Articles | Volume 16, issue 24
Atmos. Chem. Phys., 16, 15665–15688, 2016
Atmos. Chem. Phys., 16, 15665–15688, 2016

Research article 20 Dec 2016

Research article | 20 Dec 2016

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

Kerneels Jaars1, Pieter G. van Zyl1, Johan P. Beukes1, Heidi Hellén2, Ville Vakkari2, Micky Josipovic1, Andrew D. Venter1, Matti Räsänen3, Leandra Knoetze1, Dirk P. Cilliers1, Stefan J. Siebert1, Markku Kulmala3, Janne Rinne4, Alex Guenther5, Lauri Laakso1,2, and Hannele Hakola2 Kerneels Jaars et al.
  • 1Unit for Environmental Sciences and Management, North-West University, Potchefstroom, South Africa
  • 2Finnish Meteorological Institute, PL 503, 00101 Helsinki, Finland
  • 3Department of Physics, University of Helsinki, Helsinki, Finland
  • 4Department of Physical Geography and Ecosystem Science Lund University Sölvegatan 12, 223 62 Lund, Sweden
  • 5Department of Earth System Science, University of California, Irvine, CA, USA

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.

Short summary
Biogenic volatile organic compounds (BVOCs) – important in tropospheric ozone and secondary organic aerosol formation – were measured at a savannah grassland in South Africa. Results presented are the most extensive for this type of landscape. Compared to other parts of the world, monoterpene levels were similar, while very low isoprene levels led to significantly lower total BVOC levels. BVOC levels were an order of magnitude lower compared to anthropogenic VOC levels measured at Welgegund.
Final-revised paper