer et al., 2018), both coupled with a time-of-flight (TOF) mass
analyzer. With the enhanced sensitivities by a factor of
Known for strong monoterpene emitters (Simon et al., 1994), the Landes forest in southwestern France is a suitable place to investigate atmospheric terpene chemistry. A previous study at this site reported a high nocturnal monoterpene loading and suggested that monoterpene oxidations play an important role in the formation of new particles and the consequent growth of atmospheric particles (Kammer et al., 2018). To better assess the roles of BVOCs in aerosol formation, the Characterization of Emissions and Reactivity of Volatile Organic Compounds in the Landes forest (CERVOLAND) campaign took place in July 2018. The recently developed Vocus PTR-TOF was deployed during the CERVOLAND campaign to characterize terpenes and their gas-phase oxidation products, which provides the first Vocus PTR-TOF measurements in a forested environment. In this work, we present a comprehensive summary of the identified gas-phase molecules and gain insights into terpene chemistry to demonstrate the Vocus PTR-TOF capabilities and the importance of its applications in atmospheric sciences. Characterizations of isoprene, monoterpenes, sesquiterpenes, and particularly the rarely detected diterpenes are reported. By comparing the reaction rates of different formation pathways, we explore the formation mechanisms of terpene oxidation products, including both non-nitrate and organic nitrate compounds.
The Vocus PTR-TOF measurements were performed from 8 to 20 July 2018 in the
Landes forest (44
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); no dependence of the sensitivity on ambient sample humidity due to the high
water mixing ratio (10 % v/v–20 % v/v) in the FIMR; employment of a TOF mass analyzer with a longer flight tube and faster
sampling data acquisition card (mass resolving power up to 15 000 m dm an enhanced inlet and source design that minimizes contact between analyte
molecules and inlet or source walls, enabling detection of semivolatile and
low-volatility compounds in a similar manner as chemical ionization mass
spectrometer (CIMS) instruments (Liu et al., 2019).
Details about the Vocus PTR-TOF are well described by Krechmer et al. (2018). Compared to the ionization in a conventional PTR-MS at 2.0–4.0 mbar,
a nitrate CIMS at ambient pressure or an iodide CIMS at around 100 mbar,
the Vocus ionization source is generally operated at a low pressure
(Krechmer et al., 2018). In this work, we operated the Vocus ionization
source at a pressure of 1.5 mbar. During the campaign, the Vocus PTR-TOF
measurements were performed at around 2 m above ground level (a.g.l.), thus
within the canopy. Sample air was drawn in through a 1 m long PTFE tubing (10 mm o.d., 8 mm i.d.) with a flow rate of 4.5 L min
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 (
Data analysis was performed using the software package “Tofware”
(
The Vocus was calibrated twice a day during the campaign with a mixture (70 ppb each) of terpenes (
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
Variations of meteorological conditions and trace gases.
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
Mass defect plot of the ions identified by high-resolution
analysis of the Vocus PTR-TOF data set. The
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
The
The NO concentration was generally low during the campaign, below detection
limit (i.e.,
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 (“
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
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
Time series of
Diurnal cycles of
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
Scatter plots of
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 (
Comparison of ambient average high-resolution mass spectra
with those from
Due to the diverse precursors and changing environmental conditions in the
ambient air, it is challenging to retrieve all the atmospheric chemical
processes occurring within the Landes forest. To start with, we compare the
ambient data with those from
Gas-phase ozonolysis of alkenes generates OH radicals in high yields
(Rickard et al., 1999). Without an OH scavenger, both
Diurnal patterns of non-nitrate isoprene oxidation
products:
Diurnal patterns of non-nitrate monoterpene oxidation
products:
Based on the ambient observations, the non-nitrate oxidation products from
isoprene, monoterpenes, and sesquiterpenes are investigated in this study.
Isoprene gas-phase products are mainly represented by
Diurnal patterns of non-nitrate sesquiterpene oxidation
products:
Diurnal patterns of isoprene-derived organic nitrates:
Taking the evening peak of isoprene oxidation products at 20:00 as an
example, we compared the roles of
Diurnal patterns of monoterpene-derived organic nitrates:
The diurnal patterns of
To date the oxidation processes of sesquiterpenes have been rarely
investigated despite its potential significance in new particle formation
and SOA formation (Bonn and Moortgat, 2003; Winterhalter et al., 2009). In
this study, various sesquiterpene oxidation products were observed, mainly
including
Organic nitrates have been shown to represent a large fraction of submicron
aerosol nitrate at both urban and rural sites in Europe (Kiendler-Scharr et
al., 2016). During daytime, the reaction of peroxy radicals with NO can lead
to the formation of organic nitrates. At night,
Taking the peak concentration of monoterpene-derived organic nitrates at 20:00 as an example, the concentration of
This work presented the deployment of the new state-of-the-art Vocus PTR-TOF
in the French Landes forest during the CERVOLAND campaign. The Vocus PTR-TOF
capabilities are evaluated for the first time in the actual ambient
environment by the identification of the observed gas-phase molecules. With
the improved detection efficiency and measurement precision compared to
conventional PTR instruments, multiple hydrocarbons with carbon numbers
varying from 3 to 20 were observed as well as various VOC oxidation
products. Hydrocarbon signals were dominated by monoterpenes and their major
fragment ions (e.g.,
To demonstrate the importance of the Vocus PTR-TOF application in atmospheric science study, the characteristics of terpenes and their oxidation products were investigated. In addition to the observation of isoprene, monoterpenes, and sesquiterpenes, this study presented the ambient characteristics of the rarely recorded diterpenes, which are traditionally considered non-volatile species in the atmosphere. On average, the concentration of diterpenes was 1.7 ppt in the Landes forest, which was 100 to 1000 times lower than that of monoterpenes (6.0 ppb) and sesquiterpenes (64.5 ppt). However, considering their low vapor pressure and high reactivity, diterpenes may potentially play an important part in atmospheric chemistry. The diurnal variations of diterpenes showed the maximum peak at night and low levels during the day, similar to those of monoterpenes and sesquiterpenes.
With strong photochemical oxidations of terpenes during the day, the more
oxidized terpene reaction products were observed with a broad daytime peak,
whereas the less oxidized terpene reaction products showed peak
concentrations in the early morning and/or in the evening. By calculating
the reaction rates of terpenes with the main oxidants, OH radical,
Data used in this study are available from the corresponding author upon request. Please contact Haiyan Li (haiyan.li@helsinki.fi).
The supplement related to this article is available online at:
ME and MR conceived the study. MR, LH, PMF, EV, and EP conducted the field measurements. HL carried out the data analysis. MR, PR, KD, JEK, DW, MK, ME, and FB participated the data analysis. HL wrote the paper with inputs from all coauthors.
Jordan E. Krechmer and Douglas Worsnop both work for Aerodyne Research Inc.
The authors would like to thank the PRIMEQUAL program for financial support (ADEME, convention #1662C0024). This study has also been carried out with financial support from the French National Research Agency (ANR) in the frame of the “Investments for the Future” program, within the Cluster of Excellence COTE (ANR-10-LABX-45) of the University of Bordeaux. Special thanks to Elena Ormeño-Lafuente (IMBE) for the loan of the BVOC calibration gas cylinders and Christophe Chipeaux and Denis Loustau (ISPA-INRA) for their precious help in providing meteorological data and access to the ICOS station facility.
This research has been supported by the H2020 European Research Council (grant nos. ATM-GTP (742206), COALA (638703), and CHAPAs (850614)) and the Academy of Finland (grant nos. 317380, 320094).Open access funding provided by Helsinki University Library.
This paper was edited by Alex B. Guenther and reviewed by two anonymous referees.