Preprints
https://doi.org/10.5194/acp-2021-445
https://doi.org/10.5194/acp-2021-445

  17 Jun 2021

17 Jun 2021

Review status: this preprint is currently under review for the journal ACP.

Seasonal and diurnal variations of biogenic volatile organic compounds in highland and lowland ecosystems in southern Kenya

Yang Liu1, Simon Schallhart2, Ditte Taipale3, Toni Tykkä2, Matti Räsänen3, Lutz Merbold4,a, Heidi Hellén2, and Petri Pellikka1,3 Yang Liu et al.
  • 1Department of Geosciences and Geography, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 2Finnish Meteorological Institute, PL 503, 00101 Helsinki, Finland
  • 3Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
  • 4Mazingira Centre, International Livestock Research Institute, P.O. Box 30709, 00100 Nairobi, Kenya
  • anow at: Agroscope, Research Division Agroecology and Environment, Reckenholzstrasse 191, 8046, Zurich, Switzerland

Abstract. The East African lowland and highland areas consist of water-limited and humid ecosystems. The magnitude and seasonality of biogenic volatile organic compounds (BVOCs) emissions from these functionally contrasting ecosystems are limited due to a scarcity of direct observations. We measured mixing ratios of BVOCs from two contrasting ecosystems, humid highlands with agroforestry and dry lowlands with bushland, grassland, and agriculture mosaics, during both the rainy and dry seasons of 2019 in southern Kenya. We present the diurnal and seasonal characteristics of BVOC mixing ratios and their reactivity, and estimated emission factors (EFs) for certain BVOCs from the African lowland ecosystem based on field measurements. The most abundant BVOCs were isoprene and monoterpenoids (MTs), with isoprene contributing > 70 % of the total BVOC mixing ratio during daytime, while MTs accounted for > 50 % of the total BVOC mixing ratio during nighttime at both sites. The contributions of BVOCs to the local atmospheric chemistry were estimated by calculating the reactivity towards the hydroxyl radical (OH), ozone (O3), and the nitrate radical (NO3). Isoprene and MTs contributed the most to the reactivity of OH and NO3, while sesquiterpenes dominated the contribution of organic compounds to the reactivity of O3.

The mixing ratio of isoprene measured in this study was lower to that measured in the relevant ecosystems in west and south Africa, while that of monoterpenoids was similar. Isoprene mixing ratios peaked daily between 16:00 and 20:00 with a maximum mixing ratio of 809 parts per trillion by volume (pptv) and 156 pptv in the highlands, and 115 pptv and 25 pptv in the lowlands, during the rainy and dry seasons, respectively. MT mixing ratios reached their daily maximum between midnight and early morning (usually 04:00 to 08:00) with mixing ratios of 254 pptv and 56 pptv in the highlands, and 89 pptv and 7 pptv in the lowlands, in the rainy and dry seasons, respectively. The dominant species within the MT group were limonene, α-pinene, and β-pinene.

EFs for isoprene, MTs, and 2-methyl-3-buten-2-ol (MBO) were estimated using an inverse modeling approach. The estimated EFs for isoprene and β-pinene agreed very well with what is currently assumed in the world’s most extensively used biogenic emissions model, the Model of Emissions of Gases and Aerosols from Nature (MEGAN), for warm C4 grass, but the estimated EFs for MBO, α-pinene, and especially limonene, were significantly higher than that assumed in MEGAN for the relevant plant functional type. Additionally, our results indicate that the EF for limonene might be seasonally dependent in savanna ecosystems.

Yang Liu et al.

Status: open (until 01 Aug 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-445', Anonymous Referee #1, 16 Jul 2021 reply

Yang Liu et al.

Yang Liu et al.

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Short summary
We studied the mixing ratio of biogenic volatile organic compounds (BVOCs) in a humid highland and dry lowland African ecosystem in Kenya. The mixing ratio of monoterpenoids was similar to that measured in the relevant ecosystems in west and south Africa, while that of isoprene was lower. Modeling the emission factors (EFs) for BVOCs from the lowlands, the EFs for isoprene and β-pinene agreed well with what is assumed in the MEGAN, while those of α-pinene and limonene were higher.
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