Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Biogenic volatile organic compounds and formaldehyde from East African ecosystems

Auteur : Liu, Yang

Université de soutenance : University of Helsinki,

Grade : Doctoral dissertation (article-based) / Doctoral Programme in Atmospheric Sciences 2022

Résumé partiel
Biogenic volatile organic compounds (BVOCs) are highly reactive atmospheric compounds, released from vegetation, affecting atmospheric aerosols through oxidation, and contributing to climate change. Global climate change, which is accompanied by frequent extreme climate events, as well as human modification on natural landscapes, are largely changing BVOC emissions and concentrations. These trends pose challenges for BVOC inversions and estimations, particularly in regions where BVOC field measurements are scarce, such as East Africa.

This thesis consists of two major parts : firstly, regions where vegetation has a high contribution on atmospheric formaldehyde (HCHO) levels are identified over Ethiopia and Kenya, and secondly, the atmospheric BVOC mixing ratios in the afore-detected tropical ecosystems in Kenya are measured. For HCHO source partitioning, satellite data observed by Ozone Monitoring Instrument were used, and applied for detecting the HCHO spatial and temporal distributions. The data were decomposed into seasonal and annual signals, and the contribution of vegetation was estimated by the methods of Breaks for Additive Season and Trend, Max Entropy, and multiple regression. The results showed that savannas, forests, and grasses are the main contributors to local HCHO in Kenya, in particular for the northern and eastern areas.

To quantify the atmospheric BVOC mixing ratios in typical African ecosystems in Kenya, a campaign for BVOC measurements was conducted in 15 different sites, including forest, bushland, grassland, agroforestry, and plantation in the highlands and the lowlands. The highest mixing ratio of BVOCs was detected in agroforestry sites followed by montane native forests in the highlands. Fresh grasslands and dense bushland had higher mixing ratios of studied BVOCs than other lowlands ecosystems. A second field campaign was set up in a fenced station surrounded by agroforestry landscape in the highlands, and a cropland surrounded by bushes and savanna in the lowland. Samples were collected with autosamplers during the local rainy and dry seasons. The mixing ratio of monoterpenoids was similar to that measured in the relevant ecosystems in western and southern Africa, while that of isoprene was lower. Estimates of the emission factors (EFs) and emissions for some BVOCs, isoprene, and β-pinene, agreed well with EFs of Model of Emissions of Gases and Aerosols from Nature (MEGAN), while those of α-pinene and limonene were higher.

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