Biogenic volatile organic compounds (BVOCs) play an important role in
the chemistry of the troposphere, especially in the formation of
tropospheric ozone (O
Ecosystems produce and emit a large number of biogenic volatile organic
compounds (BVOCs) that are involved in plant growth and reproduction. These
species also act as defensive compounds, e.g. enhancing tolerance to heat
and oxidative stress (Sharkey and Yeh, 2001; Loreto and Schnitzler,
2010), preventing the colonization of pathogens after wounding, and
deterring insects or recruiting natural enemies of herbivores (Holopainen
and Gershenzon, 2010). The BVOC production rate in an ecosystem depends on
several physical (e.g. temperature, precipitation, moisture, solar radiation
and carbon dioxide (CO
BVOCs can contribute significantly to the carbon balance in certain
ecosystems (Kesselmeier et al., 2002; Malhi, 2002). BVOC concentrations
in ambient air depend on several factors, such as emission rates from
vegetation, atmospheric transport and mixing, as well as the chemical
composition and oxidative state of the atmosphere, which determine the sink
of these species. BVOCs are important in the formation of tropospheric ozone
(O
Various studies have indicated the link between BVOCs and the formation of SOA (Vakkari et al., 2015; Andreae and Crutzen, 1997; Ehn et al., 2014), while the influence of BVOCs on the growth of newly formed aerosol particles has also been indicated (Kulmala et al., 2004; Tunved et al., 2006). However, there are many uncertainties associated with the exact chemical reactions and physical processes involved in SOA formation and aerosol particle growth, which largely depends on regional emissions and atmospheric processes (Kulmala et al., 2013; Ehn et al., 2014). Vakkari et al. (2015) indicated the importance of volatile organic compounds (VOCs) for new particle formation and growth in clean background air in South Africa. Therefore, it is essential to understand the sources, transport and transformations of these compounds for air quality management and climate change-related studies, as well as for the modelling of atmospheric chemistry at global, regional and local scales (Laothawornkitkul et al., 2009; Peñuelas and Staudt, 2010; Peñuelas and Llusià, 2003).
Long-term ambient BVOC measurements to establish seasonal cycles have been conducted extensively in several regions, which include boreal forest (Hakola et al., 2009, 2000; Rinne et al., 2000, 2005; Rantala et al., 2015; Räisänen et al., 2009; Eerdekens et al., 2009; Lappalainen et al., 2009), hemiboreal mixed forest (Noe et al., 2012), temperate (Spirig et al., 2005; Stroud et al., 2005; Fuentes et al., 2007; Mielke et al., 2010), Mediterranean (Davison et al., 2009; Harrison et al., 2001) and tropical (Rinne et al., 2002) ecosystems. Shorter campaigns have also been conducted in western and central Africa, which include several different studies in the framework of the African Monsoon Multidisciplinary Analyses (AMMA) (Grant et al., 2008; Saxton et al., 2007) and the EXPeriment for the REgional Sources and Sinks of Oxidants (EXPRESSO) (Serca et al., 2001). Zunckel et al. (2007) and references therein indicated that limited research has been conducted on BVOC emissions in southern Africa, which consisted mainly of short campaigns measuring BVOC emission rates. Considering that BVOC emissions on a global scale are considered to be significantly higher (ca. 10 times) than the emission of anthropogenic VOCs, it is very important that longer-term BVOC measurements are conducted in southern Africa. Furthermore, a large part of the land cover in South Africa consists of a grassland bioregion, as indicated in Fig. 1. Although it is considered that grasslands cover approximately one-quarter of the Earth's land surface, relatively few studies have been conducted on BVOC emissions from grasslands, while there are no long-term BVOC studies reported for these landscapes (Bamberger et al., 2011; Ruuskanen et al., 2011; Wang et al., 2012). Therefore, the aim of this study was to quantify the ambient BVOC concentrations over different seasons at a regional background site in South Africa. In addition, the objective was also to characterize their seasonal patterns, as well as to relate BVOC concentrations measured in southern Africa to levels in other regions in the world. To the best of the authors' knowledge, this is the first record of ambient BVOC concentrations covering a full seasonal cycle in southern Africa and for a grassland bioregion anywhere in the world.
Map of southern Africa indicating the location of the Welgegund measurement station within the context of the bioregion and large point sources in the industrial hub of South Africa (Mucina and Rutherford, 2006).
Measurements were conducted at the Welgegund measurement station
(26.57
Welgegund is geographically located within the South African Highveld, which
is characterized by two distinct seasonal periods, i.e. a dry season from
May to September that predominantly coincides with winter (June to August),
and a wet season during the warmer months from October to April. The dry
period is characterized by low relative humidity, whereas the wet season is
associated with higher relative humidity and frequent rains that
predominantly occur in the form of thunderstorms. The mean annual
precipitation is approximately 500 mm with
The Welgegund measurement station is located in a grassland biome (Fig. 1), which covers 28 % of South Africa's land surface (Mucina and Rutherford, 2006). This biome has been significantly transformed, primarily as a result of cultivation, plantation forestry, urbanization and mining (Daemane et al., 2010, and references therein). It has also been severely degraded by erosion and agricultural development. The station is situated on the sandy grasslands within the Vaal–Vet rivers in the Andesite Mountain Bushveld of a savannah biome, which is prominent on nearby ridges. At present, only 0.3 % of the Vaal–Vet sandy grassland is statutorily conserved, while the rest is mostly used for grazing and crop production. In Fig. 2, a land cover map within a 60 km radius of Welgegund is presented, indicating the extent of cultivation in this region. The land cover survey was performed within a region that was estimated to represent the BVOC footprint at Welgegund, which was calculated from typical atmospheric lifetimes (Table 1) of the species measured and the general wind speed(s) (Fig. 3) at Welgegund. The immediate area surrounding Welgegund is grazed by livestock, with the remaining area covered by crop fields (mostly maize and to a lesser degree sunflower). In the demarcated 60 km radius, a further three vegetation units of the dry Highveld grassland bioregion (grassland biome) and another two of the central Bushveld bioregion (savannah biome) are also present. In addition, alluvial vegetation is found associated with major rivers and inland saline vegetation in scattered salt pans.
General vegetation map for 60 km radius of Welgegund measurement station.
Monthly variation of
The study area comprises a highly variable landscape with scattered hills and sloping, slightly irregular, undulating plains, which are dissected by prominent rocky ridges. Soil in the catchment area is heterogeneous and rocky, varying from sandy to clayey depending on the underlying rock types, such as andesite, chert, dolomite, mudstone, quartzite, sandstone and shale.
Land use within the surrounding area is divided into six major land cover types, i.e. cultivated land, grasslands, mountainous areas, plantations, urban areas and water bodies, as indicated in Fig. 2. Mountainous areas, grasslands and water bodies (riparian areas) comprised many different vegetation units. The other homogenous areas were anthropogenically altered and are no longer representative of the surrounding natural vegetation. The study area is characterized by a grassland–woodland vegetation complex, dominated by various grass and woody species, and recognized by the presence of non-native species in altered environments.
The most dominant woody species of the entire study area include the trees
The most dominant species of the grass sward in the entire study area
include
BVOC measurements were conducted for a period of more than 2 years through
a 13 month sampling campaign from February 2011 to February 2012 and a
15 month sampling campaign from December 2013 to February 2015. Samples were
collected twice a week for 2 h during the daytime (11:00 to 13:00 local
time, LT) and 2 h during the night-time (23:00 to 01:00 LT) on Tuesdays and
Saturdays. Several previous studies have demonstrated that the maximum
emissions of isoprene and monoterpene from vegetation occur around midday
(Fuentes et al., 2000; Kuhn et al., 2002). Understandably, the chosen
sampling schedule, i.e. same days each week and same hours of the day, was
prone to some bias. As mentioned by Jaars et al. (2014), considering the distance of the sampling site from the nearest town
and logistical limitations during the sampling campaigns, the sampling
schedule applied was the most feasible option that enabled the collection of
data for more than 2 years. VOCs were sampled at a height of 2 m above
ground level, with a 1.75 m long inlet. The first 1.25 m of the inlet was a
stainless steel tube (grade 304 or 316) and the second 0.5 m was Teflon. To
prevent the degradation of BVOCs by O
VOCs were collected with stainless steel adsorbent tubes (6.3 mm ED
Individual BVOCs were identified and quantified using a thermal desorption
instrument (Perkin-Elmer TurboMatrixTM 650, Waltham, USA) connected to a gas
chromatograph (Perkin-Elmer® Clarus® 600,
Waltham, USA) with a DB-5MS (60 m, 0.25 mm, 1
Ancillary measurements continuously performed at the Welgegund station were
used to interpret the measured BVOC concentrations. General meteorological
parameters, i.e. temperature (
Trace gas measurements were performed utilizing a Thermo-Electron 43S
sulfur dioxide (SO
In Table 1, the atmospheric lifetimes (
Local meteorological influences on the measured BVOC concentrations are
likely to be more significant than regional impacts of air masses due to the
short lifetimes associated with atmospheric BVOCs (Table 1). Therefore, BVOC
concentrations were only interpreted in terms of local meteorological
patterns and no back trajectory analyses were employed. In Fig. 3, the
monthly medians of the meteorological parameters – precipitation,
Micrometeorological CO
Figure 4 presents micrometeorological CO
Lifetime (
The ambient BVOC concentration for the two campaigns measured at Welgegund.
In Table 2, the median (mean) and inter-quartile range (IQR; 25th to 75th) concentrations, as well as the median (mean) daytime to night-time concentration ratios of the BVOC species determined during the two sampling campaigns at Welgegund are presented. It is evident from the median (mean) daytime to night-time concentration ratios that there were not significant differences in levels of most of the BVOCs measured during daytime and night-time at Welgegund, with the exception of isoprene measured during the first sampling campaign, as well as the monoterpenes terpinolene and bornylacetate, and the SQT aromadendrene measured during the second sampling campaign. Isoprene levels during the first sampling campaign were approximately 2 times higher during daytime, which reflect the light dependency usually associated with isoprene emissions. However, daytime to night-time concentration ratios of isoprene did not exhibited the strong light dependency typically associated with atmospheric isoprene concentrations, which could be attributed to the characteristics of sources of these species that are discussed in subsequent sections. The temperature and photoactive radiation (PAR) measurements at Welgegund were used in the MEGAN BVOC emission model, which indicated that the measurement time (11:00 to 13:00 LT) captured most of the period of maximum isoprene emission (typically about 12:00 to 02:00 LT). In addition, by assuming a typical diurnal variation in VOC oxidation rate and boundary layer height, it was also found that the isoprene concentration of the measurement time is representative of the daytime isoprene concentration (Greenberg et al., 1999). In Table 3, the concentrations of BVOC species measured during other campaigns in South Africa and the rest of the world are presented.
Ambient BVOC concentrations (pptv) as reported by Noe et al. (2012) for various ecosystems and then modified: avg – mean value, med – median value, and max – maximal value of the measurements reported.
SA: South Africa; WA: West Africa; KNP: Kruger National Park; MEF: Manitou Experimental Forest; AM: Agrafa Mountains; FNT: Floresta Nacional do Tapajos; NNNP: Nouabalé-Ndoki National Park; NC: northern Congo.
The most abundant species observed throughout the study was the monoterpene,
The annual median (IQR) isoprene concentration measured during the first
campaign was 14 (6–35) pptv, while the annual median (IQR) isoprene
concentration measured during the second sampling campaign was 14 (7–24) pptv. The highest isoprene concentration, i.e. 202 pptv, was recorded in
summer (wet season). Harley et al. (2003) reported that the maximum
isoprene concentration measured during an 8-day campaign in the wet
season at a
The annual median (IQR) MBO concentrations measured during the first and
second campaign were 7 (3–16) and 4 (3–10) pptv, respectively. MBO and
isoprene are both produced from dimethylallyl diphosphate
(Gray et al., 2011). Guenther (2013) indicated
that MBO is emitted from most isoprene emitting vegetation at an emission
rate of
Most SQTs are highly reactive species and are difficult to detect in ambient
air samples, which resulted in concentrations of these species being
frequently below the detection limit of the analytical procedure. This is
also reflected in the concentrations of these species being an order of
magnitude lower compared to the other BVOC species measured in this study.
The total annual median (IQR) SQT concentration measured during the first
sampling campaign was 8 (5–14) pptv and 4 (3–11) pptv during the second
sampling campaign. The most abundant SQT during the first sampling campaign
was longicyclene with an annual mean concentration of 4 (1–4) pptv. During
the second sampling campaign,
The lower BVOC concentrations measured at Welgegund compared to other regions can mainly be attributed to the much lower isoprene concentrations measured. However, monoterpenes that are important for SOA formation are similar to levels thereof in other environments. In an effort to explain the BVOC concentrations measured at Welgegund, a comprehensive vegetation study was conducted, as described in Sect. 2.2. The influence of the type of vegetation in the region surrounding Welgegund on ambient BVOC concentrations will be further explored.
Jaars et al. (2014) presented concentrations of aromatic VOCs measured at
Welgegund during the same two sampling campaigns discussed in this paper.
The total BVOC concentrations measured were at least an order of magnitude
lower compared to concentrations of aromatic VOCs measured at Welgegund. The
most abundant aromatic compound, toluene, had a median value of 630 pptv,
whereas the most abundant BVOC measured,
The panels on the left show monthly median concentrations of
BVOC concentration rose at Welgegund for the two sampling campaigns. Different colours represent percentiles: blue – 25 %, aquamarine – 50 %, azure – 75 %, and the black solid line represents the average.
In Fig. 5, the panels on the left show monthly median concentrations of
(a) isoprene, (b) MBO, (c) monoterpene and (d) SQT measured for the two
campaigns, while the panels on the right present the wet (October to April)
and dry (May to September) season concentrations of the respective compounds
measured for the two campaigns. As indicated in Sect. 3.2, isoprene
measured during the first sampling campaign had higher median (mean) daytime
concentrations compared to median (mean) night-time concentrations, which
reflects the light dependency expected from isoprene. All other BVOCs with
the exception of two monoterpenes and one SQT did not indicate significant
differences between daytime and night-time median (mean) concentrations.
Therefore, the seasonal plots of only isoprene were separated between daytime
and night-time median concentrations. Seasonal variations in BVOC
concentrations are expected due to the response of emissions to changes in
environmental conditions, e.g. temperature and rainfall, as discussed in
Sect. 3.1, and the associated biogenic activity. In addition, BVOC
emission is expected to be lower during the winter months (June to August),
since foliar densities rapidly decrease due to deciduous trees dropping
their leaves in winter (Otter et al., 2002). As expected, it is evident
that the concentrations of all the BVOC species, with the exception of the
isoprene (Fig. 5a), and SQT values (Fig. 5d) measured during the second
sampling campaign, were higher in the wet season. The wet season also had
more occurrences of BVOC concentrations that were higher than the range of
the box and whisker plots (whiskers indicating
The monthly median isoprene concentrations (Fig. 5a) measured during the first sampling campaign indicated the expected seasonal pattern with higher isoprene concentrations coinciding with the wet and warmer months, with the exception of April with had lower isoprene concentrations. Surprisingly, during the second sampling campaign, there was no distinct seasonal pattern observed. However, higher isoprene concentrations seem to coincide with higher wind speeds (Fig. 3d), which are observed for both sampling campaigns. This indicates that the major sources of isoprene measured at Welgegund can be considered not to be within close proximity. However, since oxidation products of isoprene (e.g. methyl vinyl ketone, methacrolein) were not measured in this study, more distant sources of isoprene could not be verified. It is evident from Fig. 2 that the region in close proximity to Welgegund in the south-western to north-eastern sectors largely comprises cultivated land, while in the north-eastern to south-western sectors the predominant land coverage is grassland and natural vegetation. It is expected that isoprene emissions from the cultivated land will be lower compared to savannah grassland (Otter et al., 2003). Therefore, if Welgegund is more frequently affected by winds from the south-western to north-eastern sectors, higher wind speeds will coincide with higher isoprene levels, since the savannah grassland fetch region is distant from Welgegund and related to the approximately 3 h atmospheric lifetime of isoprene due to OH radicals.
In Fig. 6, the wind roses for the BVOCs species measured in this study are presented. It is evident that the highest isoprene concentrations for the first sampling period were associated with winds originating from the south to south-western sector, i.e. predominantly from the grassland region in close proximity during the first sampling campaign resulting in a relatively more distinct seasonal pattern for isoprene levels. During the second sampling campaign, higher isoprene concentrations were associated with winds originating from the south-western to the northern sector, i.e. from the cultivated land area. Therefore, isoprene concentrations measured during the second sampling period coincided predominantly with stronger wind speeds from more distant fetch regions.
Distinct seasonal patterns are observed for MBO (Fig. 5b) concentrations
during both sampling campaigns, i.e. higher MBO concentrations coinciding
with wet warm months and lower levels corresponding with dry cold months
(Fig. 3). The MBO concentrations also corresponded to the seasonal
CO
No distinct seasonal pattern is observed for monoterpene and SQT concentrations, with the exception of significantly higher levels measured from February to April 2011 during the first sampling campaign. These increased monoterpene and SQT concentrations can also be attributed to the significantly higher soil moisture measured at a depth of 20 cm during the first sampling campaign (Fig. 3g), as observed for the MBO. The monoterpene and SQT concentrations measured during the first sampling campaign were generally higher compared to the second sampling campaign. In Fig. S1a and b in the Supplement, the relationship between soil moisture and monoterpene concentrations, as well as between soil moisture and SQT are presented, respectively. It is evident that higher concentrations of monoterpene and SQT are associated with higher soil moisture measured at a depth of 5 and 20 cm. Otter et al. (2002) also reported a more pronounced seasonal pattern for isoprene compared to monoterpene emissions at the Nylsvley Nature Reserve, which is approximately 200 km north-west of Welgegund.
Spearman's correlation coefficients between the BVOCs during the
wet and dry season of the first campaign
As discussed in Sect. 2.2 and indicated in Fig. 2, Welgegund is situated
in a region that has been significantly transformed through cultivation.
Cultivated land within the demarcated 60 km radius (Fig. 2) consists
mainly of maize and, to a lesser degree, sunflower production. These
cultivated lands are also typically characterized by eucalyptus trees, which
have a very high BVOC emission potential (Kesselmeier and Staudt, 1999),
planted on their peripheries as is evident in Fig. 2. The grassland region
in close proximity to Welgegund (south-western to north-eastern sector) has
a high diversity of grass and woody species, as mentioned in Sect. 2.2. In
general, it can be considered that the woody species in the grasslands are
major sources of all the BVOCs measured in this study. Otter et al. (2003) also considered woody vegetation to be the most important in terms of
BVOC emissions in southern Africa. It is generally considered that crops and
grass have very low isoprene-emitting capacities (Kesselmeier and Staudt,
1999; Guenther, 2013). However, Schuh et al. (1997) indicated that
sunflowers emit isoprene; the monoterpenes
The optimum combination of independent variables to include in a
MLR equation to calculate the dependant variable, i.e. BVOC concentrations.
The root mean square error (RMSE) difference between the calculated and
measured concentrations indicated that the inclusion of
In an effort to determine possible sources of BVOC species, concentrations roses were compiled, as presented in Fig. 6. In general, the concentration roses indicated that isoprene concentrations were higher from the western sector (indicated by the average and highest concentrations) that is considered to be a relatively clean regional background region with no large anthropogenic point sources (Fig. 1), while wind direction did not indicate any significant differences in the concentrations of the other BVOC species. On occasion, higher MBO, monoterpene and SQT concentrations were observed from the south-eastern region, which may be attributed to a large eucalyptus plantation approximately 15 km south-east of Welgegund, indicated in Fig. 2. However, high isoprene emissions are also usually associated with eucalyptus trees, which are not observed in the isoprene concentration roses. Therefore, other sources of MBO, monoterpene and SQT in these regions are most likely to be the main sources, which can possibly include the urban footprint indicated in this region. In addition, pine trees are common foreign tree species that are planted on farms in this region (Rouget, 2002), which could be potential sources of MBO and monoterpenes.
The similar concentration roses determined for monoterpenes and SQTs during
the first sampling campaign can be attributed to similar sources of these
species. However, most SQTs have short atmospheric lifetimes (
Floral emissions could also be considered a potential source of monoterpenes in this region, which could also contribute to the relative abundance of monoterpenes compared to the relatively low isoprene concentrations. Floral emissions in this region would typically occur with the onset of the wet season in October up until February. It is well-known that meadows, i.e. grazed grasslands in South Africa, in this region have a significant number of species that flower. South African grasslands are considered to be exceptionally species rich (Siebert, 2011), since it is ancient, primary grasslands, i.e. not man-made (Bond, 2016).
Of particular interest is the potential sources of 4-allylanisole
(estragole) due to its relatively substantial levels as indicated in Table 2. Bouvier-Brown et al. (2009) and Misztal et al. (2010) indicated that
this species could potentially have a significant contribution to regional
atmospheric chemistry due to relatively large estragole emissions measured
from ponderosa pine trees and oil palms, respectively. As mentioned
previously, pine trees are typically found on farms in this region as
intruder tree species (Rouget, 2002), while numerous palm trees are found in
cities/towns surrounding Welgegund (Lubbe et al., 2011). In
addition,
Although a comprehensive vegetation survey has been conducted within a 60 km radius of Welgegund, vegetation types have been identified only generally at this stage, as indicated in Sect. 2.2. Therefore, the predominant woody species in each of the regions surrounding Welgegund associated with specific BVOC emissions have not yet been characterized.
Photochemical properties of measured BVOCs during the first and second campaign at Welgegund.
Spearman's correlation analyses were applied to correlate the measured concentrations of isoprene, MBO, monoterpene and SQT measured to each other in order to substantiate sources of these species. These correlations for the two sampling campaigns are presented in Table 4, with correlations in the wet seasons listed in the lower bottom (not highlighted) and correlations in the dry season presented in the top right (bold highlighted). It is evident that MBO had good correlations with monoterpenes and SQTs in the wet season, as well as with monoterpenes in the dry season during the first sampling campaign. Although not as distinct as during the first sampling campaign, MBO did also correlate with monoterpenes during the wet and dry season, as well as with SQT in the dry season during the second sampling campaign. During the first sampling campaign, monoterpenes had a strong correlation with SQT in the wet season and moderate correlation during the dry season, while strong correlations between monoterpene and SQT were determined in the dry season and a moderate correlation during the wet season during the second sampling campaign. As indicated previously, concentration roses did indicate similar sources of monoterpene and SQT, especially during the first sampling campaign, which is signified by these correlations.
Spearman correlations between BVOCs and other parameters measured at Welgegund did not show significant correlations. However, in certain instances, good correlations were observed between soil moisture and MBO, monoterpene and SQT concentrations. This is expected, since the monthly average concentrations of these species indicated increased levels thereof that were associated with increased soil moisture from February to April 2011. Therefore, in an effort to further statistically explore the data set, explorative MLR (multilinear regression) was performed by using all ancillary measurements as input data in order to indicate parameter interdependencies on the BVOC concentrations measured. In Fig. 7, the root mean square error (RMSE) difference between the calculated and measured BVOC concentrations, as a function of the number of independent variables included in the optimum MLR solution, is presented. It is evident that interdependence between temperature, soil temperatures and PAR yielded the largest decrease in RMSE for isoprene concentrations measured. However, for MBO, monoterpene and SQT, a much more significant contribution from soil moisture is observed to decrease the RMSE differences between calculated and measured BVOC levels. It is also evident that the interdependence between soil moisture and soil temperature at 20 cm is important to estimate MBO, monoterpene and SQT concentrations. Therefore, explorative MLR indicated that temperature had the largest influence on isoprene concentrations, while soil moisture was the most significant for MBO, monoterpene and SQT levels.
It is important to evaluate the significance of BVOCs on their atmospheric
reactivity, since these species are important precursor species in the
photochemical formation of tropospheric O
The OFP of BVOCs was determined by calculating the product of the average
concentration and the maximum incremental reactivity (MIR) coefficient of
each compound, i.e. OFP
Table 5 indicates that, according to the OFP calculated with MIR
coefficients,
In Fig. 8a, the monthly mean reaction rates for the reactions between
O
The annual median concentrations of isoprene, MBO, monoterpene and SQT
determined during two sampling campaigns indicated that the sum of the
concentrations of the monoterpenes was an order of magnitude higher than the
concentrations of other BVOC species, with
Distinct seasonal patterns were observed for MBO during both sampling campaigns, which coincided with wet and warmer months. Although less pronounced, a similar seasonal trend than for MBO was observed for isoprene during the first sampling campaign, while higher isoprene concentrations during the second sampling campaign were associated with higher wind speeds that indicated a distant source region of isoprene. No distinct seasonal pattern was observed for monoterpene and SQT concentrations. However, significantly higher levels of monoterpene and SQT, as well as MBO were measured from February to April 2011 during the first sampling campaign, which were attributed to the considerably higher soil moisture measured at a depth of 20 cm resulting for the wet season preceding the first campaign and is indicative of biogenic emissions from deep-rooted plants.
Woody species in the grassland region were considered to be the main sources of BVOCs measured, while sunflower and maize crops were also considered to be potential sources for BVOCs in this region. Multilinear regression analysis indicated that soil moisture had the most significant impact on atmospheric levels of MBO, monoterpene and SQT concentrations, while temperature had the greatest influence on isoprene levels.
The O
It is important in future work that a comprehensive study on BVOC emissions
from specific plant species in the area surrounding Welgegund must be
performed in order to relate the emission capacities of vegetation types to
the atmospheric BVOCs measured. It is also recommended that the oxidation
products of BVOC species are measured in order to verify distant source
regions of BVOCs measured at Welgegund. In addition, the interactions
between anthropogenic and biogenic VOCs must also be further explored,
together with other ancillary measurements conducted at Welgegund (e.g.
SO
The data of this paper are available upon request to Pieter van Zyl (pieter.vanzyl@nwu.ac.za) or Paul Beukes (paul.beukes@nwu.ac.za).
The authors would like to acknowledge the Finnish Academy (project no. 132640), the University of Helsinki, the Finnish Meteorological Institute, the North-West University and the National Research Foundation (NRF) for financial support. Opinions expressed and conclusions arrived at are those of the authors and are not necessarily to be attributed to the NRF. Assistance with data processing from Rosa Gierens is also acknowledged. Edited by: S. E. Pusede Reviewed by: two anonymous referees