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

Abstract

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.

Seasonal influences on surface ozone variability in continental South Africa and implications for air quality

“…fossil fuel combustion, biomass burning, as well as the oxidation of methane (CH4) and VOCs 30 (Novelli et al, 1992). Anthropogenic sources of VOCs are largely due to industrial and vehicular emissions , while emissions from vegetation provide the biogenic source (Jaars et al, 2016). Regional biomass burning, which includes household combustion for space heating and cooking, agricultural waste burning and open biomass burning (wild fires), is a Atmos.…”

“…A two-year VOC dataset compiled during two sampling campaigns was available for Welgegund (Jaars et al, 2016;10 Jaars et al, 2014), which was used to calculate the instantaneous production rate of O3 as a function of NO2 levels and VOC reactivity (Geddes et al, 2009;Murphy et al, 2006).…”

“…The station is surrounded by grassland savannah (Jaars et al, 2016). The station can be considered a regionally representative background site with few local anthropogenic sources.…”

“…Air masses arriving at Welgegund from the west reflect a relatively clean regional background. However the site is, similar to the Botsalano station, at times 15 impacted by polluted air masses that are advected over major anthropogenic source regions in the interior of South Africa, which include the western Bushveld Igneous Complex, Johannesburg-Pretoria megacity, the Mpumalanga Highveld and the Vaal Triangle Jaars et al, 2016;Venter et al, 2017). In addition, Welgegund is also affected by regional savannah and grassland fires that are common in the dry season .…”

“…Africa and biogenic VOCs are relatively less abundant (Jaars et al, 2016), to fuel the O3 formation process.…”