The Amazonian rainforest is a large tropical ecosystem, which is one of the last pristine continental terrains. This ecosystem is ideally located for the study of diel and seasonal behaviour of biogenic volatile organic compounds (BVOCs) in the absence of local human interference. In this study, we report the first atmospheric BVOC measurements at the Amazonian Tall Tower Observatory (ATTO) site, located in central Amazonia. A quadrupole proton-transfer-reaction mass spectrometer (PTR-MS), with seven ambient air inlets, positioned from near ground to about 80 m (0.05, 0.5, 4, 24, 38, 53 and 79 m above the forest floor), was deployed for BVOC monitoring. We report diel and seasonal (February–March 2013 as wet season and September 2013 as dry season) ambient mixing ratios for isoprene, monoterpenes, isoprene oxidation products, acetaldehyde, acetone, methyl ethyl ketone (MEK), methanol and acetonitrile. Clear diel and seasonal patterns were observed for all compounds. In general, lower mixing ratios were observed during night, while maximum mixing ratios were observed during the wet season (February–March 2013), with the peak in solar irradiation at 12:00 LT (local time) and during the dry season (September 2013) with the peak in temperature at 16:00 LT. Isoprene and monoterpene mixing ratios were the highest within the canopy with a median of 7.6 and 1 ppb, respectively (interquartile range (IQR) of 6.1 and 0.38 ppb) during the dry season (at 24 m, from 12:00 to 15:00 LT). The increased contribution of oxygenated volatile organic compounds (OVOCs) above the canopy indicated a transition from dominating forest emissions during the wet season (when mixing ratios were higher than within the canopy), to a blend of biogenic emission, photochemical production and advection during the dry season when mixing ratios were higher above the canopy. Our observations suggest strong seasonal interactions between environmental (insolation, temperature) and biological (phenology) drivers of leaf BVOC emissions and atmospheric chemistry. Considerable differences in the magnitude of BVOC mixing ratios, as compared to other reports of Amazonian BVOC, demonstrate the need for long-term observations at different sites and more standardized measurement procedures, in order to better characterize the natural exchange of BVOCs between the Amazonian rainforest and the atmosphere.
The Amazonian rainforest is one of the last pristine continental areas. Low
atmospheric concentrations of nitrogen oxides (NO
In the Amazonian rainforest, as well as on the global scale, the compound most copiously emitted by vegetation is isoprene (Crutzen et al., 2000; Karl et al., 2004; Kuhn et al., 2010), with emissions being circa 4 times the total of all anthropogenic volatile organic compounds (VOCs) (Boucher et al., 2013). Isoprene accounts for almost half of the world's biogenic emissions and its emission represents a few percent of the assimilated carbon in tropical regions (Kesselmeier et al., 2002a). Due to its high emission rates and reactivity, isoprene is a key player in the oxidative capacity of the atmosphere (Lelieveld et al., 2008). Furthermore, evidence of SOA originating from isoprene has been observed recently in several different ecosystems (Claeys et al., 2004; Paulson et al., 2009; Carlton et al., 2009; Surratt et al., 2010; Chan et al., 2010; Pye et al., 2013). Despite the expected low particulate-mass yield of isoprene relative to other natural VOCs, such as monoterpenes and sesquiterpenes (Claeys et al., 2004; Surratt et al., 2010), its high abundance could potentially produce enough aerosols to significantly impact radiative forcing (Hallquist et al., 2009). The seasonality of isoprene in the Amazon is well established. Atmospheric mixing ratios may vary throughout the year by a factor of 4 (Kesselmeier et al., 2002b). During the wet season, the emission of isoprene is much lower, due to reduced sunshine from cloud cover and slightly lower temperatures compared to the dry season, when high temperatures and radiation stimulate stronger biogenic emissions. In addition, strong seasonal cycles of vegetation growth and growth development, as observed by dendrochronology as well as by direct monitoring of emission quality, may contribute to seasonal isoprene fluctuations (Kuhn et al., 2004a, b; Kesselmeier et al., 2009).
Monoterpenes comprise a class of well-known isoprenoid SOA precursors (Sakulyanontvittaya et al., 2008). These compounds also influence global atmospheric chemistry due to high OH reactivity (Kesselmeier and Staudt, 1999; Atkinson and Arey, 2003). Similar to isoprene, monoterpene emissions are regulated by light and temperature (Kesselmeier and Staudt, 1999), a feature which has also been specifically demonstrated for Amazonian monoterpene emitting plant species (Kuhn et al., 2002; Jardine et al., 2015).
Oxygenated volatile organic compounds (OVOCs) are also present in the Amazonian
atmosphere (Kesselmeier et al., 2002b). Their atmospheric abundance has been
observed to be determined by the balance between their sources (direct plant
emissions as well as secondary formation through oxidation) and sinks
(reaction with oxidants and surface uptake) (Karl et al., 2009). For
instance, methanol is known to have multiple sources (Folkers et al., 2008).
Its biogenic origin is primarily pectin lamella in plant cell walls during
periods of leaf growth, but it could also be released by plant wounding, and
decomposition of soil and leaf litter (Fall and Benson, 1996). Methanol is
the second, after isoprene, most highly emitted VOC in terms of
carbon (Koppmann and Wildt, 2007), and its impact on atmospheric oxidation
can be large, even influencing the HO
Acetaldehyde has already been reported to have a 4–5-fold increase from the wet to the dry season, possibly due to vegetation fires, or due to an increase of secondary biogenic sources (i.e. oxidation of other BVOC) (Kesselmeier et al., 2002b). Several biological formation mechanisms for acetaldehyde exist. During root anoxia and fermentation in plants, acetaldehyde is formed as a by-product of ethanol production and is released via the transpiration stream (Kesselmeier et al., 2009; Bracho-Nunez et al., 2012, 2013). Another source of biological production of acetaldehyde occurs during the light-to-dark transition in plants, where as an outflow mechanism, acetaldehyde is leaked from intracellular space after conversion from pyruvate (Fall, 2003; Rottenberger et al., 2004). An interesting feature is the bidirectional exchange of acetaldehyde by plants, in close relation to ambient concentrations (Rottenberger et al., 2004; Jardine et al., 2008). Furthermore, acetaldehyde has an important atmospheric source due to the oxidation of other BVOCs and long-lived VOCs such as ethane in the atmosphere (Seco et al., 2007).
Acetone is another very abundant oxygenated BVOC, with a variety of sources.
In the troposphere, acetone is relevant due to its photolytic sink, leading
to peroxyacetyl nitrate (PAN) formation under sufficient NO
Methyl vinyl ketone and methacrolein (MVK
Although acetonitrile is generally considered a biomass burning tracer (Lobert et al., 1990; Andreae and Merlet, 2001), it has also been reported to be emitted by plants (Nyalala et al., 2011, 2013) and soils (Bange and Williams, 2000). Nonetheless, it is widely considered an anthropogenic VOC given that most regional biomass burning events are man-made (Crutzen and Andreae, 1990) and the importance of biogenic sources is unknown. Its long atmospheric lifetime with respect to OH oxidation (2–3 years) allows for long-range transport (Williams et al., 2001).
The objective of this study is to provide a detailed description of BVOC mixing ratios in and above the canopy and their diurnal fluctuations, which can indicate heterogeneous sources and sinks within the canopy, and can be related to the exchange between the Amazonian forest and the atmosphere under dry and wet seasonal conditions. Measurements were performed on an 80 m tower at the site of the Amazonian Tall Tower Observatory (ATTO), and therefore represent a new data set from this very pristine area. Several (two) field campaigns enabled the determination of seasonal, diel, and vertical variation of BVOCs. This new information allows for a more comprehensive understanding of BVOC emissions and interactions within the central Amazon rainforest.
The ATTO site is located in central Amazonia, 135 km north-east from Manaus,
the closest populated area. The reserve in which ATTO is embedded comprises
424 430 hectares of mainly non-flooded closed forest vegetation
(
Compounds calibrated for PTR-MS measurements with respective
protonated mass-to-charge (
This site generally experiences dry seasonal conditions from August to October,
during which cumulative precipitation is less than 100 mm month
Measurements of VOC mixing ratios were accompanied by the determination of
total OH reactivity, nitric oxide (NO), nitrogen dioxide (NO
Measurements of VOCs were performed using a PTR-MS (Ionicon Analytic GmbH,
Austria) operated under standard conditions (2.2 mbar drift pressure, 600 V
drift voltage, 127 Td) (Lindinger et al., 1998). Each level of the vertical
profile was sampled for 2 min with six to seven cycles (ca. 20 s each) of
PTR-MS measurements. A catalytic converter (Supelco, Inc. with platinum
pellets heated to
Humidity dependent calibrations (using bubbled synthetic zero air to dilute
the standard, regulated as close as possible to ambient humidity conditions)
were performed using a gravimetrically prepared multicomponent standard,
including methanol (
Schematic diagram of the experimental inlet set-up. The various inlet heights at the tower and tripod are shown with respect to the canopy. The PTR-MS was housed in the air conditioned measurement container.
Top panels: example of PTR-MS and GC-FID time series data on 25 and 26 September 2013 at 24 m for isoprene (left) and the total monoterpenes (right). Brown points are GC-FID measurements with an error of 30% and green points are PTR-MS continuous measurements with standard deviations of the 2 min averages of the data. Bottom panel: correlation of isoprene (left) and total monoterpene (right) mixing ratios obtained by PTR-MS and GC-FID.
The PTR-MS technology allows for fast sampling at very low mixing ratios, but
the system relies solely on mass-over-charge ratios (
Similarly, there could be multiple contributors to the mass for
First, we present the cross-validation of the measurements between PTR-MS and GC-FID. Second, all measured BVOC mixing ratios are reported linking their seasonal, diel and vertical behaviour to sources, sinks and atmospheric processes. The medians were calculated from the dry (20–30 September 2013) and wet (20 February to 6 March 2013) season measurement periods. The dates expressed herein are all in local time. In all cases, variability is represented as the interquartile range (IQR) which is the 75 % percentile minus the 25 % percentile, thus enclosing 50 % of all data.
A cross-validation for isoprene and monoterpene data obtained by in situ
PTR-MS measurements was performed off-line analyzing absorbent tubes by
GC-FID (Fig. 2). The temporal variation compared well. The coefficients of
determination (
Meteorological data (2 min averages) for the intensive measurement periods at the ATTO site for the 81 m level of the tower reporting rainfall (blue), temperature (red) and net radiation (grey).
Diurnal hourly medians of isoprene during the wet season (top), isoprene during the dry season (middle) and the sum of monoterpenes during the dry season (bottom) (wet season data for monoterpenes are below LOD and not reported). Range bars represent the 25 and 75 % percentiles.
Vertical profiles for isoprene mixing ratios. Data are presented as medians over a period of 3 h between 00:00 and 03:00 LT (left) and 12:00 and 15:00 LT (right) (both local time). The 3 h medians are shown in red for the dry season and in blue for the wet season. Range bars represent the 25 and 75 % percentiles.
Diurnal hourly median isoprene oxidation product
(I
Four campaigns were performed in the course of 1 year: in November 2012,
February–March 2013, June 2013 and September 2013. The two intensive BVOC
measurement periods cover the two extremes of seasonality: 20 February to 6
March 2013 (during the wet season) and 20 to 30 September 2013 (during the
dry season). Meteorological conditions between the dry and wet season varied
significantly (Fig. 3). For September 2013, ambient temperature and net
radiation measured at the tower were higher
(
During daytime, isoprene showed the highest mixing ratios within the canopy reaching a median of 2.29 ppb (IQR 1.88) at 24 m for the period of 12:00–15:00 LT for the wet season and 7.6 ppb (IQR 6.1) for the same time and height during the dry season. During the night (median of the period of 00:00–03:00 LT), isoprene mixing ratios above the canopy were considerably higher than within the canopy (Fig. 4). Strong gradients towards the ground were seen, especially for the dry season, which could suggest a deposition towards the ground or plants (Fig. 5). The vertical profiles of isoprene revealed the highest mixing ratios for both day and night during the dry season. In addition, its variability increased during the dry season compared to the wet season, as observed in the difference of IQR of 1.9 ppb during the wet season to 6.1 ppb during the dry season, at 24 m for the period 12:00–15:00 LT. The vertical profiles for isoprene during the wet season had a relatively stronger night-to-day variability at 24 m, as compared to the dry season, despite the overall lower mixing ratios during this period. The less pronounced vertical gradient during the dry season could be due to a stronger turbulent mixing (Fig. 5).
Total monoterpene mixing ratios were extremely low during the wet season and
rarely above the LOD (LOD
Methyl vinyl ketone, methacrolein and ISOPOOH (hereafter referred to as isoprene
oxidation products, I
Average profiles of the isoprene oxidation product
(I
During the wet season, the highest median mixing ratios were found above the
canopy with 0.22 ppb (IQR 0.34) around 13:00 LT at 79 m. This coincided
with the peak of isoprene at 24 m during the same time of the day,
suggesting that isoprene oxidation product production is at its maximum when
isoprene emissions are the highest and that they are produced above the canopy
shortly after the emission of isoprene. Mixing ratios for 24 and 0.5 m
during the wet season remained almost constant during the night (
During the dry season, median mixing ratios of I
An analysis of the ratio between isoprene and the isoprene oxidation products
indicated changes in the oxidative patterns between the seasons. During the
wet season, ratios never exceeded 0.2, indicating there was not much
oxidation taking place. I
Oxygenated VOCs such as acetaldehyde, acetone and MEK were grouped together. They exhibited similar variations over the day and season. In the wet season, acetaldehyde, acetone and MEK (Fig. 8) showed the highest mixing ratios at 24 m at midday, reaching medians of 0.94 ppb (IQR 1.61), 0.98 ppb (IQR 0.93) and 0.4 ppb (IQR 0.58), respectively. With the highest mixing ratios found within the canopy, the diurnal cycle with a pronounced increase around 12:00 LT is consistent with biogenic emissions from canopy vegetation. Throughout the measurement period, the mixing ratios above the canopy and at the ground remained much lower than within the canopy (24 m), except for acetone. Of all three species, acetone was the only compound with higher mixing ratios above the canopy (up to 0.92 ppb (IQR 0.53) at 12:00–15:00 LT), compared to within the canopy. For acetone, above-canopy mixing ratios were always higher than inside the canopy during daytime except for the period of 11:00–14:00 LT, where canopy mixing ratios (24 m) dominated (Fig. 8).
Conditions during the dry season were markedly different, with mixing ratios
peaking above the canopy, instead of inside. Interestingly, the in-canopy
mixing ratios of acetaldehyde were comparable to those of the wet season,
whereas the 79 m level showed a 3-fold increase. Such differences clearly
indicate a seasonal change in the dominating sources and sinks, as well as
possible differences in the vertical mixing for this species. In a comparable
fashion, acetone was also more abundant above canopy than within during the
dry season. Similarly, MEK mixing ratios changed to comparable mixing ratios
in and above the canopy during the dry season. Despite the 4-fold increase
from the wet to the dry season at the 79 m level, mixing ratios within the
canopy remained similar (
Up to 80 % of the methanol data during the wet season were below the
LOD (LOD
Diurnal medians for the wet season (left) and dry season (right), for acetaldehyde (top), acetone (middle) and methyl ethyl ketone (bottom) at different heights. Range bars are expressed as the 25 and 75 % percentiles.
Diurnal hourly medians for methanol during the dry season at different heights. Range bars are expressed as the 25 and 75 % percentiles.
Diurnal hourly median acetonitrile mixing ratios during the wet season (left) and the dry season (right) for different heights. Range bars represent the 75 and the 25 % percentiles.
The main climatic difference between the dry and the wet season in the
central Amazonian
The forest in central Amazonia does not seem to suffer from water limitation
in any season (Restrepo-Coupe et al., 2013). High light and temperature
during the dry season corresponds to photosynthesis rates and affects the
related BVOC production pathways. Such climatic differences in the Amazon
rainforest area during wet and dry season trigger seasonal growth rates which
are mirrored in tree rings (Brienen and Zuidema, 2005; Zuidema et al., 2012;
Restrepo-Coupe et al., 2013), indicating not only stronger growth during the wet
season in
A distinct seasonality in the magnitude of mixing ratios or the diel cycle was observed for all monitored compounds. During the wet season the fluctuations and mixing ratio levels of BVOCs seemed to be dominantly driven by biological processes, resulting in higher mixing ratios for isoprene, acetaldehyde, acetone and MEK inside the canopy. The general behaviour strongly suggested the main source to be at the forest canopy top height. This was not the case for isoprene oxidation products and acetonitrile, which exhibited the highest mixing ratios above the canopy. This observation was expected, as the isoprene oxidation products are produced by the oxidation of isoprene, released by vegetation and transported to above the canopy (Kesselmeier et al., 2002b). Similarly, acetonitrile was expected to be higher above the canopy as it was likely from regional biomass burning plumes that have been transported to the site (Kuhlbusch et al., 1991; Andreae and Merlet, 2001). Surprisingly, acetonitrile showed concentrations at the canopy top to be comparable to above-canopy concentrations, both during wet and dry season, indicating a potential biogenic source (Nyalala et al., 2011, 2013). The mixing ratios of water soluble compounds, such as methanol, might also be influenced by the deposition to wet plant surfaces (Laffineur et al., 2012) emphasizing the seasonal trend that originated from reduced emissions during the wet season.
During the dry season, a different situation was observed. Higher mixing ratios of acetaldehyde, acetone, MEK, methanol, isoprene oxidation products and acetonitrile were detected above the canopy during both the day and night-time. Most likely, this increase above the canopy was due to higher production of secondary VOCs such as acetaldehyde, acetone, isoprene oxidation products or methanol via oxidation of BVOCs primarily emitted from the forest. In addition, an increase in the variability of BVOC mixing ratios above the canopy was observed in the dry season, with mixing ratios inside the canopy sometimes reaching the above-canopy levels. Most likely, this was due to the higher insolation during the dry season, which resulted in high upper canopy leaf temperatures and turbulent mixing above the canopy during the day. Thus, this higher variability might also have been influenced by changing environmental conditions, seasonal differences in plant emission potential, variation in the oxidative capacity of the atmosphere, influencing secondary BVOC production rates, and transport of air masses from other regions. This could be seen in the weak gradients above the canopy, except for those of isoprene, which was additionally chemically depleted.
Furthermore, during the dry season, long-range transport of NO
Isoprene and monoterpenes peaked together with light intensity just after
midday inside the canopy during the dry and the wet season, but monoterpenes
exhibited a slight broadening to later hours. Oxygenated compounds showed the
highest mixing ratios at 15:00–16:00 LT during the dry season. This
suggested that isoprene emissions are more closely related to radiation,
which peaks at 12:00 LT. The oxygenated compounds, including isoprene
oxidation products, produced directly by the forest plant species or by
chemical reactions in the atmosphere may have been more related to
temperature (which peaks at the same time: 15:00–16:00 LT) than to light.
During both seasons, isoprene had the highest mixing ratios during the day at
the top of the canopy where leaves received most of the radiation. After
ceasing of photosynthesis and isoprene production during the night (Fig. 5),
mixing ratios of isoprene decreased considerably towards the ground. Such a
decrease just after sunset was too rapid to be explained exclusively by
gas-phase chemistry due to the expected decrease of the OH levels which does
not have major sources in the dark (Goldan et al., 1995). Furthermore, the
ozonolysis of alkanes during the night can be neglected due to the low ozone
levels (Paulson and Orlando, 1996; Andreae et al., 2002). Moreover, the
development of the nocturnal boundary layer results in low transport rates
(with a wind speed at 19m of 0.23
Total monoterpenes presented a very strong seasonality with mixing ratios during the wet season below 0.23 ppb and during the dry season up to 1.8 ppb (maximum value). A few Amazonian tree species have been monitored for monoterpene emissions in previous studies, suggesting that more factors than solely meteorological can influence the seasonality of monoterpene emissions (Kuhn et al., 2004a, b; Bracho-Nunez et al., 2013; Jardine et al., 2015). Among those factors are the oxidative capacity of the atmosphere and phenological development, which may be accentuated during the dry season especially during the transition between seasons (Kuhn et al., 2004a, b). In addition, during the dry season, total monoterpene mixing ratios exhibited a broader peak with some relation to the temperature. However, as the PTR-MS measures the sum of monoterpenes, it is possible that the monoterpene composition changes seasonally along with their reactivities and vertical patterns (Kesselmeier et al., 2002b; Kuhn et al., 2004a, b; Jardine et al., 2015).
The ratio of isoprene oxidation products to isoprene provides an indication
of the oxidative capacity of the atmosphere. Patterns of isoprene oxidation
products clearly indicated changes in the oxidation processes at the
Amazonian rainforest over the year. During the wet season, a low isoprene
oxidation product-to-isoprene ratio, suggested a weak oxidative regime above
the canopy, in contrast to the dry season, where higher ratios indicated a
higher oxidative capacity. Even though mixing ratios of isoprene oxidation
products and isoprene rose during the dry season, they did not rise
proportionally compared to isoprene. Whereas isoprene had a 4-fold increase
from the wet to the dry season, isoprene oxidation products had a 10-fold
increase, possibly indicating a faster depletion of isoprene than isoprene
oxidation product production, as well as efficient deposition to wet surfaces
during the wet season. This faster processing could also be observed in the
more accentuated isoprene profiles despite the much higher variability
observed during the dry season (Fig. 5). The I
Regarding the vertical distributions of the other oxygenated VOCs above the
canopy, a similarity of patterns became apparent. When
I
Mixing ratios of isoprene, methanol, acetaldehyde, acetone,
acetonitrile, MEK, isoprene oxidation products (I
The vertical profiles of acetone, MEK and acetaldehyde during the wet season
showed a maximum inside the canopy at midday. Acetaldehyde is reported to be
emitted by the forest crown as a by-product of fermentation processes within
roots due to water flooding and root anoxia (Kesselmeier et al., 2009;
Bracho-Nunez et al., 2013). If rain events cause such an anoxia, this would
explain the observed mixing ratios below the LOD
(LOD
Methanol behaved similarly to other oxygenated compounds. During the wet season, mixing ratios were below detection limit. This could have been caused by lower plant production and deposition to wet surfaces (Laffineur et al., 2011). For the dry season, the overall higher mixing ratios above the canopy were consistent with both enhanced photochemical production within the troposphere and transport of biomass burning impacted air masses. However, higher mixing ratios within the canopy in the afternoon (15:00) indicated the potential of a biogenic methanol emission due to pectin lamella aging in cell walls during leaf growth (Galbally and Kirstine, 2002; Kesselmeier et al., 2009). Leaf area index measurements demonstrated a growth of the canopy during the period of July 2013 to September 2013 (G. Martins, personal communication, 2014).
Acetonitrile is currently regarded to be primarily emitted from biomass burning sources with an oceanic and photochemical sink, but a biogenic source cannot be excluded (Bange and Williams, 2000; Andreae and Merlet, 2001; Nyalala et al., 2011, 2013). Our data differed significantly between the seasons with mixing ratios 2.5 times higher during the dry season than detected in the wet season, which is consistent with an influence from biomass burning in September 2013. Distinct biomass burning plumes were not observed. This indicates that there was no biomass burning close by affecting our measurements, and impact from long-range transport may be assumed. Such burning activities can be expected during this time of the year (Karl et al., 2007). In addition, the observation that levels decrease towards the ground could indicate a potential uptake by soil bacteria, although wet deposition cannot be excluded. The variability of the measurements especially inside the canopy hides a possible influence by plant emissions which has previously been reported (Bange and Williams, 2000; Nyalala et al., 2011, 2013).
The Amazonian ecosystem is characterized by great variability of plant species, though regional considerations prove a hyperdominance by a smaller species number (Ter Steege et al., 2013). Thus, atmospheric BVOC levels and compositions exhibit similar variations. The overview of the investigated BVOCs, as given in Table 2, was assembled from literature values (for isoprene, monoterpenes, acetaldehyde, acetone, MEK, isoprene oxidation products, methanol and acetonitrile). As compared to other more reactive compounds, isoprene has been the most measured compound, due to its high abundance and the availability of techniques. Although isoprene values reported in this study generally agree with previous reports, some of our median values for isoprene in the wet season are the lowest reported. Furthermore, we measured at 0.05 and 0.5 m, which were not reported for an Amazonian ecosystem before, though it is needed to characterize the isoprene behaviour close to the ground. Monoterpene mixing ratios for this study were high, although they were far from the highest total monoterpene mixing ratio reported by Greenberg and Zimmerman (1984). The values compare to ecosystems such as boreal forests, known to strongly emit monoterpenes. For instance, a mean mixing ratio of 1.1 ppb was observed in Hyytiälä, Finland, during July, when the highest emissions occur (Raisanen et al., 2009), as compared to mean mixing ratios for monoterpenes of 1 ppb during the dry season in this study. In agreement with Kanakidou et al. (2005), further efforts on monoterpene characterization in terms of abundance and reactivity in Amazonia are needed for a better understanding of SOA growth and formation processes, regionally as well as worldwide. In addition, the oxidative capacity of the atmosphere can be studied based on the oxidation products of isoprene. Across the Amazon region mixing ratios of isoprene oxidation products are always higher above the canopy and during the dry season (Table 2), especially when measured in the mixed layer by aircrafts (Karl et al., 2007; Kuhn et al., 2007) or tethered balloons (Kesselmeier et al., 2000) and suggest a higher oxidative capacity during the dry season above the canopy.
The reported values in the literature for acetaldehyde, methanol, acetone and MEK are too scarce to make reliable conclusions. Among the studies listed in Table 2, we reported the highest mixing ratios for acetone and methanol; acetonitrile seems to match previous measurements. The lack of surface measurements of MEK does not allow for a comparison. More continuous measurements with a better representation in space and height are needed to investigate the seasonality of within- and above-canopy interactions with the atmosphere to improve our understanding of BVOC emissions, transport and chemistry over Amazonia (Greenberg and Zimmerman, 1984; Gregory et al., 1986; Rasmussen and Khalil, 1988; Zimmerman et al., 1988; Helmig et al., 1998; Kesselmeier et al., 2000, 2002b; Warneke et al., 2001; Williams et al., 2001; Rinne et al., 2002; Greenberg et al., 2004; Rottenberger et al., 2004; Pöschl, 2001; Karl et al., 2007; Kuhn et al., 2007; Rizzo et al., 2010; Jardine et al., 2011a, b,; Jardine et al., 2015).
Our first BVOC measurements at the ATTO site provide unique information about the role of these compounds in the central Amazon rainforest area within and above the canopy. Marked seasonality and diel variance was observed at the site, as seen in the BVOC mixing ratios seasonality and vertical profiles. These changes in BVOC mixing ratios were attributed to changing sources of these compounds mainly in relation to seasonal fluctuations of light, temperature and phenology. During the wet season, BVOCs seemed to be dominantly produced by vegetation, and mixing ratios are the highest within the canopy. During the dry season additional atmospheric BVOC sources became apparent. Mixing ratios increased in magnitude and presented a different diurnal pattern with respect to the wet season. This picture indicated a mixture of sources such as vegetation emissions, oxidation from primary emitted BVOCs and regional atmospheric transport as well as sinks such as deposition, photochemistry and transport.
The summary of BVOC mixing ratios in the Amazonian ecoregion (Table 2) shows an enormous variation depending on site, height and season. Year-round continuous measurements are required in order to improve our understanding of the within canopy BVOC dynamics and their interaction with the atmospheric chemistry in such biodiverse ecoregions forming the Amazonian rainforest.
We thank the Max Planck Society and the Instituto Nacional de Pesquisas da Amazonia for continuous support. Furthermore, we acknowledge the support by the ATTO project (German Federal Ministry of Education and Research, BMBF funds 01LB1001A; Brazilian Ministério da Ciência, Tecnologia e Inovação FINEP/MCTI contract 01.11.01248.00); UEA and FAPEAM, LBA/INPA and SDS/CEUC/RDS-Uatumã. We would like to especially thank all the people involved in the logistical support of the ATTO project, in particular Reiner Ditz and Hermes Braga Xavier. We acknowledge the Micrometeorological group of the INPA/LBA for their collaboration concerning the meteorological parameters, with special thanks to Marta Sá, Antonio Huxley and Leonardo Oliveira. We are grateful to Tracey W. Andreae for help with the manuscript, Guenther Schebeske for the GC-FID analysis and Nina Knothe for logistical help. We would also like to thank Thomas Klüpfel for all the great support provided with the PTR-MS operation in the laboratory as well as in the field. Lastly, we would like to acknowledge the referees of this manuscript for the extensive contributions and suggestions which helped to improve this study.The article processing charges for this open-access publication have been covered by the Max Planck Society.Edited by: R. Holzinger