Measurements of biogenic volatile organic compounds at a grazed savannah grassland agricultural landscape in South Africa

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Jaars , K , van Zyl , P G , Beukes , J P , Hellen , H , Vakkari , V , Josipovic , M , Venter , A D , Räsänen , M , Knoetze , L , Cilliers , D P , Siebert , S J , Kulmala , M , Rinne , J , Guenther , A , Laakso , L & Hakola , H 2016 , ' Measurements of biogenic volatile organic compounds at a grazed savannah grassland agricultural landscape in South Africa ' Atmospheric Chemistry and Physics , vol. 16 , no. 24 , pp. 15665-15688 . DOI: 10.5194/acp-16-15665-2016

Title: Measurements of biogenic volatile organic compounds at a grazed savannah grassland agricultural landscape in South Africa
Author: Jaars, Kerneels; van Zyl, Pieter G.; Beukes, Johan P.; Hellen, Heidi; Vakkari, Ville; Josipovic, Micky; Venter, Andrew D.; Räsänen, Matti; Knoetze, Leandra; Cilliers, Dirk P.; Siebert, Stefan J.; Kulmala, Markku; Rinne, Janne; Guenther, Alex; Laakso, Lauri; Hakola, Hannele
Contributor: University of Helsinki, Department
University of Helsinki, Institute for Atmospheric and Earth System Research
Date: 2016-12-20
Language: eng
Number of pages: 24
Belongs to series: Atmospheric Chemistry and Physics
ISSN: 1680-7316
URI: http://hdl.handle.net/10138/175008
Abstract: Biogenic volatile organic compounds (BVOCs) play an important role in the chemistry of the troposphere, especially in the formation of tropospheric ozone (O-3) 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 alpha-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 O-3 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. alpha-Pinene and limonene had the highest reaction rates with O3, whereas isoprene exhibited relatively small contributions to O3 depletion. Limonene, alpha-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.
Subject: ATMOSPHERIC BOUNDARY-LAYER
BOREAL FOREST
MONOTERPENE CONCENTRATIONS
CHEMICAL-COMPOSITION
MASS-SPECTROMETRY
FLUX MEASUREMENTS
TROPICAL FOREST
EMISSIONS
ISOPRENE
VOC
114 Physical sciences
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