measurements of biogenic volatile organic compounds at a grazed savannah grassland agricultural landscape in south africa
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2016
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Abstract
Biogenic volatile organic compounds (BVOCs) play an important role in
the chemistry of the troposphere, especially in the formation of
tropospheric ozone (O3) and secondary organic aerosols (SOA).
Ecosystems produce and emit a large number of BVOCs. It is estimated on a
global scale that approximately 90 % of annual BVOC emissions are from
terrestrial sources. In this study, measurements of BVOCs were conducted at
the Welgegund measurement station (South Africa), which is considered to be
a regionally representative background site situated in savannah grasslands.
Very few BVOC measurements exist for savannah grasslands and results
presented in this study are the most extensive for this type of landscape.
Samples were collected twice a week for 2 h during the daytime and 2 h during the night-time through two long-term sampling campaigns from
February 2011 to February 2012 and from December 2013 to February 2015, respectively.
Individual BVOCs were identified and quantified using a thermal desorption
instrument, which was connected to a gas chromatograph and a mass selective detector.
The annual median concentrations of isoprene, 2-methyl-3-butene-2-ol (MBO),
monoterpene and sesquiterpene (SQT) during the first campaign were 14, 7,
120 and 8 pptv, respectively, and 14, 4, 83 and 4 pptv, respectively, during
the second campaign. The sum of the concentrations of the monoterpenes were
at least an order of magnitude higher than the concentrations of other BVOC
species during both sampling campaigns, with α-pinene being the most
abundant species. The highest BVOC concentrations were observed during the
wet season and elevated soil moisture was associated with increased BVOC
concentrations. However, comparisons with measurements conducted at other
landscapes in southern Africa and the rest of the world that have more woody
vegetation indicated that BVOC concentrations were, in general,
significantly lower for savannah grasslands. Furthermore, BVOC
concentrations were an order of magnitude lower compared to total aromatic
concentrations measured at Welgegund. An analysis of concentrations by wind
direction indicated that isoprene concentrations were higher from the
western sector that is considered to be a relatively clean regional
background region with no large anthropogenic point sources, while wind
direction did not indicate any significant differences in the concentrations
of the other BVOC species. Statistical analysis indicated that soil moisture
had the most significant impact on atmospheric levels of MBO, monoterpene
and SQT concentrations, whereas temperature had the greatest influence on
isoprene levels. The combined O3 formation potentials of all the BVOCs
measured calculated with maximum incremental reactivity (MIR) coefficients during the first and second
campaign were 1162 and 1022 pptv, respectively. α-Pinene and
limonene had the highest reaction rates with O3, whereas isoprene
exhibited relatively small contributions to O3 depletion. Limonene,
α-pinene and terpinolene had the largest contributions to the
OH reactivity of BVOCs measured at Welgegund for all of the months during
both sampling campaigns.
| Reference Key |
jaars2016atmosphericmeasurements
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|---|---|
| Authors | ;K. Jaars;P. G. van Zyl;J. P. Beukes;H. Hellén;V. Vakkari;M. Josipovic;A. D. Venter;M. Räsänen;L. Knoetze;D. P. Cilliers;S. J. Siebert;M. Kulmala;J. Rinne;A. Guenther;L. Laakso;L. Laakso;H. Hakola |
| Journal | Journal of agricultural and food chemistry |
| Year | 2016 |
| DOI |
10.5194/acp-16-15665-2016
|
| URL | |
| Keywords |
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